Singlet-triplet energy separation and barrier for ring closure for trimethylenemethane and its complexes

Abstract

The energy separations between the 3A′2 ground state and the excited electronic 1B1 and 1A1 states of trimethylenemethane (TMM) have been studied using quantum mechanical methods. The 1B1 minimum TMM is predicted to lie 15.7 kcal/mol higher in energy than the 3A′2 ground state at the TZ2P + f CISD + Q level. This is by far the highest level of theory employed to date to the vexing TMM singlet-triplet separation. The 1A1 minimum of TMM is 1.5 kcal/mol higher in energy than the 1B1 minimum at the DZP CISD + Q level. The transition state for ring closure of TMM to methylenecyclopropane has been located, with a barrier of 1.4 kcal/mol on the 1A1 surface at the DZP CISD + Q level. The above energies relative to the 3A′2 ground state for isolated TMM are much higher than Dowd's experimental maximum (7.8 kcal/mol) for the singlet-triplet separation. The discrepancy is discussed in the light of possible solvation effects. Direct evidence for the magnitude of the solvation effect has been obtained from a study of the TMM · methylenecyclopropane (TMM · MCP), TMM · H2CO, TMM · CO, and TMM · Ne complexes. There is essentially no change of the triplet-singlet energetics upon forming the TMM · Ne complex. However, the 3A′2-1A1 energy difference is 0.24 kcal/mol lower in the TMM · H2CO, 0.20 kcal/mol lower for the TMM · MCP complex, and 0.10 kcal/mol lower in the TMM · CO complex than for isolated TMM, respectively, at the DZP CISD + Q level. There is no barrier for ring closure for the TMM · H2CO complex on the 1A1 surface at the DZP TC-CISD level, even though the barrier is 0.2 kcal/mol at the DZP TCSCF level. It has been found that the electronic states of TMM interact with surrounding molecules in the order: 1B1 < 3A′2 < 1A1 state.