Normal coordinate treatment for propane: Force constant selection through a backward elimination regression–refinement procedure

Abstract

A 26 parameter internal valence force field for propane and four deuterium isotopic species was determined by applying a backward elimination regression–refinement technique for selecting and adjusting statistically significant force constants. In comparison to computations based upon fits to harmonic frequencies, this regression–adjustment procedure generated a more satisfactory set of force constants in a fit to observed vibrational frequencies. The regression analysis for the final minimum parameter force field retains a long range methyl-methyl interaction force constant involving H–C–C in-plane bending displacement coordinates for each methyl rotor. The general valence force field for propane, whose methyl groups are tilted such that the out-of-plane CH3 hydrogen atoms approach one another, is compared to force fields for other two rotor systems in which the methyl groups are tilted in an oppostie manner.