The Raman scattering intensity parameters of acetylene

The Raman trace scattering cross-sections for CH stretching vibrations may be described in terms of derivatives of the mean molecular polarizability with respect to the CH stretching internal coordinate. There is, in general, good agreement between values for these parameters obtained from experimental absolute intensity studies and those calculated by ab initio quantum mechanical techniques. One apparent exception to this result is acetylene, for which this derivative is calculated to be significantly smaller than the experimental value when compared with the results for other molecules. We have remeasured the Raman trace and quadrupole scattering cross-sections for acetylene and re-analysed the experimental results within the double harmonic approximation. The obtained parameters fit the experimental results to a high degree of accuracy, and both the measured cross-sections and derived intensity parameters agree well with the previous results. We have also considered the contribution of additional factors to the ab initio predictions. Inclusion of electron correlation causes a significant effect, and the variation in the resulting calculated intensity parameters more nearly corresponds to that for the observed values.