Abstract: The photochemical reaction of bicyclo[4.1.0]heptane was studied at the complete active space SCF(CASSCF) level with a 6-31G* basis set. A multireference MP2 algorithm that has been implemented in the Gaussian program was used to correct the energetics for the dynamic correlation. Starting from the Franck-Condon excitation of bicyclo [4.1.0] heptane, the reaction is via two bonds' breakage to give rise to 1,6-heptdiene. One internal conversion(IC) and two intersystem crossing points(ISC) were located and are discussed separately. The reaction proceeds to its own characteristic product on the ground state.

Key words: CASSCF, Intersystem crossing, Photochemical reaction mechanism, Internal conversion, Bicyclo[4.1.0] heptane, 1-6 Heptadiene