The density functional theory(DFT) calculations were performed to investigate a typical Norrish/Yang type II photoreaction of 5-methylbicyclo[1.1.1]-pentanyl ketone.The results reveal the essential correlation between structures on the one hand and energies,on the other hand,of the reactants,transition states and products based on both singlet ground(S0) and triplet excited(T1) potential energy surfaces.The feasible mechanism indicates that an intramolecular Norrish/Yang cyclization reaction takes place via H-abstraction to obtain the sole chiral cyclobutanol photoproduct.The located crossing point plays an important role in the cyclization process,which permits intersystem crossing(ISC) from T1 to S0 state.The rate-determining step may be to experience ISC between two different potential energy surfaces,requiring sufficient time for electron spin reversion,i.e.,spin multiplicity alteration.These conclusions are further confirmed by the second-order M ller-Plesset perturbation theory(MP2) calculations. The density functional theory (DFT) calculations were performed to investigate a typical Norrish / Yang type II photoreaction of 5-methylbicyclo [1.1.1] -pentanyl ketone. The results reveal the essential correlation between structures on the one hand and energies, on the other hand, of the reactants, transition states and products based on both singlet ground (S0) and triplet excited (T1) potential energy surfaces. The feasible mechanism that that an intramolecular Norrish / Yang cyclization reaction takes place via H-abstraction to obtain the sole chiral cyclobutanol photoproduct. The located crossing point plays an important role in the cyclization process, which permits intersystem crossing (ISC) from T1 to S0 state. The rate-determining step may be to experience ISC between two different potential energy surfaces, requiring sufficient time for electron spin reversion, ie, spin multiplicity alteration.These conclusions are further confirmed by the second-order M ller-Plesset perturbation theory (M P2) calculations.