The far infrared ͑FIR͒ spectra of various isotopic species of ethanol ͑-h 6 ,-d 1 , and-d 3 ͒ are analyzed from MP4͑͑SDQT͒ ab initio calculations using models in one and two dimensions. From the calculated frequencies and intensities, previous assignments of ethanol-h 6 and-d 1 bands are reviewed. The position of several combination bands are predicted. Ethanol shows two conformers, trans and gauche, and two interacting torsional modes. The torsional barriers have been calculated to be V 3 ͑trans͒ ϭ1226.7 cm Ϫ1 , V 3 ͑gauche͒ ϭ1296.3 cm Ϫ1 , V OH (␣ϭ62°)ϭ404.1 cm Ϫ1 , and V OH (␣ ϭ180°)ϭ423.3 cm Ϫ1. The flexible models in one and two dimensions yields the same OH torsional frequencies, whereas they differ in the methyl group state calculations. The fundamental bands of the ethanol-h 6 have been evaluated at 205.5 cm Ϫ1 ͑OH torsion͒ and 257.0 cm Ϫ1 ͑CH 3 torsion͒ and their corresponding intensities to be 18.650ϫ10 Ϫ4 and 0.662ϫ10 Ϫ4 .
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
The structural and spectroscopic effects of hydrogen bonding on isolated 2,2,2-trifluoroethanol (TFE) and its molecular complexes are theoretically investigated at the MP2/aug-cc-pVDZ level. As a result, previous interpretations of the relative stability of the trans and gauche conformers of the isolated molecule are revised. We show that the prevalence of the gauche form is due to a decrease of repulsion forces, rather than to the formation of an intramolecular hydrogen bond. We find that the instability of the trans geometry is caused by repulsion forces between the oxygen electronic pair and the fluorine atom clouds, which are significantly stronger in trans-TFE. Molecular agents capable of weakening the repulsion produce stabilization. These results lead to a reinterpretation of the stabilizing factors of halogenated compounds. To analyze complexation, two small molecules (water and ammonia) have been chosen. Water can form four different molecular aggregates with TFE. The most stable corresponds to a species where H 2 O acts as hydrogen donor and TFE presents the cis-gauche conformation, forming two intramolecular hydrogen bonds. For NH 3 , the cis-gauche conformation loses stability, because of steric hindrance. In this case, TFE varies the relative stability of its conformers, with trans-TFE becoming the preferred structure. Hydrogen bond formation between NH 3 and trans-TFE produces vibrational shifts of -354, -17, and +518 cm -1 for the OH stretching, the OH bending, and the OH torsion, in agreement with the experimental findings. We found complexation to produce an important variation of the position of the infrared bands corresponding to the hydroxyl group.
AbstractsThe half-projected Hartree-Fock function (HPHF) for singlet states is defined as a linear combination of two Slater determinants, which contains only spin eigenfunctions with even quantum number. Using a self-consistent procedure based on the generalized Brillouin's theorem, the RHF, HPHF and PHF functions are deduced for the ground states of the Li-, Be, B+, and C2+ systems, in a limited basis set. I t is found that the HPHF function yields better energy values than the RHF function, very close to that of the PHF one. The HPHF scheme seems thus to be useful as a substitute for the PHF model, specially in the case of large electronic systems in which the latter method becomes unmanageable.On dtfinit la fonction Hartree-Fock Semi-Projette (HPHF)
We have calculated the potential energy hypersurface of dimethyl ether with respect to the COC bending coordinate ␣ and the torsional angles of the two methyl groups, 1 and 2 . Two sets of ab initio calculations were carried out. The first was made at the level MP2/6-31G(d,p) in which the structural coordinates were fully relaxed except for the grid points on the hypersurface. More extensive calculation were carried out with MP4 corrections for electron correlation with the same molecular structure. The torsional bending Hamiltonian matrix was symmetrized by the operations of the G 36 nonrigid group and was solved variationally. The effect of explicitly considering the bending mode in the three-dimensional treatment was determined by a comparison to the two-dimensional model in which the flexibility of the frame was absorbed into the calculation by the fully relaxed method. It was found that the three-dimensional calculation gave a much better account of the sin͑3 1 ͒sin͑ 2 ͒ intermode coupling than the two-dimensional treatment.
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