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 series of alkanes. SPASIBA is actually a hybrid of the AMBER force field and the Urey-Bradley-Shimanouchi force field. The parameters of the energy function are optimized to give a best fit to the observed vibrational 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 deuterated analog, isobutane, 2-methylbutane, neopentane, 2,2,3,3-tetramethylpentane, n-octane and n-decane. A set of 18 independent force constants is found to be sufficient to represent the vibrational frequencies accurately, and the standard deviation between the observed and calculated frequencies is 13 cm-1. Although such an accuracy was obtained using a consistent valence force field with a comparable number of adjustable 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 and highly strained energies.