The document provides an overview of the theory of nuclear magnetic resonance (NMR) spectroscopy. It discusses how nuclei with spin absorb electromagnetic radiation when placed in a magnetic field, creating distinct energy levels. When radio waves are applied at the resonance frequency, transitions between spin states occur, producing signals in the NMR spectrum. Chemical shifts arise from electrons shielding or deshielding nuclei from the magnetic field in different ways. Neighboring protons cause splitting of peaks according to spin-spin coupling rules.

1. Theory of NMR
Name -Ashis Kumar Dash
Regd no- 210705100034

2. Outline
• Introduction
• Theory of NMR
• Energy levels of nucleus in applied magnetic field
• Role of radio frequency
• Chemical shift
• Shielding and Deshielding
• Downfieild shift
• General principles for spin – spin interactions

3. Introduction
• Nuclear magnetic resonance involves absorption of electromagnetic
radiation in the radio frequency region.
• Absorption of radiowaves in the presence of magnetic field is accompanied
by a special type of nuclear transition,and for this reason we call this type of
spectroscopy nuclear magnetic resonance (NMR) spectroscopy.
• NMR was first discovered in 1946 by F.Bloch (Stanford University) and E.M
Purcell of (Havard University). They shared noble prize in 1952 for this
discovery.

4. Theory
• All the nuclei carry positive charge. Each proton and neutron inside the
nucleus have their own spin about their axis,and give rise to a resultant spin
of the nucleus called nuclear spin.
• All nuclei carry a charge , since a charged particle spinning around its axis
gives rise to a magnetic moment and generates a magnetic field.
• When such nuclei placed in an applied magnetic field, magnetic interaction
takes place (i.e –attrraction / repulsion) leading to creation of magnetic
energy levels in the nuclei.
• The proton and neutron each has spin ½ , thus the nuclear spin is the
resultant spin of protons and neutrons.
Ex- Deuteron contains one proton and neutron. (⇈) or (⇵)
Nuclear spin=1 ↲ ↳ Nuclear spin =0

5. (a) Nucleus which doesn’t have mechanical spin, i.e – Spin quantum
number(I)=0. Such nucleus has no magnetic properties.
(b) Nuclei having either an odd number of protons or an odd number of
neutrons, exhibit half integral I. i.e – I=1/2, 3/2, 5/2 .
(c) Odd proton and odd neutrons, exhibits integral spin quantum no- 1,2,3 etc.
(d) Even proton and neutrons have no angular momentum(I=0), have no
magnetic properties.

6. Energy levels of a spinning nucleus in an applied magnetic field
• Spinning nucleus (I>0) acts as a tiny magnet. When such nucleus is placed in
magnetic field , magnetic interaction takes place and magnetic energy levels are
created.
• Without applied magnetic field, the spin states of nuclei are degenerate and
energy level transition doesn’t arise.
• Magnetic nucleus can take up different energy levels or orientations with respect
to the direction of applied field.
• For a nucleus, there are (2I+1) possible orientations.
Ex- For Hydrogen the spin quantum no(I)=1/2.when it is placed in a magnetic field
of strength (Ho) have orientations (2×1/2+1)=2.(i.e.-1/2 and +1/2).
- Among these two types of orientations one is parallel orientation (low energy
state /magnetic moment is alligned parallel with the field). Another one is anti
parallel (high energy state / magnetic moment is against the applied field).

8. Role of radio frequency on the spinning
nucleus
• Spinning nucleus under the influence of the applied magnetic field rotates
about it’s axis at a definite frequency.
• When an alternating radio frequency field is applied to the spinning nucleus
at right angle to the applied field, if the radio frequency is exactly equal to
the frequency of the spinning nucleus , then these frequencies are said to be
in resonance ( resonance frequency).
• In the state of resonance, absorption or emission of the radiation takes place
by the nucleus which leads to transition between nuclear spin states. The
absorbed frequency flips the nucleus from low energy spin state to high
energy spin state. This absorption process produces a signal which is
recorded as a band in the spectrum called the NMR spectrum.

9. • Chemical shift – Nuclei are essential parts of molecules held together by
the electrons of-the chemical bonds.The magnetic field at which a free
or ‘bare’ nucleus resonates is quite different from the field at which the
same nucleus in a molecule resonates. Electrons in a molecule affects the
NMR.The electrons may shield or dishield a nucleus from the applied
magnetic field. The difference between the magnitude of magnetic field at
which a free nuclei and a molecular nuclei resonate is called chemical shift.
• Shielding and Deshielding of protons – A bare or pure proton when
exposed to a radio frequency, it flips it’s spin from lower energy level(⇈)
to a higher energy level (⇵). However all the protons don’t flip their spin.
The magnetic field “sensed” by the nucleus isn’t same as the applied
magnetic field.
• The electrons present near to the bond / protons induce their own
magnetic field, which may reinforce or oppose the applied magnetic field.

10. General rules for spin spin interactions-
• Protons of the same group don’t interact with each other to give splitting
signal.
Ex- 3 protons in the -CH3 group don’t interact with each other.
• The multiplicity of a peak of group of equivalent protons is determined by
the neighbouring protons. If “n” number of protons present in the adjacent
carbon atom then the peak split into (n+1) signals.
• The intensities of the peak are symmetric at the mid point. Pascal’s triangle
formed.
Downfield shift – most of the organic Compounds are less
shielded than those of TMS and therefore require less energy for
resonance. Thus the peaks appear at lower fields or downfields of
the TMS reference peak.

13. Thank you