COMBINED DENSITY-FUNCTIONAL THEORY AND INTRINSIC REACTION COORDINATE STUDY ON THE CONROTATORY RING-OPENING OF CYCLOBUTENE

The ring-opening of cyclobutene (a) has been studied by methods based on density functional theory (DFT). Both the simple local spin density approximation (LDA) and a more extensive nonlocal (NL) approach has b een applied. A detailed analysis of the atomic movements in the ring-o pening process was further obtained by intrinsic reaction coordinate ( IRC) calculations. The IRC study indicates that the ring-opening is a concerted process in which the rotation of the CH2 groups and the brea king of the C-C sigma-bond take place simultaneously. Attention has al so been given to the number of conformers for the 1,3-butadiene ring-o pening product. The species s-trans-1,3-butadiene (e) of C-2h symmetry was calculated to be the most stable conformer. The only other confor mer calculated to be stable was a nonplanar cis-1,3-butadiene species (c) of C-2 symmetry. Planar s-cis-1,3 butadiene was found to be a tran sition state connecting the interconversion of c to its enantiomer. St ructures, relative energies and frequencies are provided for the speci es a-e as well as the ring-opening transition state b. The DFT estimat es were further compared with results from ab initio calculations on t he species a-e. The ab initio calculations were based on up to fourth- order Moller-Plesset perturbation theory (MP4) and large basis sets of 6-311G* and 6-311+G** quality. It was found that the DFT methods pro vide as accurate estimates as the high level nb initio calculations of the properties for a-e. The DFT frequencies compare better with the e xperimental values than the MP2 figures. The LDA method affords a good estimate of the barrier height for the forward reaction a --> c. Howe ver it underestimates the heat of reaction by as much as 5 kcal/mol. T he NL corrections remove this error, and for the overall relative ener gies, the NL results are comparable to the MP4 estimates.