Organic Chemistry Laboratory
Building A Toolset
For
The Identification of Organic Compounds
Physical Properties
Melting Point
Boiling Point
Density
Solubility
Refractive Index
3/10/2016
Chemical Tests
Hydrocarbons
Alkanes
Alkenes
Alkynes
Halides
Alcohols
Aldehydes
Ketones
Spectroscopy
Mass
(Molecular Weight)
Ultraviolet
(Conjugation, Carbonyl)
Infrared
Functional Groups
NMR
(Number, Type, Location
of protons)
Gas Chromatography
(Identity, Mole %)
1
Spectroscopy
The Absorption of Electromagnetic
Radiation and the use of the Resulting
Absorption Spectra to Study the
Structure of Organic Molecules
3/10/2016
2
Spectroscopy
Spectroscopy Types:

Mass Spectrometry (MS) – Hi-Energy Electron-Beam
Bombardment
Use – Molecular Weight, Presence of Nitrogen, Halogens

Ultraviolet Spectroscopy (UV) – Electronic Energy States
Use – Conjugated Molecules; Carbonyl Group, Nitro Group

Infrared Spectroscopy (IR) – Vibrational & Rotational
Movements
Use – Functional Groups; Compound Structure

Nuclear Magnetic Resonance (NMR) – Magnetic Properties
of Nuclei
3/10/2016
Use – The number, type, and relative position of protons
(Hydrogen nuclei) and Carbon-13 nuclei
3
The Electromagnetic Spectrum
High
Frequency ()
Low
High
Energy (E)
Low
Short
Wavelength ()
Long
1.2 x 1014 Hz
Frequency
3 x 1019 Hz
3 x 1016 Hz
1.5 x 1015 Hz
3 x 108 Hz
2 x 1013 Hz
3 x 1011 Hz
1 x 109 Hz
6 x 107 Hz
4 x103cm-1
1.25 x104cm-1
Wave Number
2.5 x104cm-1
1 x109cm-1 1 x107cm-1
Cosmic
&
 Ray
X-Ray
0.01 nm
Wavelength
5 x104cm-1
667cm-1
Vacuum
UV
10 cm-1
Infrared
200 nm
400 nm
800 nm
Visible
Blue
Red
2.5 
3 cm-1
Microwave
1 mm
10 nm
Near
Ultraviolet
3/10/2016
0.002 cm-1
0.01 cm-1
Radio
30 cm
1m
Frequency
5m
15 
Vibrational
Infrared
Nuclear
Magnetic
Resonance
4
NMR
Nuclear Magnetic Resonance Spectroscopy
NMR
3/10/2016
5
NMR

3/10/2016
Nuclear Magnetic Resonance Spectroscopy (NMR)

Nuclear Spin

Nuclear Spin State

Magnetic Moments

Quantized Absorption of Radio Waves

Resonance

Chemical Shift

Chemical Equivalence

Integrals (Signal Areas)

Chemical Shift - Electronegativity Effects

Chemical Shift - Anisotropy (non-uniform) effects of pi
bonds

Spin-Spin Splitting
6
NMR

NMR




3/10/2016
NMR is an instrumental technique to determine the
number, type, and relative positions of certain Nuclei in a
molecule
NMR is concerned with the magnetic properties of these
nuclei
Many Nuclei types can be studied by NMR, but the two
most common nuclei that we will focus on are Protons
(1H1) and Carbon-13 (13C6)
The magnetic properties of NMR suitable nuclei include:
 Nuclear Magnetic Moments
 Spin Quantum Number (I)
 Nuclear Spin States
 Externally Applied Magnetic Field
 Frequency of Angular Precession
 Absorption of Radio Wave Radiation - Resonance
7
NMR

3/10/2016
The Magnetic Properties
 Many atomic nuclei have a property called “Spin”
 Since all nuclei have a charge (from the protons in the
nucleus), a spinning nuclei behaves as if it were a tiny
magnet, generating its own Magnetic field
 The Magnetic Field of such nuclei has the following
properties – Magnetic Dipole, Magnetic Moment and
Quantized Spin Angular Momentum
 The Magnetic Moment (μ) of a nuclei is a function of its
Charge and Spin and is defined as the product of the pole
strength and the distance between the poles
 Only Nuclei with Mass & Atomic number combinations of
Odd/Odd, Odd/Even, Even/Odd possess “Spin Properties,”
which are applicable to NMR
 Note: Nuclei with a Mass & Atomic number combination of
Even/Even do not have “Spin” and are not useful for
NMR
8
NMR

3/10/2016
Nuclear Spin States
 Nuclei with spin (Magnetic Moment, Quantized Spin
Angular Momentum, Magnetic Dipole) have a certain
number of “Spin States.”
 The number of “Spin States” a nuclei can have is
determined by its “Spin Quantum Number I,” a physical
constant, which is an intrinsic (inherent) property of a
spinning charged particle.
 The Spin Quantum Number (I) is a non-negative integer
or half-integer (0, 1/2, 1, 3/2, 2, etc.).
 The Spin Quantum Number value for a given nuclei is
associated with the Mass Number and Atomic Number of
the nuclei.
 Odd Mass / Odd Atomic No - 1/2, 3/2, 5/2 Spin
 Odd Mass / Even Atomic No - 1/2, 3/2, 5/2 Spin
 Even Mass / Even Atomic No - Zero (0) Spin
 Even Mass / Odd Atomic No - Integral (1, 2, 3) Spin
9
NMR

Nuclear Spin States (Con’t)
The
number of allowed Spin States for a nuclei is:
2I + 1
with integral differences ranging from +I to -I
Ex. For I = 5/2
2I + 1 = 2 * 5/2 + 1 = 5 + 1 = 6
Thus, Spin State Values = 5/2, 3/2, 1/2, -1/2, -3/2, -5/2

The Spin Quantum number (I) for either a Proton (1H1)
or a Carbon-13 (13C6) nuclei is 1/2

Thus, the number of Spin States allowed for either a
Proton (1H1) or a Carbon-13 (13C6) nuclei is:
[2 * ½ + 1 = 1 + 1 = 2]

Therefore, the two spins states for either nuclei are:
+ 1/2 & - 1/2
3/10/2016
10
NMR

Nuclear Spin States (Con’t)

In the absence of an applied Magnetic field, all the spin
states ( + ½ & - ½ ) of a given nuclei are of
equivalent energy (degenerate), equally populated, and
the spin vectors are randomly oriented

When an external Magnetic Field is applied, the
degenerate spin states are split into two opposing states
of unequal energy

3/10/2016

+ 1/2 spin state of the nuclei is aligned with the
applied magnetic field and is in a lower energy state

- 1/2 spine state of the nuclei is opposed to the
applied magnetic field and is in a higher energy state
There is a slight majority of the lower energy (+1/2)
nuclei
11
NMR

Two Allowed Spin States for a Proton
Spin +1/2
Aligned
Spin -1/2
Opposed
Ho
- 1/2 Opposed to Field
E
Direction of an
Externally Applied
Magnetic Field (Ho)
+1/2 Aligned
E
+ 1/2 Aligned with Field
No
Field
Externally Applied
Magnetic Field Ho
Eabsorbed
-1/2 Opposed
Ho
Alignments
= (E-1/2 state - E+1/2 state)
= h
E = f(Ho)
3/10/2016
The stronger the applied magnetic field (Ho),
the greater the energy difference between the spin states
12
NMR

3/10/2016
Applied Magnetic Field, Frequency of Angular Precession

Under the influence of an externally applied magnetic field,
Nuclei with “Spin Properties,” such as Protons & Carbon-13,
begin to Precess about the axis of spin with Angular
Frequency ω, similar to a toy top

The Frequency which a proton precesses is directly
proportional to the strength of the applied magnetic field

For a proton in a magnetic field of 14,100 gauss (1.41 Tesla),
the Frequency of Precession is approximately 60 MHz

That same proton, in a magnetic field of 23,500 gauss (2.35
Tesla), will have a Frequency of Precession of approximately
100 MHz

The stronger the applied magnetic field, the higher the
Frequency of Precession and the greater energy difference
between the +1/2 and -1/2 spin states
13
NMR

3/10/2016
NMR Spectrometers

NMR spectrometers are rated according to the frequency,
in MHz, at which a proton precesses - 60 MHz, 100 MHz,
300 MHz, 600 MHz, or even higher.

Continuous Wave (CW) NMR instruments are set up so
that the externally applied magnetic field strength is held
constant while a RF oscillator subjects the sample to the
full range of Radio Wave frequencies at which protons
(or C-13 nuclei) resonate.

In Fourier Transform (FT) NMR instruments, the RF
oscillator frequency is held constant and the externally
applied magnetic field strength is changed.

Most NMR instruments today are of the Continuous Wave
type
14
NMR
Typically, Continuous Wave (CV) Spectrometers are used in
which the externally applied magnetic field is held constant
and RF Radio Oscillator applies a full range of frequencies
at which protons or C-13 nuclei resonate
3/10/2016
15
NMR

Energy Absorption, Resonance

3/10/2016
If long wave radio radiation (1-5 m) is applied from a RF
Oscillator to a sample under the influence of a strong
externally applied magnetic field, and the frequency of the
oscillating electric field component of the incoming
radiation matches the Angular Frequency of Precession of
the nuclei, the two fields couple and energy is transferred
from the incoming radiation to the protons

This causes the nuclei with +1/2 spin state to absorb
energy and change to the -1/2 spin state

When Energy is absorbed at specific frequencies it is
referred to as being “Quantized”

When a proton absorbs a radio wave, whose frequency
matches its Angular Frequency of Precession, it is said
to be in “Resonance” with the incoming signal
16
NMR

3/10/2016
Electron Density, Frequency of Angular Precession

Protons exist in a variety of chemical and magnetic
environments, each represented by a unique electron
density configuration

Under the influence of a strong externally applied
magnetic field, the electrons around the proton are
induced to circulate, generating a secondary magnetic
field (local diamagnetic current), which acts in
opposition (diamagnetically) to the applied magnetic
field

This secondary field shields the proton (diamagnetic
shielding or diamagnetic anisotropy) from the influence
of the applied magnetic field

Recall from slide # 13 that the Angular Frequency of
Precession is directly proportional to the applied
Magnetic Field strength
17
NMR

3/10/2016
Electron Density, Frequency of Angular Precession (Con’t)

As the shielding of the proton increases (increased
electron density) it diminishes the net applied magnetic
field strength reaching the proton; thus the Angular
Frequency of Precession is lower

If the electron density decreases, more of the applied
magnetic field strength impacts the proton and it will
precess at a higher Angular Frequency

Thus, each proton with a unique electron density
configuration will “Resonate” at a unique “Frequency of
Angular Precession

In a 60 MHz NMR Spectrometer all protons will resonate
at a magnetic field strength of approximately 60 MHz,
but each unique proton will resonate at its own unique
frequency, with differences among unique protons of
only tens of Hertz in a field of 60 MHz
18
NMR

NMR Spectra – Fourier Transform vs. Continuous Wave

3/10/2016
Fourier Transform

In a Fourier Transform (FT) NMR, the spectrum
produced is a plot of the magnetic field strength –
representing the frequency of the resonance signal
– on the X-axis – versus the intensity of the
absorption on the Y-Axis.

Each signal – consisting of one or more peaks –
represents the “Resonance Frequency” of a
particular type of proton with a unique chemical &
magnetic (electron density) environment.
19
NMR

NMR Spectra – Fourier Transform vs. Continuous Wave

3/10/2016
Fourier Transform (Con’t)

As the pen of the recorder moves from left to right,
the value recorded on X-axis of the NMR spectrum
represents small increments of increasing magnetic
field strength.

The right side of the NMR Spectrum is referred to
as being “Upfield” (higher magnetic field strength).

The left side of the NMR Spectrum is referred to as
being “Downfield” (lower magnetic field strength).
20
NMR

NMR Spectra – Fourier Transform vs. Continuous Wave
(Con’t)

3/10/2016
Continuous Wave

In a Continuous Wave NMR, the spectrum produced
is a plot of the RF Radio Oscillator Frequency
versus the intensity of the absorption on the Y-Axis.

As before, each signal – consisting of one or more
peaks – represents the “Resonance Frequency” of a
particular type of proton with a unique chemical &
magnetic (electron density) environment.

As the pen of the recorder moves from left to right,
the value recorded on X-axis of the NMR spectrum
represents a decreasing RF Oscillator Frequency
(Resonance Frequency)
21
NMR

NMR Spectra – Fourier Transform vs. Continuous Wave
(Con’t)

3/10/2016
Continuous Wave (Con’t)

The Signals on the right side of the NMR Spectrum
represent protons (C-13 nuclei) that Resonate at
lower frequencies.

The Signals on the left side of the NMR Spectrum
represent protons (C-13 nuclei) that Resonate at
higher frequencies.
22
NMR

NMR Spectra – FT or CW: the spectrum looks the same

A FT or CW spectrometer will produce the same spectrum.

The peaks on the right side of the spectrum represent
those protons (or C-13 nuclei) that resonate at the highest
externally applied magnetic field strength and the lowest
frequency

This statement would appear to be in conflict with the
statement on Slide #13:
“The Frequency which a proton Precesses is directly
proportional to the strength of the applied magnetic field”
3/10/2016

This apparent conflict is resolved by consideration of the
influence of the secondary magnetic field set up by the
Diamagnetic Current from circulating valence electrons.

This magnetic field opposes the externally applied field
reducing the effect of the applied Magnetic Field on the
proton, which in turn lowers the Resonance Frequency
23
NMR

3/10/2016
NMR Spectra – FT or CW: the spectrum looks the same (Con’t)

The protons that resonate and produce signals on the right
side of the NMR Spectrum (up field) have higher electron
density shields than protons that resonate downfield

The net effect of the difference between the externally
applied magnetic field and the amount prevented from
actually reaching the proton results in a significantly reduced
Resonance Frequency

As the NMR spectrum moves from right to left, the electron
density about the various proton environments is
decreasing, resulting in more of the externally applied
magnetic field getting through to the proton

As this net magnetic force is increasing downfield toward
the left side of the spectrum, the Resonance Frequency
increases in conformance with the statement on Slide #13
24
NMR

NMR Spectra – Background Summary
 In a continuous Wave NMR, the
strength of the externally applied
magnetic field is held “constant”.
Signal
TMS
13
PPM
0
Applied Magnetic Field Strength – Ho is held constant
Applied Radio Frequency - RF
Shielding of Proton by Valence Electrons
Diamagnetic (Anisotropic) Magnetic Field Strength
Produced by Circulation of Valence Electrons
Net Magnetic Field Impacting Proton
Frequency of Angular Precession
(Resonance Frequency)
3/10/2016
 Protons that produce signals on the
right side of the NMR spectrum
have a higher amount of valence
electron shielding.
 The Magnetic Field produced by
circulating valence electrons
(Diamagnetic Current) opposes the
externally applied Magnetic Field.
 The Diamagnetic Field diminishes
the amount of Applied Magnetic
Field reaching the proton.
 The net amount of magnetic force
impacting the proton is reduced
resulting in a lower Resonance
Frequency.
 As the Electron Density about a
proton decreases downfield, the
Resonance Frequency increases
because more of the applied
Magnetic Field impacts the Proton.
25
NMR

3/10/2016
NMR Spectra – The Chemical Shift

The differences in the applied Magnetic Field strength
(Angular Frequency of Precession) at which the
various proton configurations in a molecule Resonate
are extremely small

The differences amount to only a few Hz (parts per
million) in a magnetic field strength of 60, 100, 300,
.... MHz (megahertz)

It is difficult to make direct precise measurements of
resonance signals in the parts per million range
26
NMR

3/10/2016
NMR Spectra – The Chemical Shift

The typical technique is to measure the difference
between the Resonance signals of various sample
nuclei and the Resonance signal of a standard
reference sample (see slides 25 & 26)

A parameter, called the “Chemical Shift” (), is
computed from the observed frequency shift
difference (in Hz) of the sample and the “standard
resonance signal” divided by the applied Magnetic
Field rating of the NMR Spectrometer (in MHz)

Thus, the Chemical Shift () is field-independent of
the Magnetic Field rating of the instrument
27
NMR

NMR Spectra – The Chemical Shift (Con’t)

The Chemical Shift is reported in units of:
“Parts Per Million” (ppm)
Chemical Shift =
Observed Shift from TMS(Hz)
Hz
=
= PPM
60 MHz
MHz
Ex: If a proton resonance was shifted downfield 100 Hz relative to
the standard in a 60 MHz machine, the chemical shift would be:
 = 100 Hz / 60 MHz = 1.7 ppm
3/10/2016

By convention, the Proton Chemical Shift values increase from
right to left, with a range of 0 – 13

In other words: Chemical Shift values decrease with increasing
Magnetic field strength or Chemical Shift values increase with
increasing Resonance frequency!
28
NMR

3/10/2016
NMR Spectra – The Internal Reference Standard

The universally accepted standard used in NMR is:
Tetramethylsilane (TMS)

The 12 protons on the four carbon atoms have the same
chemical and magnetic environment and they resonate at
the same field strength, i.e., one signal (1 peak) is
produced

The protons are highly protected from the applied
magnetic field because of high valence electron density

The strength of the Diamagnetic Field generated by the
valence electrons in TMS is greater than most other
organic compounds
29
NMR

3/10/2016
NMR Spectra – The Internal Reference Standard

Thus, little of the applied magnetic field gets through to
the TMS protons reducing the Frequency of Angular
Precession (Resonance Frequency) to a value that is
lower than most other organic compounds.

For most all other Proton environments, the electron
density is less than TMS and slightly more of the applied
magnetic field gets through to the protons resulting in a
slightly higher frequencies of Angular Precession.
30
NMR

NMR Spectra – The Internal Reference Standard

The TMS signal appears on the far right hand side of
the X-axis.

Small amount TMS in the sample produces large signal

By definition, the Chemical Shift value for TMS is
“0 ppm”

3/10/2016
Thus, most all other protons will have Chemical Shifts
> 0 and will be downfield from the TMS signal.
31
NMR

NMR Spectra – Simple Example
(6)
Ethane
Signal
(singlet)
TMS
13 12 11 10 9 8 7 6 5 4 3 2 1 0
Chemical Shift  (PPM)
Typical location (1 ppm) of resonance signal for Methyl
group protons not under the influence of an electronegative
group (see slide )
Note the “6” at the top of the signal
This is the peak integration value and represents the
electronically integrated area under the signal curve and is
proportional to the number of Protons generating the signal,
i.e., Ethane has 6 chemically and magnetically equivalent
protons
See slides 36-39 for a discussion of signal integration
3/10/2016
All six protons of Ethane are
chemically and magnetically
equivalent and all resonate at
the same frequency producing
one signal consisting of “one”
peak, i.e., a singlet.
An NMR Signal can consist of
one or more “peaks”
Multiple peaks are produced by
a phenomenon called “spin-spin
splitting”
Note – See slides 53-62 for a
discussion of Spin/Spin
Splitting
For the chemically equivalent
protons in Ethane there is no
splitting, thus the signal consists
of “one” peak, a singlet
See next slide for more NMR
Spectrum examples, showing
basic splitting patterns
32
NMR

NMR – Simple Examples
The Six (6) equivalent Methyl Protons are
represented as a Triplet at about 1 ppm.
The 3 Triplet peaks are produced by Spin-Spin
splitting based on the 2 protons attached to
the Methylene Group (n + 1 rule).
Propane
13
12
11
(2)
The Two (2) equivalent Methylene Protons
are represented as a Septet at about 2 ppm.
The 7 Septet peaks are produced by spin-spin
splitting based on the 6 protons attached to
the two Methyl groups (6 +1 = 7).
10
9
8
7
6
5
Chemical Shift  (PPM)
4
TMS
3
13
3/10/2016
12
5 unsubstituted Protons
on Benzene ring are not
equivalent, producing
complex spitting
patterns typical of the
resonance structures in
aromatic rings.
See slides 60-65.
11
10
9
8
2
1
0
(3)
(5)
Toluene
(6)
3 equivalent Protons on
Methyl Group Carbon
attached to a Benzene
ring Carbon that has no
attached protons.
Therefore, the signal is a
singlet with no splitting.
The Methyl group donates
electrons to Benzene ring
activating it. The Methyl
protons have less electron
density (deshielded), thus,
the Chemical Shift is moved
downfield.
7
6
5
4
3
2
1
0
33
NMR

Chemical Equivalence
 Protons in a molecule that are in chemically identical
environments will often show the same chemical shift
 Protons with the same chemical shift are chemically equivalent
 Chemical equivalence can be evaluated through symmetry
 Protons in different chemical environments have different
chemical shifts, i.e. a signal is produced for each.
Chemicals giving rise to 1 NMR signal
Chemicals giving rise to 2 NMR signals
H
CH3
H
H
H
H
H
H
O
H
H
CH3 C O CH3
H
Benzene
Cyclopentane
Methyl Acetate
O
C
CH3
3/10/2016
CH3
1,4 dimethyl benzene
(p-xylene)
CH3 O CH2Cl
CH3
Acetone
1-Chloro Methyl Ether
34
NMR

An Isomer Example – C5H12O
(a)
CH3
(a)
CH3
(b)
Signals

Value
Rel Area
of Signal
a
~1
9
b
c
>2
~2
2
1
(c)
C CH2OH
(a) CH3
2-Dimethyl Propanol
a
9
b
2
c
1
3
tms
2
0.9
0
9 protons on 3 Methyl groups are equivalent and are not under the influence of the
electronegative OH group.
2 protons on Methylene group are equivalent and are influenced by electronegative OH group.
The proton on OH group is concentration and hydrogen bonding dependent. Location on
spectrum variable.
Note: All signals are “singlets”, i.e., no adjacent protons to produce spin-spine asplitting.
Signals
(a)
CH3
(a)
CH3 C
O
(b)
CH3
(a)CH3
t-Butyl Methyl Ether

Value
9
Rel Area
of Signal
a
~1
9
b
>>1
3
b
3
tms
3
2
1
0
9 protons on 3 Methyl groups are equivalent and not under the influence of electronegative group.
3 protons on single Methyl groups are equivalent and are under influence of electronegative oxygen.
3/10/2016
35
NMR

Integrals (Signal Area)

An NMR spectrum also provides means of determining How
Many of each type of proton the molecule contains.

The Area under each signal is proportional to the number
of protons generating that signal.

In the Phenylacetone example below there are three (3)
chemically distinct types of protons:
Aryl (7.2 ppm), Benzyl (3.6 ppm), Methyl (2.1 ppm)

The three signals in the NMR spectrum would have Relative
Areas in the ratio of 5:2:3.

Thus, 5 Aryl protons, 2 Benzyl protons, and 3 Methyl
protons
Phenylacetone
7.2 ppm
(5 protons)
3/10/2016
3.6 ppm
(2 protons)
2.1 ppm
(3 protons)
36
NMR

NMR Spectrum – Phenylacetone (103-79-7)
Ring
5 Protons
Methyl
3 Protons
Methylene
2 Protons
C9H10O
3/10/2016
37
NMR

Integrals (Signal Area) (Con’t)





3/10/2016
NMR Spectrometer electronically integrates the area under a
signal and then traces rising vertical lines over each peak by an
amount proportional to the area under the signal – see next
slide.
The heights of vertical lines give RELATIVE numbers of each type
of hydrogen.
Integrals do not always correspond to the exact number of
protons,
e.g., integrals of 2:1 might be 2H:1H or 4H:2H or...
Computation
 Draw Horizontal lines separating the adjacent signals.
 Measure vertical distance between the Horizontal lines.
 Divide each value by the smallest value.
 Multiple each value by an integer >1 to obtain whole
numbers.
See example computation on next Slide.
38
NMR

Integrals (Signal Area) (Con’t)

NMR Spectrum – Benzylacetate (C9H10O2)
Peak 7.3 ppm (c) - (h1) 55.5 Div
Peak 5.1 ppm (b) - (h2) 22.0 Div
(c)
(b)
Peak 2.0 ppm (a) - (h3) 32.5 Div
(a)
h3
h2
55.5 div
= 2.52
22.0 div
22.0 div = 1.00
22.0 div
h1
32.5 div
= 1.48
22.0 div
2.52 : 1.00 : 1.48
5 :
2 :
3
c : b :
a
Each value multiplied by “2”
to obtain integral values
3/10/2016
39
NMR

Chemical Shift – Impact of Electronic Density

3/10/2016
Valence Electrons

In the presence of the applied magnetic field, the
valence electrons in the vicinity of the proton are
induced to circulate (Local Diamagnetic Current)
producing a small secondary magnetic field

The greater the electron density circulating about the
nuclei, the greater the induced magnetic shielding
effect

The induced magnetic field acts in opposition
(diamagnetically opposed) to the applied magnetic
field, thus shielding the proton from the effects of the
applied field in a phenomenon called Local
Diamagnetic Shielding or Diamagnetic Anisotropy

As the Diamagnetic Anisotropy increases, the amount
of the applied magnetic field reaching the proton is
diminished, decreasing the frequency of Resonance
40
NMR

3/10/2016
Chemical Shift - Anisotropy (non-uniformity)

For some proton types, the chemical shifts can be
complicated by the type of bond present

Aryl compounds (benzene rings), Alkenes (C=),
Alkynes (C ), and Aldehydes (O=CH) show
anomalous resonance effects caused by the presence
of  electrons in these structures

The movement of these electrons about the proton
generate secondary non-uniform (anisotropic)
magnetic fields

The relative shielding and deshielding of protons in
groups with  electrons is dependent on the
orientation of the molecule with respect to the applied
magnetic field
41
NMR

Chemical Shift - Anisotropy (non-uniformity) (Con’t)

The Diamagnetic Anisotropic effect diminishes with
distance

In most cases, the effect of the Diamagnetic
Anisotropic effect is to Deshield the protons,
increasing the Chemical Shift

In some cases, such as acetylene hydrogens, the
effect of the anisotropic field is to shield the
hydrogens, decreasing the Chemical Shift

3/10/2016
In a Benzene ring , the  electrons are induced to
circulate around the ring by the applied magnetic
field, creating a ring current, which in turn produces a
magnetic field further influencing the shielding of the
ring protons
42
NMR

Chemical Shift - Anisotropy (non-uniformity) (Con’t)
 The presence of ring current causes the applied
magnetic field to become non-uniform (diamagnetic
anisotropy) in the vicinity of the benzene ring
 The effect of the anisotropic field is to further deshield
the benzene protons, increasing the chemical shift
 Thus, protons attached to the benzene ring are
influenced by three (3) magnetic fields:

Strong Applied Magnetic Field

Local Diamagnetic Shielding by Valence Electrons

Anisotropic Effect from the Ring Current
 The net effect of the deshielding of the Benzene Ring
protons is to increase the Chemical Shift far downfield
to about 7.0 ppm
3/10/2016
43
NMR

3/10/2016
Electron Density and Electronegativity

Protons in a molecule exist in many different electronic
environments (Methyl group (CH3), Methylene group
(CH2),  bonds, unsubstituted Benzene ring Protons,
Amino protons (NH), Hydroxyl protons (OH), etc.)

Each proton with a unique electron density configuration
will have a unique Angular Frequency of Precession

The electron density of a given proton and thus, the
frequency of precession, can be further influenced by
the presence of electronegative groups in the vicinity of
the proton

Electronegative groups (or elements) are electron
withdrawing, pulling electron density away from the proton
44
NMR

3/10/2016
Chemical Shift – Impact of Electronegative Elements

The decrease in electron density about the proton
results in a lower secondary magnetic field, a
diminished shielding effect, an increase in the
strength of the applied magnetic field reaching the
nuclei, resulting in an increase in the precession
frequency

Electronegative elements are electron withdrawing

When added to a carbon atom with protons attached,
the Electronegative element withdraws electron
density about the proton

Reducing electron density deshields the proton from
the effect of the applied field, allowing more of the
magnetic field to impact the proton
45
NMR
Chemical Shift – Impact of Electronegative Elements
(Con’t)
3/10/2016

Recall that Deshielding the proton increases the
Resonance Frequency producing a greater chemical
shift, i.e., the resonance peak is moved downfield to
the left on the spectrum

The chemical shift increases as the electronegativity
of the attached element increases

Multiple substitutions have a stronger effect than a
single substitution

Electronegativity also affects the Chemical Shift of
Protons further down the chain. But the effect is
diminished as distance from the Electronegative
Element increases
46
NMR

Chemical Shift – Impact of Electronegative Elements
Compound CH3X
CH3F CH3OH CH3Cl CH3Br
Element X
F
O
Cl
Br
Electronegativity of X 4.0
3.5
3.1
2.8
Chemical Shift (ppm) 4.26
3.40
3.05
2.68
3.59 ppm
CH4
0.23 ppm
CH3OH
3.39 ppm
CH3CH2OH
1.18 ppm
CH3I
I
2.5
2.16
CH4 (CH3)4Si
H
Si
2.1
1.8
0.23 0
1.53 ppm
CH3CH2CH2OH
0.93 ppm
3.49 ppm
Note: The Chemical Shift of the Proton increases as the
distance from the Electronegative Oxygen increases.
3/10/2016
47
NMR
Chemical Shift values of typical
Proton environments and the effects
of Electronegative Elements on the
Chemical Shift.
-OH, -NH
TMS
12
11
10
9
8
7
6
5
H
CHCl3
Groups with variable chemical shifts
(Protons attached to elements other than Carbon)
Acids
Aldehydes
Phenols
Alcohols
Amines
Amides
Enols
3/10/2016
RCOOH
RCOH
ArOH
ROH
RNH2
RCONH2
CH=CH-OH
11.0
9.0
4.0
0.5
0.5
5.0
15.0
- 12.0 ppm
- 10.0
- 7.0
- 5.0
- 5.0
- 8.0
4
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO3
3
2
CH2Ar
CH2NR2
CH2S
C C H
CH2 C
1
0  (ppm)
C CH C Methine (1H)
C
C CH2 C Methylene (2H)
C CH3
Methyl (3H)
O
CH2
Effects of Electronegativity
F > O > Cl = N > Br > S > I
Electronegative Elements will pull electron density away
from the proton diminishing the electron density. Proton is
exposed to increased effects of the applied magnetic field,
which increases the frequency of absorbance (Chemical Shift)
moving the Resonance Signal downfield to the left.
48
NMR

General Regions of Chemical Shifts
Aliphatic Alicyclic (CH2, CH3)
-Substituted Aliphatic
Alkyne
-Monosubstituted Aliphatic
-Disubstituted Aliphatic
Alkene
Aromatic & Heteroaromatic
Carboxylic
12
3/10/2016
11
Aldehydic
TMS
10
9
8
7
6
5
4
3
2
1
0  (ppm)
49
NMR
Approximate Chemical Shifts
Protons (1H1)
Type of Proton
Chemical Shift,  (ppm)
1o Alkyl, RCH3
2o Alkyl, RCH2R
3o Alkyl, R3CH
Allylic, R2C C CH3
R
Ketone, RCCH3
O
Benzylic, ArCH2-R
Acetylenic, RC CH
Alkyl Iodide, RCH2I
Ether, ROCH2R
Alcohol, HOCH2R
Alkyl Bromide, RCH2Br
Alkyl Chloride, RCH2Cl
Vinylic, RC2 CH2
Vinylic, RC2 CH2-R
Aromatic, ArH
Aldehyde, RCH
O
Alcohol hydroxyl, ROH
Amino, R NH2
Phenolic, ArOH
Carboxylic, RCOOH
3/10/2016
Type of Carbon Atom
Chemical Shift ,  (ppm)
1o Alkyl, RCH3
2o Alkyl, RCH2R
3o Alkyl, RCHR2
Alkyl Halide or Amine, C-X
Alcohol or Ether, C-O
Alkyne, C
Alkene, C =
0.8 - 1.0
1.2 - 1.4
1.4 - 1.7
1.6 - 1.9
2.1 - 2.6
2.2 - 2.5
2.5 - 3.1
3.1 - 3.3
3.3 - 3.9
3.3 - 4.0
3.4 - 3.6
3.6 - 3.8
4.6 - 5.0
5.2 - 5.7
6.0 - 9.5
9.0 - 10.0
0.5 - 6.0a
1.0 - 5.0a
4.5 - 7.7a
11 - 12a
Carbon (13C6)
Aryl,
Ar-
C-
Nitriles, -C N O
Amides, -C - N O
Carboxylic Acids, Esters, C O
O
Aldehydes, Ketones, -C -
a
0 - 40
10 - 50
15 - 50
10 - 65
50 - 90
60 - 90
100 - 170
100 - 170
120 - 130
150 - 180
160 - 185
182 - 215
Chemical shifts of these protons
vary in different solvents and
with temperature
50
NMR
Functional
Group
Chemical
Shift, ppm
TMS (CH3)4Si
0
Cyclopropane
0 - 1.0
Alkanes
RCH3
R2CH2
R3CH
0.9
1.3
1.5
Alkenes
C=C–H
C = C – CH3
4.6 – 5.9
1.5 – 2.5
Alkynes
CC–H
C  C – CH3
1.7 – 2.7
1.6 – 2.6
3/10/2016
Functional
Group
Aromatic
AR – H
AR – C –H (benzyl)
Fluorides
F–C–H
Chlorides
Cl – C – H
Cl
Cl – C – H
Chemical
Shift, ppm
6.5 – 8.0
2.3 – 2.7
4.2 – 4.8
3.1 – 4.1
5.8
Bromides
Br – C – H
2.5 – 4.0
Iodides
I–C–H
2.0 – 4.0
Nitroalkanes
O2N – C – H
4.2 – 4.6
51
NMR
Functional
Group
Chemical
Shift, ppm
Alcohols
Functional
Group
Chemical
Shift, ppm
Carboxylic Acids
H–C–O–H
3.4 – 4
R–O–H
0.5 – 5.0
Phenols
O
H–O–C–C–H
2.1 – 2.6
O
Ar – O – H
4.0 – 7.0
Amines
R – NH2
0.5 – 4.0
Ethers
R –C–O–H
11.0 – 12.0
Ketones
O
R–O–C-H
3.2 – 3.8
R–C–C–H
2.1 – 2.4
Acetals
R–O
R
C
R–O
Aldehydes
5.3
R–C–H
H
Esters
9.0 – 10.0
Amides
O
R–O–C–C–H
3/10/2016
O
O
3.5 – 4.8
R–C–N–H
5.0 – 9.0
52
NMR

Spin – Spin Splitting
In addition to the Chemical Shift and Signal Area, the
NMR spectrum can provide information about the
number of the protons attached to a Carbon atom.
 Through a process called “Spin-Spin Splitting, a Proton
or a group of equivalent Protons can produce
“multiple” peaks (multiplets).
 Protons on a Carbon atom are affected by the
presence of Protons on nearby, generally adjacent
atoms.
 Spin - Spin splitting is the result of the interaction or
coupling of the +1/2 & -1/2 spins of the protons on
the adjacent carbon atoms.
 Spin - Spin coupling effects are transferred primarily
through the bonding electrons

3/10/2016
53
NMR

3/10/2016
Spin – Spin Splitting (Con’t)

Those Protons on the adjacent Protons aligned with
the applied magnetic field (+1/2 spin state), will
transfer Magnetic Moment to, and thus augment, the
strength of the magnetic field applied to the Proton
sensing the adjacent Protons.

This increase in the magnetic field strength affecting
the sensing Proton makes it more difficult for the
“secondary” or “diamagnetic” field produced by the
valence electrons to protect the proton; thus, the
Proton is “deshielded” causing the Chemical Shift to
increase slightly
54
NMR

3/10/2016
Spin – Spin Splitting (Con’t)

If the spins of the adjacent Protons are opposed to
the magnetic field (-1/2 spin state), the strength of
the applied magnetic field around the sensing proton
is slightly decreased

With a reduced applied magnetic field strength, the
secondary diamagnetic field is better able to “shield”
the Proton from the applied field resulting in a slight
decrease in the Chemical Shift (increased “Resonance
Frequency”)

With 2 or more Protons on the adjacent Carbon
atoms, there will be mixtures of +1/2 & -1/2 spins
states producing unique Chemical Shift effects
55
NMR
3/10/2016

Each unique Proton or group of equivalent Protons
“senses” the number of Protons on the Carbon
atom(s) next to the one it is bonded, and splits its
resonance signal into n+1 signals, where “n” is the
number of Protons on the adjacent Carbon atom(s)

The “n+1” value represents the number of unique
combinations of the +1/2 and -1/2 spin states of the
adjacent Protons
56
NMR

Spin-Spin Splitting (Con’t)
1,1,2-Trichloroethane
Tert-Butyl Methyl Ether
(a)
(a)
(b)
(a)
All protons chemically equivalent
(a) protons & (b) protons are separated by more than
three (3)  bonds
No signal splitting - 2 signals (a) & (b)
Possible spin
combinations of
adjacent protons
b
3H
a
9H
TMS
0
+1
1
3/10/2016
0
2
-1
1
+1/2
1
-1/2
1
Net Spin
Signal Intensity
57
NMR

Spin-Spin Splitting – An example
1,1,2-Trichloroethane
3/10/2016
58
NMR

Spin - Spin Splitting - Multiplet Signal Intensities
Example Spectrum: Ethyl group
Note Relative Signal Intensities
(a)
CH3
(b)
CH2
(b)
3.20
No.
Adjacent
Protons
(a)

1.83
= spin +1/2
= spin -1/2
Net Spin
Intensity
+3/2 +1/2 -1/2 -3/2
1
3
3
1
There are 3 times as many protons with
+1/2 or - 1/2 spin arrangements than +3/2 & -3/2
Therefore, the signal intensities are greater.
3/10/2016
Pascal’s Triangle
No.
Peaks
Seen
0
Singlet
1
Doublet
2
Triplet
3
Quartet
4
Quintet
5
Sextet
6
Septet1
Relative
Intensity
1
1
1
1
1
1
2
3
4
5
6
1
3
6
10
15
1
1
4
10
20
1
5
15
1
6
Intensity ratios derived from the n + 1
rule
Each entry is the sum of the two entries
above it to the left and right.
The relative intensities of the outer
signals in sextet & septet multiplets are
very weak and sometimes obscured.
59
1
NMR

Spin - Spin Splitting - Common Splitting Patterns
Singlet

Doublet
2 signals
(see 1)

Triplet

Quartet

Sextet

Septet
X
2 signals
(see 1)
3 signals
(see 2)

Quintet
No. signals produced based on the no. of adjacent protons
CH
CH
(X  Y)
CH2
X
CH2 CH2
(X  Y)
CH3
C
3 signals
(see 2)
CH3
CH2
CH3
CH3
2 signal
(see 1)
3 signals
(see 2)
CH
2 signals
(see 1)
2 signals
(see 1)
Y
Y
H
CH
3 signals
(see 2)
4 signals
(see 3)
4 signals
(see 3)
7 signals
(see 6)

3/10/2016
60
NMR

Spin - Spin Splitting - Isomer Example
(a)
(b)
(c)
CH3 CH2 CH2 Cl
1-Chloropropane
3 signals
Signal
a
a
b
c
b
c
Rel Chem
Shift
lowest
middle
highest
Rel Signal Neighbors
Multiplicity
Area
3
2
3 (Triplet)
2
5
6 (Sextet)
2
2
3 (Triplet)
0 ppm
(a)
2-Chloropropane
CH3
Cl
CH
(a)
2 signals
CH3
(b)
a
Signal
b
a
b
Rel Chem
Shift
lowest
highest
Rel Signal
Area
6
1
Neighbors
Multiplicity
1
6
2 (Doublet)
7 (Septet)
0 ppm
3/10/2016
61
NMR

Spin - Spin Splitting - Coupling Constant
 The Coupling Constant (J) is the spacing between the
component signals in a multiplet.
 The distance is measured on the same scale as the
chemical shift (Hz or cycles per second (CPS)). Note: 60
Hz = 1 ppm in a 60 MHz instrument.
 The Coupling Constant has different magnitudes for
different types of protons H
H
C
H
H
C
H
6-8 Hz
H
11-18 Hz
6-15 Hz
H
H
4-10 Hz
CH
H C
meta
0-2 Hz
H
H
C
H
O
H
H
8-10 Hz
H
CH 0-3 Hz
cis 6-12 Hz
trans 4-8 Hz
H
0-5 Hz
H
a,a 8-14 Hz
a,e 0-7 Hz
e,e 0-5 Hz
H
para
1-4
Hz
H
H
3/10/2016
H
H
H
H
H
Ortho
6-10 Hz
cis 2-5 Hz
trans 1-3 Hz
5-7 Hz
H
62
NMR

Magnetic Equivalence
In the spin-spin example of 1,1,2-Trichloroethane, the two
(geminal) protons attached to the same carbon atom (HB & HC),
do not split each other
Cl
HA
HB
C
C
Cl
HC
HA HB
Cl
Cl
C
C
Cl
HC
Cl
They behave as an integral group.
Protons attached to the same carbon atom and have the same chemical
shift do not show spin-spin splitting.
These protons are coupled to the same extent to all other protons in the
molecule.
They have the same Coupling Constant value J to the HA proton.
Protons that have the same chemical shift and are coupled equivalently
to all other protons are magnetically equivalent and do not show spinspin splitting.
3/10/2016
63
NMR

Differentiation of Chemical and Magnetic Equivalence
Br
Br
CH3
CH3
HA
HB
Cyclopropane Compound
Two geminal protons (HA & HB) are chemically equivalent, but
not magnetically equivalent
Proton HA is on same side of ring as two halogens
Proton HB is on same side of ring as the two methyls
Protons HA & HB, therefore have different chemical shifts
They couple to one another and show spin-spin splitting
Two doublets will be seen for both HA & HB
Coupling Constant J for them is about 5 Hz
3/10/2016
64
NMR

Differentiation of Chemical and Magnetic Equivalence (Con’t)
HA
HC
C=C
HB
X
Vinyl Compound
Geminal protons (HA & HB) are chemically equivalent, but not magnetically
equivalent
Protons HA & HB have different chemical shifts
Each has different coupling constant with HC
Constant JAC is a cis coupling constant
Constant JBC is a trans coupling constant
Therefore, HA & HB are not magnetically equivalent
They do not act as group to split proton HC
HB splits HC with constant JBC into a doublet
HA splits each component of doublet into doublets with coupling constant
JAC
3/10/2016
65
NMR

Proton (1H) NMR Spectrum and Splitting Analysis of Vinyl Acetate
Ha & Hb chemically equivalent, but
not magnetically equivalent.
Each has different chemical shift.
Each has different coupling constant
with Hc.
Hb splits Hc into doublet (Jbc).
Ha then splits each Jbc doublet into a
doublet.
Similary, Ha splits Hc into doublet (Jac).
Hb then splits each Jac doublet into a
doublet.
Hc splits Ha & Hb into doublets.
Ha & Hb each then split these
doublets.
3/10/2016
66
NMR

3/10/2016
Aromatic Compounds (Substituted Benzene Rings)

We have previously stated that a magnetic field
applied to an Aromatic ring becomes non-uniform
(anisotropic) by the stabilizing effect of the Benzene
Ring Current resulting in the protons being
deshielded (electron density becomes less); thus,
increasing the chemical shift. i.e., the absorption
signal (Resonance Frequency) moves to the left on
the chart – in the vicinity of 7.0 ppm.

Depending on the number and type of groups
substituted on an Aromatic ring, the NMR spectra of
the remaining protons on the ring are often complex,
with the Chemical Shift moving up field or downfield.
67
NMR
3/10/2016

Some groups, such as – Cyano, Nitro, Carboxyl,
Carbonyl – are electron-withdrawing (deactivate the
ring), decreasing the electron density, and resulting
in an increase in the Chemical Shift, i.e., resonance
frequency moves further down field.

For Electron-Withdrawing groups the Ortho & Para
protons lose more electron density that the Meta
protons; thus, are less shielded moving (increasing)
the chemical shift downfield relative to the Meta
protons.
68
NMR

Aromatic Compounds (Substituted Benzene Rings) (Con’t)

Electron-donating groups such as – Methyl, Methoxy, Amino,
Hydroxy – activate the ring and increase the electron density
resulting in a decrease in the Chemical Shift, i.e., resonance
frequency moves up field to the right.

For Electron–Donating groups, the Ortho & Para protons gain
more electron density than the Meta protons; thus are more
shielded moving (decreasing) the chemical shift up field slightly
from the Meta protons.

Mono – Substituted Aromatic Rings

When a single substituted group is neither strongly electronwithdrawing (deactivates ring by decreasing electron density
about the ring protons) nor strongly electron-donating
(activates ring by increasing the electron density) – Methyl &
Alkyl groups – , all ring protons (ortho, meta, para) have near
identical chemical shifts resulting in a slightly broad singlet
(the protons are not quite chemically equivalent).
See pattern “A” on slide 73
3/10/2016
69
NMR

3/10/2016
Aromatic Compounds (Substituted Benzene Rings) (Con’t)
 Mono – Substituted Aromatic Rings (Con’t)
 In general, electron withdrawing groups (Cyano, Nitro,
Carboxyl, Carbonyl) decrease the electron density of the
Ortho & Para protons more so than the Meta protons,
resulting in the signal for the O & P protons being slightly
more downfield than the Signal for the Meta protons as seen
in pattern “C” on slide 73).

In the case of electron withdrawing groups with double bonds
such as Nitro (NO2) and Carbonyl (C=O) groups, or other
double bonds attached directly to the ring, Magnetic
Anisotropy causes the Ortho protons to be much more
deshielded than the Para & Meta protons, resulting in the
Ortho protons having a significantly increased Chemical shift
as seen in pattern “D” on slide 73.

In the case of electron donating group such as Methyl,
Methoxy, Amino, Hydroxy, the Chemical Shift of the Ortho &
Para protons, while not exactly the same, will be distinctly up
field from the Meta protons as seen in pattern “B” on slide 73.
70
NMR

Aromatic Compounds (Substituted Benzene Rings) Con’t)

Mono – Substituted Aromatic Rings (Con’t)

For Monosubstituted Electronegative elements, such as
Halides, which are electron withdrawing due to the Dipole
effect, the electron withdrawing effect is less dominant than
the electron donating resonance effect.
Thus, the increased electron density about the Ortho & Para
protons would be increased relative to the Meta protons,
resulting in an decrease in the Chemical Shift – signal moves
up field as seen in pattern “E” on slide 64.
Note: The “m/p” signal is actually an overlapping of the “m”
and “p” signals with the “p” signal slightly up field from
the “m” signal.
The “o” proton has more electron density than the “p”
proton because of the Magnetic Anisotropy effects of the
ring current.
3/10/2016
71
NMR

Aromatic Compounds (Substituted Benzene Rings) Con’t)

3/10/2016
Para – Disubstituted Rings

P-Disubstituted patterns are generally easy to recognize.

When the Aromatic ring has two groups substituted in the
para position, three distinct patterns are possible, depending
on the relative electronegativity of the two groups.

If the two p-substituted groups are identical, the four
remaining protons on the ring are chemically and magnetically
equivalent producing a singlet as seen in pattern “F (a)” on
slide 73.

If the two p-substituted groups are different, the protons on
one side of the symmetrically ring split the protons on the
other side of the ring into a doublet.

The patterns produced by the two doublets will be different
depending on the relative electronegativity of the two
substituted groups as seen in patterns F (b) & F (c) on slide
73.
72
NMR

3/10/2016
Common Aromatic Patterns
73
NMR

“Activating” and “Deactivating” groups and the impact of
the changing electron density in the Benzene ring on
Chemical Shift of ortho, meta, para protons
 Methoxyl (0-CH3) group is Electron
Donating, activates ring by adding
electron density to o/p protons.
 Chemical Shifts, , of ring o/p
protons are moved up field, i.e.,
decreasing ppm because of
increase electron density.
 Note location of “Methyl Protons”
absorption at 3.7 ppm (without
influence of “O” it would be around
1 ppm).
o
m
p
o
m
3 Methyl Protons
m o, p
Anisole (C7H8O)
3/10/2016
74
NMR

“Activating” and “Deactivating” groups and the impact of
the changing electron density in the Benzene ring on
Chemical Shift of ortho, meta, para protons



The Amino group is Electron Donating
and Activates the ring.
Increases electron density around
Ortho & Para protons relative to Meta.
Chemical Shift, , of ring protons is up
field, decreased ppm
o
m
p
o
m
m
o/p
Aniline (C6H7N)
2 Amino
Protons
3/10/2016
75
NMR

“Activating” and “Deactivating” groups and the impact of
the changing electron density in the Benzene ring on
Chemical Shift of ortho, meta, para protons
o
 Nitro group is electron withdrawing and
deactivates the ring.
 Protons in ring are deshielded moving
Chemical Shift downfield.
p m  Magnetic Anisotropy causes the Ortho protons
to be more deshielded than the Para & Meta
protons.
o
m
p
o
m
Nitrobenzene (C6H5NO2)
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76
NMR

“Activating” and “Deactivating” groups and the impact of
the changing electron density in the Benzene ring on
Chemical Shift of ortho, meta, para protons
P-Chloroaniline (C6H6ClN)
Hb&Hb’
Ha
Ha’
Hb
Hb’
The Molecular Ion peak
from Mass Spectrometry
would have indicated the
presence of the single
Chlorine atom and Nitrogen.
Ha&Ha’






Para Di-Substituted Benzene ring
Ha & Ha’ have same Chemical Shift
Hb & Hb’ have same Chemical Shift
Ha is split into doublet by Hb
Hb is split into doublet by Ha
Two sets of peaks produced by relative
electronegativity of Amino & Cl groups
2 Amino Protons
3/10/2016
77
NMR

“Activating” and “Deactivating” groups and the impact of
the changing electron density in the Benzene ring on
Chemical Shift of ortho, meta, para protons
2,4-Dinitroanisole (C7H6N2O5)
3H


c
b
a
3/10/2016
b
a
c


The Methoxy group is
moderately activating,
while the Nitro groups are
strongly deactivating
(electron withdrawing)
Net effect is to Decrease
the electron density about
the ring protons
The a & b protons are
Ortho to the strongly
deactivating Nitro groups,
thus, they have reduced
electron density and their
Chemical Shift is down
field relative to the “c”
proton
All protons interact to
produce Spin-Spin
Coupling.
78
NMR

3/10/2016
Protons attached to atoms other than carbon atoms

Widely variable ranges of absorptions.

Protons on heteroelements, such as oxygen (hydroxyl,
carboxyl, enols), and nitrogen (amines, amides)
normally do not couple with protons on adjacent
carbon atoms to give spin- spin splitting.

Solvent effect - The absorption position is variable
because these groups undergo varying degrees of
hydrogen bonding in solutions of different
concentrations.

Amount of hydrogen bonding can radically affect the
valence electron density producing large changes in
chemical shift.
79
NMR

Protons attached to atoms other than carbon atoms
(Con’t)


Absorption signals are frequently broad relative to
other singlets, which can be used to help identify the
signal.
Protons attached to Nitrogen atoms often show
extremely broad signals and can be indistinguishable
from the base line.
Typical Ranges for Groups with Variable Chemical Shifts
3/10/2016
80
NMR

NMR Spectra at Higher Field Strengths

The 60 MHz spectrum for some compounds can be
very difficult to read because the chemical shifts of
several groups of protons are very similar and they
overlap.

Chemical shifts are dependent on the frequency of the
applied radiation (or the strength of the applied
magnetic field).
Note: Coupling Constants (J) ARE independent.

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As the field strength increases, the chemical shifts of
proton groups in also increase.
81
NMR

3/10/2016
NMR Spectra at Higher Field Strengths (Con’t)

For example, a proton group resonating at 60 Hz in a
60 Mhz instrument would resonate at 100 Hz in the
100 Mhz instrument.

This effectively stretches the X-axis scale improving
resolution.

Note, however, the  value in ppm, does not change.
82
NMR

3/10/2016
Chemical Shift Reagents

Interactions between molecules and solvents, such as
those due to hydrogen bonding can cause large
changes in resonance positions of certain types of
protons, such as hydroxy (OH) and amino (NH2).

Changes in resonance positions can also be affected
by changing from the usual NMR solvents, such as
chloroform (CCL4) and deuterochloroform (CDCl3) to
solvents like benzene which impose local anisotropic
effects on the surrounding molecules.

In some cases a solvent change allows partially
overlapping multiplets to be resolved.

Most chemical shift reagents are organic complexes of
the Lanthanide elements.
83
NMR

3/10/2016
Chemical Shift Reagents (Con’t)
 When added to a compound, these complexes
produce profound chemical shifts, sometimes up field
and sometimes downfield, depending on the metal.
 Europium, erbium, thulium, and ytterbium shift
resonances to the lower field (higher ).
 Cerium, praseodymium, neodymium, samarium,
terbium, and holmium shift resonances to the higher
field (lower ).
 Another advantage of shift reagents is that shifts
similar to those observed at higher fields can be
induced without the need to purchase an expensive
higher field instrument.
 The amount of the shift change depends on the
distance separating the lanthanide element from the
proton group and the concentration of the shift
reagent.
84
NMR

Example 1H1 NMR Spectra
Suggestions
For
Interpreting NMR Spectra
3/10/2016
85
NMR Example Spectra

3/10/2016
NMR Spectra Interpretation Procedure

The following 4 slides provide a suggested process to
follow in attempting to interpret an NMR Spectra.

The 1st slide is a typical NMR spectra showing 5
signals each consisting of one or more peaks.

Note that each signal has a number associated with it
representing the area integration, i.e., the number of
protons generating the signal.

Also note the expanded spectra of the signal at 2.6
ppm. Expanded spectra are often provided when the
signal lacks sufficient resolution to clearly display the
number of peaks being generated by the protons on
the carbon atom generating the signal.
86
NMR Example Spectra
 The 2nd slide presents interpretations of each signal
relative to the number of protons (n) on the carbon
atom generating the signal and the number of protons
attached to adjacent carbons atoms that produce the
n+1 peaks comprising the signal as a result of “spinspin” splitting.
 The 3rd slide shows how the fragments from slide 2
might fit together.
 The 4th slide ties it all together.
3/10/2016
87
NMR Example Spectra

Four slides demonstrating a process for interpreting an
NMR Spectra
Slide 1
3H
 Chemical Shift () in PPM
Note: Magnetic Field (Ho) increases 
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88
NMR Example Spectra
Slide 2
Doublet
From chemical shifts, peak
integration values, and splitting
patterns, develop substructures for
each signal.
Triplet
3H
Mono-substituted
Benzene Ring
Quintet
Sextet
2 protons see 4 protons
 5 peaks (quintet) produced
1 proton sees 5 protons
 6 peaks (sextet) produced
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3 protons see 2 protons
 3 peaks (triplet) produced
3 protons see 1 proton
 2 peaks (doublet) produced
89
NMR Example Spectra
Slide 3
Solve the Puzzle
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90
NMR Example Spectra
Slide 4
The Solution:
2-Phenylbutane (C10H14)
3H
3 protons on a
Methyl group
see 2 adjacent
protons
triplet
5 protons on a
mono-substituted
Benzene Ring
3 + 2 +1 = 6
Spin-Spin Splitting
No. of peaks in a signal is equal
to the number of protons on all
adjacent carbon atoms plus 1
(N+1 rule)
Integration value (Area under signal)
is proportional to No. of Protons
generating the signal
3/10/2016
doublet
sextet
quintet
3 protons on a
Methyl group
see 1 adjacent
proton
2 protons on a
Methylene group
see 4 adjacen
protons
1 proton on
Methine group
sees 5 adjacen
protons
91
NMR Example Spectra
Benzyl Acetate (C9H10O2)
Methyl Protons (3)
No adjacent protons;
no splitting
Aromatic Protons (5)
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Methylene Protons (2)
No adjacent protons;
no splitting
92
NMR Example Spectra
Cyclohexene (C6H10)
3 different chemical environments
Peaks actually show splitting,
but is hard to see at this
2H
resolution (90Hz)
The single protons on the
“c” carbons are equivalent;
thus, they do not split each
other.
The single protons on each
“c” carbon split the
respective adjacent “b”
proton to form two equal
overlapping triplets
n+1=2+1=3
(triplet)
b
c
c
a
c´
a´
b´
4H a
The “a” protons have a
smaller Chemical Shift
than the “b” protons
because of their
4H b
relative position to the
electron rich  bond.
The 2 protons on each of
the equivalent “b” carbons
split their respective
adjacent “protons (1 on the
“c” carbon and 2 on the “a”
carbon) to form two equal
overlapping quartets
n+1=3+1=4
(quartet)
The sets of 2 protons
on each of the adjacent
“a” carbons are
equivalent and do not
split each other.
Each 2 proton set on
an “a” carbons splits
its respective adjacent
2 “b” protons to form 2
overlapping triplets.
n+1=2+1=3
(triplet)
3/10/2016
93
NMR Example Spectra
2,5-Hexanedione (Acetonylacetone) (C6H10O2)
6H
n+1=0+1=1
(singlet)
2 Sets Methylene Protons
Chemical & Magnetically Equivalent
They do not split each other
4H
n+1=0+1=1
(singlet)
3/10/2016
94
NMR Example Spectra

Cis-Stilbene (C14H12)
Vinyl Protons (2)
Chemically & Magnetically Equivalent
Do Not Split
Aromatic Protons (10)
(very fine splitting)
3/10/2016
95
NMR Example Spectra
Methyl Phenyl Acetate (C9H10O2)
Methylene Protons (2)
n+1 = 0 + 1 =1
Chem Shift – 3.65 ppm
Methyl Protons (3)
n+1 = 0 + 1 = 1
Chem Shift – 3.60 ppm
Aromatic Protons (5)
Note: 2 Overlapping
Absorptions!! Electronegative
Carboxyl Oxygen forces shift
of Methyl Proton absorptions
downfield to about same
location as shift of Methylene
Protons forced downfield by
effects of electronegative
Carbonyl group.
3/10/2016
96
NMR Example Spectra
4-Heptanone (C7H14O)
The Methyl & Methylene group
protons on the left side of the
Molecule are Chemically & Magnetically
Equivalent to their counterparts on the
right side, thus, the Chemical Shifts are
the same and signals overlap
n+1 = 2+1=3
3/10/2016
n+1 = 5+1=6
n+1 = 2+1=3
97
NMR Example Spectra
Isopropyl Benzene (C9H12)
1 proton sees
6 protons
Producing Septet
n+1=6+1=7
6 protons see 6H
1 proton
producing
Doublet
n+1=1+1=2
5H
1H
3/10/2016
98
NMR Example Spectra
P-Nitrotoluene (C7H7NO2)
Nitro group is strongly deactivating and
the Methyl group is weakly activating.
The net withdrawing effect on benzene ring
moves the chemical shift downfield, the
Protons ortho to the Nitro group more so than
The protons ortho to the Methyl group.
3H
P- Dibsubstitution
4H
3/10/2016
99
NMR Example Spectra
Phenacetin (C10H13NO2)
3H
n+1=0+1=1
(singlet)
3H
n+1=2+1=3
(triplet)
P-Disubstitution
4H
2H
1H
3/10/2016
n+1=3+1=4
(quartet)
100
NMR Example Spectra
Isobutyric Acid (C4H8O2)
n+1=1+1= 2
(doublet)
6H
Carboxylic
Proton
1H
3/10/2016
n+1=6+1=7
(septet)
1H
101
NMR Example Spectra
4-Amino-Acetophenone (C8H9NO)
Withdrawing
Donating
3
3 Methyl protons see
0 adjacent protons
producing singlet
n+1=0+1=1
Chemical Shift moved
downfield because
of proximity to
Electronegative
Carbonyl group (C=O)
P-Disubstitution
2
2
2
3/10/2016
102
NMR Example Spectra
Butyrophenone (C10H12O)
The Chemical Shift of the Methylene group nearest the moderately
deactivating Carbonyl group is greater than the adjacent Methylene group,
because the deactivating effect diminishes with distance
3
Propoxyl group is moderately
deactivating, deshielding “o”
ring protons more so than “p”
protons (aniostropic effect)
“m” protons are deshielded least
ortho
(2)
para, meta
(3)
2
2
3/10/2016
103
NMR Example Spectra
Cyclohexanone (C6H10O)
a
a
The 4 (a) protons, probably 2 identical
Methylene groups each of which is attached to
a (b) Carbon with 2 protons.
The net effect of these equivalent structures is
that two protons see two adjacent protons
producing a triplet: 2+1 = 3.
4
b
b
c
The 4 (b) protons represent 2 identical
Methylene groups each of which is attached to
an equivalent Methylene group.
The (b) protons also see the two (c) protons.
The net effect of this is that the 4 (b) protons
see effectively 2 (a) protons and 2 (c) protons
producing a 4 +1 = 5 quintet.
The 2 (c) protons see the 4 (b) protons
producing a 4 + 1 = 5 quintet
4
2
Two quintets
overlappiing
3/10/2016
104
NMR Example Spectra
Isobutyl Acetate (C6H12O2)
3
6 protons see
1 adjacent proton
producing doublet
6
3 protons see 0 protons on
Carbonyl carbon producing a singlet
1 proton sees
8 adjacent protons
producing (8+1) 9 peaks
Singlet
2
2 protons see
1 adjacent proton
produces doublet
Triplet
3/10/2016
2
Nonet
Doublet
105
NMR (Carbon–13)

Carbon-13 Nuclear Magnetic Resonance






3/10/2016
Carbon-13 (13C6) possesses spin (I=1/2); thus it is a
candidate for NMR
Carbon-13 resonances are not easy to observe:
 Natural abundance of of C-13 is 1.08 %
 Magnetic Moment () is low
 Resonances are 6000 times weaker than those of
hydrogen
Fourier Transform instrumental techniques make it possible
to observe 13C6 nuclear magnetic resonance.
Chemical Shift is most useful parameter derived from 13C6
spectra.
Integrals (signal areas) are Unreliable and not necessarily
related to the relative number of 13C6 atoms present.
Hydrogens attached to 13C6 atoms cause spin-spin splitting,
spin-spin interaction between adjacent carbon atoms is
rare.
106
NMR (Carbon–13)

3/10/2016
Carbon-13 Nuclear Magnetic Resonance

The Chemical Shifts for 13C6 spectra are reported by the
number of ppm ( units) that the signal is shifted downfield
from TMS, just as in the proton NMR.

The 13C6  scale ranges from 0 (TMS) in the upper (higher
magnetic)_field to 225 ppm in the lower field.

Resonance signals are more distinct providing more
resolution.

Adjacent CH2 carbons have distinct resonance signals.

Unusual to find two carbon atoms in a molecule with the
same chemical shift unless they are chemically equivalent
by symmetry.
107
NMR (Carbon–13)

3/10/2016
Coupled C-13 Spectra

In coupled C-13 NMR spectra, the spectra diagram
exhibits spin-spin splitting, similar to Proton NMR, but
with a significant difference

The splitting pattern exhibited by a particular C-13
atom follows the N+1 rule, but the value of N is based
on the number of protons attached to the C-13 atom,
NOT the number of protons attached to the adjacent
carbon atoms.
108
NMR (Carbon–13)

Coupled C-13 Spectra - Example
(See Slide 113)
O
CH2
Ethyl Phenyl Acetate
C
O
CH2
CH3
1
1
1
1
1
Carbons
Aromatic Ar
Benzyl
CH2
Methylene CH2
Methyl
CH3
Carboxyl O=C-O
A. In the coupled spectra of Ethyl Phenyl Acetate (Slide 107),
the Methyl group at 14.2 ppm is split into quartet by the three
hydrogen atoms attached to the carbon itself, not the protons
on the adjacent Methylene (CH2) group
B. Each quartet line is split into a triplet by the adjacent CH2
group (not seen on chart).
3/10/2016
109
NMR (Carbon–13)

Coupled C-13 Spectra - Example (Con’t)
(See Slide 113)
O
CH2
Ethyl Phenyl Acetate
C. Two CH2 groups
C
O
CH2
CH3
1
1
1
1
1
Carbons
Aromatic Ar
Benzyl
CH2
Methylene CH2
Methyl
CH3
Carboxyl O=C-O
 The Methylene (CH2) group with the Ethyl group (60.6
ppm) is deshielded by the adjacent O and forms a triplet
because of the two attached hydrogens
 The Benzyl (CH2) group at 41.4 ppm is also a triplet
D. The carbonyl appears to be a singlet at 171.1, but it is
actually a triplet because of the adjacent -CH2 group (very
fine, not easy to see).
E. The aromatic ring carbons have resonances over the range of
127-136 ppm.
3/10/2016
110
NMR (Carbon–13)

3/10/2016
Broad-Band Decoupled
13C
6
Spectra

Simple molecules such a Ethyl Phenyl Acetate can
yield interesting and useful structural information,
namely the number of hydrogens attached to each
carbon.

However, for larger molecules the spectra can become
very complex with overlapping splitting patterns.

A broader range of 13C6 spectra can be obtained if all
the protons are decoupled from the molecule by
irradiating them simultaneously with a broad spectrum
of frequencies in the appropriate range.
111
NMR (Carbon–13)

3/10/2016
Broad-Band Decoupled
13C
6
Spectra (Con’t)

The decoupled spectra are much simpler and easier to
interpret.

Each signal represents a different carbon atom.

If two carbon atoms are represent by a single signal,
they must be equivalent by symmetry.

In the Aromatic ring of Ethyl Phenyl Acetate, the
carbons at positions 2 & 6 produce a single signal,
and the carbons at positions 3 & 5 also produce a
single signal.
112
NMR (Carbon–13)

3/10/2016
Carbon-13 Spectra for coupled and decoupled Ethyl
Phenyl Acetate
113
NMR (Carbon–13)

Chemical Shifts for Carbon-13 NMR

Chemical Shift of each Carbon indicates its type and
structural environment.

As with proton NMR, electronegativity, hybridization, and
anisotropy effects influence the chemical shift.
Correlation Chart for Carbon-13 (ppm from TMS)
3/10/2016
114
NMR (Carbon–13)

Carbon-13 Spectra (Proton Decoupled)

2,2-Dimethylbutane

Six carbon atoms, but only 4 signals + solvent
signals for CDCl3 and TMS.

Single Methyl Carbon (a), signal at 8.8 ppm.

Three Methyl Carbons (b) on quaternary Carbon
(c), signal at 28.9 ppm.

Quaternary Carbon (c), which has no hydrogens
attached, appears as a small (weak) signal at 30.4
ppm.
Con’t on next slide
3/10/2016
115
NMR (Carbon–13)

Carbon-13 Spectra (Proton Decoupled)

2,2-Dimethylbutane (Con’t)

Methylene Carbon (d), signal at 36.5 ppm.

Relative size of signals gives some idea of number
of each type of carbon
Note: Signal at 28.9 ppm for 3 carbon atoms.
2,2-Dimethylbutane
3/10/2016
116
NMR (Carbon–13)

Aromatic Ring Methyl Substitution
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
5 Carbon Types
3/10/2016
3 Carbon Types
9 Carbon Types
6 Carbon Types
117
NMR (Carbon–13)

Bromocyclohexane & Cyclohexanol

A carbon atom should be deshielded by the presence
of an electronegative element.

The ring carbon atoms will resonate at a higher field
(smaller  shift) as they are located farther away from
the electronegative element
Bromocyclohexane
Cyclohexanol
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118
NMR (Carbon–13)

Cyclohexene

Diamagnetic Anisotropy - Carbon atom attached to
the double bond (c) is deshielded by the effects of the
non-uniform magnetic field produced by the presence
of the  electrons.

Carbon atoms located farther from the double bond
resonate at higher field (less chemical shift).
Chemical Shift
(c) > (b) > (a)
Cyclohexene
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119
NMR (Carbon–13)

Toluene (Hydrogen Coupled Spectrum)

Diamagnetic Anisotropy causes the carbon atom
signals of the aromatic ring [ (e) > (d) > (c) > (b) ] to
appear at lower field strengths (higher  values).

The signal (a) for the Methyl carbon atom attached to
the ring located in the higher field ( value about 22)
illustrates little anisotropic effect of aromatic ring.
Toluene
a
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120
NMR (Carbon–13)

3/10/2016
Cyclohexanone

Strong deshielding effect of carbonyl group.

The carbonyl carbon atom (d) resonates at a very low field
value –  value about 211.3
121
NMR (Carbon–13)

Symmetry - 1,2- & 1,3-Dichlorobenzene isomers
 A plane of symmetry for 1,2-Dichlorobenzene defines
three (3) different Carbon atoms, producing three
signals.
 A plane of symmetry for 1,3-Dichlorobenzene defines
four (4) different Carbon atoms, producing four
signals
1,2-Dichlorobenzene
1,3-Dichlorobenzene
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122
NMR Summary

3/10/2016
NMR Summary Notes

NMR - An instrumental technique to determine the
number, type, and relative positions of certain atoms in
a molecule.

The technique is based on the nuclear spin properties of
the nuclei of certain elements and isotopes.

When the nuclei of these elements are placed in a
strong magnetic field and irradiated with low energy
radio waves (wavelengths of 1 - 5 meters) they absorb
energy through a process called magnetic resonance.

Under these conditions the absorption of energy is
quantized producing a characteristic spectrum for the
compound.

The absorption of energy does not occur unless the
strength of the magnetic field and the frequency of the
electromagnetic radiation are at specific values.
123
NMR Summary

NMR Summary Notes (Con’t)
 1H1
and 13C6 have odd atomic number and/or atomic
mass; thus they have spin properties and are the two
primary isotopes utilized in NMR.
 1H1
3/10/2016
and
13C
6
have two spins states (+1/2 & -1/2).

Nuclei with +1/2 spin state align with an applied
magnetic field.

Nuclei with -1/2 spin state oppose magnetic field.

Resonance - If radiofrequency (low energy, long
wavelength) waves are applied to nuclei with spin in
an applied magnetic field, the lower energy nuclei
aligned with the field absorb a quantized amount of
energy, reverse direction, and become opposed to the
field.

The stronger the magnetic field, the greater the
energy absorption (resonance).
124
NMR Summary

3/10/2016
NMR Summary Notes (Con’t)

An NMR instrument applies a constant radiofrequency
of 60, 100, or 300 MHz and applies an increasing
magnetic field strength.

A higher field strength instrument allows for cleaner
separation of overlapping signals, i.e., more
resolution.

Protons or Carbon-13 atoms of different types
(chemical environments electronegativity, anisotropy,
etc.) resonate at unique field strengths measured in
Hertz (cycles per second) producing a signal (peak)
on the chart paper.
125
NMR Summary

NMR Summary Notes (Con’t)

A parameter called the “Chemical Shift ()” has been
defined to give the position of the absorption of a
proton a quantitative value.
Observed Shift from TMS (in Hz)
Chemical Shift () =
3/10/2016
60 MHz
Hz
=
MHz
= PPM

The Chemical Shift values are report in units of “Parts
Per Million” (ppm).

Magnetic field, in Hertz, increases from left to right on
chart scale, while  increases from right to left starting
with 0 (for TMS) to 13.

Protons in molecules in chemically equivalent
environments will generally have the same chemical
shift - one signal is produced.
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NMR Summary
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NMR Summary Notes (Con’t)

The area under an NMR signal is proportional to the
number of protons generating the signal.

The NMR Spectrometer electronically integrates the
area under the signal.

The height the of traced vertical line gives the relative
numbers of each type of hydrogen.

Diamagnetic Shielding - Valence electrons shield
proton from applied magnetic field.

Electronegative elements produce an electron
withdrawing effect, deshielding the proton, resulting
in a larger chemical shift, that is, a smaller magnetic
field is required to induce resonance.
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NMR Summary
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NMR Summary Notes (Con’t)

The movement of the  electrons in aromatic rings
(benzene, etc), alkenes C=, Alkynes (C), and
aldehydes (O=CH) produce their own magnetic fields
causing the applied magnetic field to become nonuniform (diamagnetic anisotropy), which deshields the
proton increasing the chemical shift.

In some cases, such as acetylenes, the effect of the
anisotropic field is to shield the hydrogens, decreasing
the chemical shift.
Protons are affected by the presence of protons on
nearby, generally adjacent, carbon atoms.
 Each type of proton “senses” the number of
equivalent protons (n) on the carbon atom next to
the one it is bonded, and splits its resonance signal
into n+1 signals, a multiplet – Spin – Spin Splitting.

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NMR Summary
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NMR Summary Notes (Con’t)
 Coupling Constant (J) - The spacing, in Hz, between
the component signals in multiplet.
 The Coupling Constant has different magnitudes for
different types of protons.
 Protons that have the same chemical shift and are
coupled equivalently to all other protons are
magnetically equivalent and do not show spin-spin
splitting.
For example: Protons attached to the same carbon
atom that have the same chemical shift do not split
each other.
 In monosubstituted aromatic rings, all ring protons
have near identical chemical shifts resulting in a
single, but slightly broader single.
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NMR Summary
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NMR Summary Notes (Con’t)

Electron-withdrawing (electronegative) groups, such
as nitro, cyan, carboxyl, and carbonyl, deshield the
ring moving the chemical shift downfield (increase ).

Electron-donating groups, such as methoxy, amino,
increase the electron density, moving the chemical
shift up field (decrease ).

Hydrogen on heteroelements - Protons on elements
other than carbon, such as, oxygen (hydroxyl,
carboxyl, enols), nitrogen (amines, amides) do not
couple with protons on adjacent carbon atoms; thus
no spin-spin splitting.
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NMR Summary
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NMR Summary Notes (Con’t)

Solvent effect - The absorption position in variable
because these groups undergo varying degrees of
hydrogen bonding in solutions of different
concentrations.

The amount of hydrogen bonding can radically affect
the valence electron density producing large changes
in chemical shift.

Chemical Shift Reagents - Chloroform,
Deuterochloroform, Organic Complexes of Lanthanide
Elements.

When added to the compound in question, these
complexes produce profound chemical shifts,
sometimes up field and sometimes downfield,
depending on the metal.
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NMR Summary
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NMR Summary Notes (Con’t)

Europium, erbium, thulium, and ytterbium shift
resonances to the lower field (higher ).

Cerium, praseodymium, neodymium, samarium,
terbium, and holmium shift resonances to the higher
field (lower ).

Carbon-13 NMR spectra.

Carbon-13 resonances not easy to observe - natural
abundance 1.08 %, low magnetic moment, 6000
times weaker than those of hydrogen.

Integrals (signal areas) are not reliable as indicators
of the number of carbon atoms.

Chemical shift scale - 0 to 200 (higher resolution).
13C
6
has spin and produces NMR
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NMR Summary
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NMR Summary Notes (Con’t)

Higher resolution produces more distinctive signals
making it easier to resolve signal overlapping.

Unusual to find two carbon atoms with same chemical
shift unless chemically equivalent.

Except for a few simple molecules, 13C6 spectra can
become very complex, making interpretation difficult.

Irradiation of the molecules simultaneously with a
broad spectrum of frequencies decouples the
hydrogens from the molecule producing a much
simpler spectra.

Electronegativity, hybridization, and anisotropy effects
influence the chemical shiftt
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