Single-crystal Raman spectrum of bismuth pentafluoride and of antimony tetrachloride fluoride and the vibrational spectrum of vanadium, niobium, tantalum, and antimony pentafluorides

Abstract

Single-crystal Raman polarisation data on α-BiF₅ enable an unambiguous assignment to be made of the Raman-active modes of this polymer. The corresponding i.r. data define a Bi–F–Bi bridge stretching mode at 450 cm.^–1, suggesting a force constant equal to one quarter of the terminal force-constant. Single-crystal Raman studies and normal-co-ordinate calculations (based on this bridge force-constant) are used to assign the vibrational spectrum of SbCl₄F. The vibrational spectra of solid VF₅, of NbF₅ and TaF₅ in the solid and liquid states, and of liquid SbF₅ are examined to study the position of the ‘fluorine-bridging’ modes and to investigate the stereochemistry of the liquid states. An interpretation in terms of cis-fluorine-bridged polymers is satisfactory for NbF₅ and SbF₅. In the case of SbF₅ the Raman spectrum of the melt is the same at 20 and 125°. Normal-co-ordinate calculations are used to assign the vibrational spectrum of solid NbF₅.