Mak Chun C, Peslherbe Gilles H
Centre for Research in Molecular Modeling and Department of Chemistry and Biochemistry, Concordia University , Montréal, Québec H4B 1R6, Canada.
J Phys Chem A. 2014 Jun 26;118(25):4494-501. doi: 10.1021/jp503216m. Epub 2014 Jun 12.
Upon photoexcitation of iodide-methanol clusters, I(-)(CH3OH)n, to a charge-transfer-to-solvent (CTTS) excited state, extensive relaxation was found to occur, accompanied by a convoluted modulation of the stability of the excited electron, which ultimately decreases substantially. In order to develop a molecular-level understanding of the relaxation processes of CTTS excited I(-)(CH3OH)n, high-level quantum chemical calculations are first used to investigate the ground, excited, and ionized states of I(-)(CH3OH)n (n = 2). Because of the relatively small size of I(-)(CH3OH)2, it was possible to characterize the contributions of solvent-solvent interactions to the stability of the CTTS excited cluster relative to dissociation into methanol, iodine, and a free electron, which exhibits a substantial dependence on the cluster geometric configuration. Ab initio molecular dynamics simulations of CTTS excited I(-)(CH3OH)3 are then performed to shed some light onto the nature of the relaxation pathways involved in the modulation of the stability of the excited electron in larger clusters. Simulation results suggest that separation of I and (CH3OH)3(-) accompanied by solvent reorganization in the latter can initially stabilize the excited electron, while gradual cluster fragmentation to I, (CH3OH)2(-), and CH3OH ultimately destabilizes it. This work shows, for the first time, that the inability of small CTTS excited I(-)(CH3OH)n to retain a solvated electron may be attributed to the limited hydrogen-bonding capacity of CH3OH, which increases the propensity for fragmentation to smaller clusters with lower excess-electron binding energies, and highlights the critical role of intricate molecular interactions in the electron solvation process.
当碘化物 - 甲醇簇I(-)(CH3OH)n被光激发至电荷转移到溶剂（CTTS）激发态时，发现会发生广泛的弛豫，同时伴随着激发电子稳定性的复杂调制，最终激发电子稳定性大幅下降。为了从分子水平理解CTTS激发的I(-)(CH3OH)n的弛豫过程，首先使用高水平量子化学计算来研究I(-)(CH3OH)n（n = 2）的基态、激发态和离子化态。由于I(-)(CH3OH)2尺寸相对较小，有可能表征溶剂 - 溶剂相互作用对CTTS激发簇相对于解离成甲醇、碘和自由电子的稳定性的贡献，这表现出对簇几何构型的显著依赖性。然后对CTTS激发的I(-)(CH3OH)3进行从头算分子动力学模拟，以揭示较大簇中激发电子稳定性调制所涉及的弛豫途径的本质。模拟结果表明，I与(CH3OH)3(-)的分离以及后者中的溶剂重组最初可以稳定激发电子，而簇逐渐碎片化形成I、(CH3OH)2(-)和CH3OH最终会使其不稳定。这项工作首次表明，小尺寸的CTTS激发的I(-)(CH3OH)n无法保留溶剂化电子可能归因于CH3OH有限的氢键能力，这增加了碎片化形成具有较低过剩电子结合能的较小簇的倾向，并突出了复杂分子相互作用在电子溶剂化过程中的关键作用。
