SCALED QUANTUM-MECHANICAL (SQM) VIBRATIONAL ANALYSIS OF ALCL3, AL2CL6 AND THE FREE-ION ALCL4-

A series of basis sets are evaluated for AlCl3 and AlCl4- and a common scaling factor derived for these compounds was transferred to Al2Cl6. 6-31G* calculations, scaled with a common factor of 0.92, give a good fit to all vibrational frequencies for AlCl3, AlCl4 and Al2Cl6, and there is also a qualitative fit between calculated and observed IR intensities. Extending the basis set does not lead to significant improvements, either for the geometry or for the vibrational frequencies. The calculations verify earlier assignments based on general valence force fields and predict frequencies for some unobserved bands. The SQM force fields for aluminium chlorides show large similarities with the isoelectronic thiophosphates; the main deviations can be attributed to more ionic bonds in the aluminium chlorides. A significant influence on the out-of-plane vibration in AlCl3 by the basis set on the d-orbitals may indicate that pi(d-p) bonding plays a role. A remarkable and unexpected basis set sensitivity is found for the nu6(B1g) ring vibration in Al2Cl6.