Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna (Austria).
Chemphyschem. 2013 Sep 16;14(13):2920-31. doi: 10.1002/cphc.201300370. Epub 2013 Jul 25.
The photoinduced excited-state dynamics of the keto and enol forms of cytosine have been investigated by using ab initio surface-hopping to gain an understanding of the outcome of molecular beam femtosecond pump-probe photoionisation spectroscopy experiments. Both singlet and triplet states were included in the dynamics. The results show that triplet states play a significant role in the relaxation of the keto tautomer, whereas they are less important in the enol tautomer. In both forms, the T1 state minimum was found to be too low in energy to be detected in standard photoionisation spectroscopy experiments and therefore experimental decay times should arise from simultaneous relaxation to the ground state and additional intersystem crossing followed by internal conversion to the T1 state. In agreement with available experimental lifetimes, we observed three decay constants of 7, 270 and 1900 fs, the first two coming from the keto tautomer and the third from the enol form. Deactivation of the enol tautomer is due to internal conversion to the ground state through two ethylenic-type S1/S0 conical intersections.
已通过使用从头算表面跳跃来研究胞嘧啶的酮式和烯醇式两种形式的光致激发态动力学，以了解分子束飞秒泵浦探针光致电离光谱实验的结果。动力学中包括单重态和三重态。结果表明，三重态在酮式互变异构体的弛豫中起着重要作用，而在烯醇式互变异构体中则不那么重要。在这两种形式中，T1 态的最小值能量太低，无法在标准光致电离光谱实验中检测到，因此实验衰减时间应该来自同时弛豫到基态和额外的系间穿越，然后通过内部转换到 T1 态。与可用的实验寿命一致，我们观察到三个衰减常数，分别为 7、270 和 1900 fs，前两个来自酮式互变异构体，第三个来自烯醇式。烯醇式互变异构体的失活是由于通过两个乙烯型 S1/S0 锥形交叉点内部转换到基态。
