NORMAL COORDINATE TREATMENT FOR PROPANE - FORCE-CONSTANT SELECTION THROUGH A BACKWARD ELIMINATION REGRESSION-REFINEMENT PROCEDURE

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. copy; 1979 American Institute of Physics.