This document discusses chemical shift in NMR spectroscopy. It begins by defining chemical shift as the shift in the NMR signal resulting from shielding and deshielding by electrons. Protons near electronegative atoms experience deshielding and absorb at lower fields, while protons near electropositive atoms experience shielding and absorb at higher fields. Tetramethylsilane (TMS) is commonly used as an internal reference standard due to its non-reactivity and single peak. Factors that influence chemical shift include electronegativity, anisotropy, hydrogen bonding, and molecular structure. Common isotopes used in NMR include 1H, 13C, 19F, and 31P. Reference standards are necessary for quantitative NMR and include T
2. CHEMICAL SHIFT
The shift in the position of the NMR region resulting
from the shielding and deshielding by electrons is called
chemical shift.
When a proton is present inside the magnetic field more
close to an electro positive atom more applied magnetic
field is required to cause excitation. This effect is called
shielding effect.
When a proton is present outside the magnetic field
close to a electronegative atom less applied magnetic
field is required to cause excitation . This effect is called
deshielding effect 2
3. Greater the electron density around the
proton greater will be the induced secondary
magnetic field.
[ local diamagnetic effect]
currents induced by fixed magnetic field
result in secondary fields which can either
enhance or decrease the field to given a
proton responds.
3
4. under the influence of the magnetic field electrons
bonding the protons tends to process around the
nucleus in a plain perpendicular to magnetic field
4
5. 5
The position of the peaks in an NMR spectrum relative to the
reference peak is expressed in terms of the chemical shift
ᵟ= H0(reference)-H0(sample) X 106PPM
H0(reference)
The value of H0 for the reference is usually greater than H0 for the
sample,so subtraction in the direction indicated gives a positive ᵟ.In
terms of frequency unit ᵟ takes the form
ᵟ = ѵ(sample)- ѵ(reference) X 106ppm
ѵ(reference)
6. chemical shift is dimension less
expressed in parts per million (ppm ) 10
Alternative system used us tau scale
τ=10-ᵟ
scale the position of TMS signal is taken as 0.0 ppm
most chemical shifts value ranges from 0-10
6
7. a small value if tau represents low field absorption.
high value represents high field absorption.
7
greater the deshielding of protons larger the value of
delta
8. Reason for chemical shift
8
Positive shielding :resonance position moves upfield.
Negative shielding: resonance position moves
downfield.
9. Measurement of Chemical Shift
In order to measure the magnitude of chemical shifts of
different kinds of protons,
There must be some standard signal .
0.5%Tetra methylsilane (TMS)(ch3)4si is used as
reference or standard compound.
Chemical shift is represented by δ
Δ scale: 0 to -10 scale
TMS as zero markers.
Dimensionless expression; negative for most protons.9
11. Reason for TMS as reference std
TETRA METHYL SILANE(TMS)
Accepted internal standard.
TMS has 12 equivalent protons and gives an intense
single signal.
Electro negativity of silicon is very low so the shielding
of equivalent protons in TMS is more than other
compound so all the signal arrives in a down field
direction.
11
12. chemically inert
low boiling point
so it can be easily removed by evaporation after the
spectrum has been recorded
so the sample can easily recovered.
TMS is not suitable in aqueous solution so DSS ( 2,2-
dimethyl-2silapentane-5 sulphate) used as reference.
protons in the methyl group of DSS gives a strong
line.
12
13. FACTORS AFFECTING CHEMICAL SHIFT
1. ELECTRONEGATIVITY AND INDUCTIVE EFFECT
The proton is said to be deshielded if its attached with an
electronegative atom/group. Greater the electro negativity of
atom greater is the deshielding caused to proton. If the
deshielding is more, then δ value also more.
Electronegative atoms like Halogens Oxygen and Nitrogen
deshield the protons
There for the absorption occurs downfield.
The deshielding is directly proportional to the halogens
oxygen or nitrogen
13
14. + i effect
- i effect
An electron withdrawing group is able to
reduce electron density around the proton
and Deshields the proton.
An electron releasing group increase the
electron density around the proton and give
rise to its shielding.
14
16. 2. Anisotropic effect(space effect)
Shielding and deshielding can be
determined
By location of proton in space
Space effect
Downfield or paramagnetic shift of protons
attached to C = C, aldehydic, aromatic proton is
experienced by the molecular magnetic field
induced by an action of applied field Ho on pi
electrons, this magnetic field induced by pi
electrons are directional or unsymmetrical and
this directional measurement is called
ANISOTROPY 16
20. 3. VANDERWAALS DESHIELDING
The electron cloud of a bulkier group will
tend to repel the electron cloud
surrounding the proton.
Thus such a proton will be deshielded
will resonate at slightly higher value of δ
than expected in the absence of this effect.
20
21. 4. HYDROGEN BONDING
if an atom exhibits hydrogen boding
in a compound .
it will deshielded due to the strongly
electronegative atom attached to it
so absorption in shifted downfield.
Intra molecular hydrogen bonding
Inter molecular hydrogen bonding 21
22. ISOTOPIC NUCLEI
Many isotopes of chemical elements can be used for
NMR analysis
1h
Most commonly used spin 1/2 nucleus in NMR
Most sensitive nucleus.
Produce narrow chemical shift with sharp signal
Fast result is possible due to short relaxation time
22
23. 2H
A spin 1nucleus commonly utilize as signal free
medium
Deuterium is used
Commonly used in high resolution NMR.
To monitor drifts in the magnetic field strength
To improve the homogenicity of the external magnetic
field
3h
Very sensitive to nmr 23
24. 11B
-More sensitive than 10 B
-Yields sharper signals
-Quarts tubes are used(borosilicate glass interfere
with measurements)
13C
-Spin 1/2
-Widely used, stable to nuclear decay
-low sensitivity
-Wide chemical shift
-Yields sharp signals 24
25. 14N
Spin 1
Medium sensitivity
Wide chemical shift
Limited use to small molecules and functional
group.
17 O
Spin 5/2
Low sensitivity
Low natural abundance
Wide chemical shift
25
26. 19f
Spin 1/2
Yields sharp signal
-Wide chemical shifts
31P
Spin 1/2
Medium chemical shift
Yield sharp line
26
27. 43ca
Used in biochemistry to study Calcium binding
to DNA protein
Moderately sensitive
Very low natural abundance
195 P t
eg:-6Li,7 Li
9 Be,19F
59Co,61Ni,77 Se etc.........
27
28. REFERENCE STANDARDS USED IN
NMR1. Tetra Methylsilane(tms)
2. DSS( 4,4-dimethyl-4-silapentane-1-sulfonic Acid)
Used In Proton And Carbon Related Nmr
High Water Solubility
High Intensity Signals
Almost All Peaks Found In Naturally Occurring Org
Molecule.
Show Minor Peaks At 3.1ppm - Triplet
28
29. 3. TRIMETHYLSILYL PROPIONIC ACID
Used for aqueous solvents
(3-(trimethylsilyl)-2,2',3,3'-tetradeutero propionic
acid ) Or tmsp-d4
4. DEUTERATED DMSO
Show no peaks in 1+1
H1 NMR signal is observed at 2.20 ppm
13C chemical shift of DMSO - 39.52ppm
29
30. REFERENCE
Instrumental Methods Of Chemical Analysis By
Gurdeep R. Chatwal ,Sham K Anand, Pg No:
2.192-2.196
Www. Wikipedia.Org
30
Spectroscopy of organic compounds by P.S. Kalsi,
page no.453-490.
Spectroscopy of organic compound by Y.R.
Sharma.
