Chemistry spectroscopy

1. Dr. Altaf Hussain Pandith
Professor
Department of Chemistry, University of Kashmir
Srinagar 190006, India.
NMR: Chemical Shift
4/19/2024 BY Prof Altaf Hussain Pandith

2. 1. Local Diamagnetic Shielding
2. Local Paramagnetic Shielding
3. Shielding Due to Neighbouring Group effects
 Magnetic Anisotropy due to chemical Bond
 Ring current Effects
4. Hydrogen Bonding
NMR: Contributions to Chemical Shift
4/19/2024 BY Prof Altaf Hussain Pandith

3.  An atom with spherical electron
density distribution in a magnetic field:
Diamagnetic susceptibility identical
in all directions Isotropic Susceptibility
 Atom with Isotropic Susceptibility in a magnetic field
induces electron circulation so as to generate a magnetic
field opposed to the external field , the nucleus is shielded
 Induced field is proportional to applied field and the
electron density surrounding the nucleus.
 Magnitude of the diamagnetic current and shielding is
given by ground state electronic wave function of atoms
or molecules; depends sensitively on the electron density
close to the nucleus
Local Diamagnetic Shielding
4/19/2024 BY Prof Altaf Hussain Pandith

4. • CH3-X
4/19/2024 BY Prof Altaf Hussain Pandith
ELECTRONEGATIVITY
-X   (ppm)
F 3.98 4.26
Cl 3.16 3.05
Br 2.96 2.68
I 2.66 2.16
= 0.684( CH2 -  CH3) + 1.78
For CH3-CH2-X
Atom  Compound  1H-
NMR
Li 1.0 CH3-Li -1.94
Si 1.9 CH3-SiMe3 0.00
N 3.0 CH3-NH2 2.41
O 3.4 CH3-OH 3.50
F 4.0 CH3-F 4.27
CH CH2 CH3
H 1.7 1.3 0.9
Values to be added
1ppm 
Alkenes, aryl, carbonyl. Nitrile,
Sulphur, Nitrogen
2ppm
Oxygen, halogens, nitro groups

5. 4/19/2024 BY Prof Altaf Hussain Pandith
OTHER EFFECTS ; ELECTRON WITHDRAWING AND DONATING
SUBSTITUENTS

6. 4/19/2024 BY Prof Altaf Hussain Pandith
NMR: Local Paramagnetic Shielding
Local paramagnetic contribution arises from
the anisotropy in the electron distribution
about the atom whose chemical shift is being
measured.
Electron circulation develops a magnetic
field in the same direction as the applied
field, which reinforces the applied field.
This paramagnetic field is induced because
of restricted of hindered circulation of
electron density which is locked by virtue of
difference of the energy of orbitals involved
in the circulation of electron density.
This anisotropy is described by a mixing in,
under the influence of the applied field, of
the ground electronic state an a low lying
excited state of appropriate symmetry.
For an electron to have orbital angular momentum, it must be able to
circulate about an axis. This requires the availability of an orbital
additional to the orbital containing the electron , having same shape,
energy, symmetry and being able to be supper imposed with the first on
by a rotation about the axis of electron circulation.

7. .
Local Paramagnetic Effect
E
n
e
r
g
y
E= h (UV--Visible
Zero
Field
4/23/2020
BY: Prof. Altaf Hussain Pandith, Deptt. of
Chemistry, KU
t2g
eg
dxy dxz dyz
dx2-y2 dz2
d6
ligand Field
E
n
e
r
g
y dxz
dxy
dyz
dx2-y2
dz2
Orbital Energies in a
d6 system in Oh Field
Mixing in of ground
state with excited
state in Magnetic
Field
Orbital Energies in a
d1 system (VO2+ ion)
in an axial symmetry.
Mixing in of ground
state with excited
state in Magnetic
Field

8. • The paramagnetic contribution to shielding may be estimated by
finding the way in which the electronic wave functions are
modified by the magnetic field.
• When a field is applied, new energy term appear and the
electronic wave function calculated in the absence of the field
are no longer eign-functions of the electronic Hamiltonian. The
new electronic wave functions may be obtained as linear
combination of the eign-functions, for the field absent case.
• Thus, field can be considered to mix excited electronic states
into the ground state wave function. The resulting expression
for the paramagnetic contribution to shielding involves a
summation over excited electronic states, with the energies of
excitation in the denominator.
4/19/2024 BY Prof Altaf Hussain Pandith
Local Paramagnetic Effect

9. 4/19/2024 BY Prof Altaf Hussain Pandith
Local Paramagnetic Effect
Where the  is the
energy difference
between ground state
and excited state and R
is the average distance
between the nucleus and
the electron. The
electron in bonded
orbitaks () bonds may
also give rise to
paramagnetic effect due
their hindered circulation
of electron density.