Nonrigid Molecule Effects on the Rovibronic Energy Levels and Spectra of Phosphorous Pentafluoride

摘要

Although phosphorous pentafluoride is known from electron diffraction, infrared, and Raman data to have aD3htrigonal bipyramid equilibrium configuration, NMR data shows only one fluorine resonance. Although this is almost certainly due to a fast intramolecular rearrangement process occurring via a Berry pseudo rotation, the detailed mechanism (e.g., classical thermal rearrangement or quantum mechanical tunneling in low vibrational levels) is unknown. A theoretical study has been made of the effects of quantum mechanical tunneling on the rovibronic levels, i.e., treating phosphorous pentafluoride as a nonrigid molecule. The levels are shown to be split in a very characteristic manner. Simple results for the relative intensities of the resultant fine structure to the original rovibronic transitions have also been obtained. This predicted, characteristic fine structure could be searched for experimentally in order to further our understanding of the intramolecular rearrangement process occurring in phosphorous pentafluoride. A theoretical study on a suitable millimeter wave spectroscopic experiment shows that such an experiment is at present on the border line of accessibility. A new general method of treating nonrigid molecule energy levels is illustrated by this application to the specific case of phosphorous pentafluoride.