Engineering Chemistry S Datta

INSTRUMENTAL METHODS OF ANALYSIS 531
δ =
60
40 × 10 6
× 10 6 = 1.5 ppm.
6
90 × 10
δ =
6
= 1.5 ppm.
60 × 10
It is now becoming common practice to express chemical shifts in τ (tau) values, defined by:
τ = 10-δ.
10 ppm is assigned to the line of TMS in nmr spectrum. Most protons have positive τ
value i.e., δ < 10. Strong acidic protons have negative τ value i.e., δ > 10.
Position of signals (chemical shifts)
The greater the shielding of nucleus the larger is τ value (the smaller is δ). Since the
degree of shielding depends on the electron density surrounding the proton any structural
feature that decreases this electron density causes a decrease in shielding (deshielding) with
consequent lowering of the τ value (the chemical shifts) movesdown field. Thus electronegativity
τ(ppm) τ(ppm)
CH 3
I 7.83 CH 3
—C 9.12
CH 3
Br 7.35 CH 3
—N 7.85
CH 3
Cl 6.98 CH 3
—O 6.70
CH 3
F 5.70
of the atoms attached in the compounds in the column gradually increases as such there is
gradual deshielding and τ value decreases in the column.
Si is less electronegative than carbon, so the protons of CH 3
groups in TMS are more
shielded than those in C—CH 3
.
Examples of Shielding and Deshielding
H
-electron circulating
at right angles
to the field
C
C
R
H
C
O
Induced field
opposing the applied
field, circulating
electrons act to
shield the protons.
This effect is called
diamagnetic
anisotropy.
H
H 0 H 0
(a)
(b)
Fig. 24.16 (a) Shielding of an acetylenic proton and (b) deshielding of an aldehydic
proton in terms of diamagnetic anisotropic effects.