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J. Phys. Chem. A 2010, 114, 730–734
Conical Intersection Is Responsible for the Fluorescence Disappearance below 365 nm in Cyclopropanone Ganglong Cui, Yuejie Ai, and Weihai Fang* Chemistry College, Beijing Normal UniVersity, Beijing 100875, P. R. China ReceiVed: September 15, 2009; ReVised Manuscript ReceiVed: October 16, 2009
The photodissociation dynamics of cyclopropanone was explored with the complete active space self-consistent field (CASSCF) calculations and ab initio nonadiabatic molecular dynamics simulations. The related minima, transition state (TS) and minimum-energy conical intersections (MECIs) were obtained as well as energetics. In the static CASSCF calculations, one MECI was found to be responsible for the fluorescence disappearance below 365 nm because the ultrafast internal conversion (IC) via this MECI deprived the opportunity of the fluorescence emission. Further evidence of this ultrafast IC event came from the subsequent ab initio nonadiabatic molecular dynamics simulations. Introduction Tuning and controlling photophysical and photochemical processes in polyatomic molecules is one dream of chemical and physical scientists; however, we should first understand the mechanisms of photophysics and photochemistry of these polyatomic molecules, which have achieved substantial progress in small- and medium-sized molecules and is striding ahead in biology, material, and so on.1-8 To arrive at these targets, we have extensively studied the various aliphatic and aromatic carbonyl compounds in the past decade, including the conjugated, halogen-substituted, sulfur-substituted derivatives of them, and so on. Compared with aliphatic and aromatic carbonyl compounds, cyclic carbonyl compounds gain few studies in photodissociation dynamics.9-14 Cyclopropanone, the simplest cyclic carbonyl compound, has a unique characteristic, strong ring-tension, and thus the ringopening reaction can easily occur not only on the ground state but also on the excited states. This characteristic increases the difficulty of understanding its photodissociation dynamics because this strong ring tension might significantly change the photodissociation mechanism, making it different from those of aliphatic and aromatic carbonyl compounds. For this reason, it is significant for us to understand this ring-tension effect on the photodissociation dynamics of cyclopropanone. First, Bedoer et al. synthesized cyclopropanone in the 1960s;15 then, Pochan et al. studied its ground-state structure using microwave spectroscopy techniques and found that it has a C2V symmetry in the ground state.16,17 Later, Thomas et al. explored cyclopropanone photodissocation dynamics in the gas phase in the wavelength range of 292-365 nm.18-20 They observed that carbon monoxide (CO) and ethylene (C2H4) were the exclusive photoproducts, and their quantum yield increased when the irradiation wavelength decreased; the electronically first singlet state (S1) was assigned to an n f π* transition whose band origin was estimated near 395 nm. In addition, because of the appearance of diffuse vibrational spectroscopy in cyclopropanone irradiated at 292 nm, they predicted that the photochemical reactions might have proceeded at this wavelength. More interestingly, the fluorescence disappears when the wavelength is
