AB-INITIO STUDIES OF THE CONFORMATION DEPENDENCE OF THE SPECTRA OF STABLE CONFORMERS OF N-PENTANE AND N-HEXANE

Scaled ab initio force fields were used to calculate normal-mode frequencies of the 14 stable conformers of n-pentane and n-hexane. Force constants were obtained from the 6-31G basis set because it did not exhibit the predictive problems of several others that were examined. Twelve local symmetry coordinate scale factors were optimized by fitting calculated modes to the well-assigned bands of trans-n-pentane. The rms deviation for 61 observed bands below 1500 cm(-1) of all trans-n-pentane and n-hexane is 4.8 cm(-1). Assignments of some of these bands differ from those derived from empirical force fields. These 14 force fields provide the basis for developing a molecular mechanics potential that embodies a conformation-dependent force field for vibrational analysis as well as a reliable energy function for molecular dynamics simulations.