The vibrational motion of 'polymeric' BeCl2

A recently introduced polarizable-ion model for BeCl2 is compared to high quality ab initio calculations for the geometries of isolated chains [BeCl2](n). The relaxed geometries and vibrational frequencies (calculated using an instantaneous normal mode (INM) approach) are found to agree well. The density of stales for the melt, calculated using the INMs, is shown to be very structured compared with typical molten salts, corresponding to a more 'molecular' spectrum. The band frequencies are seen to change little between the melt and the isolated chains further promoting the description of the liquid as an ensemble of such 'polymeric' chains. The calculated Raman spectrum is compared with experiment and shown to be fully consistent with the model as long as they are considered in terms of the symmetry of the molecular chains themselves. The dangers of assigning experimental bands in terms of the motions of local coordination polyhedra are emphasized.