Comparison of the IR and Raman vibrational frequencies and intensities of alkanes using the AMBER and SPASIBA force fields. Application to ethane, and gauche- and trans-n-butane

Abstract

The SPASIBA and AMBER force fields were used to study the IR and Raman frequencies and intensities of ethane and trans- and gauche-n-butane. The intensity parameters came from ab initio molecular orbital calculations. A total of nine basis sets at the Hartree—Fock level of theory was picked out: STO-3G, 3-–21 G, 6–21 G, 6–31 G, 3–21 G *, 6–31 G *, 6–21 G **, D95 and D95 **. Some of the calculations were repeated with electron correlation, included through Møller—Plesset perturbation to second-order (MP2). A better fit of the experimental IR and Raman spectra of n-butane is obtained with the present force field than with the AMBER force field. The discrepancies come mainly from the difference in the vibrational frequencies and then in the form of the normal modes of vibration. Although ab initio methods bear high degrees of uncertainty, the calculated intensities using SPASIBA are closer to the experimental ones. The Urey—Bradley potential, which is commonly omitted from molecular mechanics force fields, seems to be required for improving the goodnes-of-fit of the Raman intensities.