Photoisomerization mechanism of 4-methylpyridine explored by electronic structure calculations and nonadiabatic dynamics simulations

Journal Article · Fri Jan 28 00:00:00 EST 2011 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.3547207· OSTI ID:21559987

Jun, Cao; Weihai, Fang ^[1]; Qiu, Fang ^[2]

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1. College of Chemistry, Beijing Normal University, Beijing 100875 (China)
2. Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, S-10691 Stockholm (Sweden)

In the present paper, different electronic structure methods have been used to determine stationary and intersection structures on the ground (S{sub 0}) and {sup 1}{pi}{pi}* (S{sub 2}) states of 4-methylpyridine, which is followed by adiabatic and nonadiabatic dynamics simulations to explore the mechanistic photoisomerization of 4-methylpyridine. Photoisomerization starts from the S{sub 2}({sup 1}{pi}{pi}*) state and overcomes a small barrier, leading to formation of the prefulvene isomer in the S{sub 0} state via a S{sub 2}/S{sub 0} conical intersection. The ultrafast S{sub 2}{yields} S{sub 0} nonradiative decay and low quantum yield for the photoisomerization reaction were well reproduced by the combined electronic structure calculation and dynamics simulation. The prefulvene isomer was assigned as a long-lived intermediate and suggested to isomerize to 4-methylpyridine directly in the previous study, which is not supported by the present calculation. The nonadiabatic dynamics simulation and electronic structure calculation reveal that the prefulvene isomer is a short-lived intermediate and isomerizes to benzvalene form very easily. The benzvalene form was predicted as the stable isomer in the present study and is probably the long-lived intermediate observed experimentally. A consecutive light and thermal isomerization cycle via Dewar isomer was determined and this cycle mechanism is different from that reported in the previous study. It should be pointed out that formation of Dewar isomer from the S{sub 2}({sup 1}{pi}{pi}*) state is not in competition with the isomerization to the prefulvene form. The Dewar structure observed experimentally may originate from other excited states.

• OSTI ID: 21559987
• Journal Information: Journal of Chemical Physics, Vol. 134, Issue 4; Other Information: DOI: 10.1063/1.3547207; (c) 2011 American Institute of Physics; ISSN 0021-9606
• Country of Publication: United States
• Language: English

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