DFT studies of Al–O Raman vibrational frequencies for aquated aluminium species

Raman frequency calculations of Al(H2O)63+ and Al(OH)4- species in different models were carried out with density functional theory (DFT). Gas-phase model (GP), polarizable continuum model (PCM), supermolecule model (SM) and supermolecule-polarizable continuum model (SM-PCM) were considered and frequencies over the wavenumber range from 0 to 1000 cm−1 were discussed. The calculated frequencies of the two species in SM-PCM model are in fair agreement with the observed frequencies in aqueous solution and the average deviation is 6 cm−1. Based on the results of Al(H2O)63+ and Al(OH)4-, the DFT method and the SM-PCM model were used to study the Al–O Raman bands for some other monomeric and dimeric aluminiums. The monomeric aluminiums Al(OH)2+, Al(OH)2+ and Al(OH)63- as well as the dimer Al2O(OH)62- were included. As a result, we predict the main Al–O skeleton vibrations of Al(OH)(H2O)52+ at 685 cm−1, trans  -Al(OH)2(H2O)4+ at 542 cm−1, Al(OH)63- at 510 cm−1 and Al2O(OH)62- at 510 and 699 cm−1 respectively in Raman spectra of aqueous solution. It can also be concluded that the calculated frequencies of Al2O(OH)62- are close to the experimental values and the existence of Al(OH)63- in aluminate solution cannot be ruled out.