Article pubs.acs.org/JPCA
Photodecarbonylation Mechanism of Cyclopropenone in the Gas Phase: Electronic Structure Calculation and AIMS Dynamics Simulation Lihong Liu, Shuhua Xia, and Wei-Hai Fang* Key Laboratory of Theoretical and Computational Photochemistry Ministry of Education College of Chemistry, Beijing Normal University, Beijing 100875, China S Supporting Information *
ABSTRACT: In this article, structures and energies of cyclopropenone in the low-lying electronic states have been determined by the CASSCF and MS-CASPT2 calculations with different basis sets. Two minimum-energy conical intersections (CI-1 and CI-2) between S0 and S1 were obtained and their topographic characters were characterized by the SA4-CAS(10,9) calculated energy gradients and nonadiabatic coupling vectors. The AIMS method was used to carry out nonadiabatic dynamics simulation with ab initio calculation performed at the SA4-CAS(10,9) level. On the basis of time evolution of wave functions simulated here, the S1 lifetime is fitted to be 125 fs with a pure exponential decay for the S1 electronic population. The CI-1 intersection is mainly responsible for ultrafast S1→S0 nonadiabatic transition and the photoinduced decarbonylation is a sequential process, where the first CC bond is broken in the S1 state and fission of the second CC bond occurs in the S0 state as a result of the S1→S0 internal conversion via the CI-1 region. As a minor channel through the CI-2 region, the decarbonylation proceeds in an asynchronous concerted way. Effects of the S1 excess energies and the S1− S0 energy gap on the nonadiabatic dynamics were examined, which reveals that the S1→S0 nonadiabatic transition occurs within a small energy gap and high-energy conical intersection regions can play an important role. The present study provides new insights into mechanistic photochemistry of cyclopropenones and reveals that the AIMS dynamics simulation at a high-accuracy ab initio level is a powerful tool for exploring a mechanism of an ultrafast photochemical reaction.
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INTRODUCTION Cyclopropenone (CP) is the simplest organic molecule that contains the conjugated CC and CO double bonds. Cyclopropenone and its derivative are molecules of great experimental and theoretical interests.1−40 Cyclopropenone was first synthesized in 1967 and its properties were subsequently reported by Breslow and co-workers.4,6,7,11 Because of the conjugation interaction between the CC and CO bonds, cyclopropenone exhibits the aromatic character.32,41 In comparison with cyclopropanone, cyclopropenone has higher thermal stability. However, cyclopropenone and its derivatives can dissociate into acetylene and carbon monoxide under relatively high temperature, which has been a subject of numerous experimental studies.2,3,6,11,16,21,41 The key issue on the pyrolysis mechanism is whether the decarbonylation of cyclopropenones proceeds in a concerted or stepwise way. In order to solve this issue, a number of electronic structure calculations have been performed at different levels of theory.21,36,38,39 Up to now, the thermal decarbonylation was well established as a stepwise process by the DFT, MP2, CASSCF, and CCSD(T) calculations. A short-lived intermediate was involved in the sequential cleavage of the two single bonds in the three-membered ring and the intermediate was determined to have a resonance structure between a carbene and a zwitterion.21,36,38 © XXXX American Chemical Society
The photoinduced decarbonylation reaction of cyclopropenones has been investigated with laser flash photolysis, pump− probe transient absorption, and other spectroscopic detections.6,11,13,14,16,18−21,41 However, the underlying mechanism is still unclear up to now. Rapid formation (
