THE STRUCTURES AND VIBRATIONAL FREQUENCIES OF A SERIES OF ALKANES USING THE SPASIBA FORCE-FIELD

The spectroscopic potential energy function SPASIBA is applied to a se ries of alkanes. SPASIBA is actually a hybrid of the AMBER force field and the Urey-Bradley-Shimanouchi force field. The parameters of the e nergy function are optimized to give a best fit to the observed vibrat ional spectra and structures of the series of alkanes, including a few deuterated analogs. The test molecules are methane, ethane and ethane -d6, propane, trans- and gauche-n-butane, cyclohexane and its fully de uterated analog, isobutane, 2-methylbutane, neopentane, 2,2,3,3-tetram ethylpentane, n-octane and n-decane. A set of 18 independent force con stants is found to be sufficient to represent the vibrational frequenc ies accurately, and the standard deviation between the observed and ca lculated frequencies is 13 cm-1. Although such an accuracy was obtaine d using a consistent valence force field with a comparable number of a djustable parameters, 21, this potential has the merit to be expressed in a more physical way and, consequently, can be applied very easily and successfully to all the molecules without delocalized electrons an d highly strained energies.