AB-INITIO QUANTUM-MECHANICAL VIBRATIONAL ANALYSIS OF THE DIMERIC A(2)X(6) MOLECULES (A=AL, GA, X=CL, BR, I)

Geometry, vibrational frequencies (SQM = Scaled Quantum Mechanical) and IR intensities are calculated for Al2Cl6, Al2Br6, Al(2)l(6), Ga2Cl6, Ga2Br6, and Ga(2)l(6) at the SCF level with extended basis sets. All molecules are found to be of perfect D-2h symmetry. The calculated geometries hint at inaccuracies in the experimental geometries for some of the compounds, mainly for ring angles and in particular for Al(2)l(6). The calculations mainly verify earlier assignments, but impose a few important corrections. For nu(6) there are significant differences (up to 47%) between the calculated and observed values, which suggests errors in the assignments, but a large basis set sensitivity for this mode makes the conclusions uncertain. Energy evaluations show that the mixed D-2h-symmetric compounds Al2Cl2Br4 and Al2Br2Cl4 are probably slightly unstable towards disproportionation, but should appear in detectable amounts in mixtures of Al2Cl4 and Al2Br6. Predictions are given for their vibrational frequencies and IR intensities.