AB-INITIO STUDY OF S-TRANS-1,3-BUTADIENE USING VARIOUS LEVELS OF BASIS-SET AND ELECTRON CORRELATION - FORCE-CONSTANTS AND EXPONENTIALIY SCALED VIBRATIONAL FREQUENCIES

The structure, vibrational spectra, and force constants of s-trans-1,3-butadiene have been studied with ab initio Hartree-Fock (HF), Moller-Plesset second-order perturbation (MP2), and Moller-Plesset fourth order perturbation (MP4) calculations using various basis sets. The vibrational frequencies show a very sensitive dependence on basis sets and electron correlation. The root mean square (rms) frequency deviation between experiment and theories of MP2/6-311G(2d,p) and MP4/6-31G* is 22 cm(-1). The scaled frequencies with exponentially varying weight agrees better with experiment than those with constant weight, because higher frequencies tend to be more overestimated. The exponentially scaled rms frequency deviation by MP2/6-311G(2d,p) reduces down to 12 cm(-1), which is in surprisingly good agreement with experiment. The error due to the incompleteness of basis sets is found to be more important at the MP2 level than the error due to the electron correlation. The backbone stretching force constants are in excellent agreement with the experimental values. The C-C/C=C force constant is not so large as anticipated for the construction of reliable polyacetylene force fields. Otherwise, the force constant in polyacetylene may be different from that in butadiene.