Ab initio quantum mechanical study of [gamma]-AlOOH Boehmite: structure and vibrational spectrum
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The structure and vibrational spectrum of boehmite have been in- vestigated 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 corre- spond 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 ?? 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 (800-1300 cm-1) an absolute dif- ferences of 50-100 cm-1 is observed; for the OH stretching modes (3200-3500 cm-1) it increases to 120-200 cm-1: anharmonicity is large because OH groupsare involved in strong hydrogen bonds.
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