NMR.pptx

Dr.P.GOVINDARAJ
Associate Professor & Head , Department of Chemistry
SAIVA BHANU KSHATRIYA COLLEGE
ARUPPUKOTTAI - 626101
Virudhunagar District, Tamil Nadu, India
NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

Nuclear Magnetic Resonance Spectroscopy
Definition
• The interaction between radiofrequency waves and nuclear spin energy levels of a molecule
in an applied magnetic field causes transition between nuclear spin energy levels called NMR
spectra
Condition
• Nuclei with I  ½ are given NMR spectra
• Nuclei with I =0 are not given NMR
spectra Where I is the total spin
Example : 1H, 13P,19F and 13C are given NMR spectra

Explanation
• Consider a nucleus with I = ½ (example 1H)
• The angular momentum due to the spinning of the nucleus
𝐼= 𝐼 1 + 𝐼 X
ℎ
2𝜋
--------(1)
• The spinning nucleus generate magnetic field of magnetic moment
 = 𝑔𝑁 𝑁
X Angular momentum --------(2)
• Where
𝑔𝑁 is the nuclear g – factor
𝑁 is the nuclear magneton

• When the nucleus is placed in an external magnetic field (B0), applied in the z-direction,
the energy of interaction is
E = -  𝐵 ----------(3)
• Substitute equation (2) in (3) we get
E = -𝑔𝑁 𝑁
𝐵𝑧 𝐼→ ----------(4)
• The orientation of the nuclear spin (I =1/2 ) in the presence of an external magnetic field
B0 is
mI = I, I-1,…….-I
When I = ½ , mI = +1/2 and -1/2 i.e., mI =  ½

• Equation (4) may be written as
EmI = -𝑔𝑁 𝑁
𝐵𝑧 𝑚𝐼
2 𝑁 𝑁 0 I
i.e., E1/2 = - 1
𝑔 𝐵 for m = +1/2
(-1/2)
E =
2
1
𝑔
𝑁 𝑁 0 I
𝐵 for m = -1/2
• This indicates that the energy of the proton (nucleus) with mI = +1/2 and mI =-1/2
are same in the absence of external field

• But the degeneracy (same energy) is removed in the presence of external magnetic field
this is shown as

• Since the energy difference ∆E will be matched by the energy of the radiofrequency
radiation.
• Radiofrequency radiation of energy flip from upward direction (↑) to the downward
direction(↓)
∆E = 𝑔𝑁 𝑁
𝐵0 = h
 = ∆E = 𝑔𝑁 𝑁 𝐵 0
ℎ ℎ

• This transition will be recorded as NMR spectra and it will be recorded in the two ways
i) by increasing  at constant B0
ii) by increasing B0 at constant 

Relaxation process
Relaxation processes involve some non radiative transitions by which a nucleus in an
upper transition state returns to the lower spin state . Two kinds of relaxation processes are
1. Spin – spin relaxation
2. Spin – lattice relaxation (Longitudinal relaxation)
3. Quadrupole relaxation

Spin – spin relaxation
• It is due to the mutual exchange of spins by two precessing nuclei which are in
close proximity to each other.
• Weknow that each precessing nucleus is associated with a magnetic vector componenet
rotating in a plane perpendicular to the field.
• If this small rotating magnetic field is the same as is required to induce a transition in the
neighbouring proton, then mutual exchange of spin takes place
• That is , it involves the transfer of energy from one nucleus to the other. There is no net loss
of energy

Spin – spin relaxation
• The spread of energy among the nuclei concerned results in line broadening of NMR spectra

Spin – lattice relaxation
• It involves the transfer of energy from the nucleus in its higher energy state to the molecular
lattice. The energy is transferred to the components of the lattice as the additional translational,
vibrational and rotational energy
• The total energy of the system remains the same. An efficient relaxation process involves a
short time and results in the broadening of absorption peaks.
• Smaller the time of the excited state, greater is the line width. This mechanism is not effective
in solids. This process keeps the excess of nuclei in the lower energy state which is a
necessary condition for nuclear magnetic resonance phenomenon

Quadrupole relaxation
• It is a relaxation process for nuclei having I > ½ . The nuclei (such as
14N, 17O, 11B etc)due to anisotropic interaction between non spherical,
electrically quadrupole nuclei and the electric field gradients at the
nucleus caused by electric environments possess an asymmetric positive
charge distribution on the nuclei .
• Hence, these nuclei exhibit electric quadrupole moments and relax
rapidly as display very broad signals. This process of deactivation of
nuclei is called electric quadrupole relaxation
QUADRUPOLE

Significance of NMR:
• The NMR spectrum of a molecule gives information about the kind of protons present in a
molecule
i.e., one signal means one kind of proton
two signal means two kind of protons
three signal means three kind of protons
• The number of protons of each kind will be getting from the intensities of the respective
signals

Example
i) NMR spectrum of ethanol :
• Three signals of the NMR spectrum of ethanol indicates three kinds of proton present
in the ethanol
i.e., one is –OH proton
second is –CH2 proton
third is –CH3 proton

Chemical Shift
• A chemical shift is defined as the difference in parts per million (ppm) between the
resonance frequency of the observed proton and tetramethylsilane (TMS) hydrogen
• TMS is the most common reference compound in NMR, it is set as  = 0 ppm

Chemical Shift
• A chemical shift is defined as the difference in parts per million (ppm) between the
resonance frequency of the observed proton and tetramethylsilane (TMS) hydrogen
• TMS is the most common reference compound in NMR, it is set as  = 0 ppm
• Chemical shift is expressed in two scales:  scale and  scale
• The formula for chemical shift interms of  scale is
• The relationship between  scale and  scale is
 = 10 - 

Relaxation times
• There are two relaxation time
1. Spin - lattice (or longitudinal) relaxation time (T1)
2. Spin - spin (or transverse) relaxation time (T2)
Spin - lattice (or longitudinal) relaxation time (T1)
• The Spin - lattice (or longitudinal) relaxation time (T1) is defined as the time by which
the energy from the nuclear spin system transfer to the neighboring molecule (the lattice
or environment). This relaxation takes place in the z-direction and leads to restoring the
Boltzmann equilibrium

• The rate of change of net magnetization vector MZ related
to the T1 by the equation
𝑑𝑀𝑧
= 𝑀0−𝑀𝑍
𝑑𝑡 𝑇1
• Where
𝑀0 is the net magnetization at equilibrium
𝑀𝑍 is the net magnetization vector along z-axis
at various time interval
• Spin – lattice (or longitudinal) relaxation is graphically
represented as

Spin - spin (or transverse) relaxation time (T2)
• The Spin - spin (or transverse) relaxation time (T2) is defined as the time by which
the energy from the nuclear spin system of the excited state transfer to the nuclear spin
system of the lower energy state and the time T2 is used to quantify the rate of the decay
of the magnetization within the x(y) plane
i.e., the rate of decay of the magnetization within the x(y) plane is related to the T2 by the
equation
=
𝑑𝑀𝑥(𝑦) −𝑀𝑥(𝑦)
𝑑𝑡 𝑇2

• The decay of the magnetization within the x(y) plane is represented as
• The negative sign indicates that the decrease of
magnetization Mx(y) with respect to time

Line shape and line width
Line shape
• The shape of the spectral line is called line shape of the spectral line. The broadening
of the line shape is mainly due to the spin – spin relaxation (T2) and it is obtained by
operating FT ( Fourier Transform mathematical tool) on the signal intensity resulted
by T2 relaxation

Line width
• Line width is defined as the full width at half-height of the resonance line shape shown
in the diagram

Double resonance technique
Definition
• It is a technique in which protons (or) set of protons of a compound is irradiated by
intense radiofrequency radiation to eliminates spin-spin coupling amongst protons
Example:
Ethanol : CH3 (c) CH2(b)OH (a)
• Weknow that the NMR spectra of ethanol is due to spin-spin coupling is
a
b
c

i.e,
Ha
Hb
singlet
quartet, due to spin – spin coupling with three protons
of CH3 group
Triplet, due to spin – spin coupling with two protons of
CH2 group
Hc
• During the irradiation of the protons with intense radio frequency radiation the spin
of these protons will be rapidly changed and its life time will be very short. So that the
average of the spin of these protons will itself be coupled with the neighboring protons
no neighboring
proton
three neighboring
proton
two neighboring
proton

i.e., Ha
Hb
Hc
singlet
quartet
triplet
Ha
Hb
No effect because no neighboring protons, so resulted singlet signals
Average spin of the neighboring three protons (-CH3) is zero , so its
coupling with Hb proton is eliminated and resulted singlet signal
due to Hb itself
[𝗍 𝗍 𝗍 + 3
+ 1
2 2
𝗍 𝗍 ↓ 𝗍 ↓𝗍 ↓𝗍 𝗍 ↓↓↓ − 3
2
𝗍 ↓ ↓ ↓𝗍 ↓↓↓𝗍 − 1
]
2
Irradiation

Hc Average spin of the neighboring two protons (-CH2) is zero , so its
coupling with Hc proton is eliminated and resulted singlet signal
due to Hc itself
[ 𝗍𝗍 +1 𝗍↓ (0) ↓𝗍 (0) ↓↓ −1 ]
• After double resonance technique the spectra of ethanol and it is very simple spectra
• In this technique, intense radio frequency radiation is used in addition to normal radio
frequency radiation. So this technique is named as double resonance technique
• This technique is powerful tool for simplifying the spectrum of complex structured
compounds

ENDOR (Electron Nuclear Double Resonance) Experiment
Definition
• Electron nuclear double resonance (ENDOR) is a magnetic resonance technique for
elucidating the molecular and electronic structure of paramagnetic species by
irradiating the nuclear transitions to eliminates the hyperfine splittings
Explanation
• Consider an energy level diagram for a system shown
in the figure which has a single electron spin and a
nucleus with spin ½

• There are two allowed spin transitions, labelled E1 and E2 involving the electron spins
and the two transitions, labelled N1 and N2 involving the nuclear spins
• In ENDOR experiment , the transition E1 or E2 monitored and the frequencies N1 and N2
are irradiated. Their frequencies depend on the magnetogyric ratio of the nucleus
• ENDOR is used
* Identification of nuclei responsible for particular hyperfine splittings
* T
o simplify the complexed ESR spectrum of a compound
ENDOR (Electron Nuclear Double Resonance) Experiment

Nuclear Overhauser Effect (NOE)
Definition
• NOE is the change in the integrated intensity (positive or negative) of the NMR signal
of one proton that occurs when another proton, which close proximity in space to the
former proton, is saturated by irradiating with an radiofrequency radiation
Explanation
• Consider a molecule in which two protons Ha and Hb are in close proximity in space
within the molecule

• If Hb is irradiated, then this proton gets stimulated and the stimulation is transferred
through space to the relaxation mechanism of Ha
• Thus, due to the increase in the spin lattice relaxation of Ha, its signal will appear more
intense by 15-50%
• Thus the intensity of absorption of Ha signal is increased by double irradiating Hb
• NOE is used to find out the close proximity nature of protons in space of a given
complex molecule
Nuclear Overhauser Effect (NOE)

Lanthanide Shift Reagents
Definition
• Lanthanide shift reagents are paramagnetic compounds and have the ability to induces
the neighboring nuclear spins signals of molecular system towards enormous shifts to
higher frequency with which they interact by modifies the magnetic field experienced
by the proton
Example
Eu(dpm)3 and Eu(fod)3

Lanthanide Shift Reagents
Explanation
• The shift reagent coordinates with electronegative atoms in the substrate and thus
modifies the magnetic field experienced by neighboring protons
• Unpaired electron spin in the paramagnetic ion [e.g: Eu(III)] is partially transferred
through the intervening bonds to the protons of the organic substrate
• The spinning paramagnetic ion also generates magnetic vectors which operates
through space, creating secondary magnetic field around the protons
• The extent of the lanthanide shift reagent is dependent on the basicity of the
functional group and on the nature, purity and concentration of the shift reagent

FT-NMR
Definition
• It is a method to collect NMR spectrum of a sample in which a pulse of radio frequency
energy is used to excite all nuclei of a sample in a short period of time and the excited
nuclei relax to the ground state by the emission of radio frequency radiation resulted time
domain free induction decay (FID) signal then this signal is converted into
frequency domain signal by Fourier transform software

FT-NMR
Explanation
• Consider a sample containing three types of
nuclei having different spin energy levels of
frequency 1,2 and 3 in the presence of
applied magnetic filed B0
• On irradiating the sample with a pulse of
radiofrequency radiation in a short period
of time, all types of nuclei excited to the
higher spin energy levels from the ground
state energy levels

FT-NMR
• The higher energy spin states of all types of nuclei immediately relax to ground state
by emitting radio-frequency radiation and are collected in the detector as time
domain
signal called free induction decay signal shown in the diagram
• Further the time domain signal is converted into frequency domain signal using
Fourier transform technique and resulted signal are called as FT-NMR spectra of
the sample
• Further the time domain signal is converted into frequency domain signal using
Fourier transform technique and resulted signal are called as FT-NMR spectra of
the sample

FT-NMR
Advantage of FT-NMR
• FT-NMR spectra is totally free from noise signal
• It is used to detect less sensitive or less abundance nuclei like 13C, 19F,31P etc.,
• FT-NMR spectra recording time is very short compared to CW NMR spectra

NMR Imaging (NMRI)
Definition
• Nuclear magnetic resonance imaging is a diagnostic technique used in the medical field
and biomedical research
Explanation
• NMRI is based on the fundamental relation
ω =  B0
where
ω is the angular velocity of the magnetic moment vector 
 is the gyromagnetic ratio
B0 is the applied magnetic fielf

NMR Imaging (NMRI)
• In NMRI, the net magnetization at each locations of the object is
measured in order to get the resonance of normal tissues and the
pathological state of the tissues over a region of interest
• This is done by spatial encoding and it is achieved by the
use of magnetic gradients produced by suitably placed
electrical coils
• When the magnetic field varies systematically across an object,
the resonance frequency of a normal nucleus and the one
affected by disease appear at different magnetic fields as
shown in the figure

NMR.pptx

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