Mean amplitudes of vibration of the novel XeF −5 anion

Abstract

The first example of a chemical species adopting the unusual pentagonal planar geometry was the anion XeFj, reported in 1991 [1], Recently, Christe et al. [2] reported the existence of the IF|~ dianion, which is only the second known example of such a molecular geometry. In order to advance in the characterization of this very interesting species, we have now undertaken a calculation of its mean amplitudes of vibration and compared the results with those obtained for some related molecules and ions. The calculations were performed using the method of characteristic vibrations of Muller and co-workers [3-5], The vibrational frequencies were taken from [2] and an F I F angle of 72° was assumed. The mean amplitudes of vibration calculated for IF|~, in the temperature range between 0 and 1000 K, are presented in Table 1. For comparative purposes, Table 2 shows the corresponding values for the isoelectronic XeFJ anion [6] and other closely related species. The amplitude value for the I F bond in IFf~ is, as expected, appreciably higher than that of the X e F bond in XeFJ, in agreement with the additional negative charge present over the iodine species and its lower nuclear charge. These two factors evidently produce an enhancement of the I F bond polarity, generating a weakening of these bonds [2], which can also be visualized from the strong temperature dependence of its amplitude values (see Table 1), which in this case is yet more important than in XeFj [6]. On the other hand, the very strong temperature dependence also observed for the non-bonded in-plane