AB-INITIO DETERMINATION OF THE ROTO-TORSIONAL ENERGY-LEVELS OF TRANS-1,3-BUTADIENE

In this paper, the flexible model based on relaxed ab initio calculati ons, which has been several times employed for vibrational calculation s, is extended to the analysis of the rotational structures starting b y the roto-torsional bands of trans-1,3-butadiene. For this purpose, t he potential energy surface and the kinetic energy parameters of the n u(13) vibrational mode of butadiene are obtained with the Moller-Pless et perturbation theory up to the second order and the 6-31G(d, p), 6-3 1G(df, p), 6-311G(d, p), 6-311G(df, p), and 6-311G(df, ps) basis sets. The torsional levels of the -h(6), -d(4), and -d(6) isotopic species are calculated variationally and are compared with experimental data. It may be concluded that the one-dimensional model appears sufficientl y accurate for butadiene-h(6) and -d(4), whereas a large kinetic inter action with the lowest wagging mode is observed for butadiene-d(6). Th e rotational levels corresponding to the first vibrational states of t he -h(6) and -d(4) species are determined variationally up to J = 17 a nd J = 11 from the ab initio spectroscopic parameters which have been expanded as functions of the torsional coordinate using symmetry adapt ed series. The torsional wavefunction is contracted to reduce the size of the Hamiltonian matrix. A good agreement with the observed transit ions is obtained for the first states nu = 0 and nu = 1. As is expecte d, the K doubling obtained is relatively small. For this reason, the q uartic and sextic centrifugal distortion constants are obtained from t he least-square fit of the variational levels to the perturbation theo ry equations for the symmetric top. (C) 1998 Academic Press.