CHEMICAL SHIFT

1. CHEMICAL SHIFTS
Presented by;
Aiswarya.A.T,
First year M.Pharm.,
Dept. of Pharmacy Practice,
Grace College of Pharmacy.

2. Shielding shifts the
absorption
At higher Magnetic field
strength
Up
field
Deshielding shifts the
absorption
At lower Magnetic field
strength
Down
field
TO GET AN EFFECTIVE FIELD STRENGTH
NECESSARY FOR ABSORPTION

3. Such shifts in the positions of NMR absorption which
arise due to Shielding / Deshielding of Protons by
the Electrons is called as “CHEMICAL SHIFTS”
It is the difference b/w the absorption position
of a sample proton & that of a reference
proton.

4. Measured in DELTA & TAU scales
The value of chemical shift is expressed in terms of “ppm”
REFERNCE STANDARDS
Chemical inertness
Magnetic isotropy
Give a single sharp peak
Easily recognizable peak
Miscible with wide range of solvents
Volatility to facilitate recovery
from valuable samples

5. Si
CH3
CH3
CH3
CH3
Reference std.
TETRA METHYL SILANE (TMS)
at 0.5% conc.
Added to the sample, so it is called as
“internal standard”
12 protons
Uniformly shielded
High electro +ve nature of Si
at the centre
Sharp peak at 0 delta; requires
max. MF than protons of most of
the organic compounds

6. DELTA
100
TAU
TMS

7. FACTORS AFFECTING CHEMICAL SHIFT
INDUCTIVE EFFECT
VANDERWAALS
DESHIELDING
ANISOTROPIC EFFECT
HYDROGEN BONDING

8. INDUCTIVE EFFECT
Greater the electronegativity
Greater is the deshielding
Delta value will be more

9. VANDERWAALS DESHIELDING
Electron crowd of a bulky group will tend
to repel the electron cloud (in over crowded
molecules) surrounding the proton , & the
proton is deshielded.

10. ANISOTROPIC EFFECT
ALKENES
H+ deshielded
Induced MF – diamagnetic, act paramagnetic in
the region of alkene proton.
H+ feels greater field strength
ALKYNES
H+ shielded
Induced field opposes the applied field
H+ feels smaller field strength
Low delta value

11. HYDROGEN BONDING
Downfield shift depends upon the strength
of H-bonding
Intra-molecular H-bonding doesn’t show any
shifting absorption