CHEMICAL SHIFTS

Presented by;

Aiswarya.A.T,First year M.Pharm.,

Dept. of Pharmacy Practice,

Grace College of Pharmacy.

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

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.

Measured in DELTA & TAU scales

The value of chemical shift is expressed in terms of “ppm”

REFERNCE STANDARDS

Chemical inertnessMagnetic isotropyGive a single sharp peakEasily recognizable peakMiscible with wide range of solventsVolatility to facilitate recovery

from valuable samples

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 protonsUniformly shielded

High electro +ve nature of Siat the centre

Sharp peak at 0 delta; requires max. MF than protons of most of the organic compounds

DELTA

100

TAU

TMS

FACTORS AFFECTING CHEMICAL SHIFT

INDUCTIVE EFFECT

VANDERWAALS DESHIELDING

ANISOTROPIC EFFECT

HYDROGEN BONDING

INDUCTIVE EFFECT

Greater the electronegativity

Greater is the deshielding

Delta value will be more

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.

ANISOTROPIC EFFECT

ALKENESH+ deshieldedInduced MF – diamagnetic, act paramagnetic in

the region of alkene proton.H+ feels greater field strength

ALKYNESH+ shieldedInduced field opposes the applied fieldH+ feels smaller field strengthLow delta value

HYDROGEN BONDING

Downfield shift depends upon the strength of H-bonding

Intra-molecular H-bonding doesn’t show any shifting absorption