CHEMISTRY - BE-I Year (July’ 2014), Sem -A

Nuclear Magnetic Resonance (NMR) spectroscopy

NUCLEAR MAGNETIC RESONANCE (NMR)
Nuclear Magnetic Resonance (NMR) is a spectroscopic technique that reveals information about the
environment of magnetically active nuclei. Under proper conditions, such nuclei absorb electromagnetic
radiation in the radio-frequency region at frequencies governed by their chemical environment. This
environment is influenced by chemical bonds, molecular conformations, and dynamic processes. By
measuring the frequencies at which these absorption's occur and their strengths, it is usually possible to
deduce facts about the structure of the molecule being examined.

NMR is commonly used in organic chemistry to elucidate molecular structures and conformations by
studying 1H and 13C nuclei. NMR is sensitive to many other nuclei, however, and is not restricted to
these uses. The field of NMR continues to grow at a prodigious rate and applications of NMR can be
found in virtually every field of chemistry. NMR has even lead to the development of Magnetic
Resonance Imaging (MRI), an important medical imaging technique.

SPIN
All nuclei have a property called spin. As moving charge produces electrical
field as well as magnetic field and nucleus is considered as moving or spinning
charge particle and hence produces magnetic field. Spin describes the nature of
that nucleus' magnetic field. Spin is characterized by a spin number, I, which
can be zero or some positive integer multiple of 1/2 (e.g. 1/2, 2/2, 3/2, etc.).
Nuclei whose spin number, I, equals zero have no magnetic field, while nuclei
with non-zero spin numbers do have magnetic fields. Higher values of spin
number imply more complex magnetic fields.
Some common nuclei, such as 12C and 16O, have I=0 and have no magnetic
field. This makes these nuclei invisible to NMR spectroscopy. Other elements
such as 1H, 13C, 19F and 31P have I=1/2, while others have even higher spin
numbers:
I=1 e.g. 14N, 2H
I=3/2 e.g. 11B, 35Cl, 37Cl, 79Br, 81Br.
As the values for I increase, the shapes of the magnetic fields become
progressively more and more complex.

While the shape of a nucleus' magnetic field is described by I, it's strength is

described by , the magnetogyric ratio. A nucleus with a large magnetogyric ratio has a stronger
magnetic field than a nucleus with a small magnetogyric ratio. The strongest magnetic field is
conveniently possessed by 1H, the most common isotope of the most common element. These simple
facts explain why hydrogen NMR (also called proton NMR) is the most popular form of NMR today.
Carbon-13 and phosporus-31 NMR are probably close seconds.

EFFECT OF EXTERNAL MAGNETIC FIELD

If two magnets are brought near each other they will exert a force on each other and will try to align
themselves. For simple bar magnets, the favoured alignment is parallel (north pole of one magnet faces
the south pole of the second).

Similarly, when a magnetic nucleus (I>0) is placed between the poles of an external magnet, it too will
try to align itself with respect to this externally applied magnetic field (Ho). In the macroscopic world,
two magnets can be aligned in an infinite number of orientations . At the atomic level, these alignments
are quantized. There are only a finite number of alignments a nucleus can take against an external
magnetic field. This number depends on the shape of the nucleus' magnetic field and therefore depends
on the value of its spin number I. Each possible alignment is assigned a value called Iz which ranges
from -I to +I in steps of 1. These orientations are referred to as spin states. In general, a nucleus will
have (2I+1) orientations (spin states) with respect to the external magnetic field.

DR. JITENDRA SINGH, IET-DAVV, INDORE 1

CHEMISTRY - BE-I Year (July’ 2014), Sem -A

The diagram illustrates the possible spin states for a spin 1/2 and a spin 1 nucleus.

Nuclei with I=1/2 have a simple magnetic field and can align With (Iz = 1/2) or Against (Iz = -1/2) the
external magnetic field yielding two spin states. Nuclei with I=1 have a more complex magnetic field
and can have one of three orientations (Iz = 1, 0, -1).

PRECESSION
Classical magnets, when brought together, will align exactly parallel to
each other and will maintain this alignment in a static fashion. Magnetic
nuclei, due to restrictions described by quantum mechanics, do not align
exactly parallel to or against the external magnetic field but rather, they
align at an angle. This has an important consequence that can be illustrated
by considering a gyroscope.

A gyroscope is first encountered by many people as a child's toy. A

gyroscope is typically a spinning mass supported in a frame. A spinning
gyroscope, when placed in a specific orientation, will tend to hold that
orientation despite the effects of external forces like gravity.

In a vertical gravity field (gravity pulling straight down) a gyroscope placed vertically will maintain this
orientation motionlessly. If the gyroscope is placed at an angle to the gravity field, it will rotate about an
axis parallel to this field demonstrating something call precession. The frequency of this precession
depends on two factors, the force exerted by the gravity field, and the force exerted by the gyroscope.
This can be illustrated in two ways. A mass placed on the upper tip of a gyroscope will increase the
force exerted by the gravity field and the precession frequency will increase. Increasing the speed or
mass of the gyroscope's spinner will increase the force it exerts and, again, the precession frequency
will increase.

A magnetic nucleus in an external magnetic field behaves very much like a gyroscope and precesses
about the external magnetic field. The angular frequency at which this precession occurs is given by

and is called the Larmor frequency. The value, , is the magnetogyric ratio and is characteristic for
each type of nucleus. It relates to the strength of the nucleus' magnetic field. H is the strength of the
externally applied magnetic field. For example, a 1H atom in a magnetic field H=1.41 Tesla has a
Larmor frequency of 60 megahertz (MHz).

DR. JITENDRA SINGH, IET-DAVV, INDORE 2

CHEMISTRY - BE-I Year (July’ 2014), Sem -A

ENERGIES
The orientations a magnetic nucleus can take
against an external magnetic are not of equal
energy. Spin states which are oriented more nearly
parallel to the external field are lower in energy
than in the absence of an external field. In contrast,
spin states whose orientations more nearly oppose
the external field are higher in energy than in the
absence of an external field.

Where an energy separation exists there is a

possibility to induce a transition between the
various spin states. By irradiating the nucleus with
electromagnetic radiation of the correct energy (as
determined by its frequency), a nucleus with a low
energy orientation can be induced to "jump" to a
higher energy orientation. The absorption of energy
during this transition forms the basis of the NMR
method.

When discussing NMR you will find that spin state energy separations are often characterized by the
frequency required to induce a transition between the states. While frequency is not a measure of energy
of these states, the simple relationship E=hv (where E=energy, h=Planks constant, and v=frequency)
makes this substitution understandable. The statement "the transition (peak) shifted to higher
frequencies" should be read as "the energy separation increased".

Comparison of various spectroscopic methods

NMR ~60x106 to 600x106 Hz Probes nucleus' magnetic field
ESR ~1x109 to 30x109 Hz Probes electron's magnetic field
Microwave ~1x109 to 600x109 Hz Probes molecular rotation
Infrared ~6x1011 to 4000x1011 Hz Probes bond vibrations and bending
Ultraviolet/Visible ~7.5x1014 to 300x1014 Hz Probes outer core electron transitions
Mössbauer ~3x1016 to 300x1016 Hz Probes inner core electron transitions
Probes transitions within the nucleus

Population Distribution
In a given sample of a specific nucleus, the nuclei will be distributed throughout the various spin states
available. Because the energy separation between these states is comparatively small, energy from
thermal collisions is sufficient to place many nuclei into higher energy spin states. The numbers of
nuclei in each spin state are described by the Boltzmann distribution

where the N values are the numbers of nuclei in the respective spin states, is the magnetogyric ratio, h
is Planck's constant, H is the external magnetic field strength, k is the Boltzmann constant, and T is the
temperature. For example, given a sample of 1H nuclei in an external magnetic field of 1.41 Tesla
Ratio of populations = e((-2.67519x108 rad.s-1.T-1 * 1.41T * 6.626176x10-34 J.s) / (1.380662x10-23 J.K-1 *K 293))
= 0.9999382
At room temperature, the ratio of the upper to lower energy populations is 0.9999382. In other words,
the upper and lower energy spin states are almost equally populated with only a very small excess in the
lower energy state.
When a system is irradiated with a frequency, whose energy matches that separating the levels,
transitions will be induced not only from the lower energy level to the higher, but also in the reverse
direction. Upward transitions absorb energy while downward transitions release energy.

DR. JITENDRA SINGH, IET-DAVV, INDORE 3

CHEMISTRY - BE-I Year (July’ 2014), Sem -A

NMR EXPERIMENT
When a sample of magnetically active nuclei is placed into an external magnetic field, the magnetic
fields of these nuclei align themselves with the external field into various orientations. Each of these
spin-states will be nearly equally populated with a slight excess in lower energy levels. During the
experiment, electromagnetic radiation is applied to the sample with energy exactly equivalent to the
energy separation of two adjacent spin states. Some of the energy is absorbed and the alignment of one
nucleus' magnetic field reorients from a lower energy to a higher energy alignment (spin transition). By
sweeping the frequency, and hence the energy, of the applied electromagnetic radiation, a plot of
frequency versus energy absorption can be generated. This is the NMR spectrum.

In a homogeneous system with only one kind of

nucleus, the NMR spectrum will show only a
single peak at a characteristic frequency. In real
samples the nucleus is influenced by its
environment. Some environments will increase
the energy separation of the spin-states giving a
spin transition at a higher frequency. Others will
lower the separation consequently lowering the
frequency at which the spin transition occurs.
These changes in frequency are called the
chemical shift of the nucleus and will be
examined in more detail. By examining the exact
frequencies (chemical shift) at which the spin
transitions occur conclusions about the nature of
the various environments can be made.

In a simply type of experiment, where the frequency is swept across a range, is known as a continuous
wave (CW) experiment. One simple variation on this experiment is to hold the frequency of the
electromagnetic radiation constant and to sweep the strength of the applied magnetic field instead. The
energy separation of the spin states will increase as the external field becomes stronger. At some point,
this energy separation matches the energy of the electromagnetic radiation and absorption occurs.
Plotting energy absorption versus external magnetic field strength produces the identical NMR
spectrum.

In fact, the NMR spectrum obtained by plotting

magnetic field increasing to the right will be a
mirror image of the spectrum where frequency
is plotted increasing to the right. Low energy
transitions (to the left) in a frequency swept
experiment will not occur until very high
magnetic fields (to the right) in a magnetic field
swept experiment. Early NMR spectrometers
swept the magnetic field since it was too
difficult to build the very stable swept RF
sources that NMR required. Even today where
this is no longer required, NMR spectra are still
plotted with magnetic field increasing to the
right.

Technological advances have made the CW

experiment obsolete and today virtually all
NMR experiments are conducted using pulse methods. These methods are inherently much more
sensitive and this explains part of their popularity.

DR. JITENDRA SINGH, IET-DAVV, INDORE 4

CHEMISTRY - BE-I Year (July’ 2014), Sem -A

THE NMR SPECTRUM - INTRODUCTORY COMMENTS

The magnetic nuclei are influenced by their chemical environments. In this section we will explore how
the chemical environment influences the nuclei and what effects will be produced in the NMR
spectrum. By understanding these effects, we will be able to interpret the features of an NMR spectrum
and make predictions about the structure of the compound being examined.

SHIELDING AND DESHIELDING -

CHEMICAL SHIFT
Electrons, like nuclei, have an inherent magnetic
field (Electrons have spin = 1/2). The strength of this
magnetic field is, however, several thousands of
times stronger. Also, because the electron's charge is
negative rather than positive, the polarity of the
magnetic field generated by an electron is opposite
that of the nucleus.

In a molecule, the nucleus is always surrounded by

an electron cloud. As a result, the nucleus will
experience an effective magnetic field (Heff) which is
a combination of the externally applied magnetic
field and the magnetic field generated by the electron
cloud surrounding the nucleus. Since the electron's
magnetic field opposes the external magnetic field,
the nucleus is "shielded" from the full force of the
external magnetic field. Heff is normally less than Ho.

Within a molecule there are factors which can

increase or decrease the electron density surrounding
a nucleus. Factors which reduce the electron density
are said to deshield the nucleus since Heff at the nucleus increases. Similarly, factors which increase the
electron density are said to "shield" the nucleus since Heff will decrease.

Previously we saw that the energy separation of a nucleus' spin states is proportional to the strength of
the magnetic field that the nucleus is exposed to. If the magnetic field strength is reduced by shielding,
the spin state energy separation decreases and a lower frequency of RF can induce a spin transition.
Analogously, if the nucleus is de-shielded, the Heff at the nucleus is larger, the spin state energy
separation increases, and a higher frequency of RF is needed to induce a spin transition. This change in
absorption frequency due to shielding is called the Chemical Shift.

The chemical shift of a nucleus is the difference between the resonance frequency of the nucleus and a
standard, relative to the standard. This quantity is reported in ppm and given the symbol delta, δ.

νsample - ν reference
δ= × 106 (ppm)
ν0
Hsample - H reference
OR δ = × 106 (ppm)
H0
1
Consider H NMR. On a spectrometer where the proton transition frequencies are nominally 60 MHz,
the chemical shift frequency changes may be only hundreds of hertz; about a million times smaller than
the resonance frequency. Because of how the electron shielding and deshielding effects arise, they are
proportional in strength to that of the externally applied magnetic field. Because of this relationship,
chemical shifts are typically reported as a fraction of the nominal resonance frequency. Due to their
small size, parts-per-million (ppm) are used.

DR. JITENDRA SINGH, IET-DAVV, INDORE 5

CHEMISTRY - BE-I Year (July’ 2014), Sem -A

STANDARD FOR NMR SPECTROSCOPY

The following compounds are generally used as internal standard (reference materials )in NMR
spectroscopy-
1. Tetramethylsilane (TMS)-
TMS is generally employed as internal standard for measuring the position of 1H and 13C in the NMR
spectrum because of the following facts:
1. It gives single sharp line from twelve magnetically equivalent protons.
2. It is chemically inert and miscible with a large range of solvents.
3. It, being highly volatile, can easily be removed if the sample has to be recovered.
4. It does not involve in intermolecular association with the sample.
5. It has the added advantage that its resonance position is far removed from the absorptions due to
protons in most organic molecules. Therefore, by arbitrarily assigning TMS = 0, it is possible to
device a scale in which most proton resonance will be of the same sign.

2. Sodium salt of 3-(trymethylsylyl)propane sulphonate

This water soluble compound is commonly used as an internal standard for running the PMR spectra of
water soluble materials in deuterium oxide solvents.
CH3
H3 C

Si SO 3 Na
H3C Si CH3 H3 C
CH3
CH3

Tetramethylsilane (TMS) Sodium salt of 3-(trymethylsylyl)propane sulphonate

FACTORS AFFECTING THE VALUE OF CHEMICAL SHIFT –

1. Inductive effect (Electronegativity)
2. Anisotropic effects
3. van der Waal’s deshielding
4. Hydrogen bonding
5. Hybridization

1. Inductive effect (Electronegativity)-

A nucleus is shielded or deshielded whenever it is influenced by the magnetic fields of nearby electrons.
The closest electrons to a nucleus are those that bond the nucleus to its neighbouring atoms. Any factor
that effects the distribution of these bonding electrons will also effect the degree of shielding the
nucleus experiences. Electronegativity is one important factor that will alter the distribution of bonding
electrons.

Electronegative atoms have an affinity for electrons. The more electronegative the atom is, the stronger
this affinity. Consider the following two cases; an 1H atom bonded to a carbon; an 1H atom bonded to
an oxygen. Carbon is less electronegative than oxygen. In an oxygen-hydrogen bond, the bonding
electrons will be drawn towards the oxygen. The electron density around the hydrogen atom will be
reduced in comparison to the same hydrogen bonded to a carbon atom. In the case of an O-H bond,
hydrogen has a lower electron density surrounding it and is, therefore, less shielded. Electronegative
atoms or electron withdrawing functional groups are considered to be deshielding. Electropositive
atoms or electron donating functional groups are considered to be shielding.

As the electronegativity of atom increases the proton becomes

more deshielded and the peak is shifted towards down field
direction (left hand side away from TMS signal). The inductive
effect decreases with distance and hence deshielding decreases as
the distance increases.

DR. JITENDRA SINGH, IET-DAVV, INDORE 6

CHEMISTRY - BE-I Year (July’ 2014), Sem -A

2. Anisotropic effects (Pi-System Ring Currents)-

The magnetic fields associated with the electron circulations induced by applied magnetic field H0, in
molecules containing pi bonds , are anisotropic, that is, they so not sum to zero in overall possible
orientations of the molecule with respect to applied magnetic field H0 . This phenomenon is, therefore,
called anisotropic effect. The effect is particularly important in molecule containing pi bonds for
example alkenes, alkynes, aromatic systems and carbonyl compounds.

The bonding electrons always produce a shielding effect. The strength of this effect will vary depending
on the exact nature of the bonded atoms but the resulting Heff will always be less then Ho. There are,
however, some effects which can actually produce a magnetic field that reinforces Ho. One of these
effects is a ring current.

The primary shielding effect comes from the electron's inherent

magnetic field. We know from physics, however, that a moving
charge (current) can also produce a magnetic field by virtue of
its motion. Most organic compounds contain a carbon atom
skeleton. When the atoms of this skeleton are sp2 hybridized,
there is a p-orbital available on each carbon. There is a strong
tendency for p-orbitals on adjacent carbon atoms to align and
when they do a π-system is produced. Organic compounds
containing π-system permit this type of electron "motion".
Because the benzene ring is the classic example of this type of
system, this electron "motion" is termed a ring current.

As in the case of an individual electron, the magnetic field

generated by this ring current opposes the externally applied
magnetic field, Ho. The aryl hydrogens, however, are located in
the "return" portion of the ring current magnetic field and
experience a magnetic field which reinforces Ho. Ring currents
strongly deshield the aryl hydrogens. This can be seen more
clearly in the cut-away view of the aryl -system. The direction
of the ring current magnetic field is shown by the black arrows.

The situation is exactly analogous for other π-system. The vinyl

system is the other commonly encountered π-system. As seen
below, the ring current generates exactly the same type of field
and, again, the vinyl hydrogens are located in the "return" region
of the magnetic field. These hydrogens are strongly deshielded
although to a lesser extent when compared to aryl hydrogens.

In some rigid systems it is possible for hydrogens to be located

directly above or below the -system. Thus they will be shielded
by the -system's magnetic field. While these cases are
uncommon, one should be aware of them as the effect can be
pronounced.

3. van der Waal’s deshielding-

In overcrowded molecule, when the molecular geometry constrains two groups of protons, in a rigid
molecule, to approach more closely than the sum of their van der Waals radii, weak repulsive forces,
called van der Waals forces operate between them. These force result in the decrease of electron density
around the nucleus causing deshielding of the nucleus (downfield shift). Although the intramolecular
van der Waals deshielding effects are are small usually less than one ppm however valuable
conformational information may be derived from such chemical shifts.

DR. JITENDRA SINGH, IET-DAVV, INDORE 7

CHEMISTRY - BE-I Year (July’ 2014), Sem -A

4. Hydrogen bonding-
Hydrogen bonding involves electron cloud transfer from hydrogen atom to neighbouring
electronegative atoms. The electron withdrawal from a proton, involved in hydrogen bonding, thus
causes deshieling of the proton. Greater is the degree of hydrogen bonding of a proton, the greater the
downfield shift (higher delta value) of its resonance.

5. Hybridization-
As the s-character of hybrid orbital increases the electronegativity of carbon also increases and hence
the electron cloud shifted towards carbon. This results in the downfield shift of peak in NMR spectrum.

SPIN- SPIN COUPLING

Spin-Spin coupling (splitting of NMR signals) arises
due to interaction of magnetic moment of particular
nuclei with magnetic moments of nearby nuclei. The
Heff that a nucleus experiences is primarily a
combination of the externally applied magnetic field
and the shielding and deshielding effects of bonding
electrons & nearby ring currents. A smaller, but very
important, effect is produced by the magnetic fields of
neighbouring nuclei. While these fields are very small,
they can produce an observable effect if the nuclei are
brought close enough together. Typically, "close"
means within 3 bonds.

To understand this effect, consider CHbCl2CHaBr2

molecule having two types of suitably close hydrogen
atoms , A and B, that have different chemical shifts.
Hydrogen A has two spin states, IZ=+1/2 and IZ= -1/2.
and in the absence of hydrogen B, it will give a single
NMR peak. It's neighbour, hydrogen B, has a small
magnetic field, however, which is either aligned with
or against the external field. We know that the two
orientations have about the same probability.

When hydrogen B's field is aligned with Ho, hydrogen

A will be deshielded and Heff will become slighter
larger. When hydrogen B's field is aligned against Ho,
hydrogen A will be shielded and Heff will become
slightly smaller.

In our sample, half of the molecules will have hydrogen B's magnetic field aligned with Ho. Hydrogen
A in these molecules will resonate at a higher frequency. The other half of the molecules will have
hydrogen B's magnetic field aligned against Ho and hydrogen A will resonate at a lower frequency. The
presence of hydrogen B, therefore, causes the single peak for hydrogen A to split into a doublet (two
peaks).

The frequency separation between these two peaks is called J, the dipolar coupling constant, and
typically is between 0 and 15 Hz. Whereas, the chemical shift is dependent on the strength of Ho and is
reported in ppm, the coupling constant is independent of Ho and is reported in Hz. An important point
to note is that if hydrogen B splits the peak for hydrogen A by J Hz, the the reverse is true too.
Hydrogen A will split the peak for hydrogen B by J Hz as well. This fact is very useful for identifying
which hydrogens are coupled to each other.

DR. JITENDRA SINGH, IET-DAVV, INDORE 8

CHEMISTRY - BE-I Year (July’ 2014), Sem -A

As the number of coupled hydrogens increases, the splitting pattern becomes more complicated.
Consider a hydrogen (A) located adjacent to a methyl group.

The diagram, to the right, illustrates the possible alignments that the methyl hydrogen magnetic fields
can have and the effect this has on hydrogen A's spin state energies. For three identical hydrogens there
are eight possible combinations of Up/Down spin. The net shielding or deshielding experienced at
hydrogen A depends on the sum of the three hydrogen's magnetic
fields. The most shielding (lowest frequency) occurs when all three
hydrogens oppose Ho (DDD). The next most shielded case occurs
when two of the three hydrogens oppose Ho. There are three
combinations of magnetic fields that can produce this effect (DDU
DUD UDD).

All of the combinations have the same probability of occurring (1/8th). Since only one combination can
produce the lowest frequency NMR peak compared to three combinations producing the next higher
frequency peak, the relative intensities of these peaks will be 1 to 3. The situation is exactly analogous
for the remaining two cases, one hydrogen opposing Ho and no hydrogens opposing Ho. Overall, the set
of four NMR peaks have the intensity ratio of 1:3:3:1.

INTENSITIES OF MUTIPLATES:

Where a single coupled hydrogen produces a doublet, three identical coupled hydrogens produce a
quartet. In general, N identical coupled nuclei will produce (2N+1) lines. The intensities of the lines can
be predicted (for N identical coupled spin 1/2 nuclei) by using Pascal's triangle.
SINGLET 1 no coupled nuclei
/ \
DOUBLET 1 1 1 coupled spin 1/2 nucleus
/ \ / \
TRIPLET 1 2 1 2 coupled spin 1/2 nuclei
/ \ / \ / \
QUARTET 1 3 3 1 3 coupled spin 1/2 nuclei
/ \ / \ / \ / \
QUINTET 1 4 6 4 1 4 coupled spin 1/2 nuclei

In Pascal's triangle, a number is always the sum of the two numbers in the preceding row that straddle it.

DR. JITENDRA SINGH, IET-DAVV, INDORE 9