Knowledge Bank

A complete re-examination of the vapor-phase vibrational spectra of the much-studied octahedral hexafluoride molecules has been undertaken. Some novel features of this study are the following: (1) Observation of infrared spectra at path lengths of up to 10 m and Raman spectra in multipass cells has enabled many overtone and combination bands to be measured; the resulting anharmonicity constants have yielded the first good estimates of the harmonic frequencies of these molecules. (2) Infrared band contours of the fundamentals have been studied at temperatures down to $190\circ K$, thus suppressing the hot-band structure sufficiently to allow accurate measurement of the P-R branch spacing. The improved Coriolis constants obtained in this manner have been used in fixing the general quadratic force fields. (3) Centralatom isotope shifts have been observed either from resolution of the Q branches at low temperatures or from the spectra of isotopically-enriched samples, thus providing additional data for the force field constraint. (4) A careful analysis of sources of error has led to estimates of 90% confidence intervals on the final general quadratic symmetry and valence force constants. Results will be presented for $WF6,UF6$, and several of the Group VI A hexafluorides.