Engineering Chemistry S Datta

532 ENGINEERING CHEMISTRY
Ring current
H
H
Resultant
paramagnetic
field
Induced
diamagnetic
field
H 0
Fig. 24.17 The deshielding of aromatic protons due to a ring current effect.
The nuclear magnetic spectrum
At a given radio frequency, all the protons will absorb at a same effective field strength,
but they also absorb at different applied field strengths. The applied field strength is measured
and plotted against the absorption of radiation; the results are signals and the spectrum is
called a nuclear magnetic resonance spectrum (Fig. 24.18(a) and (b)).
Downfield
Absorption
Lowfield
Upfield
(a)
(b)
Fig. 24.18 (a) NMR spectrum for ethanol at low resolution, (b) a signal.
Informations predicted by the spectrum
• The number of signals indicate the different types or set of protons present in the
molecule.
• The positions of signals predict something about the electronic environment of each
kind of proton.
• The intensities of signals indicate how many protons of each kind is present.
• The splitting of the signals into several peaks indicates the environment of a proton
with respect to other nearby protons.