Characterisation of octahedral hexahalo compounds.

We have recently investigated a series of compounds by inelastic neutron scattering, (INS) and conventional optical spectroscopies, to generate complete and unambiguous assignments for all the modes. One of the aims of the work was to see if there are any systematic trends that can be exploited to estimate the (usually) optically inactive mode ?6, if the other modes are known. A plot of ?5 vs ?6 shows that there is generally a good correlation between the two mode energies. We propose to measure the INS spectra of WF6 and MoF6, which will provide an unambiguous assignment of ?6 because of the absence of selection rules. We also propose to measure SF6 and WCl6, the former because it will provide confirmation of existing assignments and the latter because it will allow a comparison with WF6. In all four molecules, ?6 has only been observed indirectly from overtone and combination bands.