FACTORS INFLUENCING CHEMICAL SHIFT

Presented by: B.Kavita Under the guidance of Dr.K.Shanta Kumari, M.Pharm,Ph.D

The shifts in the positions of NMR signals

resulting from the Shielding / Deshielding by the

Electrons is called as “CHEMICAL SHIFTS”

As the shift is due to difference in the

chemical environment ,it is known as chemical

shift.

Shielding shift

At higher Magnetic field strength(300MHZ,500MHZ)

Up field

Deshielding shift

At lower Magnetic field strength(100MHZ,60MHZ)

Down field

10

TMS

Measured in DELTA & TAU scales.

The value of chemical shift is expressed in terms of “ppm”(parts per million).

DELTA

TAU 0

Chemical shift, ppm δ = Shift in (Hz) Spectrometer frequency (in MHz)

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

The Electron cloud of the bulky group will tend to

repel,the electron cloud (in over crowded molecules)

surrounding the proton , & the proton is deshielded .

ANISOTROPIC EFFECT

The opposite of isotropy which means uniformity in all directions.

So, anisotropy is non-uniformity.Now for different compounds this anisotropy is

different as different distribution of electrons around nuclei.

Anisotropy effect of Alkene Alkyne Benzene ketone/Aldehyde

ALKENES: protons adjacent to alkene (C=C-H)are

deshielded by anisotropy effect,so, chemical shift will be induced.

ALKYNES: All groups in a molecule having πelectrons generate

secondary anisotropic field. In alkynes the system will shields the proton,thus

causing decrease in chemical shift of alkynes. Also the proton next to the alkynes carbon experience

the anisotropic effect towards shielded side.

HYDROGEN BONDING

Downfield shift depends upon the strength of H-bonding

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

APPROXIMETE CHEMICAL SHIFT RANGE (ppm) FOR SELECTED

TYPES OF PROTONS

REFERENCE

Organic spectroscopy by WILLIAM KEMP. Spectroscopy by SILVERSTEIN. Instrumental methods of chemical analysis by

GURUDEEP R.CHARWAL. Introduction to spectroscopy by DONALD L .

PAVIA.

Thank you