Ab initio calcuations have been carried out on the singlet and triplet states of the lowest energy conformation of the homotrimethylenemethane diradical 28 (HTMM) at the CASSCF, 4-31G level with full geometry optimization. At this level the lowest energy triplet state is only 0.77 kcal/mol-1 lower in energy than the singlet state, and the geometries are essentially identical, indicating that there is no interaction between the two radical centers in the intermediate. Geometry optimization calculations at the UHF 6-31G* level on the HTMM energy surface have located two other minimum-energy conformations 31 and 29 lying 0.56 and 1.61 kcal mol-1 higher in energy. Optimization calculations at the 6-31G* level indicate that the energy barriers for rotation about the C1-C2 and C1-C9 bonds in 28 are approximately 1.68 and 1.62 kcal mol-1, respectively. The effect of the values of these energy barriers on the stereochemistry of the (2 + 2) cycloaddition reactions of optically active 1,3-dimethylallene are discussed. Calculations have been also carried out on the methyl-substituted HTMM's 24 and 25 at the 4-31G level as models for the substituted diradical intermediates formed in the (2 + 2) cycloaddition reactions of substituted allenes with variously substituted radicophiles, and rotational energy barriers for racemization processes in the diradical intermediates have been estimated. The results of these calculations are compared with the proposed structures of substituted HTMM's formed in the (2 + 2) cycloaddition reactions of substituted allenes and in the methylenecyclobutane rearrangement.
