Cauët Emilie, Dehareng Dominique, Liévin Jacques
Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles, CP 160/09, 50 Avenue F. D. Roosevelt, B-1050 Bruxelles, Belgium.
J Phys Chem A. 2006 Jul 27;110(29):9200-11. doi: 10.1021/jp0617625.
The ionization of the four DNA bases is investigated by means of ab initio calculations. Accurate values of the gas-phase vertical and adiabatic ionization potentials (IP) are obtained at the MP2/6-31G(2d(0.8,alpha(d)),p) level of theory. The need of introducing extra polarization to the standard 6-31G(d,p) basis set is demonstrated by test calculations and an optimal value of alpha(d) = 0.1 is obtained. Ionization to electronically excited radical cations is also considered. The low-lying excited states of the cations are characterized for the first time. The topology of the corresponding potential energy surfaces is qualitatively described in terms of the stationary points (minima and saddle points) located on these surfaces. A conical intersection is characterized for the first time on the ground-state potential energy surface of all cations. It arises from the crossing of the adiabatic surfaces of the ground and first excited state at planar geometries. A nonplanar minimum is observed for the cytosine cation only. The geometry and electronic changes occurring along these surfaces are analyzed, leading to a comparison between the different nucleobase cations. The study of larger ionized systems related to DNA is rendered possible thanks to the optimized medium size basis set proposed in this work, as exemplified by the calculation of the IP of a stacked dimer of guanines.
通过从头算方法研究了四种DNA碱基的电离。在MP2/6-31G(2d(0.8,α(d)),p)理论水平上获得了气相垂直和绝热电离势(IP)的精确值。通过测试计算证明了在标准6-31G(d,p)基组中引入额外极化的必要性，并获得了α(d)=0.1的最佳值。还考虑了电离生成电子激发的自由基阳离子。首次对阳离子的低激发态进行了表征。根据位于这些表面上的驻点(极小值和鞍点)定性描述了相应势能面的拓扑结构。首次在所有阳离子的基态势能面上表征了一个锥形交叉点。它源于基态和第一激发态的绝热表面在平面几何构型处的交叉。仅在胞嘧啶阳离子中观察到一个非平面极小值。分析了沿这些表面发生的几何和电子变化，从而对不同的核碱基阳离子进行了比较。由于本工作中提出的优化的中等大小基组，使得对与DNA相关的更大电离系统的研究成为可能，如鸟嘌呤堆叠二聚体的IP计算所示。
