Internal vibrations and hindered rotation of ReH 2? 9 by neutron scattering spectroscopy

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Abstract
The hydrogen amplitude weighted frequency spectra for the whole ion librations and for intra-ion hydrogen vibrations of ReH2– 9 have been measured by inelastic neutron scattering spectroscopy for K2ReH9 and Na2ReH9 at 90 K. Librations about both non-equivalent molecular axes have been detected and assigned for each of the two distinct crystallographic sites in K2ReH9. In the sodium salt two librational frequencies only were observed, suggesting a single site for ReH2– 9 in the structure of this crystal. Observation of librations about distinct molecular axes allows the barriers to hindered molecular rotation to be calculated for these axes in each crystallographic site of K2ReH9. N.m.r. measurements contrast these data and indicate that rotational diffusion is isotropic for the “a” sites at 90 K and becomes so for the “d” sites at about 152 K. The different result arises from the longer observation time scale of n.m.r. relaxation. From K2ReH9 the barriers to hindered rotation calculated from the E and A symmetry libration frequencies, using a six-fold cosine potential and the harmonic approximation are, 4.1; 7.2 and 8.8; 11.7 kcal mol–1(17.1; 30.1 and 36.8; 48.9 kJ mol–1) for the two-fold, and singly degenerate rotation axes in sites “a”, “d” respectively. For Na2ReH9 the activation energies for these axes and the single site are reversed in magnitude compared to K2ReH9. The intramolecular vibrational spectrum shows three peaks for both salts and the lowest of these (ca. 650 cm–1 in the potassium salt and ca. 600 cm–1 in the sodium salt) is tentatively assigned to the vibrational species, which may lead to pseudo-rotation in solution.

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