2. THEORY
Nuclear magnetic resonance (NMR) spectroscopy is the study of molecules by
recording the interaction of radiofrequency (Rf), electromagnetic radiations with the
nuclei of molecules placed in a strong magnetic field.
“Zeeman first observed this type of activity in the end of 19th century, but practical
use of the so called ZEEMAN EFFECT was only made in 1950s when NMR
spectrometers are commercially available.
3. PRINCIPLE OF NMR SPECTROSCOPY
• All nuclei are electrically charged and many have spin.
• Transfer of energy is possible from base energy to higher energy levels when an
external magnetic field is applied.
• The transfer of energy occurs at a wavelength that coincides with the radio
frequency.
• Also, energy is emitted at the same frequency when the spin comes back to its
base level.
• Therefore, by measuring the signal which matches this transfer the processing of
the NMR spectrum for the concerned nucleus is yield.
4. This instrument consists of nine major parts:
1. Sample holder – It is a glass tube which is 8.5 cm long and 0.3 cm in diameter.
2. Magnetic coils – Magnetic coil generates magnetic field whenever current flows
through it
3. Permanent magnet – It helps in providing a homogenous magnetic field at 60 – 100
MHZ
4. Sweep generator – Modifies the strength of the magnetic field which is already
applied.
5. Radiofrequency transmitter – It produces a powerful but short pulse of the radio
waves.
INSTRUMENTATION
5. WORKING OF NMR
• Place the sample in a magnetic field.
• Excite the nuclei sample into nuclear magnetic resonance with the help of
waves to produce NMR signals.
• These NMR signals are detected with sensitive radio receivers.
• The resonance frequency of an atom in a molecule is changed by the
intramolecular magnetic field surrounding it.
• This gives details of a molecule’s individual functional groups and its electronic
structure.
• Nuclear magnetic resonance spectroscopy is a conclusive method of
monomolecular organic compounds.
• This method provides details of the reaction state, structure, chemical
environment and dynamics of a molecule.
6. CHEMICAL SHIFT
The chemical shift tells us about the magnetic field that the nucleus feels.
The chemical shift in absolute terms is defined by the frequency of the resonance expressed with
reference to a standard compound which is defined to be at 0 ppm. The scale is made more
manageable by expressing it in parts per million (ppm) and is independent of the spectrometer
frequency.
It is often convenient to describe the relative positions of the resonances in an NMR spectrum. For
example, a peak at a chemical shift, δ, of 10 ppm is said to be downfield or deshielded with respect to
a peak at 5 ppm, or the peak at 5 ppm is upfield or shielded with respect to the peak at 10 ppm.
8. ELECTRONEGATIVITY & INDUCTIVE EFFECT
• Greater the electronegativity, Greater is the deshielding
• Delta value will be more
HYDROGEN BONDING
• Downfield shift depends upon the strength of H-bonding
• Intra molecular H-bonding doesn’t show any shifting
absorption
VANDER WAALS DESHIELDING
• Electron crowd of a bulky group will tend to repel the electron
cloud (in over crowded molecules) surrounding the proton, &
the proton is deshielded
9. ANISOTROPIC (SPACE) EFFECT
• In a compound containing = or ≡ bond circulation of pi electrons about nearby nuclei generate
an induced field which can either oppose or reinforce the location of proton or the space
occupied by the proton.
• The occurrence of shielding or deshielding can be determined by the location of proton in the
space, that's why this effect is known as space effect.
• Anisotropy is non-uniformity.
• For different compounds this anisotropy is different as different distribution of electrons around
nuclei.
ALKENES
• H+ deshielded.
• Induced magnetic fluctuation (MF) – diamagnetic- act paramagnetic in the region of alkene
proton.
• H+ feels greater field strength.
ALKYNES
• H+ shielded.
• Induced field opposes the applied field.
• H+ feels smaller field strength.
10. 3.SHIELDING AND DESHIELDING
The basic principle of NMR is to apply an external magnetic field called B0 and
measure the frequency at which the nucleus achieves resonance.
Electrons orbiting around the nucleus generate a small magnetic field that opposes
B0. In this case we say that electrons are shielding the nucleus from B0.
Shielding:
The higher the electron density around the nucleus, the higher the opposing
magnetic field to B0 from the electrons, the greater the shielding. Because the
proton experiences lower external magnetic field, it needs a lower frequency to
achieve resonance, and therefore, the chemical shift shifts upfield (lower ppms) .
11. Deshielding:
If the electron density around a nucleus decreases, the opposing magnetic field becomes
small and therefore, the nucleus feels more the external magnetic field B0, and therefore it
said to be deshielded. Because the proton experiences higher external magnetic field, it
needs a higher frequency to achieve resonance, and therefore, the chemical shift shifts
downfield (higher ppms) .
12. Comparison of the chemical shift of CH4 protons and CH3Cl
protons:
Chlorine atom is an electronegative atom that will pull the electron density
toward it(electron withdrawing), resulting in a deshielding of the hydrogen
nucleus;
So it will fell higher external magnetic field B0 increasing the resonance
frequency and therefore, shifting to higher ppms.
Hydrogen nucleus is shielded in the case of CH4 and therefore, the peak
appears on the lower ppm side.
13. SOLVENTS USED IN NMR
CHARACTERSITICS OF SOLVENTS
• Chemically inert
• Solvents should be magnetically isotropic in nature
• Free from any hydrogen(1
1H) atom
• Solvent should be able to dissolve the molecule/sample in a reasonable quantity(approx.
or more)
For dissolving polar compound – Polar solvent is used
For dissolving non-polar compound – Non-polar solvent is used.
Commonly used solvents are:-
The following solvents are normally used in which hydrogen may be replaced by deuterium.
1. Carbon tetrachloride – CCl4
2. Carbon disulphide – CS2
3. Deuterochloroform – CDCl3
4. Hexachloroacetone – {(CCl3)2CO}
5. Deuterobenzene – C6D6
6. Deuterium oxide – D2O
14. NMR SIGNAL FOR THE GIVEN COMPOUNDS
a) C8H18 – Octane
CH3 – (CH2)6 – CH3 NOW IT HAS ONLY ONE SIGNAL
(B) (A) (B)
Octane has two signals.
b) C2H6O – Ethanol
NOW IT HAS ONLY ONE SIGNAL
Ethanol has 3 signals
c) C5H12 – Pentane
CH3 – (CH2)3 – CH3 NOW IT HAS ONLY ONE SIGNAL
(B) (A) (B)
Pentane has two signals.
d) C4H12Si – Tetramethylsilane
It has One signal.
C
C H 3
C H 3
C
H 3 C C H 3
C H 3
C H 3
2 , 2 , 3 , 3 - t e t r a m e t h y l b u t a n e
O C
H
3
H
3
C
m
e
t
h
o
x
y
m
e
t
h
a
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C
C H 3
C H 3
C H 3
H 3 C
2 , 2 - d i m e t h y l p r o p a n e
S i
C H 3
C H 3
C H 3
H 3 C
t e t r a m e t h y l s i l a n e
O
H
15. REASON FOR DIFFERENT FREQUENCY OF PROTON MAGNETIC RESONANCE
SIGNALS
Since electrons are charged particles, they move in response to the external magnetic field (Bo) so as
generate a secondary field that opposes the much stronger applied field. This secondary field shields
the nucleus from the applied field, so Bo must be increased in order to achieve resonance (absorption
of rf energy).
Bo must be increased to compensate for the induced shielding field.
The resonance condition ΔE = hν is satisfied when ΔE =fγB where B is the value of the magnetic field
the H nucleus being observed.
B can be regarded as having three components:
1. B0 the external applied magnetic field;
2. B1 the field due to electrons circulating near the nucleus being observed (i.e., chemical shift δ);
3. B2 the local magnetic field due to other magnetic nuclei (e.g. other H nuclei) near the observed H
nucleus (i.e., J or spin-spin coupling).
So in general differently situated H nuclei resonate at different ν values.
*(Note:- v = nu)
16. SPIN-SPIN COUPLING PATTERN FOR THE GIVEN MOLECULES ARE :-
1.CH3CHCl2 - There are two different chemical shifts for the H nuclei in this compound.
The n+1 rule tells that for the CH3 group the single H nucleus in the adjacent CHCl2 group
will
split its signal into a 1:1 doublet (2 equal peaks).
For the CHCl2 group the 3 protons in the adjacent CH3 group will split its signal into a
quartet:
four peaks of relative intensities 1:3:3:1.
The total intensities of the two signals will be in the ratio 3:1.
2.CH3CHClCH3 - For the CHCl group the 6 equivalent H nuclei in the two adjacent and
equivalent CH3 groups will give rise to a 7 peak signal ( a septet).
For the two equivalent CH3 groups the single H nucleus in the CHCl group will give rise to an
equal doublet signal.
The total intensities of the two signals will be in the ratio of 1:6.
3.CH3OCH2CH3 - The ether oxygen with its bonds means that the magnetic effects of the sets
of
H nuclei on the two sides of the ether O are not transmitted through it.
Thus the 3 H nuclei in the CH3O group give rise to one unsplit signal.
For the OCH2CH3 group we expect a 1:3:3:1 quartet for the CH2 group and a 1:2:1 triplet for
the CH3 group.
17. DIFFERENT CHEMICAL SHIFTS FOR THE GIVEN MOLECULES ARE :-
1. CH4 All H nuclei are equivalent since the molecule has a regular tetrahedral shape.
Equivalent protons have the same chemical shift and do not give rise to J splitting.
Only one chemical shift is expected.
2. CH3CH3 The chemical arrangement of molecule is the same as for CH4 and only one shift is
expected.
3. CH3CH2CH3 The two terminal CH3 groups are magnetically and chemically equivalent and
their hydrogen atoms will all have the same chemical shift.
The central CH2 group is different and will give a resonance signal at a different chemical
So two different chemical shifts are expected.
4. H2C=CH2 All H nuclei in this planar molecule are equivalent.
Only one chemical shift is expected.
5. H2C=CHBr Each H nucleus in this planar molecule is different, it has three different
molecules.
So three chemical shifts are expected.
18. APPLICATION OF NMR SPECTROSCOPY
• SOLUTION STRUCTURE
• MOLECULAR DYNAMICS
• PROTEIN FOLDING
• IONIZATION STATE
• PROTEIN HYDRATION
• HYDROGEN BONDING
• DRUG SCREENING AND DESIGN – Particularly useful for identifying drug leads and
determining the conformations of the compounds bound to enzymes, receptors, and
other proteins.
• METABOLITE ANALYSIS – A very powerful technology for metabolite analysis.
• CHEMICAL ANALYSIS – A matured technique for chemical identification and
conformational analysis of chemicals whether synthetic or natural.
