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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Radiationless decay mechanism of cytosine: An ab initio study with comparisons to the fluorescent analogue 5-methyl-2-pyrimidinone
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Radiationless decay mechanism of cytosine: An ab initio study with comparisons to the fluorescent analogue 5-methyl-2-pyrimidinone

机译：胞嘧啶的无辐射衰变机理：从头算研究与荧光类似物5-甲基-2-嘧啶酮的比较

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摘要

The ultrafast radiationless decay mechanism of photoexcited cytosine has been theoretically supported by exploring the important potential energy surfaces using multireference configuration-interaction ab initio methods for the gas-phase keto-tautomer free base. At vertical excitation, the bright state is S-1 (pi pi*) at 5.14 eV, with S-2 (n(N)pi*) and S-3 (n(O)pi*) being dark states at 5.29 and 5.93 eV, respectively. Minimum energy paths connect the Franck-Condon region to a shallow minimum on the pi pi* surface at 4.31 eV. Two different energetically accessible conical intersections with the ground state surface are shown to be connected to this minimum. One pathway involves N-3 distorting out of plane in a sofa conformation, and the other pathway involves a dihedral twist about the C-5-C-6 bond. Each of these pathways from the minimum contains a low barrier of 0.14 eV, easily accessed by low vibronic levels. The path involving the N-3 sofa distortion leads to a conical intersection with the ground state at 4.27 eV. The other pathway leads to an intersection with the ground state at 3.98 eV, lower than the minimum by about 0.3 eV. Comparisons with our previously reported study of the fluorescent cytosine analogue 5-methyl-2-pyrimidinone (5M2P) reveal remarkably similar conformational distortions throughout the decay pathways of both bases. The different photophysical behavior between the two molecules is attributed to energetic differences. Vertical excitation in cytosine occurs at a much higher energy initially, creating more vibrational energy than 5M2P in the Franck-Condon region, and the minimum S-1 energy for 5M2P is too low to access an intersection with the ground state, causing population trapping and fluorescence. Calculations of vertical excitation energies of 5-amino-2-pyrimidinone and 2-pyrimidinone reveal that the higher excitation energy of cytosine is likely due to the presence of the amino group at the 4-position.
机译：通过使用多参考构型-相互作用从头算方法研究气相酮-互变异构体游离碱，探索了重要的势能表面，从理论上支持了光激发胞嘧啶的超快无辐射衰变机理。在垂直激励下，亮态为5.14 eV时的S-1（pi pi *），S-2（n（N）pi *）和S-3（n（O）pi *）为5.29和暗状态。分别为5.93 eV。最小能级路径将Franck-Condon区连接到pi pi *表面上的浅最小值（4.31 eV）。示出了与基态表面的两个不同的在能量上可接近的圆锥形相交点被连接到该最小值。一个途径涉及N-3在沙发构型中偏离平面，而另一途径涉及围绕C-5-C-6键的二面角扭曲。这些路径中的每一个从最小值开始都包含0.14 eV的低势垒，低振动水平很容易访问。涉及N-3沙发变形的路径导致在4.27 eV处与基态呈圆锥形相交。另一条路径导致在3.98 eV处与基态相交，比最小值低约0.3 eV。与我们先前报道的对荧光胞嘧啶类似物5-甲基-2-嘧啶酮（5M2P）的研究相比较，发现在两个碱基的衰变途径中，构象畸变都非常相似。两个分子之间不同的光物理行为归因于能量差异。最初，胞嘧啶中的垂直激发以更高的能量发生，比Franck-Condon地区的5M2P产生更多的振动能量，并且5M2P的最小S-1能量太低，无法进入与基态的相交点，从而导致种群被困和荧光。 5-氨基-2-嘧啶酮和2-嘧啶酮的垂直激发能的计算表明，胞嘧啶的较高激发能可能是由于4-位氨基的存在。

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