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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

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.

We have calculated the potential surface with respect to the torsional angles of the methyl groups, the torsional energy level structure, and the infrared and overtone Raman intensities by ab initio methods using extended basis sets. The conclusions that were drawn from a correlation to the observed spectra are the following, (i) The modest basis set MP2/6-31G(d, p) yields a value of 238.32 cm−1 for the v16 gearing mode in reasonable agreement with the observed value of 241.0 cm−1. Extension of the basis set to MP2/6-311G(df, p) marginally improves this value to 239.36 cm−1. A somewhat larger basis set, RHF/6-311+G(3df, 3p) was needed for a satisfactory description of the components of the polarizability and the dipole moment. (ii) The technique of full relaxation of the coordinates of the top and the frame was unable to account for the strong resonance between the 2v12 overtone gearing level and the v7 COC in-plane bending mode. (iii) The asymmetry along the diagonals of the potential surface that determines the sign and magnitude of the sine.sine gearing term was found to display complex behaviour and was positive in the region of the eclipsed-eclipsed equilibrium conformation and negative for the staggered–staggered conformation.

50) Eberhardt, W.; Sham, T. K.; Carr, R.; Krummacher, S.; Strongin, M.; Weng, S. L.; Wesner, D. A fluorescence excitation spectrum of formic acid monomer (HCOOH) has been recorded in the 268-257-nm region and has been attributed to an n -+ K* electron promotion in the anti conformer. The So -SI electronic origins of the HCOOH/HCOOD/DCOOH/DCOOD isotopomers were assigned to weak bands observed at 37 431.5/37 461.5/ 37 445.5/37 479.3 cm-I. Four vibrational modes, u3(c=o), v7(o-c=o), u8(C-HaId), and u9(0-Hh d), were observed in (3) Brouard. M.f O'Mahony, J. Chem Phys . i e t t . 1988. 149, 45. is expected to be nonplanar as well. The SI states of the

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Motivated by our recent finding that the singlet-triplet bands of selenoformaldehyde involve an upper state with large zero field splittings, we have extended the theory and written a program for predicting and fitting such rotationally resolved spectra. Triplet state matrix elements for a case ͑A͒ basis have been developed, including corrections for centrifugal and spin-centrifugal distortion. The full Hamiltonian matrix has been symmetry adapted, simplifying the problem to four individual matrices of approximately equal size for molecules of orthorhombic symmetry. Diagonalization of these matrices yields triplet state energies that are in agreement with previous treatments using a basis in which the spin splittings are small relative to the rotational intervals. Methods have been developed for sorting the eigenvalues and assigning quantum labels regardless of the magnitude of the spin splittings. The calculation of the relative intensities of the rotational lines within a band has been programmed using transition moment matrix elements from the literature. The selection rules for various upper state symmetries have been developed in a form useful for the analysis of spectra. Band contour predictions of spectra for various coupling cases have been presented.

The electronic spectra of H 2 CO, D 2 CO, HDCO, and D2 \3CO have been recorded between 1250 and 2000 A and the vibrational fine structure associated with the n-> 3 s, 3 p" 3 py, 3 p" 3 d transitions assigned. The electronic origin bands in the Rydberg transitions show quite unusual isotope effects, with the v(D 2 CO)-v(H 2 CO) shift for the n-> 3 d transition being somewhat smaller than the limiting value of the ion while the lower energy n->3s and 3py transitions have anomalously large values. As these effects cannot be accommodated by the changes in the totally symmetric in-plane frequencies on excitation, they are attributed to large differences in Vs and V6 in the upper and lower electronic states. Direct information concerning the reduction in V6 on excitation comes from the identification of a series of low frequency bands in the n-> 3 Py system of D 2 CO which were assigned to quanta in V6' The potential function which was derived from this data was found to be very anharmonic and could contain a double minimum. The reduction in V( Q,;) at large Q,; was attributed to a coupling between the Rydberg IA1(n, 3py) state and a second IB2 state which lies at higher frequencies. An estimate of the coupling between these states lead to a value of 1480 em ~ I for the vibronic matrix element. While this value is entirely compatible with what would be expected for valence-valence coupling, it is somewhat larger than what would have been anticipated for vibronic coupling between Rydberg states. RESULTS Electronic structureThe electronic structure of formaldehyde in its ground and higher electronic states may be derived from molecular orbital theory as a linear combination