Knowledge Bank

In most normal coordinate investigations of hydrocarbons and related molecules, the valence force field (VFF) has been shown to yield better agreement between calculated and observed frequencies than the Urey-Bradley force field (UBFF). Often the UBFF has been modified by the addition of several VFF terms, a procedure which has yielded better frequency fits, but which has resulted in a less physically meaningful Urey-Bradley approximation. By increasing the scope of the UBFF to include non-bonded interactions between four-atom (cis) and five-atom (supercis) configurations, the need for introduction of VFF parameters into the UBFF is diminished. Normal coordinate calculations have been made for acetone and propane and their deuterated analogs using the full UBFF, and the role of these configurations has been investigated. Correlations have been found between the supercis non-bonded force constant and the splitting in symmetrical methyl deformation vibrations in molecules containing the gem-dimethyl grouping. This study constitutes the first attempt to make use of the five-atom configuration in a normal coordinate calculation.