Ehrler Oli T, Neumark Daniel M
Department of Chemistry, University of California, Berkeley, California 94720, USA.
Acc Chem Res. 2009 Jun 16;42(6):769-77. doi: 10.1021/ar800263z.
Solvated electrons, and hydrated electrons in particular, are important species in condensed phase chemistry, physics, and biology. Many studies have examined the formation mechanism, reactivity, spectroscopy, and dynamics of electrons in aqueous solution and other solvents, leading to a fundamental understanding of the electron-solvent interaction. However, key aspects of solvated electrons remain controversial, and the interaction between hydrated electrons and water is of central interest. For example, although researchers generally accept that hydrated electrons, eaq-, occupy solvent cavities, another picture suggests that the electron resides in a diffuse orbital localized on a H3O radical. In addition, researchers have proposed two physically distinct models for the relaxation mechanism when the electron is excited. These models, formulated to interpret condensed phase experiments, have markedly different timescales for the internal conversion from the excited p state to the ground s state.Studies of negatively charged clusters, such as (H2O)n- and I-(H2O)n, offer a complementary perspective for studying aqueous electron solvation. In this Account, we use time-resolved photoelectron spectroscopy (TRPES), a femtosecond pump-probe technique in which mass-selected anions are electronically excited and then photodetached at a series of delay times, to focus on time-resolved dynamics in these and similar species. In (H2O)n-,TRPES gives evidence for ultrafast internal conversion in clusters up to n=100. Extrapolation of these results yields a p-state lifetime of 50 fs in the bulk limit. This is in good agreement with the nonadiabatic solvation model, one of the models proposed for relaxation of eaq-. Similarly, experiments on (MeOH)n- up to n=450 give an extrapolated p-state lifetime of 150fs. TRPES investigations of I-(H2O)n and I-(CH3CN)n probe a different aspect of electron solvation dynamics. In these experiments,an ultraviolet pump pulse excites the cluster analog of the charge-transfer-to-solvent (CTTS) band, ejecting an electron from the iodide into the solvent network. The probe pulse then monitors the solvent response to this excess electron,specifically its stabilization via solvent rearrangement. In I-(H2O)n, the iodide sits outside the solvent network, as does the excess electron initially formed by CTTS excitation. However, the iodide in I-(CH3CN)n is internally solvated, and both experimental and theoretical evidence indicate that electrons in (CH3CN)n- are internally solvated. Hence, these experiments reflect the complex dynamics that ensue when the electron is photo detached within a highly confined solvent cavity.
溶剂化电子,尤其是水合电子,在凝聚相化学、物理和生物学中是重要的物种。许多研究考察了水溶液和其他溶剂中电子的形成机制、反应性、光谱学和动力学,从而对电子 - 溶剂相互作用有了基本的理解。然而,溶剂化电子的关键方面仍存在争议,水合电子与水之间的相互作用是核心关注点。例如,尽管研究人员普遍认为水合电子(eaq^-)占据溶剂空穴,但另一种观点认为电子存在于局域在(H_3O)自由基上的弥散轨道中。此外,研究人员针对电子被激发时的弛豫机制提出了两种物理上不同的模型。这些为解释凝聚相实验而构建的模型,对于从激发的(p)态到基态(s)态的内转换具有明显不同的时间尺度。
对带负电的团簇,如((H_2O)_n^-)和(I^-(H_2O)_n)的研究,为研究水合电子溶剂化提供了一个互补的视角。在本综述中,我们使用时间分辨光电子能谱(TRPES),这是一种飞秒泵浦 - 探测技术,其中对质量选择的阴离子进行电子激发,然后在一系列延迟时间进行光解离,以关注这些及类似物种中的时间分辨动力学。在((H_2O)_n^-)中,TRPES为(n)高达100的团簇中的超快内转换提供了证据。这些结果的外推在体相极限下得出(p)态寿命为50飞秒。这与为(eaq^-)弛豫提出的非绝热溶剂化模型之一很好地吻合。同样,对(n)高达450的((MeOH)_n^-)的实验给出外推的(p)态寿命为150飞秒。对(I^-(H_2O)_n)和(I^-(CH_3CN)_n)的TRPES研究探测了电子溶剂化动力学的不同方面。在这些实验中,一个紫外泵浦脉冲激发电荷转移到溶剂(CTTS)带的团簇类似物,将一个电子从碘化物喷射到溶剂网络中。然后探测脉冲监测溶剂对这个多余电子的响应,特别是通过溶剂重排使其稳定。在(I^-(H_2O)_n)中,碘化物位于溶剂网络之外,CTTS激发最初形成的多余电子也是如此。然而,(I^-(CH_3CN)_n)中的碘化物是内部溶剂化的,实验和理论证据都表明((CH_3CN)_n^-)中的电子是内部溶剂化的。因此,这些实验反映了当电子在高度受限的溶剂空穴内光解离时所产生的复杂动力学。