The Infrared Photodissociation Spectra and the Internal Mobility of SF6-, SiF4- and SiH4-Dimers
We present an analysis of the couplings originating from different intermolecular interactions (electrostatic, exchange, dispersion, induction) which split and shift the frequencies of the vibrational transitions in Van der Waals dimers, and determine their intensities. Model potential calculations illustrate the importance of the various contributions in (SF6)2, (SiF4)2 and (SiH4)2 and their dependence on the monomer orientations. The results, in conjunction with calculated equilibrium structures, barriers to internal rotation and (harmonic) Van der Waals vibrational frequencies, lead to several observations which are relevant for the interpretation of the infrared photodissociation spectra of these complexes. We confirm that in (SF6)2 and (SiF4)2 (orientation-independent) resonant dipole-dipole coupling dominates the appearance of the spectra. For (SiH4)2 we conclude, however, that other than electrostatic terms are not negligible and, moreover, that the electrostatic coupling leads to orientation-dependent vibrational frequencies and intensities. This orientational dependence is related to the large displacements of the hydrogen atoms in the v4 mode of SiH4. We also find that the internal rotations in (SF6)2 and (SiF4)2 are more strongly locked than those in (SiH4)2. Especially the geared internal rotations in the latter dimer could easily occur at the experimental molecular beam temperatures.
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