| Abstract
| - The vibrational spectrum of α-AlOOH diaspore has been calculated at the B3LYP level of theory with adouble-ζ quality Gaussian-type basis set by using the periodic ab initio CRYSTAL code. Harmonic frequenciesat the Γ point and the corresponding 48 normal modes are analyzed and classified in terms of simple models(octahedra modes, hydrogen stretching, bending, rotations) by direct inspection of eigenvectors, graphicalrepresentation, and isotopic substitution. Hydrogen modes are fully separated from the octahedra modesappearing under 800 cm-1; bending modes are located in the range of 1040−1290 cm-1, whereas stretchingmodes appear at 3130−3170 cm-1. The available experimental IR and Raman spectra are characterized bybroad bands, in some cases as large as 800 cm-1, and individual peaks are obtained by decomposing thesebands in terms of Lorentz−Gauss product functions; such a fitting procedure is affected by a relatively largedegree of arbitrariness. The comparison of our calculated data with the most complete sets of experimentaldata shows, nevertheless, a relatively good agreement for all but the H modes; the mean absolute differencesfor modes not involving H are 10.9 and 7.2 cm-1 for the IR and the Raman spectra, respectively, the maximumdifferences being 15.5 and 18.2 cm-1. For the H bending modes, differences increase to 30 and 37 cm-1, andfor the stretching modes, the calculated frequencies are about 200 cm-1 higher than the experimental ones;this is not surprising, as anharmonicity is expected to red shift the OH stretching by about 150 cm-1 inisolated OH groups and even more when the latter is involved in strong hydrogen bonds, as is the case here.
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