Ab Initio CI Calculations of the Energy Difference between Trimethylenemethane and Butadiene

Abstract

AbstractAb initio configuration interaction (CI) calculations have been carried out in order to determine the energy difference between 3A î2 trimethylenemethane (TMM) and 1Ag butadiene. The pi CI value of 38.3 kcal/mol is very similar to the CI values obtained when excitations of the sigma electrons are permitted. This finding suggests that correlation effects involving sigma orbitals make at most a small contribution to the energy differences between non‐Kekulé hydrocarbon diradicals and their closed‐shell isomers. Possible complications in the experimental measurement of this energy difference are discussed. It is pointed out that methylenecyclopropane might be formed in attempts to deprotonate the 2‐methallyl cation, without the involvement of TMM. From the experimental heats of formation of methylenecyclopropane and butadiene, an energy difference of 14–16 kcal/mol between 3A î2 TMM and methylenecyclopropane is estimated. This is in excellent agreement with a previous energy difference obtained by direct calculation.