Spectroscopically determined force fields for macromolecules. Part 3: Alkene chains

A spectroscopically determined force field (SDFF) has been obtained for hydrocarbon chains with olefinic unsaturation. The parametrization of this potential energy function was based on the SDFF transformation of ab initio structures and experimentally scaled force constants, which were optimized to assigned vibrational frequencies of ethene, propene, skew- and syn-1-butene, trans- and cis-2-butene, and isobutene. The Bell torsion coordinate was used to describe torsion about the C=C bond. In addition to good agreement with ab initio structures and energies, this SDFF gives an average rms error for non-CH stretch frequencies of the above molecules of 9.3 cm⁻¹, with substantially correct reproduction of ab initio eigenvectors.