1) Protons experience different amounts of shielding depending on their chemical environment and electron densities around them. 2) The chemical shift value provides a number independent of the NMR instrument used to measure it. 3) Factors like electronegativity of nearby atoms, hybridization, hydrogen bonding, and anisotropic effects influence the chemical shift values of protons in a molecule.

1. FACTORS AFFECTING CHEMICAL
SHIFT
Dr. V. S. Tambe
Professor
PES Modern College of Pharmacy (For Ladies),
Moshi

2. All different types of protons in a molecule have a different amounts of
shielding (DIFFERENT ELECTRON DENSITIES). They all respond differently to
the applied magnetic field and appear at different places in the spectrum.
The position of signal is dependent of the applied magnetic field and
chemical environment of proton. Hence, each instrument with different
magnetic field would give different positions for the same proton. So as to
remove this ambiguity, chemical shift term is used. This gives a number
independent of the instrument used. A particular proton in a given molecule
will always have the same chemical shift (constant value). Hence, position of
signal will be only dependent of the chemical environment of proton.

3. UPFIELD
Highly shielded
protons appear here.
DOWNFIELD
Less shielded protons
appear here.
SPECTRUM
TMS has assigned a value
of Zero ppm. But there are
protons more shielded
than TMS and they are
assigned negative value
Chemical Shift (ppm)
Rather than measure the exact resonance
position of a peak, we measure how far
downfield it is shifted from TMS. The
variation of NMR frequencies of the same
kind of nucleus,due to variations in the
electron distribution is called chemical shift.

4. hn = Bo
g
2p
constants
frequency
field
strength
Stronger magnetic fields (Bo) cause
the instrument to operate at higher
frequencies (n).
NMR Field
Strength
1H Operating
Frequency
60 Mhz
100 MHz
300 MHz7.05 T
2.35 T
1.41 T
n = ( K) Bo

5. chemical
shift
= d =
shift in Hz
spectrometer frequency in MHz
= ppm
This division gives a number independent
of the instrument used.
parts per
million
The shifts from TMS in Hz are bigger in higher field instruments
(300 MHz, 500 MHz) than they are in the lower field instruments
(100 MHz, 60 MHz).
We can adjust the shift to a field-independent value,
the “chemical shift” in the following way:
A particular proton in a given molecule will always come
at the same chemical shift (constant value).
6

6. It is expressed in δ (delta, PPM) as mentioned and τ (tau)
scale.
τ = 10 - δ

7. 01234567 ppm
Hz Equivalent
of 1 ppm
1H Operating
Frequency
60 MHz 60 Hz
100 MHz 100 Hz
300 MHz 300 Hz
Each ppm unit represents either a 1 ppm change in
Bo (magnetic field strength, Tesla) or a 1 ppm change
in the precessional frequency (MHz).
Value of ppm is dependent on the magnetic field strength

8. NMR Correlation Chart
12 11 10 9 8 7 6 5 4 3 2 1 0
-OH -NH
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
CH2Ar
CH2NR2
CH2S
C C-H
C=C-CH2
CH2- C-
O
C-CH-C
C
C-CH2-C
C-CH3
RCOOH RCHO C=C
H
TMS
HCHCl3 ,
d (ppm)
DOWNFIELD UPFIELD
DESHIELDED SHIELDED

9. aliphatic
C-H
CH on C
next to
pi bonds
C-H where C is
attached to an
electronega-tive
atom
alkene
=C-H
benzene
CH
aldehyde
CHO
acid
COOH
234691012 0
X-C-H
X=C-C-H
Chemical shift scale

10. FACTORS AFFECTING CHEMICAL
SHIFT

11. INDUCTIVE EFFECT
Mesomeric Effect
ANISOTROPIC EFFECT
HYDROGEN BONDINGVANDER WAAL DESHIELDING
HYBRIDIZATION

12. Compound CH3X CH3F CH3OH CH3Cl CH3Br CH3I CH4 CH3)4Si
Element X F O Cl Br I H Si
Electronegativity of X 4.0 3.5 3.1 2.8 2.5 2.1 1.8
Chemical shift δ 4.26 3.40 3.05 2.68 2.16 0.23 0
INDUCTIVEEFFECT:
•It has involvement of sigma electrons
• Caused by electron withdrawing or electron donating groups attached to the 1H
•Electron withdrawing groups such as NH,COO-,Cl etc. Shift the 1H chemical
shift to downfield regions.
Cl←C←H
Chlorine “deshields” the proton, it takes electron density away from carbon,
which in turn takes more density away from the proton.
EXAMPLES: Dependence of the chemical shift of CH3X on the element X

13. Substitution Effects on Chemical Shift
CHCl3 CH2Cl2 CH3Cl
7.27 5.30 3.05 ppm
-CH2-Br -CH2-CH2Br -CH2-CH2CH2Br
3.30 1.69 1.25 ppm
most
deshielded
most
deshielded
The effect decreases
with incresing distance.
The effect
increases with
greater numbers
of electronegative
atoms.

14. Silicon is electropositive. Hence, proton in TMS is highly shielded and come to
the resonance upfield. Its value is fixed to zero and the relative positions of all
protons is measured.
TETRA METHYL SILANE (TMS)
•Added to the sample, so it is called “internal standard”
•TMS has 12 equivalent proton and gives an intense single unsplit signal.
•Chemically inert
•Low boiling point, So it can be easily removed by evaporation after the
spectrum has been recorded.
TMS is the common reference compound in NMR,it is set at δ=0 ppm
•Its miscible in most organic solvents

15. More deshielded proton due to
higher electronegativity of Carbon
compared to hydrogen

16. HYBRIDIZATION:
Hybridization of the carbon to which the proton is attached influences the
electron density at 1H attached.
As the proportion of ‘s’ character increases from sp3 to sp2 to sp orbitals,
bonding electrons move closer to carbon and away from the protons, which
become deshielded.
EXAMPLE
SP3 hybridization=25% sp2hybridization=33.33% S character
CH4 0.23 ppm CH2=CH2 5.28ppm
CH3-CH3 0.86ppm

17. SP Hybridization=50% S character

18. Compound Type of
Hybridization
% of S
character
Chemical Shift
(ppm)
Justification
ALKANE SP3 25 0.9-1.3 Less S character,
more shielded proton
ALKENE SP2 33.3 4-6 More S character,
more deshielded
proton
ALKYNE SP 50 2.1 Deshielding due to
hybridization
But shielding due to
anisotropy.

19. Anisotropic Effect
In compound containing double or triple bond circulation of pi
electrons about nearby nuclei generate an induced field which can
either oppose or reinforce at 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 and so, this effect is known as space
effect.
Phenomenon (spatial variation) of shielding and deshielding
depending on orientation of molecule w.r.t applied magnetic field is
called Anisotropy
Anisotropy is property of molecule in different orientations which
show variations in physical properties along different axes of
molecule

20. Aromatic protons come to resonance at 7.1 ppm due to this
deshielding
RING CURRENT: The π electrons in a
compound, when placed in a
magnetic field, will move and
generate their own magnetic field.

21. In benzene, the π-electrons are delocalized above and below the plane of the ring
forming electron cloud. When a magnetic field is applied perpendicular to the
plane of the aromatic ring, the circulation of π-electrons produces a ring current.
The induced magnetic field is in the opposite direction (diamagnetic effect) as the
applied field at the centre of the ring, but outside the ring it is in the same
direction (paramagnetic effect) of the applied field.
The Aromatic protons experience larger magnetic field and resonate at higher δ
values (deshielding). Protons which are present above or below to the plane of the
aromatic ring resonate at low δ values (Shielded).

22. An alkene molecule is oriented with
the plane of the double bond
perpendicular to the applied
magnetic field.
The circulation of π-electrons is
perpendicular to the carbon-
hydrogen framework of the molecule.
As a result the induced magnetic field
caused by the circulation of π-
electrons is in the opposite direction
of applied magnetic field
(diamagnetic) around the carbon
atoms and in the direction of applied
magnetic field (paramagnetic) in the
region of olefenic protons.
Therefore olefenic protons
experience greater field strength and
consequently resonate at larger value
of chemical shift.
ANISOTROPIC FIELD IN AN ALKENE

23. An alkyne molecule when placed in an external magnetic field is so oriented that
the plane of the triple bond lies parallel to the direction of applied magnetic
field. The induced magnetic field caused by the circulation of π-electrons of the
triple bond is in the opposite direction of applied field (diamagnetic) in the
region of acetylenic protons. Therefore acetylenic protons are shielded and
experience lesser field strength and consequently resonate at smaller value of
chemical shift.
ANISOTROPIC FIELD FOR AN ALKYNE

24. Carbonyl protons (Aldehydes)
Oriented with the plane of the carbon-oxygen double bond is perpendicular to the applied
field.
The circulation of π-electrons generates an induced magnetic field which is in the direction
of applied magnetic field at the aldehydic protons
Therefore, aldehydic protons experience greater field strength (diamagnetic effect) and
consequently resonate at larger value of chemical shift (deshielding). In addition, the high
electronegativity of oxygen atom also contributes to the higher δ value of aldehydic
proton.

25. Magnetic Anisotropy

26. Mesomeric effect
Because of the higher electron density at the ortho and para positions due to
mesomeric effect, the hydrogen atom appears shielded with a chemical shift value of
6.5 ppm and upfield shifted compared to the other hydrogens at the meta position,
where it is de-shielded.

27. NH, SH & OH protons NMR signals are moved on changing to
solvents of different polarity
Higher temperatures reduces intermolecular hydrogen bonding
means lower δ values
Intramolecular hydrogen bonding is unchanged by dilution
Hydrogen bonding shifts resonance signal proton to lower field
( higher frequency ) – deshields protons & lengthens –OH bond
Hydrogen bonding

28. HYDROGEN BONDING DESHIELDS PROTONS
O H
R
O R
HHO
R
The chemical shift depends
on how much hydrogen bonding
is taking place.
Alcohols vary in chemical shift
from 0.5 ppm (free OH) to about
5.0 ppm (lots of H bonding).
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.

29. O
C
O
R
H
H
C
O
O
R
Carboxylic acids have strong
hydrogen bonding - they
form dimers. With carboxylic acids
the O-H absorptions are found
between 10 and 12 ppm very far
downfield.
O
O
O
H
CH3
In methyl salicylate, which has strong
internal hydrogen bonding, the NMR
absortion for O-H is at about 14 ppm,
way, way downfield.
A 6-membered ring is formed.

30. Vanderwaal deshielding
In a rigid molecule it is possible for a proton to occupy a sterically hindered
position, and in consequence the electron cloud of the hindering group will
tend to repel, by electrostatic repulsion, the electron cloud surrounding the
proton. The proton will be deshielded and appear at higher δ values than
would be predicted in the absence of the effect. Although this influence is
small (usually less than 1 ppm),it must be borne in mind when predicting the
chemical shift position in overcrowded molecules such as highly substituted
steroids or alkaloids.

31. Type of proton Compound Chemical
Shift
Justification
Primary alkyl RCH3 0.8-1.0 Shielded due to more of protons and less of carbons
Secondary alkyl R2CH2 1.2-1.4
Tertiary alkyl R3CH 1.4-1.7 Deshielded due to less of electropositive protons and more
of electronegative carbons
benzylic ArCH3 2.2-2.5 Deshielded due to anisotropy and electron withdrawal by
benzene
Alkyl fluoride RCH2F 4.0 -4.5 Deshielding due to Highest negative inductive effect of
Fluorine
Alkyl chloride RCH2Cl 3.6-3.8 Deshielding due to inductive effect of halogen
Alkyl bromide RCH2Br 3.4-3.6 Deshielding due to inductive effect of halogen
Alkyl iodide RCH2I 3.1-3.3 Least Deshielding in halogen series due to less electron
withdrawing inductive effect of Iodine
Ether ROCH2R 3.3-3.9 Negative inductive effect of oxygen
Alcohol HOCH2R 3.3-4.0 Negative inductive effect of Oxygen
Ketone RCOCH3 2.1-2.6 Negative inductive effect of Carbonyl

32. Type of proton Compound Chemical
Shift
Justification
Aldehyde RCHO 9.0-10.0 Deshielding due to Anisotropy of Carbonyl and Negative
inductive effect of Carbonyl
Vinvlic R1C=CH2 4.6 -5.0 Deshielding due to Anisotropy of Carbonyl
Aromatic ArH 6.0-9.0 Deshielding due to circulation of pie electrons and ring
Current, electron withdrawal by aromatic ring
Alcohol
Hydroxyl
ROH 0.5-6.0 Hydrogen bonding
Carboxylic RCOOH 10-13 Deshielding due to Anisotropy of Carbonyl and Negative
inductive effect of Carboxyl
Phenolic ArOH 4.5-7.7 Hydrogen bonding and anisotropy of aromatic ring
Amino RNH2 1.0-5.0 Hydrogen bonding
Amide RCONHR 5.0-9.0 Hydrogen bonding