Excited states and photochemistry of bicyclo[1.1.0]butane.

Calculations on the excited states of bicyclo[1.1.0]butane in the gas phase by different theoretical methods using several basis sets were performed. In general, the agreement between calculated and experimental excitation energies for bicyclo[1.1.0]butane in the gas phase is very good. Reviews of the solution-phase photochemistry of bicyclo[1.1.0]butane as well as previous calculations on the ground and excited states of bicyclo[1.1.0]butane are given to provide a necessary perspective of the photochemistry of bicyclo[1.1.0]butane in solution. To simulate the solution-phase photochemistry of bicyclo[1.1.0]butane, a well potential is added to the Kirkwood-Onsager model for obtaining solvation energies of molecules in solution. The addition of the well potential gives rise to a blue-shift of all gas-phase excitation energies in solution. However, there is also the very important added effect of providing an increase in Rydberg-valence mixing of solution-phase excited states. It is this mixing of antibonding valence character into the solution-phase excited states that is necessary to explain the solution-phase photochemistry of bicyclo[1.1.0]butane through bond-breaking and the formation of a conical intersection intermediate.