factors influencing chemical shift
TRANSCRIPT
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