chemical shift
TRANSCRIPT
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
FACTORS AFFECTING CHEMICAL SHIFT
INDUCTIVE EFFECT
VANDERWAALS DESHIELDING
ANISOTROPIC EFFECT
HYDROGEN BONDING
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