INFRARED-SPECTRA OF CHLOROETHANE SPECIES, A SCALED AB-INITIO FORCE-FIELD, INFRARED INTENSITIES, ATOMIC POLAR TENSORS, AND EFFECTIVE CHARGES

Infrared wavenumbers are reported for C2H5Cl (gas), CH3CD2Cl, and CHD2CD2Cl (gas, solution) and infrared intensities for C2H5Cl (gas). Following a number of Fermi resonance corrections, a definitive set of vibrational assignments is made. An SCF/6-31G* force field is fitted to the observed frequencies with 15 scale factors. Imperfections in the fit are attributed to limitations of the conventional scaling procedure used. The ab initio derived atomic polar tensors are used in conjunction with the unsealed force field to predict infrared intensities for all bands of all the species studied. For CH stretching vibrations these are nearly twice as seat as those observed experimentally, but the trends found in the series CH4-C2H6-C2H5Cl are well reproduced and can in part be related to changes in Mulliken atomic charges and King effective atomic charges. Additivity of CH stretching intensity, per CH bond, is confirmed by the ab initio calculations. Directions of the dipole derivatives partial derivative mu/partial derivative r for the C-H and CCl bonds are derived from rotated atomic polar tensors. A comparison of infrared intensities calculated from partial derivative mu/partial derivative r in a diatomic approximation with those from a full treatment indicates the presence of rotational contributions.