Sujit 1 h nmr ppt

1H NUCLEAR
MAGNETIC RESONANCE (NMR)
Sujitlal Bhakta
Department of Chemistry
RavenshawUniversity
Cuttack, Odisha, 753003
Sunday, January 21, 2018 1

2
• Nuclear magnetic resonance spectroscopy is a powerful analytical technique used to characterize
organic molecules by identifying carbon-hydrogen frameworks within molecules.
• Two common types of NMR spectroscopy are used to characterize organic structure: 1H NMR is
used to determine the type and number of H atoms in a molecule; 13C NMR is used to determine the
type of carbon atoms in the molecule.
• The source of energy in NMR is radio waves which have long wavelengths and thus low energy and
frequency.
• When low-energy radio waves interact with a molecule, they can change the nuclear spins of some
elements, including 1H and 13C.
INTRODUCTION TO NMR SPECTROSCOPY
Sunday, January 21, 2018

3
• Protons in different environments absorb at slightly different frequencies, so they are distinguishable
by NMR.
• The frequency at which a particular proton absorbs is determined by its electronic environment.
• The size of the magnetic field generated by the electrons around a proton determines where it
absorbs.
• Modern NMR spectrometers use a constant magnetic field strength B0, and then a narrow range of
frequencies is applied to achieve the resonance of all protons.
• Only nuclei that contain odd mass numbers (such as 1H, 13C, 19F and 31P) or odd atomic numbers
(such as 2H and 14N) give rise to NMR signals.

4
• When a charged particle such as a proton spins on its axis, it creates a magnetic field. Thus, the
nucleus can be considered to be a tiny bar magnet.
• Normally, these tiny bar magnets are randomly oriented in space. However, in the presence of a
magnetic field B0, they are oriented with or against this applied field. More nuclei are oriented with
the applied field because this arrangement is lower in energy.
• The energy difference between these two states is very small (<0.1 cal).

Bo = 0 Bo > 0
Randomly oriented Highly oriented
Bo
ENSEMBLE OF NUCLEAR SPINS
N
S
Each nucleus behaves like
a bar magnet.

The nuclei of some atoms have a property called “SPIN”.
NUCLEAR SPIN
These nuclei behave as if
they were spinning.
This is like the spin property
of an electron, which can have
two spins: +1/2 and -1/2 .
Each spin-active nucleus has a number of spins defined by
its spin quantum number, I.
….. we don’t know if they actually do spin!
The spin quantum numbers of some common nuclei follow …..

Element 1H 2H 12C 13C 14N 16O 17O 19F
Nuclear Spin
Quantum No 1/2 1 0 1/2 1 0 5/2 1/2
( I )
No. of Spin 2 3 0 2 3 0 6 2
States
Spin Quantum Numbers of Some Common Nuclei
Elements with either odd mass or odd atomic number
have the property of nuclear “spin”.
The number of spin states is 2I + 1,
where I is the spin quantum number.
The most abundant isotopes of C and O do not have spin.

Nuclear Magnetic Resonance
Nuclear spin
m = g I h
m - magnetic moment
g - gyromagnetic ratio
I - spin quantum number
h - Planck’s constant
m
I is a property of the nucleus
Mass # Atomic # I
Odd Even or odd 1/2, 3/2, 5/2,…
Even Even 0
Even Odd 1, 2, 3

Nucleus Spin
Quantum
Number
(I)
Natural
Abundanc
e (%)
Gyromagnetic
Ratio
(10-7 rad/T
sec)
Sensitivity†
(% vs. 1H)
Electric
Quadrupu
le
Moment
(Q)
(e·1024
cm2)
1H
2H
13C
15N
19F
31P
1/2
1
1/2
1/2
1/2
1/2
99.9844
0.0156
1.108
0.365
100
100
26.7520
4.1067
6.7265
-2.7108
25.167
10.829
100.0
0.965
1.59
0.104
83.3
6.63
—————
0.00277
—————
—————
—————
—————
Nuclear Magnetic Resonance

THE CHEMICAL SHIFT AND SHIELDING
δ = (Shift in Hz)
(Spectrometer frequency in MHz)

1H CHEMICAL SHIFTS
0123456789101112
downfield upfield
TMS
CH3
CH2
CH
C C
C H
C
CH3
O
Ar CH3
C C H
C
X
HC
O H
C C
H
Aromatic H
C
H
O
C
OH
O
1H Chemical shift ()

Chemical Shift, δ
 is defined in parts per million, ppm.
13C Chemical shifts are most affected by:
hybridization state of carbon
Electronegativity of groups attached to carbon
Magnetic Anisotropy
n MHz ( ) = n Hz

THE COUPLING CONSTANT(J)
H
H
H
H
H
H
O
H
H
H
H
H
H
C C
HH
H
H
HH
H
H
6 to 8
11 to 18
6 to 15
4 to 10
ortho 6 to 10
8 to 11 5 to 7
a,a 8 to 14
a,e 0 to 7
e,e 0 to 5
cis 6 to 12
trans 4 to 8
cis 2 to 5
trans 1 to 3
H
meta 2 to 3
H
para 0 to 1
H
H

A Simplified 60 MHz
NMR Spectrometer
Transmitter
Receiver
Probe
hn
SN
RF
Detector
Recorder
RF (60 MHz)
Oscillator
~ 1.41 Tesla
(+/-) a few ppm
absorption
signal
MAGNETMAGNET

Free Induction Decay
The signals decay away due to interactions with the surroundings.
A free induction decay, FID, is the result.
Fourier transformation, FT, of this time domain signal
produces a frequency domain signal.
FT
Time
Frequency

A COMPARISON OF NMR SPECTRAAT LOW- AND HIGH-FIELD STRENGTHS

Chemical Shift, δ
 is defined in parts per million, ppm.
13C Chemical shifts are most affected by:
hybridization state of carbon
Electronegativity of groups attached to carbon
Magnetic Anisotropy
n MHz ( ) = n Hz

The local magnetic fields can either oppose or augment the external
magnetic field.
If the field created by the electron oppose the external field, nuclei
‘experience’ an effective field which is smaller than the external field and it is
said to be SHIELDED.
If the field created by the electron augments the external field, nuclei
‘experience’ an effective field which is larger than the external field. It is said
to be DE-SHIELDED.
Shielding and Deshielding

The electrons around the proton create a magnetic field that opposes the
applied field. This reduces the field experienced at the nucleus and there-
fore decreases the frequency required for the absorption.
Therefore the chemical shift (delta /ppm) will change depending on the
electron density around the proton. Since Electronegative groups decrease
the electron density, there will be less shielding (i.e., deshielding) and the
chemical shift will increase.
Deshielding:
Chemical Shifts
Electronegativity is a measure of the tendency of
an atom to attract a bonding pair of electrons.

highly shielded
protons appear
at high field
“deshielded“
protons appear
at low field
deshielding moves proton
resonance to lower field
C HCl
Chlorine “deshields” the proton,
that is, it takes valence electron
density away from carbon, which
in turn takes more density from
hydrogen deshielding the proton.electronegative
element
DESHIELDING BY AN ELECTRONEGATIVE ELE
NMR CHART
- +
- +

Magnetic Anisotropy
The word "anisotropic" means "non-uniform". So magnetic anisotropy means
that there is a "non-uniform magnetic field". Electrons in π systems
(e.g. aromatics, alkenes, alkynes, carbonyls etc.) interact with the applied field
which induces a magnetic field that causes the anisotropy. As a result, the
nearby protons will experience 3 fields: the applied field, the shielding field of
the valence electrons and the field due to the π system. Depending on the
position of the proton in this third field, it can be either shielded (smaller d) or
deshielded (larger d), which implies that the energy required for, and the
frequency of the absorption will change.
Anisotropic fields
usually due to
pi-bonded
electrons
in the
molecule.
NOTE:

Secondary magnetic field
generated by circulating 
electrons deshields aromatic
protons
Circulating  electrons
Ring Current in BenzeneRing Current in Benzene
Bo
Deshielded
H H fields add together
Benzene rings have the greatest anisotropic effect.

C=C
HH
H H
Bo
ANISOTROPIC FIELD IN AN ALKENE
protons are
deshielded
shifted
downfield
secondary
magnetic
(anisotropic)
field lines
Deshielded
fields add

Bo
secondary
magnetic
(anisotropic)
field
H
H
C
C
ANISOTROPIC FIELD FOR AN ALKYNE
hydrogens
are shielded
Shielded
fields subtract

Solvent
1H NMR
Chemical Shift
13C NMR
Chemical Shift
Acetic Acid 11.65 (1) , 2.04 (5) 179.0 (1) , 20.0 (7)
Acetone 2.05 (5) 206.7 (13) , 29.9 (7)
Acetonitrile 1.94 (5) 118.7 (1) , 1.39 (7)
Benzene 7.16 (1) 128.4 (3)
Chloroform 7.26 (1) 77.2 (3)
Dimethyl Sulfoxide 2.50 (5) 39.5 (7)
Methanol 4.87 (1) , 3.31 (5) 49.1 (7)
Methylene Chloride 5.32 (3) 54.00 (5)
Pyridine 8.74 (1) , 7.58 (1) , 7.22 (1) 150.3 (1) , 135.9 (3) , 123.9 (5)
Water (D2O) 4.8
NMR SOLVENTSIGNALS
The
chemical
shifts (d)of
solvent
signals
observed
for1H NMR
and
13C NMR
spectra
Are listed
in the
following
table.
The
multiplicity
is shown in
parentheses
as 1 for
singlet, 2
for
doublet,
3 for triplet,
etc.

Solvent Chemical Shift of H2O (or HOD)
Acetone 2.8
Acetonitrile 2.1
Benzene 0.4
Chloroform 1.6
Dimethyl Sulfoxide 3.3
Methanol 4.8
Methylene Chloride 1.5
Pyridine 4.9
Water (D2O) 4.8
Signals for water occur at different frequencies in 1H NMR spectra depending on the solvent
used. Listed below are the chemical shift positions of the water signal in several common
solvents. Note that H2O is seen in aprotic solvents, while HOD is seen in protic solvents due
to exchange with the solvent deuteriums.
NMR WATER Signals

DIFFERENCE BETWEEN NMR & MRI
Nuclear Magnetic Resonance is an important tool in chemical
analysis. As the name implies, it uses the spin magnetic moments
of nuclei (particularly hydrogen) and resonant excitation.
Magnetic Resonance Imaging uses the same principle
to get an image (e.g. of the inside of the body).
ESR = Electron Spin Resonance is also a resonance phenomenon,
except in this case it is the spin of an unpaired electron that is in
resonance, rather than a nuclear spin.

Sujit 1 h nmr ppt

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Sujit 1 h nmr ppt

Similar to Sujit 1 h nmr ppt (20)

Recently uploaded

Recently uploaded (20)

Sujit 1 h nmr ppt