Dr. Farshid Zand
Organic Chemistry II – 233
Department of Chemistry
San Diego Mesa College
CHAPTER 13: Nuclear Magnetic Resonance
(1)
General Information
Spectroscopy: a technique for analyzing molecules based on how they absorb radiation.
In organic chemistry we use: NMR, IR, UV, MS for molecules identification.
Organic molecules absorb energy in discrete packages called: quanta.
excited
state
The absorbed energy produces motion: excitation.
The absorbed energy ∆E=hν (ν: Frequency, h: Planck’s constant)
and ν= c/λ (c: light velocity, λ: wavelength)
∆E=hν
ground
state
NMR:
Nuclei of atoms with an odd mass number or odd atomic number have nuclear spin and are
assigned a spin quantum number: I. Only atoms with I≠0 give rise to NMR. In this case the
nucleus behaves as a small magnet and is allowed to exist in 2I+1 energy levels.
1H
2H
and
13C
have an I=1/2 and they have (2∙1/2+1) = 2 energy levels
and
14N
have an I=1 and they have (2∙1+1) = 3 energy levels
12C and 16O have an I=0 and they have (2∙0=1) = 1 energy levels (NMR inactive)
Dr. Farshid Zand
Organic Chemistry II – 233
Department of Chemistry
San Diego Mesa College
CHAPTER 13: Nuclear Magnetic Resonance
(2)
General observations obtained from NMR spectra:
1. The number of different signals indicates how many different (non equivalent) nuclei are
in a molecule and provides information about the overall symmetry of the molecule.
2. In the 1H NMR the integration of peaks shows the relative amounts of protons/peak.
(Integration of a carbon spectrum is, however, problematic and not always reliable).
3. The chemical shift of each nuclei depends on their electronic environment and provides
information about the presence of electron donating or electron withdrawing groups.
4. The splitting pattern reveals which nuclei are close enough so they can interact under the
influence of a magnet (spin-spin coupling).
Integration of peaks (only for 1H NMR)
________________________________
The NMR absorption of nuclei is cumulative. This means that the more the protons of the
same family the bigger the overall area of the peak as compared to other peaks. By
integrating the peaks we can estimate the ratio of equivalent protons. The integration is
reliable only for protons but not for carbons.
Chemical shift
_______________
Chemical shift is the relative position of a peak using (CH 3)4Si=TMS as reference.
 The sift if reported in ppm values (or δ values) relative to TMS (0 ppm) and the higher the
number the more to the left of the spectrum the peak is.
 The δ value of a proton is independent of the spectrometer with which it was recorded and
depends only on the electronic (magnetic) environment of the nucleus that we observe.
 Electron density around a nucleus shields the nucleus from the effect of the magnet (it
reduces the effect of the external magnet) and shifts the corresponding peak to the right
of the NMR spectrum (higher field). In contrast, removing el. density deshields the
nuclei and causes resonance at lower fields (left on the NMR spectrum).
 A 1H NMR is usually recorded between 0-10 ppm, while a
13C
NMR is recorded between 0-
220 ppm Note that there is a better resolution for the carbon NMR.
Dr. Farshid Zand
Organic Chemistry II – 233
Department of Chemistry
San Diego Mesa College
CHAPTER 13: Nuclear Magnetic Resonance
(3)
Chemical shift: effects of electron density and relative electronegativity
_________________________________________________________

Adding electron density around a nucleus shields the nucleus from the effect of the
magnet and shifts the peak to the right of the NMR spectrum (higher field or upfield shift).

In contrast, removing electron density deshields the nuclei and cuases resonance at
lower fields (left on the NMR spectrum) also referred to as downfield shift.

In general, attaching more electronegative atoms on a nucleus decreases its electron density
and shifts its NMR signal downfield.
The effect of electron density is
cumulative which means that more
electron withdrawing groups enhance the
deshielding effect of a nucleus
The 1H chemical shift of hydrogens bound to
oxygen varies depending on the extent of the
hydrogen bonding, which depends on the
concentration. In general acid hydrogens are
more deshielded.
Dr. Farshid Zand
Organic Chemistry II – 233
Department of Chemistry
San Diego Mesa College
CHAPTER 13: Nuclear Magnetic Resonance
(4)
Chemical shift: Effect of multiple bounds
Double bonds: Elements found on sp2-hybridized carbon centers (double bonds or aromatic
rings) are much more deshielded than would be expected based only on electronegativity
arguments. Explanation: The motion of electrons of a double bond creates an induced
magnetic field that reinforces the external magnet, producing an overall deshielding effect.
In the aromatic rings this electronic motion is referred to as ring current.
Triple bonds: Elements found on sp-hybridized carbon centers (triple bonds) are much
more shielded than would be expected based only on electronegativity arguments.
Explanation: The motion of electrons of a triple bond creates an induced magnetic field
that weakens the effect of the external magnet, producing an overall shielding effect.
Dr. Farshid Zand
Organic Chemistry II – 233
Department of Chemistry
San Diego Mesa College
CHAPTER 13: Nuclear Magnetic Resonance
(5)
Spin-spin coupling and splitting patterns (The N+1 rule)
Nuclear spin active atoms (I≠0) whose nuclei are in close proximity (1-3 bonds apart) interact with each
other. This effect is transmitted through covalent bonds and depends on the number and type of bonds
that separate the nuclei as well as on their stereochemical relationship.
These interactions are called spin-spin couplings and produce different splitting patterns which produce
information on the number of atoms that split the observed nuclei and their stereochemical relation.
The measure of interaction is called coupling constant J (given in Hertz).
For example each proton (I=1/2; 2 energy levels can exist in two different energy levels in a ratio
statistically 1:1. As such it can split the proton on the next carbon in 2 peaks of equal intensity (1:1 ratio)
producing a doublet with a well defined coupling constant (J). In a HNMR the splitting between H-H
is observed 3 bonds apart. In a CNMR there is only splitting between H-C and is observed 1 bond
apart (H-coupled CNMR).
If 2 equivalent protons split the neighboring proton they will produce two partially overlapping doublets
of equal J value (because they are equivalent), which will show as a triple of 1:2:1 ratio.
If 3 equivalent protons are attached on the same carbon they will produce three partially overlapping
doublets of equal J value, which will show as a quartet of 1:3:3:1 ratio.
Note: In the case of aliphatic chains, although the protons may not belong in the same family, they split
the neighboring protons with the same constant coupling (usually 7H z). In these cases, they are
considered identical and they are subject to the N+1 rule.
Dr. Farshid Zand
Organic Chemistry II – 233
Department of Chemistry
San Diego Mesa College
CHAPTER 13: Nuclear Magnetic Resonance
(6)
Spin-spin coupling and splitting patterns (The N+1 rule)
For the same reason a proton (2 spin energy levels) can split its neighboring carbon
producing a doublet of 1:1 ratio which can be observed in a “proton-coupled” carbon
spectrum.
One could, however, excite all protons to the highest energy level and then record the 13C
NMR spectrum. In this case the carbon peaks will become singlets. This experiment is
referred to as a “proton-decoupled” carbon spectrum.
Example
C NMR spectrum of compound A under “proton decoupled conditions”:
The C1 carbon should appear at 71 ppm (due to the 2 Cl atoms) as a singlet. For similar
reasons, the C2 carbon will be a singlet at 50 ppm.
13
C NMR spectrum of a compound A under “proton coupled conditions”:
The C1 carbon should appear at ca 71 ppm (due to the 2 Cl atoms) and should be a
doublet of equal intensity, due to the H1 spin-spin coupling. (Spin-spin couplings between
carbons cannot be detected and the H2 and H3 atoms are too far..). The C2 carbon is split
by 2 hydrogens (H2 and H3), so it should appear as a triplet at ca 50 ppm.
13
The 1H NMR spectrum of compound A:
The H1 is attached to a carbon having two Cl groups. It should be deshielded and appear
at ca 5 8 ppm. Its peak integration should be 1. It is split by two equivalent protons (H2
and H3) so it should be a triplet.
The H2 and H3 protons are equivalent and are attached to a carbon having one Cl group.
They should appear at ca 4 ppm and should integrate for 2. Both of them are split by one
proton (H1) so they should appear as a doublet.