通过激光实验和计算确定胞嘧啶锥形交叉点
Locating Cytosine Conical Intersections by Laser Experiments and Calculations.
作者信息
Trachsel Maria A, Blaser Susan, Lobsiger Simon, Siffert Luca, Frey Hans-Martin, Blancafort Lluís, Leutwyler Samuel
机构信息
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
Institut de Quı́mica Computacional i Catàlisi and Departament de Quı́mica, Universitat de Girona, C/M.A. Capmany 69, 17003 Girona, Spain.
出版信息
J Phys Chem Lett. 2020 May 7;11(9):3203-3210. doi: 10.1021/acs.jpclett.0c00779. Epub 2020 Apr 10.
The decay mechanism of S → S excited cytosine (Cyt) and the effect of substitution are studied combining jet-cooled spectroscopy (nanosecond resonant two-photon ionization (R2PI) and picosecond lifetime measurements) with CASPT2//CASSCF computations for eight derivatives. For Cyt and five derivatives substituted at N1, C5, and C6, rapid internal conversion sets in at 250-1200 cm above the 0 bands. The break-off in the spectra correlates with the calculated barriers toward the "C5-C6 twist" conical intersection, which unambiguously establishes the decay mechanism at low S state vibrational energies. The barriers increase with substituents that stabilize the charge shifts at C4, C5, and C6 following (*) excitation. The R2PI spectra of the clamped derivatives 5,6-trimethyleneCyt (TMCyt) and 1-methyl-TMCyt (1M-TMCyt), which decay along an N3 out-of-plane coordinate, extend up to +3500 and +4500 cm.
结合喷射冷却光谱技术（纳秒共振双光子电离（R2PI）和皮秒寿命测量）以及对八种衍生物的CASPT2//CASSCF计算，研究了S→S激发态胞嘧啶（Cyt）的衰变机制及其取代效应。对于胞嘧啶以及在N1、C5和C6处被取代的五种衍生物，在高于0带250 - 1200厘米处开始快速内部转换。光谱中的中断与计算得出的朝向“C5 - C6扭曲”锥形交叉点的势垒相关，这明确确定了低S态振动能量下的衰变机制。这些势垒随着取代基的增加而增大，这些取代基在（*）激发后稳定了C4、C5和C6处的电荷转移。沿N3面外坐标衰变的钳位衍生物5,6 - 三亚甲基胞嘧啶（TMCyt）和1 - 甲基 - TMCyt（1M - TMCyt）的R2PI光谱延伸至 +3500和 +4500厘米。
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