NUCLEAR MAGNETIC RESONANCE
(NMR) SPECTROSCOPY
INTRODUCTION
Nuclear magnetic resonance spectroscopy(NMR)
•
is a sensitive, non-destructive method for elucidating the
structure of organic molecules.
•
is a powerful analytical technique used to characterize organic
molecules by identifying
carbon-hydrogen
frameworks
within
molecules.
•
is a research technique that exploits the magnetic properties of
certain atomic nuclei.
NMR is the most powerful tool for organic structure
determination.
• used to study a wide variety of nuclei
• EMR used in NMR have long wavelength (10 7 to 108 μ)
• And extremely low energy
• These radiations are able to interact with the nuclei of certain
atoms which are exposed to a strong magnetic field
Chapter 13
3
Nuclear Spin : the total angular momentum of a nucleus
denoted by 
Mass Num.
Odd
Even
Even
Atomic Num.
Odd or Even
Even
Odd
Spin Quantum Num. Example
1H, 13C, 19F
½, 3/2, 5/2… etc
12C, 16O, 32S
0
1, 2, 3…
The number of possible orientations = 2I + 1.
So in case of 1H, (I = 1/2), possible orientation will be two.
Chapter 13
4
•
Two common types of NMR spectroscopy are used to characterize
organic structure:
•
1H
NMR:- Used to determine the type and number of H atoms in
a molecule
•
13C
NMR:- Used to determine the type of carbon atoms in the
molecule.
Principle of NMR
• The Principle behind NMR comes from the spin
of a nucleus and it generates a magnetic field.
Without an external applied magnetic field, the
nuclear spin are random in directions. But when
an external magnetic field is present the nuclei
align themselves either with or against the field
of the external magnet.
• If an external magnetic field is applied, an energy transfer
(ΔE) is possible between ground state to excited state.
• When the spin returns to
its ground state level, the
absorbed radiofrequency energy is emitted at the same
frequency level.
• The emitted radiofrequency signal that gives the NMR of the
concerned nucleus.
Principle of NMR
• The emitted radiofrequency is directly proportional to the
strength of the applied field,
ꝨBₒ
Ʋ=
2𝜋
Where, Bₒ = External magnetic field experienced by proton
Ꝩ = Magnetogyric ration (the ratio between the nucear
magnetic moment and angular moment)
The nucleus of each hydrogen atom
behaves like a tiny magnet, which
usually lines up with an applied
magnetic field.
If a molecule containing hydrogen is placed in a strong magnetic field,
the hydrogen nucleus can line up with the field or line up against it!
N S N
S
N
S
N
S
Nucleus spin aligned with
the field – Low energy!
Nucleus spin aligned against
the field – High energy!
A photon (from radio frequency region) with the right
amount of energy can be absorbed and cause the spinning
proton to flip.
The NMR Spectrometer
Instrumentation
The NMR spectrophotometer consists of following components;
Magnet:- A strong magnet provides stable and homogenous field. The magnet size is 15
inches in diameter and capable of producing strong fields up to 23,500 gauss for 100MHz.
Sample and sample holder:- A 1 – 30 mg sample is used in the form of dilute solution (2 –
10%) and solvent doesn’t contain hydrogen of its own ions. The sample holder is glass tube
about 5mm in diameter and 15 – 20 cm in length.
Radiofrequency oscillator:- The RF oscillator is installed perpendicular to magnetic field and
transmits radio waves of some mixed frequency such as 60, 100, 220, 300 MHz. a sweep
generator is installed to supply dc current to sec. magnet.
RF detector or Receiver:- It is installed perpendicular to both magnetic field and the oscillator
coil and is tuned to the same frequency as transmitter. When precession frequency is match
with RF the nuclei induces (emf) in detector coil and this signal is amplified and sent to
recorder.
Recorder :- The recorder gives a spectrum as a plot of strength resonance signal on Y axis &
strength of magnetic field on X axis. The strength of resonance signal is directly proportional
to number of nuclei resonating at that particular field strength.
Working of Instrumentation
• In NMR spectrophotometer, the sample is dissolved in Deuterated Solvent and placed in
long cylindrical glass tube especially made for NMR and also add small amount of TMS as
internal reference.
• Placed the sample tube in gap between two magnetic poles where coil is attached to a
specific RF generator (e.g. 60 MHz). This coil supply EMR energy required to change spin
orientation of proton.
• Then radiofrequency generator irradiates the sample with short pulse of radiation causing
resonance. As magnetic field strength increase, the precessional frequency of all protons
increase and when this frequency proton reaches 60 MHz, the resonance occurs.
• As magnetic field increase linearly the recorder pen travels from left to right, thus protons
which achieve resonance faster i.e. (Deshielded) appears on left side (downfield), where as
those protons (Shielded) appears on right side (upfield) of chart in the form of peaks.
• The superconducting magnet in modern NMR spectrometers have coils that are cooled in
liquid helium and conduct electricity with no resistance.
The NMR Graph
NMR graph is characterized by
• The number of the signals and their splitting
• The location of the signals(chemical shift)
• The intensity of the signal
Chapter 13
16
The number of signals shows how many different kinds of
protons are present.
Splitting tells us about the neighboring protons.
The intensity of the signal shows the number of protons of
that type.
The location of the signals shows how shielded or deshielded
the proton is.
Internal Standard: Tetramethylsilane (TMS)
• TMS (Tetra methyl silane) is most commonly used as Internal
standard for measuring the position of 1H, 13C and 29Si in NMR
spectroscopy. Due to following reasons;
➢It is chemically inert and miscible with a large range of solvents.
➢Its twelve protons are all magnetically equivalent.
➢Its protons are highly shielded and gives a
strong peak even in small quantity.
CH3
CH3
Si
CH3
CH3
Tetrameth yls ilane (TMS)
➢Si is less electronegative than carbon hence TMS protons are
highly shielded.
➢It is highly volatile and can be easily removed to get back
sample.
➢It does not take part in intermolecular associations with
sample.
➢Organic protons absorb downfield (to the left) of the TMS signal.
TMS is shown at a peak value of δ = 0 ppm.
Solvents Used
• The solvent used for dissolving sample should have following properties;
➢ Should not contain proton,
➢ Inexpensive
➢ Low boiling point and non polar in nature.
• Generally deuterated chloroform CDCl3 is used as solvent.
• If sample is soluble in polar solvent, then deuterium oxide (D 2O), DMSO,
CCl4, CS2, CF3, COOH are used as solvent.
Number of signals
• Equivalent and non equivalent protons.
• Each set of magnetically equivalent protons will give rise to a
NMR signal.
• Number of signals in NMR = number of different sets of
equivalent protons
• Magnetically equivalent protons are chemically equivalent
Examples:
CH4
all 4 hydrogens are identical i.e. in same environment
hence ONE signal
CH3COCH3
ONE signal
CH3CH2Cl
ethyl chloride
CH3OH
methanol
CH3CH2CH3 propane
H
CH3
C
H
C
CH3
isobutylene
TWO signals
• CH3CH2CH2Cl
• CH3CH2NH2
THREE signals
• CH3CH(OH)CH3
ClHC
CH2
Trans to Cl
H
H
C
Cis to Cl
H
C
Cl
Splitting of signals
• The splitting of the signal is related to the number of protons
in neighboring groups.
• Let us illustrate it by taking the example of ethyl bromide
a
CH3
b
CH2
Br
Two signals will be obtained.
Splitting = (n + 1) rule
N = number of neighboring protons
Number of
Neighboring
protons
0
Splitting of
peaks
(n + l) rule
1
Termed as
Intensity
distribution
Singlet
1
1
2
doublet
1:1
2
3
triplet
1:2:1
3
4
quartet
1:3:3:1
4
5
Pentate
(multiplet)
1:4:6:4:1
Chemical Shift (Position of Signals)
• The utility of NMR is that all protons do not show resonance at same frequency, because, it
is surrounded by slightly different electronic environment from one another. Thus they
absorb at different field strength.
• When molecule placed in magnetic field, so its surrounding electron circulate & generates
counter field which opposes the applied magnetic field on proton. If the resulting field feels
by proton is high and that proton called as the Shielded proton. and if resulting field is low
then that proton called as the Shielded proton.
• Shielded proton shifts absorption signal to right side (upfield) and deshielded proton shifts
absorption signal to left side (down field) of spectrum.
• Such shifting in position of NMR absorption signals which arise due to the shielding or
deshielding of proton by surrounding electrons are called as Chemical shift.
• Position of signals in spectrum help us to know nature of protons i.e. aromatic, aliphatic,
acetylinic, vinylic, adjacent to electron releasing or withdrawing grp.
Chemical Shift
Chemical shift, ppm  =
Shift downfield from TMS (Hz)
Spectrometer frequency (MHz)
• Measured in parts per million.
• Ratio of shift downfield from TMS (Hz) to total spectrometer
frequency (MHz).
• Same value for 60, 100, or 300 MHz machine.
• Called the delta scale.
Factors affecting Chemical Shift
• Shielding and Deshielding effects
• Electronegativity
CH3F
CH3Cl
CH3Br
CH3I
TMS
Shielded proton
Higher Field (up field)
lower value of δ
Deshielded proton
Lower Field (down field)
Higher value of δ
Shielding or Deshielding Protons
Shielding or Deshielding Protons
Shielding or Deshielding Protons
Chemical Shift (Position of Signals)
Chemical shift depends upon following parameters:
2. Hybridization of adjacent atoms
1. Electro negativity of nearby atoms
CH3 -X
Electron egativity of X
Chemical
Shif t ()
CH3 F
CH3 OH
CH3 Cl
CH3 Br
CH3 I
4.0
3.5
3.1
2.8
2.5
4.26
3.47
3.05
2.68
2.16
( CH3 ) 4 C
( CH3 ) 4 Si
2.1
1.8
0.86
0.00
Type of Hydrogen
(R = alkyl)
N ame of
Hydrogen
Chemical
Sh ift ()
RCH3 , R2 CH2 , R3 CH
Alk yl
0.8 - 1.7
R2 C=C(R)CHR2
Allylic
1.6 - 2.6
RC CH
Acetylen ic
2.0 - 3.0
R2 C=CHR, R2 C=CH2
Vin ylic
4.6 - 5.7
RCHO
Ald ehydic
9.5-10.1
3. Diamagnetic effects from adjacent pi bonds
Type of H
RCH3
RC CH
R2 C=CH2
N ame
Alk yl
Acetylenic
Vin ylic
Chemical
Shift ()
0.8- 1.0
2.0 - 3.0
4.6 - 5.7
Delta Scale
Chapter 13
34
Location of Signals
• More electronegative atoms deshield
more and give larger shift values.
• Effect decreases with distance.
• Additional electronegative atoms cause
increase in chemical shift.
Predict the number of signals and multiplicity of respective
signals in the following:
CH3CH3
CH2CH3
ClCH2CH2CH2Cl
Cl2CHCH2Cl
A compound has molecular formulae C10H14. It gives following
NMR data:
0.88 δ (9 H, singlet)
Assign the structure.
7.28 δ (5 H, singlet, ar. protons)
NMR spectra of Ethane
NMR spectra of Propane
NMR spectra of Butane