9 —
Molecular Spectrometry
The nature of molecular energy is complex, comprising contributions from translational, rotational and
vibrational motions, from electrons occupying molecular orbitals, and from nuclear spins. The range of
these various energies covers a large part of the electromagnetic spectrum so that the techniques of
molecular spectrometry are both varied and comprehensive in the analytical information they provide.
The separation between quantized levels for each type of molecular energy also varies, the relative
magnitudes being in the order
Translational levels are too closely spaced to be considered quantized, while the very small differences
between nuclear spin levels arise only when the molecules are subjected to a strong magnetic field.
Both absorption and emission may be observed in each region of the spectrum, but in practice only
absorption spectra are studied extensively. Three techniques are important for analytical purposes:
visible and ultraviolet spectrometry (electronic), infrared spectrometry (vibrational) and nuclear
magnetic resonance spectrometry (nuclear spin). The characteristic spectra associated with each of
these techniques differ appreciably in their complexity and intensity. Changes in electronic energy are
accompanied by simultaneous transitions between vibrational and rotational levels and result in broad-
band spectra. Vibrational spectra have somewhat broadened bands because of simultaneous changes in
rotational energy, whilst nuclear magnetic resonance spectra are characterized by narrow bands.
A fourth technique used for the characterization of molecules is mass spectrometry. It is included in this
chapter because the structural information it provides is similar to that obtained from the other
techniques although the principle is entirely different. It is a destructive method in which the
fragmentation pattern of sample molecules is used to determine empirical formulae and molecular
weights, and to identify structural features.