Potential function for axial–equatorial fluorine atom exchange in PF5, AsF5, and VF5

Abstract

Gas-phase Raman spectra of the ν7 fundamentals of AsF5 and VF5 were recorded for spectral resolutions approaching 1.5 cm−1. The vibrational transitions associated with ν7 for these systems, as well as for PF5, were interpreted in terms of a two-dimensional anharmonic potential function constrained to a double minimum form for the motions leading to axial–equatorial fluorine atom exchange. The intramolecular exchange barrier heights, determined by the double minimum potentials, lie between 1139 and 995 cm−1 (3.26 to 2.84 kcal/mol where 1 kcal/mol=4.184 kJ/mol) for PF5, 864 and 755 cm−1 (2.47 and 2.16 kcal/mol) for AsF5, and 593 and 428 cm−1 (1.54 and 1.22 kcal/mol) for VF5. A discussion of the dynamics of the fluorine atom interchange pathways suggests that these trigonal bipyramidal (D3h) molecules form C4v intermediates by initially displacing the equatorial fluorine atoms and then by mixing in the axial fluorine distortions as the intramolecular exchange proceeds.