ABINITIO STUDIES OF POLYENES .1. 1,3-BUTADIENE

The potential energy function about the C-C single bond for the ground state 1,3-butadiene has been derived from ab initio calculations at both the Hartree-Fock (HF) level with 6-31G, 6-31G*, and 6-311G** basis sets and the second-order Moller-Plesset perturbation (MP2) level with 6-31G* basis set with the complete geometry optimizations at each of 15 fixed CCCC dihedral angles; the total energies and optimized geometries for the s-trans, gauche, and s-cis conformers were also determined at MP2 level with 6-311G* basis set and the third-order Moller-Plesset perturbation (MP3) level with 6-31G* basis set. The second stable conformer of the butadiene is predicted to be a gauche structure from all the calculations with a CCCC dihedral angle between 35-degrees and 40-degrees and a barrier of 0.5-1.0 kcal/mol to the s-cis transition state, and the theoretical torsional potentials are in good agreement with the experimental potential function of trans-gauche-gauche case derived by During et al.; by contrast, the theoretical torsional components differ significantly from the experimental results obtained from a trans-cis model. Vibrational frequencies and force field for s-trans and gauche conformers of 1,3-butadiene are determined at the Hartree-Fock and MP2 levels with 6-31G, 6-31G*, 6-311G, and 6-311G* basis sets. The mean absolute percentage deviations of the calculated frequencies from the experimental values (not corrected for anharmonicity) are approximately 10%-13% and 3%-6% for the Hartree-Fock and MP2 methods, respectively. The effects of polarization functions and electron correlation on the force fields are studied, and the additivity of correlation and d function effects are discussed. Comparisons are made with other force fields, including experimental and previous ab initio results.