Department of Chemistry, 600 South Mathews Avenue, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
J Phys Chem B. 2013 Jan 10;117(1):316-25. doi: 10.1021/jp3107292. Epub 2012 Dec 31.
Proton-coupled electron transfer can occur through concerted (electron-proton transfer, EPT) or sequential mechanisms, but this distinction becomes less well-defined for photoinduced reactions. These issues have been examined with transient absorption experiments on a hydrogen-bonded complex consisting of p-nitrophenylphenol and t-butylamine. These experiments revealed two spectroscopically distinct states: the higher-energy excited state was interpreted to be a conventional intramolecular charge transfer (ICT) state within the p-nitrophenylphenol, whereas the lower-energy state was interpreted to be an ICT-EPT state, where photoexcitation resulted in both ICT and the shifting of electronic density corresponding to effective proton transfer from the phenol to the amine. In the present work, the singlet excited states of the hydrogen-bonded p-nitrophenylphenol-methylamine complex in 1,2-dichloroethane are studied with time-dependent density functional theory and higher-level ab initio methods. The calculations suggest that the ππ* state, which is the S(1) state at the Franck-Condon geometry, corresponds to the state denoted ICT-EPT in the experimental analysis, whereas the nπ* state, which is the S(2) state at this geometry, likely corresponds to the state denoted ICT in the experimental analysis. According to the calculations, the ππ* state has charge-transfer character, as well as a change in electronic density on the amine, with the minimum-energy structure corresponding to the proton bonded to the nitrogen acceptor, consistent with proton transfer. The nπ* state has little charge-transfer character, as well as negligible change in electronic density on the amine, with the minimum-energy structure corresponding to the proton bonded to the oxygen donor. The calculations also provide evidence of an avoided crossing between these two states located energetically close to the Franck-Condon point. These calculations provide the foundation for future nonadiabatic molecular dynamics studies of the relaxation process.
质子耦合电子转移可以通过协同（电子-质子转移，EPT）或顺序机制发生，但对于光诱导反应，这种区别变得不那么明确。这些问题已经通过对由对硝基苯酚和叔丁胺组成的氢键复合物的瞬态吸收实验进行了检查。这些实验揭示了两个光谱上明显不同的状态：高能激发态被解释为对硝基苯酚内的常规分子内电荷转移（ICT）态，而低能态被解释为 ICT-EPT 态，其中光激发导致 ICT 和电子密度的转移，对应于酚到胺的有效质子转移。在目前的工作中，用时间相关的密度泛函理论和更高水平的从头算方法研究了 1,2-二氯乙烷中氢键结合的对硝基苯酚-甲胺复合物的单重激发态。计算表明，ππ态（在 Franck-Condon 几何形状下是 S(1)态）对应于实验分析中表示 ICT-EPT 的态，而 nπ态（在该几何形状下是 S(2)态）可能对应于实验分析中表示 ICT 的态。根据计算，ππ态具有电荷转移特征，以及胺上电子密度的变化，最低能量结构对应于质子与氮受体结合，与质子转移一致。nπ态几乎没有电荷转移特征，以及胺上电子密度的微小变化，最低能量结构对应于质子与氧供体结合。计算还提供了这些两个状态之间的避免交叉的证据，该交叉位于与 Franck-Condon 点能量相近的位置。这些计算为未来非绝热分子动力学研究弛豫过程提供了基础。
