Lifetime of Trimethylene Calculated by Variational Unimolecular Rate Theory
journal contributionposted on 29.02.1996, 00:00 by Charles Doubleday
The calculation was undertaken to establish an RRKM prediction of the lifetime of trimethylene for comparison to the experimental result of Zewail and co-workers, 120 ± 20 fs. Nine stationary points on the C3H6 potential energy surface were located using 2,2-CASSCF or 4,4-CASSCF and a polarized valence triple-zeta basis set consisting of [4s3p2d] on C and [3s1p] on H. These included two minima and four saddle points associated with trimethylene previously reported by others. Two saddle points associated with propene formation were located, involving direct 1,2-H transfer and 1,3-H transfer to ethylcarbene, which was also optimized. Three intrinsic reaction coordinate paths were computed for cyclization to cyclopropane using 2,2-CASSCF/6-31G*, and microcanonical variational transition states were located with RRKM theory at seven values of the total energy. Under experimental conditions the lifetime is computed to be 118 fs. The match to experiment suggests that variational RRKM theory may be useful for biradicals even on this short time scale.