Bachorz Rafał A, Klopper Wim, Gutowski Maciej, Li Xiang, Bowen Kit H
Center for Functional Nanostructures (CFN) and Lehrstuhl für Theoretische Chemie, Universität Karlsruhe (TH), D-76128 Karlsruhe, Germany.
J Chem Phys. 2008 Aug 7;129(5):054309. doi: 10.1063/1.2965128.
The photoelectron spectrum (PES) of the uracil anion is reported and discussed from the perspective of quantum chemical calculations of the vertical detachment energies (VDEs) of the anions of various tautomers of uracil. The PES peak maximum is found at an electron binding energy of 2.4 eV, and the width of the main feature suggests that the parent anions are in a valence rather than a dipole-bound state. The canonical tautomer as well as four tautomers that result from proton transfer from an NH group to a C atom were investigated computationally. At the Hartree-Fock and second-order Moller-Plesset perturbation theory levels, the adiabatic electron affinity (AEA) and the VDE have been converged to the limit of a complete basis set to within +/-1 meV. Post-MP2 electron-correlation effects have been determined at the coupled-cluster level of theory including single, double, and noniterative triple excitations. The quantum chemical calculations suggest that the most stable valence anion of uracil is the anion of a tautomer that results from a proton transfer from N1H to C5. It is characterized by an AEA of 135 meV and a VDE of 1.38 eV. The peak maximum is as much as 1 eV larger, however, and the photoelectron intensity is only very weak at 1.38 eV. The PES does not lend support either to the valence anion of the canonical tautomer, which is the second most stable anion, and whose VDE is computed at about 0.60 eV. Agreement between the peak maximum and the computed VDE is only found for the third most stable tautomer, which shows an AEA of approximately -0.1 eV and a VDE of 2.58 eV. This tautomer results from a proton transfer from N3H to C5. The results illustrate that the characteristics of biomolecular anions are highly dependent on their tautomeric form. If indeed the third most stable anion is observed in the experiment, then it remains an open question why and how this species is formed under the given conditions.
本文报道并讨论了尿嘧啶阴离子的光电子能谱（PES），该讨论基于对尿嘧啶各种互变异构体阴离子垂直脱附能（VDE）的量子化学计算。PES峰的最大值出现在电子结合能为2.4 eV处，主要特征的宽度表明母体阴离子处于价态而非偶极束缚态。通过计算研究了标准互变异构体以及由NH基团向C原子质子转移产生的四种互变异构体。在哈特里-福克和二级莫勒-普莱塞特微扰理论水平下，绝热电子亲和势（AEA）和VDE已收敛到完整基组极限，误差在±1 meV以内。后MP2电子相关效应已在耦合簇理论水平下确定，包括单、双和非迭代三激发。量子化学计算表明，尿嘧啶最稳定的价态阴离子是由N1H向C5质子转移产生的互变异构体的阴离子。其特征在于AEA为135 meV，VDE为1.38 eV。然而，峰的最大值要大1 eV之多，且在1.38 eV处光电子强度非常弱。PES既不支持标准互变异构体的价态阴离子（其为第二稳定的阴离子，VDE计算值约为0.60 eV），也不支持第二稳定的阴离子。仅在第三稳定的互变异构体中发现峰的最大值与计算得到的VDE之间存在一致性，该互变异构体的AEA约为-0.1 eV，VDE为2.58 eV。这种互变异构体是由N3H向C5质子转移产生的。结果表明，生物分子阴离子的特性高度依赖于其互变异构形式。如果在实验中确实观察到了第三稳定的阴离子，那么在给定条件下该物种为何以及如何形成仍是一个悬而未决的问题。
