INDERPRASTHA ENGINEERING COLLEGE, GHAZIABAD

DEPARTMENT OF APPLIED SCIENCES

Nuclear Magnetic Resonance (NMR) Spectroscopy

• NMR spectroscopy involves transition of nucleus from one spin state to another with
absorption of Electromagnetic radiation (Radio waves) when placed in strong magnetic
field.
• NMR spectroscopy reveals information about the number and types of protons and
carbons (and also other elements like nitrogen, fluorine etc.) present in the molecule.
• the absorption of radio-frequency photon promotes a nuclear spin from its ground state to
its excited state.

Selection rule for NMR

The nuclei having a nuclear spin (I) not equal to zero is spin active nuclei.

NMR Active Nuclei

The number of allowed orientations is given by (2I + 1), where I is spin quantum number of
the nucleus.
Example for 1H, I=1/2 [2I+1= 2x½ + 1= 2 Orientations] either aligned or opposed to the field.
Principle
Nucleus of hydrogen (proton) behaves as a tiny spinning magnet bar. It possesses both
electric charge and mechanical spin. As any spinning charged body will generate a magnetic
field, so the nucleus of hydrogen also generates its own magnetic field.

Effect of external magnetic field

In presence of an external magnetic field, protons respond to the influence of the external
magnetic field and tend to align itself with that field. Proton adopts two orientations with respect
to an external field:

Spin rotation is known as precessional motion or precessional frequency of proton which is

directly related to the external magnetic field.
ν ∝ 𝐵𝑜
 INDERPRASTHA ENGINEERING COLLEGE, GHAZIABAD
DEPARTMENT OF APPLIED SCIENCES
Flipping of protons
It is possible to induce transition between these two states i.e. proton processing in the aligned
orientation can pass into opposed orientation by absorbing energy. Also, it can come back
from the high energy, antiparallel alignment by losing energy. This transition from one energy
state to the other is called Flipping of the protons.

Chemical shift
Shift in the position of NMR spectrum as compared to standard reference, due to chemical
environment of proton is called chemical shift (δ-value)
Or
Chemical shift is the separation between the peak of reference standard (TMS) and any other
peak in NMR spectrum. This difference in the absorption position of the proton with respect to
TMS signal is called as chemical-shift value.

Standard Reference for NMR

The universally accepted standard is NMR signal of tetramethyl silane (TMS). (Why TMS used
as standard reference)

1. TMS has 12 protons in magnetically equivalent positions so it gives intense sharp signal.
2. It is chemically inert and soluble in organic solvent used
3. It is highly volatile (low boiling) and is readily removed from system.
4. It does not take part in intermolecular association with sample. (δ-value for TMS equal
to zero).
 INDERPRASTHA ENGINEERING COLLEGE, GHAZIABAD
DEPARTMENT OF APPLIED SCIENCES
Units Used in NMR
The scale is commonly expressed as parts per million (ppm) which is independent of the
spectrometer frequency. The scale is the delta (δ) scale and Tau (τ).

The τ scale is calculated using the equation τ = 10 - δ

Solvents for NMR spectroscopy

NMR spectra are usually measured using solutions of the substance being investigated. It is
important that the solvent itself does not contain any simple hydrogen atoms, because they
would produce confusing peaks in the spectrum. There are two ways of avoiding this.
1. Use a solvent such as tetrachloromethane, CCl4, which does not contain any
hydrogen.
2. Use a solvent in which any ordinary hydrogen atoms are replaced by its isotope,
deuterium - for example, CDCl3 instead of CHCl3.
Deuterium atoms have sufficiently different magnetic properties from ordinary hydrogen that
they do not produce peaks in the area of the spectrum that we are looking at.
Shielding and Deshielding Effects
Circulation of electron around the protons generates a induced field, if that opposes the
applied field then the field experienced by proton is diminished and proton is said to be
shielded from influence of applied field.
Greater the electron density around the proton greater will be shielding and greater will be
external field required to overcome shield effect.
 INDERPRASTHA ENGINEERING COLLEGE, GHAZIABAD
DEPARTMENT OF APPLIED SCIENCES
Deshielding effect
Circulation of electrons around the proton generates a field reinforce the applied field the
proton experiences a higher field strength as compared to applied field and is said to be
deshielded. Thus, smaller external field is required to bring the deshielded proton to
resonance.

Instrumentation
It relies on the phenomenon of nuclear magnetic resonance and provides detailed information
about the structure, dynamics, reaction state, and chemical environment of molecules.

NMR Spectroscopy Instruments

This instrument consists of nine major parts. They are discussed below:
Sample holder – It is a glass tube which is 8.5 cm long and 0.3 cm in diameter.
Magnetic coils – Magnetic coil generates magnetic field whenever current flows through it
Permanent magnet – It helps in providing a homogenous magnetic field at 60 – 100 MHZ
Sweep generator – Modifies the strength of the magnetic field which is already applied.
Radiofrequency (RF) transmitter – It produces a powerful but short pulse of the radio waves.
Radiofrequency – It helps in detecting receiver radio frequencies and RF detectors helps in
determining unabsorbed radio frequencies.
Recorder – It records the NMR signals which are received by the RF detector and record data
in computer system.
 INDERPRASTHA ENGINEERING COLLEGE, GHAZIABAD
DEPARTMENT OF APPLIED SCIENCES
Interpretation of NMR spectra
Number of signals in the NMR spectrum give the information about the number of different
sets of equivalent protons in a molecule.
Equivalent and Non-Equivalent Protons
Chemically equivalent protons are those protons which reside in the same magnetic
environment. This type of protons absorbs in the same δ value at a certain applied field
strength i.e they give only one signal.
On the other hand, chemically non-equivalent protons are those protons which reside in
different magnetic environment. This type of protons absorb in different δ value at a certain
applied field strength i.e they give more than one signal, depending on the number of protons.

The protons on the same carbon are always equivalent. The answer of this question is
simply no.
Examples of compounds shows equivalent and non-equivalent NMR signals

Multiplicity of lines in NMR signal for a group of protons

Spin-spin coupling: Means coupling interaction between the neighbour protons. It is
responsible for splitting of signals in proton NMR Spectra. Multiplicity of the signals tells which
atoms are present in neighbouring groups. If n neighbouring protons will be present then
multiplicity of signal is [n+1].
 INDERPRASTHA ENGINEERING COLLEGE, GHAZIABAD
DEPARTMENT OF APPLIED SCIENCES
Splitting of the NMR signals in high resolution is given by Pascal’s triangle shown in given
table
 INDERPRASTHA ENGINEERING COLLEGE, GHAZIABAD
DEPARTMENT OF APPLIED SCIENCES
Influencing factor for chemical shift
There are several factors on which chemical shifts depends.
1. Electronegativity
Greater the electronegativity of the atom (X) near the proton to be investigated, greater
is the chemical shift for the proton. Thus, for the following molecules the δ values are

Since the electronegativity of the halogens is in the order I < Br < Cl < F, so, shielding
effect of the protons in the compounds follows the order CH3I > CH3Br > CH3Cl > CH3F,
hence the order of δ values follows.
As these effects are transmitted through a chain, so, chemical shift decreases with
increase in chain length.

2. Hydrogen Bonding
The hydrogen bonded proton being attached to a highly electronegative atom, is less
shielded and the field felt by such a proton is more. As a result, resonance occur
downfield (higher δ value). Downfield shift depends upon the strength of H-bonding.

Intermolecular H-bonding show a downfield shift of absorption, while

Intramolecular H-bonding does not show any shift in absorption.

3. Anisotropic effect
When a magnetic is applied to a molecule containing π electrons, then these electrons
begin to circulate perpendicularly to the direction of the applied field, which results in
the production of induced magnetic field. The effect of this induced magnetic field on
nearby proton depends upon the orientation of the proton with respect to the π
electrons that produces the induced magnetic field.
 INDERPRASTHA ENGINEERING COLLEGE, GHAZIABAD
DEPARTMENT OF APPLIED SCIENCES
Applications of NMR spectroscopy

1. Structure determination: different types of proton, number of signals, intensity area,

chemical shift-shielded or deshielded, multiplicity with neighbouring proton.
2. Widely used in MRI and medical diagnosis.
3. Determination of hydrogen bonding
4. Drug screening and designing