General considerations and theoretical background
Nomenclature
In the UV and visible spectrum, the energy of photons associated with electronic transitions lies in the range 147–630 kJ/mol. This energy (∆E) can be expressed in terms of the principal parameters that define electromagnetic radiation, namely frequency l (Hz), wavelength k (nm) and wavenumber m v (cm–1):
where h is Planck’s constant, and c is the velocity of radiation in vacuo. The positions of peaks are sometimes described in terms of wavenumber, which has the advantage of being a linear function of energy, but this term is much more frequently used in infrared spectrophotometry. The prac tical unit most often used in UV and visible spectrophotometry is wavelength (k), usually expressed in nanometres (nm). The older units of wavelength, millimicron (ml) and ångström (Å) are not recommended terms. The position of maximum absorption of a peak is designated λmax.
The wavelength span is conventionally divided into two ranges: the UV extends from 200 nm to about 400 nm; the visible range extends from about 400 nm to 800 nm. Outside these limits, the ‘far UV’ or ‘vacuum UV’ extends from 10 nm to 200 nm, and the ‘near infrared’ from 0.8 μm to about 3 μm (see Fig. 1).
Figure 1 Radiation spectrum highlighting the visible region of the spectrum.
A molecular grouping specifically responsible for absorption is termed a chromophore, and is usually a conjugated system with extensive delo calisation of electron density, for example the benzene ring. Any saturated group, with little or no intrinsic absorption of its own, that modifies the absorption spectrum when attached directly to a chromophore is described as an auxochrome, examples being 1OR, 1NR2, 1SR. Auxochromes are considered to exert their effect through partial conjugation of their polarisable lone-pair electrons with those of the adjacent chromophore. However, if the lone pair of electrons is involved in bonding, as for example in the case of a protonated quater nary ammonium group, the auxo chromic effect vanishes. This property can be used for molecular characterisation, as discussed below. When developing analytical methods, it is important that an analyst can recognise whether or not a molecule will absorb in the UV-visible region from a consideration of the chemical structure of the substance.
