Raman and IR spectra of butane: Anharmonic calculations and interpretation of room temperature spectra

First-principles anharmonic calculations are carried out for the IR and Raman spectra of the CH stretching bands in butane. The calculations use the Vibrational Self-Consistent Field (VSCF) algorithm. The results are compared with gas-state experiments. Very good agreement between the computed and experimental results is found. Theory is successful also in computing a weak peak which is caused by combination transitions. The B3LYP potential surface is found superior to MP2, though both methods give good accord with experiment. The theoretical results provide an understanding of the role of different modes in the spectra of hydrocarbons.