A spectroscopically determined force field (SDFF) has been obtained for hyd
rocarbon chains with olefinic unsaturation. The parametrization of this pot
ential energy function was based on the SDFF transformation of ab initio st
ructures and experimentally scaled force constants, which were optimized to
assigned vibrational frequencies of ethene, propene, skew- and syn-1-buten
e, 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 erro
r for non-CH stretch frequencies of the above molecules of 9.3 cm(-1), with
substantially correct reproduction of ab initio eigenvectors. (C) 2000 Els
evier Science B.V. All rights reserved.