Department of Chemistry and Cherry L. Emerson Center for Scientific Computation , Emory University , Atlanta , Georgia 30322 , United States.
Department of Chemistry, James Frank Institute, and Institute for Biophysical Dynamics , The University of Chicago , Chicago , Illinois 60637 , United States.
J Phys Chem B. 2019 Aug 22;123(33):7214-7224. doi: 10.1021/acs.jpcb.9b05723. Epub 2019 Aug 13.
The hydrated excess proton is a common species in aqueous chemistry, which complexes with water in a variety of structures. The infrared spectrum of the aqueous proton is particularly sensitive to this array of structures, which manifests as continuous IR absorption from 1000 to 3000 cm known as the "proton continuum". Because of the extreme breadth of the continuum and strong anharmonicity of the involved vibrational modes, this spectrum has eluded straightforward interpretation and simulation. Using protonated water hexamer clusters from reactive molecular dynamics trajectories, and focusing on their central H(HO) structures' spectral contribution, we reproduce the linear IR spectrum of the aqueous proton with a high-level local monomer quantum method and highly accurate many-body potential energy surface. The accuracy of this approach is first verified in the vibrational spectra of the two isomers of the protonated water hexamer in the gas phase. We then apply this approach to 800 H(HO) clusters, also written as H(HO), drawn from multistate empirical valence bond simulations of the bulk liquid to calculate the infrared spectrum of the aqueous proton complex. Incorporation of anharmonic effects to the vibrational potential and quantum mechanical treatment of the proton produces a better agreement to the infrared spectrum compared to that of the double-harmonic approximation. We assess the correlation of the proton stretching mode with different atomistic coordinates, finding the best correlation with ⟨⟩, the expectation value of the proton-oxygen distance . We also decompose the IR spectrum based on normal mode vibrations and ⟨⟩ to provide insight on how different frequency regions in the continuum report on different configurations, vibrational modes, and mode couplings.
水合过量质子是水溶液化学中的常见物种，它以多种结构与水络合。水合质子的红外光谱对这种结构阵列特别敏感，表现为从 1000 到 3000 cm 的连续红外吸收，称为“质子连续体”。由于连续体的极端宽度和所涉及振动模式的强烈非谐性，该光谱难以进行直接解释和模拟。本研究使用反应分子动力学轨迹中的质子化水六聚体簇，并聚焦于其中心 H(HO)结构的光谱贡献，使用高精度局部单体量子方法和高度准确的多体势能面再现水合质子的线性红外光谱。该方法的准确性首先在气相中质子化水六聚体两种异构体的振动光谱中得到验证。然后，我们将该方法应用于 800 个 H(HO)簇，也可表示为 H(HO)，这些簇是从大块液体的多态经验价键模拟中提取的，用于计算水合质子络合物的红外光谱。与双谐波近似相比，将非谐效应纳入振动势能和质子的量子力学处理可以更好地符合红外光谱。我们评估质子伸缩模式与不同原子坐标的相关性，发现与 ⟨⟩（质子-氧距离的期望值）的相关性最好。我们还根据正则模振动和 ⟨⟩对 IR 光谱进行分解，以深入了解连续体中不同频率区域如何报告不同的构型、振动模式和模式耦合。
