Potential function of internal rotation for isoprene from ab initio calculations and experimental data

Ab initio computations of the potential energy curve of internal rotation around the central single C---C bond of isoprene have been performed at the Hartree—Fock level with a 3·21G basis set. The similarity of the slope of the curve obtained and the potential energy curve calculated for a more complete basis set (7s3p/4s2p) [Kavana-S2ebø, J. Mol. Struct. 106 (1984) 259] is discussed. The values of the Pitzer function F(φ), its Fourier expansion coefficients, and coefficients of the potential energy expansion were calculated from data given in the above reference. The correction of the potential energy expansion coefficients was carried out from frequencies of torsional “hot” bands of isoprene and torsional overtone of its second rotational isometric form. It was shown that the isoprene second isomer is realized as a gauche-form. The potential energy expansion coefficients were obtained as follows: V₁ = 399.9, V₂ = 1330.22, V₃ = 781.8 and V₄ = −175.8 cm⁻¹.