SPECTROSCOPICALLY DETERMINED FORCE-FIELDS FOR MACROMOLECULES - 2 - SATURATED-HYDROCARBON CHAINS

Our methodology for producing a spectroscopically determined force fie ld (SDFF) (i.e., a molecular mechanics energy function) that, in addit ion to structures and relative energies, reproduces vibrational freque ncies to spectroscopic standards, has been extended from its previous implementation on the linear to include branched saturated hydrocarbon chains. To the ab initio force fields of the 14 stable conformers of n-pentane and n-hexane, we have now added those of the 7 stable confor mers of isopentane, 3-methylpentane, and neopentane, plus specific for ce constants from a secondary set of branched molecules, to optimize t he parameters of the SDFF. This SDFF reproduces 791 ab initio non-CH s tretch frequencies with a root-mean-square deviation of 6.2 cm(-1). Wh en applied to other molecules not in the optimization set, viz., cyclo butane, cyclohexane, isobutane, tri-tert-butylmethane, and tetra-tert- butylmethane, not only are ab initio as well as experimental geometrie s and frequencies well reproduced, but the correct (reassigned) tertia ry CII stretching frequency in tri-tert-butylmethane is satisfactorily predicted. The larger frequency deviations for other modes of this mo lecule provide an unusual insight into sensitive features of the nonbo nded interaction terms in the potential function.