Thermochemical and spectroscopic studies of bond energies in simple molecules

• Main Author: Pugh, A. C. P.
• Published: University of Oxford 1957
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.734660

The gaseous alkali halide molecules are of interest because they constitute a group particularly susceptible to theoretical interpretation. It does not seem to be too far from the truth to regard them as systems of simple polarizable ions. An important molecular property is the dissociation energy, D(MX). In the case of the alkali halides, this may be determined either by spectroscopic or by thermochemical methods. However, although analyses of their ultraviolet absorption spectra have given fair estimates of the dissociation energies, the method of analysis is here liable to systematic errors and is best used to get relative values. The most reliable estimates of the energies of dissociation are therefore to be obtained from the thermochemical cycle which may be written D(MX) = - Q_f(MX) - Δ_sH(MX) + Δ_sH(M) + ½D(X ) ... (1) The quantities least well known in equations (1) for the alkali halides are often the heats of sublimation, Δ_sH₂₉₈(MX), of the salts. There is at present available in several cases information only on the vapour pressure above the liquids at relatively high temperatures, and the corrections to give Δ_sH₂₉₈ are large and uncertain, and more so since the only values commonly known for the heats of fusion are derived from rather scanty information on freezing points in binary systems. On these grounds, the merits of determining Δ_sH₂₉₈ by measuring the vapour pressures above the crystal at relatively low temperatures are obvious, in addition it has recently become clear that the interpretation of measurements at higher pressures is not simple. There is now good evidence, both from electron impact studies, and from the observations of molecular beams, that the vapours of the alkali halides consist by no means exclusively of monomers, MX, but also contain appreciable proportions of polymers, especially of (MX)₂. It would be expected that at the low vapour pressures obtaining above the crystal, dimerisation would be relatively unimportant, and that reliable values of Δ_sH₂₉₈ for the monomer species might be obtained by simple treatments. In fact, the results given below may be taken to indicate that this is the situation for caesium and rubidium fluorides, but that in lithium, sodium, and, to a larger extent in potassium fluorides, dimerisation is significant even at pressures in the range 10^-4 to 10^-2 mm.