Factors affecting chemical shift

Factors Affecting Chemical Shift in proton NMR spectroscopy.
Factors Includes: Hydrogen bonding, Anisotropy effect, Elcertonegativity and inductive effect and vanderwaal's dishelding

1. Mr. Taher Patel
Assistant professor
Gland Institute of Pharmaceutical Sciences

2. Number of signal in NMR spectrum tells the
number of sets of proton in a molecule.
The difference between the absorption position
of a proton and the absorption position of a
reference compound is known as the chemical
shift of that particular proton.
The position of signal in the spectrum help us to
know the nature of proton ex. Aromatic,aliphatic
or adjuscent to electro withdrawing group or
electro donating group etc.



Chemical shift

3. Protons in a Molecule
Depending on their chemical environment,
protons in a molecule are shielded by
different amounts.
=>

5. Factor
affecting
chemical
shift
Electronegativity
and inductive
effect
Hydrogen
bonding
Vanderwaal’s
dishielding
Anisotrophy
effect (space
effect)

6. C H Cl
electronegative
element
d- d+
d- d+
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.
“highly shielded”
protons appear
at upfield
(lower d)
“deshielded“
protons appear
at downfield
(higher d)
deshielding moves proton
resonance to lower field and higher d
NMR CHART

7. Dependence of the Chemical Shift of CH
3
X on
the Element X

CH
3
F CH
3
OH CH
3
Cl CH
3
Br CH
3
I CH
4
(CH
3
)
4
Si
Compound
CH
3X
Element X
Electron
egativity
of X
Chemical
shift d
F O Cl Br I H Si
4.0 3.5 3.1 2.8 2.5 2.1 1.8
4.26 3.40 3.05 2.68 2.16 0.23 0
most
deshielded deshielding increases with the
electronegativity of atom X
TMS

8. 8
CHCl
3 CH
2Cl
2 CH
3Cl
7.27 5.30 3.05 ppm
most
deshielded
The effect
increases with
greater numbers
of electronegative
atoms.
SUBSTITUTION EFFECTS ON CHEMICAL SHIFT
Cumulative Effect
CH
2
Br CH
2
CH
2
Br  CH
2
CH
2
CH
2
Br
3.30 1.69 1.25 ppm
most
deshielded
The effect decreases with increasing
distance from the electronegative atom.
The effect completely vanished at the
fourth bond from the electronegative
atom.
X

9. In compound contaning double or triplebond
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 occurance of shielding or deshielding can be
determined by the location of proton in the space &
so, this effect is known as space effect.

10. Applied
field Ho

11. Aromatic protons
d = 7-8 ppm

12. Vinyl (Olefinic) protons,
d = 5-6 ppm

13. Aldehyde proton

14. Acetylene protons
d̃ ≈ 2.5 ppm

15. HYDROGEN BONDING DESHIELDS PROTONS

O H
R
O R
H H O
R
The chemical shift depends
on how much hydrogen bonding
is taking place (observed in high
concentrated solutions).
Hydrogen bonding lengthens the
O-H bond and reduces the valence
electron density around the proton

It is deshielded and shifted
downfield in the NMR spectrum.
Alcohols vary in chemical shift
from 0.5 ppm (free OH) to about
5.0 ppm (lots of H bonding).
D
2O-exchangeable (peak for OH
proton in alcohol and NH in amines
disappears upon shaking with D
2O)

16. If H-
bonding
increases
Deshielding
increases
Absorption
of
downfield
Incereases

17. Decreases

18. In overcrowded molecules, it is possible that some proton may be
occupying stearically hinderals position.
Clearly electronegative cloud of bulky group(hindering group) will tend to
repell the electron cloud surrounding proton.
Thus such a proton will be deshielded and will resonate a slightly higher
values of delta then expected in the absence of this effect.This is
considered as vanderwaal’s deshielding.