Ab initio quantum mechanical study of gamma-AlOOH boehimite: 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, Cmc21, P21/c. Cmcm turns out to correspond to a transition state, whereas Cmc21 and P21/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 Γ 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