Engineering Chemistry S Datta

530 ENGINEERING CHEMISTRY
Each set of equivalent protons in a compound will have a slightly different electronic
environment from other set of protons and hence will absorb at a slightly different applied
field strength to produce the same effective field strength which can cause the ‘resonance’ to
occur.
Shielding causes a shift of the resonance frequency to higher values of the applied field
(H o
), that is , the shift is upfield. On the other hand, deshielding causes a shift of the resonance
frequency to lower values of the applied field, that is, the shift is downfield. The magnitude of
this shift is called chemical shift. Since the value of the applied field experienced by the
organic compound cannot be determined accurately, chemical shifts are measured relative to
some standard compound which contains the nucleus under consideration. Tetramethylsilane
[TMS, (CH 3
) 4
Si] is particularly useful for proton magnetic resonance (pmr). TMS contains
twelve equivalent protons. The pmr spectrum of this compound shows a single sharp line
which occurs at higher field than any other protons in most of the common organic compounds.
That is, most pmr signals occur downfield with respect to TMS.
CH 3
CH —Si—CH 3
3
CH 3
(TMS)
Fig. 24.15 Shielding of an acetylenic proton in terms of paramagnetic effects.
The chemical shift may be reported in various ways. The resonance frequency of the
proton of the compound is dependent on the strength of the applied magnetic field. The shift
may be reported as units of the magnetic field in milligauss. However, the magnetic field can
also be expressed as Hertz (Hz) or cycles per second (cps). The separation in Hz is also
proportional to the frequency of the oscillator, e.g., if the separation between a proton signal
and signal of TMS is 60 Hz at 40 MHz the separation at 60 MHz will be 90 Hz. Therefore it is
desirable to repart chemical shifts in such units that are independent of the operating conditions
of the spectrophotometer. This can be done very easily by defining chemical shift δ by the
expression
Separation in Hz
δ =
Oscillator frequency × 106
The factor 10 6 is introduced in order to record the chemical shift as a convenient value.
This is usually in the range 1-10 and is expressed in ppm.
The dependence of the chemical shift of the oscillator frequency is shown in the following
example,