Fernández-Ramos Antonio, Martínez-Núñez Emilio, Vázquez Saulo A, Ríos Miguel A, Estévez Carlos M, Merchán Manuela, Serrano-Andrés Luis
Departamento de Química Física, Facultade de Química, Universidade de Santiago de Compostela, Santiago de Compostela, Spain.
J Phys Chem A. 2007 Jul 5;111(26):5907-12. doi: 10.1021/jp072575p. Epub 2007 Jun 13.
Multiconfigurational CASSCF and CASPT2 calculations were performed to investigate the enol --> keto tautomerization in the lowest singlet excited state of the 7-hydroxyquinoline.(NH3)3 cluster. Two different reaction mechanisms were explored. The first one corresponds to that proposed previously by Tanner et al. (Science 2003, 302, 1736) on the basis of experimental observations and CASSCF optimizations under Cs-symmetry constraints. This mechanism comprises four consecutive steps and involves nonadiabatic transitions between the valence 1pipi* state and a pisigma* Rydberg-type state, resulting in hydrogen-atom transfer. Single-point CASPT2 calculations corroborate that for Cs-symmetry pathways hydrogen-atom transfer is clearly preferred over proton transfer. The second mechanism, predicted by CASSCF optimizations without constraints, implies proton transfer along a pathway on the 1pipi* surface in which one or more ammonia molecules depart significantly from the molecular plane defined by the hydroxyquinoline ring. The results suggest that both mechanisms may be competitive with proton transfer being somewhat favorable over hydrogen-atom transfer.
进行了多组态CASSCF和CASPT2计算，以研究7-羟基喹啉·(NH3)3团簇最低单重激发态中的烯醇→酮互变异构。探索了两种不同的反应机制。第一种机制与Tanner等人（《科学》，2003年，302卷，1736页）先前基于实验观察和在Cs对称约束下的CASSCF优化所提出的机制相对应。该机制包括四个连续步骤，涉及价态1ππ态和πσ里德堡型态之间的非绝热跃迁，导致氢原子转移。单点CASPT2计算证实，对于Cs对称途径，氢原子转移明显优于质子转移。第二种机制是在无约束的CASSCF优化中预测的，它意味着质子沿着1ππ*表面上的一条途径转移，其中一个或多个氨分子显著偏离由羟基喹啉环定义的分子平面。结果表明，两种机制可能具有竞争性，质子转移比氢原子转移略占优势。
