Ab-initio determination of the force field of dichloromethane, verified by gas-phase infrared frequencies and intensities and applied to a combined electron diffraction and microwave investigation of geometry

For CH2Cl2, using 6-31G*/66-31G* basis sets, the complete geometry-relaxed force field and the dipole moment derivatives along internal coordinates were calculated. The study of the force field scaling procedure yielded (i) agreement with experimental IR frequencies with a root mean square deviation of 9.6 cm-1 and maximum deviation of 18 cm-1, (ii) guidelines to select the number of scale factors and the way symmetry coordinates should be distributed over the scale factor groups, and (iii) the recognition that deviations of the scale factors from unity may be used to measure the performance of the basis set. Absolute IR band intensities were calculated and had the same sequence as the gas-phase intensities. Ab initio intensity calculations may thus become a valuable tool in the assignment of IR spectra. For example, it was proved that the 1467 cm-1 band is not the CH2-scissoring mode. Furthermore, support was found for the model of Evans and Lo, rationalizing the remarkable intensity changes of inter ali...