Torsional potential for the "weak" C(sp²)-C(sp²) bond in butadiene, biphenyl and stryrene

The transferability of the torsional potential of the C(sp²)-C(sp²) weak double bond in the molecular mechanics method was investigated using a set of simple model molecules: biphenyl, styrene, α-methylstyrene, cis-β-methylstyrene, 1,3-butadiene, isoprene, and 2,3-dimethylbutadiene. Individual torsional potentials for these molecules as well as different general potentials were derived. In the optimization of the torsional parameters, conformational energies from ab initio calculations at the Hartree-Fock level, and well-assigned torsional vibrational frequencies, were used.