Mean Amplitudes of Vibration of the

Although salts of the PBr4 and PC14 anions are well-known [1-3], a salt of the corresponding fluoroanion could only recently be prepared by reaction between N(CH3)4F and PF3 using either CH3CN, CHF3 or excess PF3 as a solvent [4], and the novel PF4 anion is stabilized in the form of its tetramethylammonium salt, N(CH3)4PF4. It possesses a pseudo-trigonal bipyramidal structure with two longer (1.74 Ä) axial bonds and an equatorial plane containing two shorter P-F bonds (1.60 Ä) and a sterically active free electron pair. As a continuation of our studies concerning vibrational and bond properties of simple novel inorganic species, we have now calculated the mean amplitudes of vibration of this interesting anion in order to attain a wider insight into its bond characteristics and peculiarities. For comparative purposes we have also undertaken a recalculation for the isoelectronic SF4 molecule. The calculations were performed with the method of the characteristic vibrations of Müller and co-workers [5-7]. The necessary vibrational and structural data for PF4 were taken from [4]. For SF4, the structural parameters were taken from [8] and the vibrational data from [9]. The obtained values, in the temperature range between 0 and 1000 K, are shown in Table 1 for PF4 and in Table 2 for SF4. A comparison of the results obtained for SF4 with those previously calculated for this molecule [10] shows Table 1. Calculated mean amplitudes of vibration (in A) for the PF4 anion.