Kaupp Martin, Remenyi Christian, Vaara Juha, Malkina Olga L, Malkin Vladimir G
Institut für Anorganische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.
J Am Chem Soc. 2002 Mar 20;124(11):2709-22. doi: 10.1021/ja0162764.
A recently developed density functional approach has been used to carry out a systematic computational study of electronic g-tensors for a series of 1,4-semiquinone radical anions. Good agreement with high-field EPR data in frozen 2-propanol is achieved only after taking into account the significant reduction of g-tensor anisotropy caused by hydrogen bonding to solvent molecules. The comparison of various model systems for the first solvation shell suggests two hydrogen bonds from 2-propanol molecules to each of the carbonyl groups of the radical anions, and one additional hydrogen bond to each of the methoxy groups in ubiquinone systems. 2-Propanol makes stronger hydrogen bonds than water and thus influences g-tensor anisotropy more strongly. Substituent effects at the semiquinone are reproduced quantitatively by the calculations. The g-tensor anisotropy is influenced significantly by the conformations of methyl and methoxy substituents, with opposite contributions. Analyses and interpretations of the interrelations between structure, bonding, and spectroscopic data are provided. The relevance of the computational results for the EPR spectroscopy of semiquinone radical anions in photosynthetic reaction centers is discussed.
最近开发的一种密度泛函方法已被用于对一系列1,4-半醌自由基阴离子的电子g张量进行系统的计算研究。只有在考虑到与溶剂分子形成氢键导致g张量各向异性显著降低之后，才能实现与冷冻2-丙醇中的高场电子顺磁共振（EPR）数据的良好吻合。对第一溶剂化层的各种模型系统的比较表明，2-丙醇分子与自由基阴离子的每个羰基形成两个氢键，在泛醌系统中，每个甲氧基还形成一个额外的氢键。2-丙醇形成的氢键比水更强，因此对g张量各向异性的影响更强。计算定量地再现了半醌上的取代基效应。甲基和甲氧基取代基的构象对g张量各向异性有显著影响，且贡献相反。文章提供了对结构、键合和光谱数据之间相互关系的分析和解释。讨论了计算结果对光合反应中心中半醌自由基阴离子的EPR光谱学的相关性。
