CH-01- Module-1 NMR Spectroscopy

1
B. Sc. Third Year
Academic Year 2020-2021
1
CH-01:- Module-1 : NMR Spectroscopy
Dr. B. N. Gawade
M. Sc. CSIR-UGC-NET (JRF), Ph. D.
Assistant Professor
Department of Chemistry
Anandrao Dhonde Alias Babaji Mahavidyalaya, Kada.
Tal. Ashti. Dist. Beed - 414 202, Maharashtra, INDIA
CH-01: NMR Spectroscopy
Welcome

Contents…
Nuclear magnetic resonance (NMR) spectroscopy.
 Proton magnetic resonance (1H NMR) spectroscopy,
 nuclear shielding and deshielding,
chemical shift and molecular structure,
 spin-spin splitting and coupling constants,
 areas of signals,
CH-01: Spectroscopy
2
BNG-Chem Study Channel

 interpretation of PMR spectra of simple organic molecules such as
ethyl bromide, ethanol, acetaldehyde, 1, 2, 2 tribromoethane, ethyl
acetate, toluene and Acetophenone.
 Problems pertaining to the structure elucidation of simple organic
compounds using UV, IR and PMR spectroscopic techniques.
(Combine and single λ max using woodword fischer rule)
3

Organic chemistry has been revolutionized in the past 50-60yrs by
the use of Spectroscopic Methods for analysis and structure
determination.
4

Spectroscopic methods have three major advantages over most chemical
methods.
 Spectroscopic methods are easier and faster than most chemical
reactions.
 Spectroscopic methods provide far more information about molecular
structure. Practically all functional groups and structural features can be
detected with very small amounts of sample.
 Spectroscopic methods are non-
destructive and, if necessary,
entire sample can be recovered.
5

 Spectroscopy is the study of the interaction of matter and energy, or
more generally, electro magnetic radiation.
 Atoms and molecules interact with electromagnetic radiation (EMR)
in a wide variety of ways.
 Atoms and molecules may absorb and/or emit EMR.
 Absorption of EMR stimulates
different types of motion in
atoms and/or molecules.
What is Spectroscopy ?
6

• The patterns of absorption (wavelengths absorbed and to what
extent) and/or emission (wavelengths emitted and their
respective intensities) are called ‘spectra’.
• The field of spectroscopy is concerned with the interpretation of
spectra in terms of atomic and molecular structure (and
environment).
7

Energy, Frequency and Wavelength of EMR are related by the
following expression.
• E - is energy
•  - is frequency
•  - Wave length
• h - Planck’s constant
its value is 6.626  10-34 Js
• c - velocity of light = 2.998  108 ms-1

hc
hv
E 

8

Wavelength
Energy
1

Frequency
Energy 
9

The Electromagnetic Spectrum
•Light comes in different colors
•No matter what part of range, there are some features in common,
that you should know.
B
A
Propagation of e,b fields, time
Amplitude
Photon , E= h
 is frequency, Hz, 1/sec
 = c/; | c = 3 x 1010 cm/sec
h = Planck’s constant = 6.624 x10-34 J.sec
/c = 1/  = wavenumber, cm-1
1 wavelength,  is a distance….
Peaks per time;
frequency
10

The Electromagnetic Spectrum
11

THE ELECTROMAGNETIC SPECTRUM
12

X-ray
UV/Visible
Infrared
Microwave
Radio Frequency
Bond-breaking
Electronic
Vibrational
Rotational
Nuclear and
Electronic Spin
REGION ENERGY TRANSITIONS
Types of Energy Transitions in Each Region
of the Electromagnetic Spectrum
(NMR)
13

Spectroscopy overview
• Spectroscopy relies on the absorption of energy to gain information
about the atoms in or structure of molecules
 UV/VIS – electron transition energy :200 – 750 nm
visible region (400 -750nm); Near uv (200-400nm)
 IR – bond vibrational energy: 5000-667 cm-1( 2µm).
 NMR – magnetic spin of certain nuclei
(Radio Frequency)
 MS - kinetic energy mass/charge ratio
14

 Nuclear magnetic resonance spectroscopy is one of the most
useful methods for structure elucidation.
 It was first observed in 1946 by Felix Bloch, Edward M. Purcell
(awarded Nobel Prize) and is based on the magnetic properties
exhibited by certain nuclei.
15

The Nobel Prize in Physics 1952 was awarded jointly to Felix Bloch
and Edward Mills Purcell…
"for their development of new methods for nuclear magnetic
precision measurements and discoveries in connection therewith"
Felix Bloch
Born: 23 October 1905,
Zurich, Switzerland
Edward Mills Purcell
Born: 30 August 1912,
Taylorville, IL, USA
16

 Many atomic nuclei behave as if they are spinning and hence are
said to have nuclear spin.
 The circulation or spinning of the nuclear charge generates a
magnetic dipole whose magnitude is given by nuclear magnetic
moment.
The spinning charged nucleus generates a magnetic field.
17

The magnetic properties occur in nuclei which have
1. Odd atomic and odd mass numbers,
e.g. 1H1, 15N7, 19F9, 31P15.
2. Odd atomic and even mass numbers,
e.g. 2H1, 14N7.
3. Odd mass numbers and even atomic number,
e.g. 13C6.
18

• Similar to Electron spin, the nuclear spin is given by nuclear spin
quantum number, I, which can have values 0, 1/2, 1, 3/2, 2, etc.
• The spin quantum number, I, has contributions both from protons
and the neutrons present in the nucleus.
• If sum of protons and neutrons is even then, I has Zero or integral
values, i.e., I = 0, 1, 2,…etc. If this sum is odd, then I has half integral
values,
i.e., I = 1/2, 3/2, 5/2, etc.
• When number of both the protons
and neutrons is even, then, I is zero.
19

• Spin, Charge and Magnetism are trios. Where there are two of
these, the third one appears automatically.
• Any charged spinning body behaves as a magnet.
• If we consider a proton, it is charged electrically and is spinning. It
behaves as tiny bar magnet or a magnetic needle.
20

What is the effect of external magnet on a needle of magnetic
compass?
 The needle will align along the applied external magnetic field.
What happens when proton is placed in an external magnetic field?
 It must also align along the applied external magnetic field.
21

Proton is different from that of compass needle
1. Proton is a very tiny particle
2. Proton is not static like needle instead it is dynamic. Because it
is a tiny particle.
3. It is governed by quantum restrictions.
22

23
Proton which is a very tiny magnet not only aligns parallel with the
external magnetic field like needle, it can also align in antiparallel way
to the external magnetic field
B0

Spin States
• Nuclei with an odd mass, an odd number of protons, or both,
are said to have spin angular momentum
• The number of allowed spin states is quantized, and is
determined by its spin quantum number, I
• There are 2I + 1 allowed spin states
24

BNG-Chem Study Channel 25
A spinning nucleus with a spin quantum number of ½ has 2 possible
spin states.
2I+1 = 2 (1/2) + 1 = 2
Nuclei with I = 0 have only one spin state and are NMR inactive.
These include 12C and 16O, two of the most common nuclei in
organic compounds.

Element 1H 2H 12C 13C 14N 16O 17O 19F
Nuclear Spin
Quantum No 1/2 1 0 1/2 1 0 5/2 1/2
( I )
No. of Spin 2 3 0 2 3 0 6 2
States
Spin Quantum Numbers of Some Common Nuclei
Elements with either odd mass or odd atomic
number have the property of nuclear “spin”.
The number of spin states is 2I + 1,
where, I is the spin quantum number.
The most abundant isotopes of C and O do not have spin.
26

The Most Interesting Elements (to us) All Have 2 Allowed Spin States
These are
•1H
•13C
•19F
•31P
Deuterium 2H is spin active with I = 1!
2 (1) + 1 = 3 spin states for deuterium
27

28
The spinning of the nuclei causes them to behave like magnets.
These nuclear magnets are influenced by other magnetic fields. These
other magnetic fields may be externally applied or they can be
generated by other nearby nuclei or electrons in the molecule.
Externally applied magnetic fields may result from the magnet that the
sample is placed in or from irradiation by radio frequency light.

In an Applied Magnetic Field
• Nuclei with 2 allowed spin states can
align either with or against the field,
with slight excess of nuclei aligned
with the field
The nuclei precess about an axis
parallel to the applied magnetic
field, with a frequency called the
Larmor Frequency (w)
29

I = -1/2
I = +1/2
more populated
less populated
Bo
In an applied magnetic field the spin states have different energies
and therefore different populations.
Transitions may occur between these
energy states which allows NMR
signals to be observed.
The greater the difference in
population, the stronger the NMR
signal.

Larmor Frequency is Proportional to the Applied Magnetic Field
Slow precession in
small magnetic field
Faster precession in
larger magnetic field

The difference in energy between the 2 spin states is proportional
to the Larmor frequency

Rf Energy Can Be Absorbed
• Precessing nuclei generates an
oscillating electric field of the
same frequency
• Rf energy with the same
frequency as the Larmor
frequency can be applied to
the system and absorbed by
the nuclei

Old CW Instruments
• We held the Rf energy
constant and varied the
strength of the magnetic
field. When they matched,
energy was absorbed and
that change was observed
by the instrument.

How does our NMR observe the signals?
1) The sample tube is placed in a strong magnetic field to
produce the primary splitting of the energy levels and
create the necessary population imbalance.
2) The sample is irradiated with a range of radio frequency
light to transfer nuclei from the lower to the higher energy
state.

3) The oscillating magnetic fields produced by the nuclei are observed
using the same coil that was used for the irradiation. A complex,
decaying signal is observed that contains all of the information about
the nuclei. This is called the free induction decay (FID)
4) A Fourier transform is performed on the FID to produce an NMR
spectrum with each signal represented by a peak at its relative Larmar
frequency which is the frequency with which it wobbles as it spins.
This is actually done several times and the results are added to increase
the signal to noise ratio.

This can be pictured with vectors
• There are an assembly of nuclei,
almost 50% in each spin state.
There is a slight excess (1,000,048 vs
1,000,000 for protons in a 300 MHz
instrument) in the lower energy
state that causes a small net
magnetization in the z direction.

How Does this Happen in the Instrument?
Red arrow represents net magnetization (there is an
excess in the low spin state. The applied Rf energy causes
the net magnetization for all types of the nuclei to tip to
the x-y plane (90 degree pulse). It should be noted that all
nuclei of a given type are in synch with one another.

Nuclei relax back to their original state, emitting electromagnetic
radiation at their original Larmor frequencies
The data we get can be complex: it is a superimposed
combination of all the frequencies
(Note: this is the difference between the applied frequency and
the Larmor frequencies of the nuclei.)

The following features lead to the nmr phenomenon
• A spinning charge generates a magnetic field, The resulting
spin-magnet has a magnetic moment (μ) proportional to the
spin.

• Every spinning body precesses in external field
• Spinning proton not only align parallel or antiparallel way it also
precesses.

Nuclear Magnetic Resonance
• If the precessing nucleus is irradiated with electromagnetic radiation of
the same frequency as the rate of precession,
 the two frequencies couple,
 energy is absorbed, and
 the nuclear spin is flipped from spin state +1/2
(with the applied field) to -1/2
(against the applied field)
44

N
S
w
Nuclei precess at
frequency w when
placed in a strong
magnetic field.
hn
If n = w then
energy will be
absorbed and
the spin will
invert.
NUCLEAR
MAGNETIC
RESONANCE
NMR
RADIOFREQUENCY
40 - 600 MHz

Absorption of Energy
+1/2
-1/2
+1/2
-1/2
DE = hn
DE
quantized
Radio
frequency
Applied
Field
Bo
Aligned
Opposed

• Higher the applied field, the greater will be the energy difference
between two spin states of nucleus.
• Therefore, the larger frequency of radiation absorbed by the nuclei
and hence larger will be the separation between the absorption
signals leading to a higher resolution and more clear spectrum.

DE
+ 1/2
- 1/2
= kBo = hn
degenerate
at Bo = 0
Bo increasing magnetic field strength
The Energy Separation Depends on Bo

Resonance
• When Energy of source matches with the energy difference
between the nuclear spin states, then nuclei are said to be in
resonance with electromagnetic radiation.
• The energy difference between two spin states, ΔE, can be
quantitatively expressed as:
• Where, h is Planck’s constant,
B0 is applied magnetic field
strength at the nucleus and
 magnetogyric ratio
o
B
2π
h
hν
ΔE 



It is the ratio of the angular momentum (due to rotating nuclear mass)
and the magnetic moment (arising from rotating nuclear charge). This
ratio has a characteristic and different value for each nucleus.
Gyromagnetic ratio, , is a constant for each nucleus
(26,753 s-1gauss-1 for H).
In a 14,092 gauss field, a 60 MHz photon is required to flip a proton.
Low energy, radio frequency.
Gyromagnetic Ratio

It is the number of revolutions per second made by the magnetic
moment vector of the nucleus around the external magnetic field B0.
OR
It is defined as the frequency of the spinning bar magnetic equal to the
frequency of EMR in mega cycles per second necessary to induce a
transition from one spin to another.
Precessional frequency (υ)

There are two frequencies for the proton in external magnetic field;
1. Spinning Frequency
It remains constant irrespective of applied magnetic field.
2. Precessional frequency
It directly varies with the strength of the magnetic field.
Simple NMR experiment involves the measurement of precessional
frequency of the nucleus directly or indirectly.

• Resonance: In NMR spectroscopy, resonance is the absorption of
electromagnetic radiation by a precessing nucleus and the
resulting “flip” of its nuclear spin from a lower energy state to a
higher energy state

 Nuclear magnetic resonance involves the interaction between an
oscillating magnetic field of electromagnetic radiation and the
magnetic energy of hydrogen nucleus or some other type of nuclei
when these are placed in an external static magnetic field.

When nuclei with a spin quantum number of 1/2 are placed in an applied
field, a small majority of nuclear spins are aligned with the applied
field in the lower energy state
The nucleus begins to precess and traces out a cone-shaped surface, in
much the same way a spinning top or gyroscope traces
out a cone-shaped surface as it precesses in the earth’s
gravitational field
We express the rate of precession as a frequency in hertz

• Magnets varying in field strengths ranging from 1.4 to 7.1 are
employed in various instruments.
• The corresponding frequency values needed to observe resonance lie
between 60MHz and 300MHz.
• For proton when magnetic field of 1.4 is applied, a radiation of
frequency 60MHz is required.

The strength of the NMR signal depends on the Population Difference
of the two spin states
Resonance
induced
emission
excess
population
Radiation
induces both
upward and
downward
transitions.
For a net positive signal
there must be an excess
of spins in the lower state.
Saturation = equal populations = no signal
Population and Signal Strength

NMR Signals
 Signal: a recording in an NMR spectrum of a nuclear magnetic
resonance
 The number of signals shows how many different kinds of protons
are present.
 The location of the signals shows how shielded or deshielded the
proton is.
 The intensity of the signal shows the number of
protons of that type.
 Signal splitting shows the number of protons
on adjacent atoms.

The NMR Spectrometer

• The instrument used to detect the coupling of precession frequency
and electromagnetic radiation records it as a signal
• The sample absorbs electromagnetic radiations in radio wave region
at different frequencies since absorption depends upon the type of
protons or certain nuclei contained in sample.

• Essentials of an NMR spectrometer are a powerful magnet, a radio
frequency generator and a radio frequency detector
• The sample is dissolved in a solvent, most commonly CDCl3 or D2O and
placed in a sample tube which is then suspended in the magnetic field
and set spinning

Interpretation of NMR Spectra
At a given radio frequency, all protons in an organic molecule may
give NMR signal at different applied field strength that is measured
and against which the absorption is plotted.
The result is a spectrum showing many absorption peaks, whose
relative positions can give detailed information about the molecular
structure.

 The number of signals, which tells us how many different kinds
of protons in the molecule.
 The positions of signals tells us something about the electronic
environment of each kind of proton.
 The intensities of signals, which tells us how many protons of
each kind there are.
 The splitting of a signal into several peaks, which tells us about
the environment of proton with respect to other nearby
protons.

The NMR Graph

Number of Signals
Each signal in NMR spectrum corresponds to a set of equivalent protons.
1. Magnetically equivalent protons are chemically equivalent.
C
H
H
H H
one signal
C = O
CH3
CH3
one signal one signal
CH3-CH2-Cl
Two signals
CH3-CH2-NH2
Three signals

2. Chemically equivalent equivalent protons must also
stereo-chemically equivalent
C C
H
H
H
CH3
Four signals
C C
H
H
Cl
Three signals
CH3
CH3 CHCl
Cl Cl
C C
CH3
H H
O
Cyclopent-
anone
(2 signals)
1,1-Dichloro-
ethane
(2 signals)
(Z)-1-Chloro-
propene
(3 signals)
Cyclohexene
(3 signals)

Shielding and Deshielding
In Organic molecules, the hydrogen atom is bonded to another atom
by a covalent bond. When an external magnetic field is applied,
electrons forming covalent bond generate secondary magnetic field
i.e. induced magnetic field which opposes the applied magnetic field.
Hence, the magnetic field felt by the proton is less than the applied
magnetic field and is said to be shielded proton

 Electrons shield the
“spin” nuclei from
the magnetic field
 Different chemical
environments lead
to different amounts
of shielding
Shielding
 Degree of shielding depends on the amount of
electron density surrounding the nucleus.

 Circulation of electrons, specially Pi electrons, about nearby nuclei
generates a field that can either oppose or reinforce the applied field
at the proton, depending on location of proton.
 If the induced field opposes the applied field, the proton is shielded.
 If the induced field reinforce the applied field, then the field felt by
the proton is said to be Deshielded.
Deshielding

Compared with a naked proton,
– A shielded proton requires a higher applied field strength and
– Deshielded proton requires a lower applied field strength
– The shielding and deshielding of protons by electrons cause
CHEMICAL SHIFT.

Shielded Protons
Magnetic field strength must be increased for a shielded proton
to flip at the same frequency.

Protons in a Molecule
Depending on their chemical environment,
protons in a molecule are shielded by different amounts.

The NMR Graph

CHEMICAL SHIFT
The shifts (compared with a standard reference) in the positions of
NMR absorption which arise due to the shielding or deshielding of
protons by electrons are called chemical shifts.
OR
The separation in the positions of the spectral signals of hydrogen
atoms in different chemically environments from that of some
arbitrarily chosen standard is called the chemical shift.

Chemical Shift
 Measured in parts per million.
 Ratio of shift downfield from TMS (Hz) to total spectrometer
frequency (Hz).
 Same value for 60, 100, or 300 MHz machine.
 Called the delta scale.

80
PEAKS ARE MEASURED RELATIVE TO TMS
TMS
shift in Hz
0
Si CH3
CH3
CH3
CH3
Tetramethylsilane
“TMS”
Reference compound
n
Rather than measure the exact resonance position of a peak, we measure how
far downfield it is shifted from TMS.
Highly shielded
protons appear
way upfield.
Chemists originally
thought no other
compound would
come at a higher
field than TMS.
downfield

Delta (δ)-scale
On Tau(τ)-scale, the position of the TMS signal is taken as 10.0 ppm and most
chemical shift have values between 0 and 10 ppm.
A small numerically value of ‘τ’ represents a low field
absorption and high value indicates a high field absorption.
Tau (τ)-scale
On delta (δ)- cale, the position of the TMS signal is taken as 0.0 ppm and most
chemical shift have values between 0 and 10 ppm.
A small numerically value of ‘δ’ indicates a small downfield shift while large value
indicates a large down field shift.

h =
Bo

2p
constants
frequency field
strength
Stronger magnetic fields (Bo) cause
the instrument to operate at higher
frequencies ().
Remember From Our Earlier Discussion
NMR Field
Strength
1H Operating
Frequency
60 Mhz
100 MHz
300 MHz
7.05 T
2.35 T
1.41 T
 = ( K) Bo

TMS
shift in Hz
0
n
downfield
The shift observed for a given proton
in Hz also depends on the frequency
of the instrument used. Higher frequencies
= larger shifts in Hz.
Higher Frequencies Give Larger Shifts

chemical
shift
= d =
shift in Hz
spectrometer frequency in MHz
= ppm
This division gives a number independent
of the instrument used.
parts per
million
THE CHEMICAL SHIFT
The shifts from TMS in Hz are bigger in higher field instruments (300 MHz, 500
MHz) than they are in the lower field instruments (100 MHz, 60 MHz).
We can adjust the shift to a field-independent value, the “chemical shift” in the
following way
A particular proton in a given molecule will always
come at the same chemical shift (constant value).

0
1
2
3
4
5
6
7 ppm
Hz Equivalent
of 1 ppm
1H Operating
Frequency
60 Mhz 60 Hz
100 MHz 100 Hz
300 MHz 300 Hz
Hertz Equivalence of 1 ppm
Each ppm unit represents either a 1 ppm change in Bo
(magnetic field strength, Tesla) or a 1 ppm change in
the precessional frequency (MHz).
1 part per million
of n MHz is n Hz
n MHz = n Hz
1
106
( )
What does a ppm represent?

NMR Correlation Chart

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.
87

Typical Values

Aromatic Protons, d7-d8

Vinyl Protons, d5-d6

Acetylenic Protons, d 2.5

Aldehyde Proton, d9-d10
Electronegative
oxygen atom

O-H and N-H Signals
 Chemical shift depends on concentration.
 Hydrogen bonding in concentrated solutions deshield the
protons, so signal is around d 3.5 for N-H and d 4.5 for O-H.
 Proton exchanges between the molecules broaden the peak.

Carboxylic Acid Proton, d 10+

Number of Signals
Equivalent hydrogens have the same chemical shift.

Intensity of Signals
 The area under each peak is proportional to the number of protons.
 Shown by integral trace.

How Many Hydrogens?
When the molecular formula is known, each integral rise can be
assigned to a particular number of hydrogens.

SPIN-SPIN SPLITTING
How it Happens ?

C C
H H
C C
H H
A A
upfield
downfield
Bo
The Chemical Shift of Proton HA is
Affected by the Spin of its Neighbors
50 % of
molecules
50 % of
molecules
At any given time about half of the molecules in solution will
have spin +1/2 and the other half will have spin -1/2.
aligned with Bo opposed to Bo
neighbor aligned neighbor opposed
+1/2 -1/2

n + 1 RULE
1,1,2-Trichloroethane
C C
H
Cl
Cl H
H
Cl
integral = 2
integral = 1
Where do these multiples come from ?
….. interaction with neighbors

C C
H H
H
C C
H H
H
Two neighbors
n+1 = 3
triplet
One neighbor
n+1 = 2
doublet
singlet
doublet
triplet
quartet
quintet
sextet
septet
MULTIPLETS
This hydrogen peak
is split by its two
neighbors
These hydrogens are
split by their single
neighbor

C C
H H
C C
H H
one neighbor
n+1 = 2
doublet
one neighbor
n+1 = 2
doublet
SPIN ARRANGEMENTS
yellow spins
blue spins
The resonance positions (splitting) of a given
hydrogen is affected by the possible spins of its
neighbor.

C C
H H
H
C C
H H
H
two neighbors
n+1 = 3
triplet
one neighbor
n+1 = 2
doublet
SPIN ARRANGEMENTS
methylene spins
methine spins

Three neighbors
n+1 = 4
quartet
Two neighbors
n+1 = 3
triplet
SPIN ARRANGEMENTS
C C
H H
H
H
H
C C
H H
H
H
H
Methyl
spins
Methylene spins

Often a group of hydrogens will appear as a multiplet rather than as a single peak.
SPIN-SPIN SPLITTING
Multiplets are named as follows:
Singlet Sextet
Doublet Septet
Triplet Octet
Quartet Nonet
Quintet
This happens because of interaction with neighboring
hydrogens and is called SPIN-SPIN SPLITTING

integral = 2
integral = 1
triplet doublet
1,1,2-Trichloroethane
The two kinds of hydrogens do not appear as single peaks, rather there is a “triplet”
and a “doublet”.
The sub peaks are due to spin-spin splitting
and are predicted by the n+1 rule.

EXCEPTIONS TO THE N+1 RULE
IMPORTANT
1) Protons that are equivalent by symmetry usually do not split one another
CH CH
X Y CH2 CH2
X Y
no splitting if x=y no splitting if x=y
2) Protons in the same group usually do not split one another
C
H
H
H C
H
H
or

3) The n+1 rule applies principally to protons in aliphatic (saturated) chains
or on saturated rings.
CH2CH2CH2CH2CH3
CH3
H
or
but does not apply (in the simple way shown here) to protons on double bonds
or on benzene rings.
CH3
H
H
H
CH3
NO NO
YES YES

CH2 CH2
X Y
CH CH
X Y
CH2 CH
CH3 CH
CH3 CH2
CH3
CH
CH3
( x = y )
( x = y )
SOME COMMON SPLITTING PATTERNS

SOME EXAMPLE SPECTRA WITH SPLITTING
NMR Spectrum of Bromoethane
CH2CH3
Br

NMR Spectrum of 2-Nitropropane
1:6:15:20:16:6:1
in higher multiplets the outer peaks
are often nearly lost in the baseline

NMR Spectrum of Acetaldehyde
offset = 2.0 ppm
C
CH3
O
H

INTENSITIES OF MULTIPLET PEAKS
1 2 1
PASCAL’S TRIANGLE
1
1 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
singlet
doublet
triplet
quartet
quintet
sextet
septet
octet
The interior
entries are
the sums of
the two
numbers
immediately
above.
Intensities of
multiplet peaks

J J
J
J J
THE COUPLING CONSTANT
 The coupling constant is the distance J (measured in Hz)
between the peaks in a multiplet.
 J is a measure of the amount of interaction between
the two sets of hydrogens creating the multiplet.
C
H
H
C H
H
H
J

100 MHz
200 MHz
1
2
3
4
1
2
3
100 Hz
200 Hz
200 Hz
400 Hz
J = 7.5 Hz
J = 7.5 Hz
7.5 Hz
7.5 Hz
Coupling constants are
constant - they do not
change at different
field strengths
The shift is
dependant
on the field
ppm
FIELD COMPARISON
Separation
is larger

100 MHz
200 MHz
1
2
3
4
1
100 Hz
200 Hz
J = 7.5 Hz
J =7.5 Hz
ppm
2
200 Hz
400 Hz
3
4
J = 7.5 Hz
Note the compression of
Multiplets in the 200 MHz
spectrum when it is plotted
on the same scale as the
100 MHz spectrum instead
of on a chart which is twice
as wide.
Separation
is larger

1
2
1
2
100 MHz
200 MHz
Why buy a higher
field instrument?
Spectra are
simplified!
Overlapping
multiplets are
separated.
Second-order
effects are
minimized.
1
2
50 MHz
J = 7.5 Hz
J = 7.5 Hz
J = 7.5 Hz

NOTATION FOR COUPLING CONSTANTS
The most commonly encountered type of coupling is between hydrogens on adjacent
carbon atoms.
C C
H
H This is sometimes called vicinal coupling.
It is designated 3J since three bonds intervene between the two
hydrogens.
Another type of coupling that can also occur in special cases is
C H
H
2J or geminal coupling
Geminal coupling does not occur when
the two hydrogens are equivalent due to
rotations around the other two bonds.
( most often 2J = 0 )
3J
2J

Couplings larger than 2J or 3J also exist, but operate only in special situations.
Couplings larger than 3J (e.g., 4J, 5J, etc) are
usually called “long-range coupling.”
C
C
C
H H
4J , for instance, occurs mainly when the hydrogens are forced to adopt this
“W” conformation (as in bicyclic compounds).
LONG RANGE COUPLINGS

C C
H H
C C
H
H
C C
H
H
C
H
H
6 to 8 Hz
11 to 18 Hz
6 to 15 Hz
0 to 5 Hz
three bond 3J
two bond 2J
three bond 3J
three bond 3J
SOME REPRESENTATIVE COUPLING CONSTANTS
Hax
Hax
Heq
Heq
Hax, Hax = 8 to 14
Hax, Heq = 0 to 7
Heq, Heq = 0 to 5
three bond 3J
trans
cis
geminal
vicinal

C
H
C H
4 to 10 Hz
H C C C
H
0 to 3 Hz four bond 4J
three bond 3J
C C
C H
H
0 to 3 Hz four bond 4J
H
H
cis
trans
6 to 12 Hz
4 to 8 Hz
three bond 3J
Couplings that occur at distances greater than three
bonds are called long-range couplings and they are
usually small (<3 Hz) and frequently nonexistent (0 Hz).

Spin-Spin Splitting
 Nonequivalent protons on adjacent carbons have magnetic fields that may
align with or oppose the external field.
 This magnetic coupling causes the proton to absorb slightly downfield
when the external field is reinforced and slightly up field when the external
field is opposed.
 All possibilities exist, so signal is split.

1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.

Doublet: 1 Adjacent Proton

Triplet: 2 Adjacent Protons

Range of Magnetic Coupling
 Equivalent protons do not split each other.
 Protons bonded to the same carbon will split each other only if they are
not equivalent.
 Protons on adjacent carbons normally will couple.
 Protons separated by four or more bonds
will not couple.

Splitting for Ethyl Groups

Splitting for Isopropyl Groups

Values for Coupling Constants

Splitting Tree

Spectrum for Styrene

Stereochemical Nonequivalence
 Usually, two protons on the same C are equivalent and do not
split each other.
 If the replacement of each of the protons of a -CH2- group with
an imaginary “Z” gives stereoisomers, then the protons are non-
equivalent and will split each other.

Some Nonequivalent Protons

Time Dependence
 Molecules are tumbling relative to the magnetic field, so NMR is an
averaged spectrum of all the orientations.
 Axial and equatorial protons on cyclohexane interconvert so rapidly that
they give a single signal.
 Proton transfers for OH and NH may occur so quickly that the proton is not
split by adjacent protons in the molecule.

Hydroxyl Proton
 Ultrapure samples of ethanol
show splitting.
 Ethanol with a small amount of
acidic or basic impurities will
not show splitting.
135

N-H Proton
* Moderate rate of exchange Peak may be broad

CH-01- Module-1 NMR Spectroscopy

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CH-01- Module-1 NMR Spectroscopy