Ab initio MO calculations on the Structure and Raman and Infrared Spectra of the [Al4O2Cl10]2- oxide species in chloroaluminate melts

The oxide complexation chemistry in molten tetrachloroaluminate salts and ionic liquids is discussed with respect to what possible structures that may be formed in addition to [AlCl4]-: [Al2OCl6]2-, [Al3OCl8]-, [Al2O2Cl4]2-, [Al3O2Cl6]- and [Al4O2Cl10]2-. Ab initio Molecular Orbital (MO) calculations were carried out on these various aluminium chloride and oxochloride ions, in assumed isolated gaseous free ionic state, by use of the Gaussian-03W program at the restricted Hartree-Fock (HF) level and with the 6-31+G(d,p) basis set [1].

Without any pre-assumed symmetries and with tight optimization convergence criteria and by using the modified GDIIS algorithm, the model calculations generally converged. The structures and their binding energies are presented. The expected geometries are supported, with one exception perhaps being the [Al2OCl6]2- ion, that gave a linear Al-O-Al bonding system of staggered (approximate D3d symmetry), in analogy with the linear Al-O-Al geometry of the analogous [Al2OF6]2- ion, found previously.

The calculations included determination of the vibrational harmonic normal modes and the infrared and Raman spectra, (vibrational band wavenumbers and intensities), without any empiric adjustments of the harmonic force constants, using constants directly predicted from the Gaussian 03W program. Previously obtained IR absorption and Raman scattering spectra of melts are assigned, by comparing to the ab initio quantum mechanical vibrational analysis results.

It is concluded that the small oxide content commonly found in basic and neutral tetrachloroaluminate melts, most probably consist of [Al4O2Cl10]2- ions and the vibrational spectra are given.