PhotodecarbonylationMechanism of Cyclopropenone inthe Gas Phase: Electronic Structure Calculation and AIMS DynamicsSimulation.

In this article, structures and energiesof cyclopropenone in thelow-lying electronic states have been determined by the CASSCF andMS-CASPT2 calculations with different basis sets. Two minimum-energyconical intersections (CI-1 and CI-2) between S0and S1were obtained and their topographic characters were characterizedby the SA4-CAS(10,9) calculated energy gradients and nonadiabaticcoupling vectors. The AIMS method was used to carry out nonadiabaticdynamics simulation with ab initio calculation performed at the SA4-CAS(10,9)level. On the basis of time evolution of wave functions simulatedhere, the S1lifetime is fitted to be 125 fs with a pureexponential decay for the S1electronic population. TheCI-1 intersection is mainly responsible for ultrafast S1→S0nonadiabatic transition and the photoinduceddecarbonylation is a sequential process, where the first CCbond is broken in the S1state and fission of the secondCC bond occurs in the S0state as a result of theS1→S0internal conversion via the CI-1region. As a minor channel through the CI-2 region, the decarbonylationproceeds in an asynchronous concerted way. Effects of the S1excess energies and the S1–S0energygap on the nonadiabatic dynamics were examined, which reveals thatthe S1→S0nonadiabatic transition occurswithin a small energy gap and high-energy conical intersection regionscan play an important role. The present study provides new insightsinto mechanistic photochemistry of cyclopropenones and reveals thatthe AIMS dynamics simulation at a high-accuracy ab initio level isa powerful tool for exploring a mechanism of an ultrafast photochemicalreaction. [ABSTRACT FROM AUTHOR]