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
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DOI：
10.1039/d0cp05216g
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发表时间：
2020-12-21
影响因子：
3.3
通讯作者：
Herbert, John M.
中科院分区：
文献类型：
--
作者：
Rana, Bhaskar;Herbert, John M.
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。广义梯度近似预测的半键结构的优势仍然存在于模拟中使用的混合泛函B3 LYP和PBE 0，但减少到一个较小的人口，如果确切的交换分数增加到50%。半键人口也是小的模拟采用远程校正功能LRC-ω PBE。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。事实证明，该自由基的半键构型中强烈的1b（2）（H2O）-> 2 p pi（OH）电荷转移跃迁导致了230 nm处的吸收特征，该吸收特征相对于307 nm处的气相吸收发生了强烈的位移，但这种强烈的特征在不存在半键的水性几何结构中基本上消失了。虽然还不足以定量地建立人口的半键OH（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-omega PBE. 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 1b(2)(H2O) -> 2p pi(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.