This document provides an introduction to nuclear magnetic resonance spectroscopy (NMR). It discusses the basic principles of NMR, including nuclear magnetic moments, how nuclei absorb energy in an external magnetic field, and the instrumentation of NMR spectrometers. It also covers concepts like chemical equivalence, chemical shift, spin-spin splitting, and applications of NMR including structure elucidation and detecting functional groups.
3. Purcell
Bloch
Introduction
The phenomenon of nuclear magnetic
resonance was first enunciated by American
physicist Felix Bloch of the Stanford university and
Edward Purcell of the Harvard University in 1946
for which they shared the Nobel Prize in 1952.The
subject has grown so fast it has become one of the
most useful methods for structural elucidation
available to the organic chemist.
4. Nuclear Magnetic Moments :-
Proton being positively charge species
bearing spin quantum number I = ½. and number
of spin state is 2.
Fig :- proton acts as bar magnet.
5. Nuclear spins are oriented randomly in the absence (a) of an
external magnetic field but have a specific orientation in the
presence (b) of an external field, B0
• Some nuclear spins are aligned parallel to the external field
Lower energy orientation
More likely
• Some nuclear spins are aligned antiparallel to the external field
Higher energy orientation
Less likely
Nuclear Magnetic Resonance Spectroscopy
6. The energy difference DE between nuclear spin states
depends on the strength of the applied magnetic field
• Absorption of energy with frequency n converts a nucleus from
a lower to a higher spin state
- DE = 8.0 x 10-5 kJ/mol for magnetic field strength of 4.7 T a
• For field strength of 4.7 T a radiofrequency (rf) of n = 200 MHz
is required to bring 1H nuclei into resonance
Nuclear Magnetic Resonance Spectroscopy
8. 1] Shielded Proton :-
Circulation of electrons about the proton itself generates a field
aligned in such a way that, at the proton it opposes the applied field. The
field felt by the proton is thus diminished, & the proton is said to be
shielded
2] Dishielded Proton :-
Circulation of electron specifically π electron about nearby nuclei
generates a field that can either oppose or reinforce the applied field at
the proton, depending upon the proton’s location. If the induced field
reinforce the applied field. The proton said to be deshielded.
Interpretation of NMR (PMR) Spectrum :
9. 3] Chemical Shift :-
Compared with a naked proton, a shielded proton requires a higher
applied field strength & a deshielded proton requires a lower applied field
strength to provide the particular effective field strength at which absorption
downfield such shift in the position of NMR absorption, arising from
shielding & deshielding by electrons are called chemical shift.
Effect of Electronegativity :-
The chemical shift simply increases as the electro negatively of
attached element increases.
Comp (CH3X) CH3Cl CH3Br
Element (X) Cl Br
Electro negativity of X 3.1 2.8
Chemical Shift δ 3.05 2.68
10. Magnetic Anisotropy :-
In Benzene, secondary magnetic field generated by circulating π
electrons deshields aromatic protons that gives the benzene proton a
chemical that is greater than expected.
Fig. Magnetic Anisotropy Of Benzene And Acetylene
In Acetylene π electron generate secondary anisotropic field. The
magnetic field generated by induced circulation of π electron has a
geometry such that the acetylene hydrogen are shielded.
11. Chemical Equivalence :-
i] Chemically Equivalent Protons :-
All proton are found in chemically identical environments within a
molecule are chemically equivalent & they often same chemical shift.
ii] Chemically non-equivalent Proton :-
A molecule that has set of proton that are chemically distinct from one
another may give rise to a different absorption peak from each set the sets of
proton called as chemically non-equivalent proton.
12. Spin-Spin Splitting .
The multiplicity of the absorption signal of a set of equivalent proton is
given by
(n+1) rule :- If all the adjacent non-equivalent protons are equivalent where is
number of adjacent non-equivalent protons.
(na+1) (nc+1) rule :- If the adjacent non-equivalent protons are of two different
kinds.
Where,
na is the number of adjacent non-equivalent protons of one kind
nc is the number of adjacent non-equivalent of different kind.
ex :-
13. The N + 1 Rule
If a signal is split by N equivalent protons,
it is split into N + 1 peaks.
14. Consider 1H NMR Spectrum of ethyl iodide by N + 1 group
The -CH2 hydrogen are spilt into 3 +1 = 4 lines
The -CH3 hydrogen are spilt into 2 + 1 = 3 lines
17. Applications :-
It helps to find out different kinds of protons in a
molecule.
The peak area under the signal gives us idea about the
proton ratio in the compound.
It is used to detect the presence of – OH group when H of
– OH group is replaced by D. The peak of – OH group in
the spectrum gets disappear.
It is used to detect hydrogen bonding inter & intra
molecular bonding shifts the absorption of protons
downfield (toward higher δ – value)
