Structure and vibrational assignment of gouche-1,3-butadiene

Abstract The completely optimized structures and harmonic force fields of the s-trans(anti) - and gouche -isomers of 1,3-butadiene have been computed at the ab initio Hartree—Fock level using the 6·31G basis set. The gouche dihedral angle was found to be 34.8° from the planar s-cis(syn) -configuration of the 1,3-butadiene molecule. Seven scale factors for correcting the theoretical force constants of the trans -isomer were calculated from the experimental frequencies of light and heavy trans -1,3-butadienes. The correction of the gouche -butadiene force field was then carried out using these scale factors, and the vibrational problems were solved for gouche forms of C 4 H 6 , CD 2 CHCHCD 2 , C 4 D 6 , 13 CH 2 CHCH 13 CH 2 , and 12 CH 2 CHCHCH 2 . The total assignment of the experimental vibrational frequencies of these isotopomers is given. The conclusion drawn is that the quantum mechanical geometry and the scaled quantum mechanical force field correctly simulate the structure and the experimental frequencies of the butadiene gouche -conformer.