HARMONIC FORCE-FIELD AND RAMAN-SCATTERING INTENSITY PARAMETERS OF NORMAL-BUTANE

Raman and infrared spectra of gaseous n-butane-h10, n-butane-d10, n-butane-1,1,2,2,3,3,4,4,4-d9, and n-butane-1,1,1,2,3,3,4,4,4-d9 are reported. Quadratic vibrational force fields for the trans and gauche conformers have been obtained by scaling the ab initio force fields, calculated by using a 6-31G basis set, to fit well-determined vibrational frequencies that have been adjusted for anharmonic effects by means of an empirical correction. Absolute Raman trace scattering cross sections have been measured for the same isotopic species, and isotopically independent partial-alphaBAR/partial S(j) intensity parameters were determined for the two conformers from the experimental cross sections. The mole fraction of the trans conformer, an additional parameter in this calculation, was found to be 0.62 +/- 0.04, in agreement with other recent measurements. Local intensity patterns could not be reproduced as accurately as for smaller molecules that had been considered previously. This is attributed to a greater ease of sharing intensity among the more closely spaced fundamental and multi-quantum levels in these larger molecules.