Role of hemibonding in the structure and ultraviolet spectroscopy of the aqueous hydroxyl radical.
Role of hemibonding in the structure and ultraviolet spectroscopy of the aqueous hydroxyl radical.
复制标题
半键合在水性羟基自由基的结构和紫外光谱中的作用。
DOI：
10.26434/chemrxiv.13054022
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发表时间：
2020
期刊：
影响因子：
--
通讯作者：
J. Herbert
中科院分区：
文献类型：
--
作者：
Bhaskar Rana;J. Herbert
半键的存在下，在当地的溶剂化结构的水羟基自由基一直有争议，因为它的外观在从头计算模拟的基础上密度泛函理论是敏感的自相互作用的错误（有利于一个两个中心，三电子半键），但也有限尺寸的影响。这里报道的模拟使用混合量子力学/分子力学（QM/MM）的框架中的一个非常大的周期性的模拟单元，以避免有限尺寸的文物，并方便测试各种密度泛函，以探测离域误差的影响。在使用混合泛函B3 LYP和PBE 0的模拟中，广义梯度近似预测的半键结构的优势仍然存在，但如果精确交换的比例增加到50%，则减少到较小的种群。在使用长程修正的泛函LRC-ωPBE的模拟中，半键的布居也很小。电子光谱计算使用含时密度泛函理论，并从这些计算中出现了一个共识的图片，其中半键的配置发挥了巨大的作用，吸收光谱，即使作为少数物种存在。自由基半键构型中强烈的1b 2（H2O）→ 2 p π（stecOH）电荷转移跃迁被证明是230 nm处的吸收特征相对于307 nm处的气相吸收强烈偏移的原因，但这种强烈的特征在不存在半键的水溶液几何构型中基本上被减弱。虽然还不足以定量地建立人口的半键stecOH（aq），这些模拟表明，它的存在是揭示了强烈移动的紫外吸收光谱的水自由基。
The presence of a hemibond in the local solvation structure of the aqueous hydroxyl radical has long been debated, as its appearance in ab initio simulations based on density functional theory is sensitive to self-interaction error (favoring a two-center, three-electron hemibond) but also to finite-size effects. Simulations reported here use a mixed quantum mechanics/molecular mechanics (QM/MM) framework in a very large periodic simulation cell, in order to avoid finite-size artifacts and to facilitate testing of various density functionals, in order to probe the effects of delocalization error. The preponderance of hemibonded structures predicted by generalized gradient approximations persists in simulations using the hybrid functionals B3LYP and PBE0, but is reduced to a minor population if the fraction of exact exchange is increased to 50%. The hemibonded population is also small in simulations employing the long-range corrected functional LRC-ωPBE. Electronic spectra are computed using time-dependent density functional theory, and from these calculations emerges a consensus picture in which hemibonded configurations play an outsized role in the absorption spectrum, even when present as a minority species. An intense 1b2(H2O) → 2pπ(˙OH) charge-transfer transition in hemibonded configurations of the radical proves to be responsible for an absorption feature at 230 nm that is strongly shifted with respect to the gas-phase absorption at 307 nm, but this intense feature is substantially diminished in aqueous geometries where the hemibond is absent. Although not yet sufficient to quantitatively establish the population of hemibonded ˙OH(aq), these simulations do suggest that its presence is revealed by the strongly shifted ultraviolet absorption spectrum of the aqueous radical.