The far infrared torsional spectra of acetone (CH 3 hCO and (CD 3 hCO have been determined from ab initio calculations, and the main features of the experimental data assigned. For this purpose, the potential energy surface for the double methyl rotation was determined with fully relaxed geometry into the RHF and RHF + MP2 approximations using a 6-31G(p,d) basis set. The energy values, as well as the kinetic parameters obtained from the optimized geometry, were fitted to double Fourier expansions as functions of the rotational angles in seven terms. The torsional solutions were developed on the basis of the symmetry eigenvectors of the G 36 nonrigid group, which factorize the Hamiltonian matrix into 16 boxes. The energy levels and torsional wave functions for each symmetry specie were then obtained diagonalizing each blocks separately. Intensities were obtained from the calculated electric dipole moment variations and the nuclear statistical weights, and were combined with the torsional frequencies to predict the spectra. The calculated band patterns show a multiplet structure and reproduce the main features of the experimental data. The torsional bands of the infrared active V17 mode were found to be clustered into quartets, (A

No abstract

Crystals of nitric acid hydrates have been studied theoretically by means of a recently developed ab initio method (SIESTA). Using as input data the X-ray structures of crystals of nitric acid monohydrate (NAM), dihydrate (NAD), and trihydrate (NAT), the atomic geometry within each unit cell has been refined. The calculated geometrical structure of these atmospherically relevant systems allows the study of further physicochemical properties. In this article, their vibrational normal modes have also been evaluated. These have been used to propose the assignment of the observed spectra of the hydrate crystals, with good overall agreement between experiment and calculation. This is the first time that this type of calculation has been carried out for NAD, whose recently observed two phases (I and II) have been studied here.

No abstract