Ab initio quantum mechanical study of γ-AlOOH boehmite: structure and vibrational spectrum

The structure and vibrational spectrum of boehmite have been investigated at the quantum-mechanical level with the CRYSTAL code, using a Gaussian-type basis set and the B3LYP Hamiltonian. Three space groups are considered in this study: Cmcm, Cmc2(1), P2(1)/c. Cmcm turns out to correspond to a transition state, whereas Cmc2(1) and P2(1)/c are minimum energy structures. The difference among them is the position of H atoms only, the Al-O frame being essentially the same. Harmonic frequencies at the I" point have been computed. The comparison between calculated and experimental frequencies shows a good agreement for the Al-O part of the spectrum (under 790 cm(-1)). For the Al-OH bending modes (800-1,300 cm(-1)) an absolute differences of 50-100 cm(-1) is observed; for the OH stretching modes (3,200-3,500 cm(-1)) it increases to 120-200 cm(-1): anharmonicity is large because OH groups are involved in strong hydrogen bonds.