Decay dynamics of 6‐azauracil from the light absorbing S 2 (ππ*) state
Decay dynamics of 6‐azauracil from the light absorbing S 2 (ππ*) state
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DOI：
10.1002/jrs.5518
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发表时间：
2019-03
影响因子：
2.5
通讯作者：
Pengli Li;J. Xue;Xuming Zheng
中科院分区：
文献类型：
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作者：
Pengli Li;J. Xue;Xuming Zheng
利用共振拉曼光谱、布朗振子模型中的时变波包理论和完全主动空间自洽场（CASSCF）协议研究了6AU在s2态的初始衰变动力学。基于傅里叶变换(FT) -拉曼、FT -红外测量、密度泛函理论计算和正态模态分析，分配了振动光谱和紫外吸收带。利用与时间相关的波包理论，在布朗振子模型中同时模拟了吸收截面和绝对共振拉曼截面。然后将得到的Franck-Condon主动模态的法向模态位移转换为易于可视化的内部坐标中的短时间结构动力学。通过比较短期结构动力学和CASSCF/CASTP2计算的FC与s2s1之间或FC与S2T3之间的结构变化，评估了S2→s1内部转换和S2→t3系统间交叉这两条通路的作用。结果表明，S2态的Franck-Condon区结构动力学主要向S2/ s1锥形相交方向发展，这支持了宽带fs - TA光谱显示的S2→s1内部转换主导6AU初始衰变路径的结论，而S2→T3系统间无障碍穿越过程由于自旋轨道耦合较弱和后续的T3→T2→t1衰变不利，可以忽略不计。分别提出了S2(ππ*)/S1（nπ*）平面和S2(ππ*)/ so0严重畸变圆锥交点的反应坐标，并阐明了溶剂对6AU、尿嘧啶和胸腺嘧啶初始衰变机制的影响。
The initial decay dynamics of 6AU in the S2state was investigated by using resonance Raman spectroscopy, the time‐dependent wave‐packet theory in a Brownian oscillator model, and complete‐active space self‐consistent field (CASSCF) protocol. The vibrational spectra and the ultraviolet absorption bands were assigned on the basis of the Fourier transform (FT)‐Raman, FT‐infrared measurements, the density‐functional theory computations, and the normal mode analysis. The absorption cross section and the absolute resonance Raman cross sections were simulated simultaneously by using the time‐dependent wave‐packet theory in a Brownian oscillator model. The obtained normal mode displacements of the Franck–Condon active modes were then converted to the short‐time structural dynamics in easy‐to‐visualize internal coordinates. The roles of two pathways via the S2→ S1internal conversion and the S2→ T3intersystem crossing were evaluated through the comparison between the short‐time structural dynamics and the CASSCF/CASTP2 calculated structural changes between FC and S2S1or between FC and S2T3. The results indicate that the structural dynamics in the Franck–Condon region of the S2state is mostly toward the S2/S1conical intersection, which supports the conclusion that the S2→ S1internal conversion dominants the initial decay pathway of 6AU as revealed by the broadband fs TA spectroscopy, whereas the barrierless S2→ T3intersystem‐crossing process is negligible owing to weak spin‐orbital coupling and unfavorable subsequent T3→ T2→ T1decay. The reaction coordinates towards the planar S2(ππ*)/S1(nπ*) and severely distorted S2(ππ*)/S0conical intersection points are proposed respectively, and the solvent effect on the initial decay mechanisms of 6AU, uracil, and thymine is clarified.