Nmr theory

NUCLEAR MAGNETIC RESONANCE
Page 1
SPECRTOSCOPY
INTRODUCTION
The phenomenon of nuclear magnetic resonance (NMR) was first
observed in 1946 and it has been routinely applied in organic
chemistry since about 1960.NMR spectroscopy is one of the most
useful technique for structural elucidation available to organic
chemist. In NMR we are going to study the interaction of the
magnetic component with the certain nuclei hence the name NMR
spectroscopy is given.
Princple
The NMR spectroscopy deals with nucleus of an atom that
possesses magnetic moment. The nucleus of an atom is
positively charged. Like electron the nuclei of certain atom also
spin about their axis . The spinning of these charged particle
generate a magnetic moment along the axis of the spin, so that
these nuclei act like tiny bar magnet. Individual protons &
neutrons have spin quantum no. +1/2 & -1/2 . Therefore
depending on number of nucleons (protons+ neutrons) certain
nuclei also possesses spin. The total spin quantum no. ‘I’ is a
characteristic of nucleus. The nuclei that have spin no. greater
than zero possesses spin.

Magnetic and Nonmagnetic Nuclei
To find out nuclear spin quantum no. I following rules are generally
applied.
a) Nucleus with I>0 when spins about its own axis generates a
magnetic moment and are known as magnetic nuclei.
b) The Nucleus with I=0 does not produce magnetic moment and
are known as nonmagnetic nuclei . The nuclei which have even
mass number and even atomic number have zero spin quantum
no. are nonmagnetic
c) The nuclei with I>0 possesses spin angular momentum and
produce magnetic field are known as magnetic nuclei.
1) The nuclei which have odd mass no. and odd or even
atomic no. have half integral spin like ½, 3/2, 5/2,
and possesses spin quantum no. I=1/2. These are
magnetic nuclei.
2) The nuclei having even mass no. and odd atomic no.
have integral spin quantum no. such as 1, 2, 3 etc.
possesses spin quantum no. I=1. These are also
magnetic nuclei

The number of orientations of the spin state is given by
(2I+1) where I is the spin quantum no. We know that proton
(H) has I=1/2 and it has only two orientations.
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No. of orientations for H =(2I+1)
=[2*1/2+1]
=2
Transition from one possible orientation to another may be
achieved by absorption or emission of electromagnetic
radiation.
The energy required for flipping depends upon strength
of applied magnetic field.
Processional motion of nucleus in applied magnetic
field.
Processional motion of nucleus in applied
magnetic field

Shielding and Deshielding

The induced magnetic field tries to protection the nucleus
from applied magnetic field H0 . This type of
protection of nuclei from H0 is called as diamagnetic
shielding . When nuclei is not protected by induced
magnetic field then Deshielding takes place .Thus the
shielding and Deshielding depends upon the electron
density around the proton.
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a) More the electron density around the proton more is
induced magnetic field and therefore more shielding
of proton from applied magnetic field.

Shielding
For example:-Acetylene molecule
Deshielding
Example:-Benzene molecule

CHEMICAL SHIFT
Each nucleus (proton) in different
environment requires a slightly different applied
magnetic field H0 for resonance and peaks occur
in different regions of the spectrum . For proton
NMR spectra of organic compound the single
resonance peak if the methyl groups in ‘Tetra
methyl silane’ (TMS) is taken as a internal
reference standard.
The distance in ‘d’ values from TMS to
each signal (absorption or peak ) in the spectrum
is called the chemical shift for the proton or
proton giving that signal.

Advantages of TMS
1. It is chemically inert.
2. It gives unique line position.
3. It is symmetrical molecule and gives single
and strong , sharp absorption peak as all 12
protons are equivalent.
4. Methyl protons of TMS are strongly shielded
and hence absorption occurs at high field . It
is taken as zero ppm.
5. It is soluble in most of organic solvents.
6. It is volatile(b.p.270c)and hence the recovery
of sample is possible.

Measurement of Chemical shift.
chemical shift is measured in frequency unit ‘Hertz’.
Most of routine instruments operate at 60, 90, 100
MHz.
More sophisticated instruments operate as high as
600MHz. The chemical shift recorded in Hz may vary
with the spectrometer. To avoid this complication the
chemical shift values are expressed in terms of delta or
tau scale. Which are independent of field strength.
Chemical shift in delta scale are expressed in parts per
million (ppm).
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Typical Values
Functional Group Chemical
Shift
Alkane 0.8-1.2
1.6
Benzyl 2.3
Carbonyl 2.2
Amine 2.3
Alcohol 3.3
Alkyl Halide 3.6
Alkene 4.5-6.0
Benzene 6.0-9.0
Alcohol 0.5-4.5
Very Broad
Carbox. acid 9.0-15.0
R CH3
C
HC
CH3
CH3
R C CH3
O
R N CH3
HO CH3
H3C Cl
H2C CH2
H
R OH
R
O
OH

Spin-SSppiinn CCoouupplliinngg((SSpplliittttiinngg))
The NMR spectrum at low resolution shows a no. of broad
absorption peaks (signals) corresponding to the protons in
different chemical environment. Area under each peak is
proportional to the no. of protons. However at high resolution
these NMR signal shows multiplicity. That is they get split
into several signals. It must be noted that area under signals
remain the same. This multiplicity of lines is due to spin-spin
coupling, which arise from small magnetic interactions that
occur between the nuclei of neighboring atoms. The nuclei are
said to be coupled and resulting NMR spectral pattern is
known as spin-spin splitting.

The N+1 Rule
If the signal is spit by N equivalent protons, it
is spit into N+1 peaks.
n n + 1 Pascal pattern:
0 singlet 1
1 doublet 2
2 triplet 3
3 quartet 4
4 quintet 5

Range of magnetic coupling
Equivalent protons do not split each other.
Protons bonded to the same carbon will split each
other only if they are not equivalent.
Protons on adjacent carbons normally will couple.
Protons separated by four or more bonds will not
couple.
Coupling Constant
The distance in Hz between adjacent peaks of a
multiplet .
 It is measured in Hz.
 Range o-8 Hz.
 Independent of magnetic field strength.
 Where as the chemical shift in Hz is directly
proportional to the field strength.

Other Techniques for NMR
Property Value
Spin ½
Natural abundance 100%
Chemical shift range 700 ppm, from -300 to 400
Frequency ratio (Ξ) 94.094011%
Reference compound CFCl3 = 0 ppm
Linewidth of reference
T1 of reference
Receptivity rel. to 1H at
natural abundance 0.83
Receptivity rel. to 1H when
enriched 0.83
Receptivity rel. to 13C at
natural abundance 4716
Receptivity rel. to 13C when
enriched 4716
19F NMR
19Fluorine is a sensitive nucleus which yields sharp signals and has a
wide chemical shift range.
A typical analysis of a 19F NMR spectrum may proceed similarly to
that of Proton (1H). Our NMR service provides 19F NMR along with
many other NMR techniques. The number of fluorines of each type in
the spectrum of a pure sample can be obtained directly from the
integrals of each multiplet provided that the multiplets are well
separated which is very likely to the large chemical shift range. A
routine NMR spectrum yields integrals with an accuracy of ±10%.
Accuracies of ±1% can be achieved by increasing the relaxation delay
to five times the longitudinal relaxation times (T1) of the signals of
interest.

13C NMR
The 1D 13Carbon NMR experiment is much less sensitive
than Proton (1H) but has a much larger chemical shift
range. Its low natural abundance (1.108%) and proton
decoupling means that spin-spin couplings are seldom
observed. This greatly simplifies the spectrum and
makes it less crowded. 13C is a low sensitivity nucleus
that yields sharp signals and has a wide chemical shift
range.
A typical analysis of a 13C NMR spectrum consists of
matching expected chemical shifts to the expected
moieties. Our NMR service provides 13C NMR along with
many other NMR techniques Each type of signal has a
characteristic chemical shift range that can be used for
assignment
Property Value
Spin ½
Natural abundance 1.108%
Chemical shift range 200 ppm, from 0 to 200
Reference compound TMS < 1% in CDCl3 = 0 ppm
Linewidth of reference 0.19 Hz
T1 of reference 9 s
Receptivity rel. to 1H at natural
abundance 1.70×10-4
enriched 0.0159
enriched 93.5

Property Value
Spin ½
Natural abundance 100%
Chemical shift range 430 ppm, from -180 to 250
Reference compound 85% H3PO4 in H2O = 0 ppm
Linewidth of reference 1 Hz
T1 of reference 0.5 s
Receptivity rel. to 1H at
natural abundance 6.63 × 10-3
enriched 6.63 × 10-3
enriched 37.7
31P NMR
The 1D 31Phosphorus NMR experiment is much less
sensitive than Proton (1H) but more sensitive
than 13Carbon. 31Phosphorus is a medium sensitivity nucleus
that yields sharp lines (fig. 1) and has a wide chemical shift
range. It is usually acquired with 1Hdecoupling (fig. 2)
means that spin-spin couplings are seldom observed. This
greatly simplifies the spectrum and makes it less crowded.
Where there are one-bond 31P-1H couplings present then
the decoupling power needs to be at lest twice that needed
for 13C because of the large coupling constant.

Conclusion
• 1) To find out different kinds of protons in
the molecule.
• 2) The intensity of a signal that is area under
the signal gives idea about the proton ratio in
the compound.
• 3) Position of signals tells us about the
electronic environment of proton.
• 4)The proton in the vicinity of aromatic ring
has higher delta value (7-9)
• 5)Multiplicity of signal tells us about
information of neighbouring protons.
• 6) Detection of hydrogen bonding.
• 7) Study of geometrical isomerism as well as
conformation.
• 8) J helps to study the NMR spectrum of
more complex compound.

Bibliography:-
 NMR, NQR, EPR and Mossbauer
Spectroscopy in Inorganic Chemistry,
.V. Parish, Ellis Haywood.
 Practical NMR Spectroscopy, M.L.
Martin. J.J. Deepish and G.J. Martin,
Heyden.
 Spectrometric Identification of
Organic Compounds, R.M. Silverstein,
G.C. Bassler adn T.C. Morrill, John
Wiley.
 Introduction to NMR
spectroscopy, R.J. Abraham, J.
Fisher and P. Loftus, Wiley.
 Application of Spectroscopy of
Organic Compounds, J.R. Dyer
Prentice Hall.
 Spectroscopic Methods in Organic
Chemistry D.H. Williams, I. Fleming,
Tata McGraw-Hill.

Nmr theory

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