水合钠离子在不同温度下的结构和光谱及团簇稳定性规则,Journal of Chemical Theory and Computation

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水合钠离子在不同温度下的结构和光谱及团簇稳定性规则
Journal of Chemical Theory and Computation ( IF 5.7 ) Pub Date : 2016-03-21 00:00:00 , DOI: 10.1021/acs.jctc.6b00038
Jean Jules Fifen _{1,

2} , Noam Agmon ₁

Affiliation

钠阳离子在几个生理过程中起着重要作用。理解其溶剂化可能有助于理解钠通道中对神经冲动至关重要的离子选择性。本文对气相Na ⁺（H ₂ O）_{n = 1–8}团簇的75个以上异构体进行了深入研究，其最佳结构，能量和（谐波）振动频率是在完整MP2 / 6的情况下通过量子力学计算得到的-31 ++ G（d，p）的理论水平。根据这些数据，我们计算了温度对团簇热力学函数的影响，从而计算了每个n的平衡玻尔兹曼分布。。对于选定数量的异构体，我们已经对基集叠加误差（BSSE）的计算进行了修正，以获取准确的聚类能量，与实验非常吻合。与大量液态水相反，计算得出的簇绝大多数都是4配位的，而钠离子被认为主要是5或6配位的。为了解释这一点，我们建议使用“簇稳定性规则”，这是一组可通过一次BSSE校正获得的依赖于配位数的氢键（HB）强度。假定可加性和可转移性，它们再现了我们大多数计算异构体的相对稳定性。这些规则使我们能够阐明HB强度的趋势，概述了簇稳定性的主要决定因素。对于n= 4和5，我们还进行了非谐振动计算（VPT2），以与这些气相团簇的可用光解离红外光谱进行比较。比较结果表明，实验实际上监测的是主要由3个配位的异构体组成的混合物，该混合物与计算出的玻耳兹曼分布非常不同，尤其是在低温下。出乎意料的是，对于这些实验，水蒸发途径可以使非平衡异构体分布合理化。反过来，平衡的异构体分布则通过“悬空的”水分子的内部旋转熵来合理化。

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Structure and Spectroscopy of Hydrated Sodium Ions at Different Temperatures and the Cluster Stability Rules

The sodium cation plays an important role in several physiological processes. Understanding its solvation may help understanding ion selectivity in sodium channels that are pivotal for nerve impulses. This paper presents a thorough investigation of over 75 isomers of gas-phase Na⁺(H₂O)_n=1–8 clusters, whose optimized structures, energies, and (harmonic) vibrational frequencies were computed quantum mechanically at the full MP2/6-31++G(d,p) level of theory. From these data, we have calculated the temperature effects on the cluster thermodynamic functions, and thus the equilibrium Boltzmann distribution for each n. For a selected number of isomers, we have corrected the calculations for basis set superposition error (BSSE) to obtain accurate clustering energies, in excellent agreement with experiment. The computed clusters are overwhelmingly 4-coordinated, as opposed to bulk liquid water, where sodium cations are believed to be mostly 5- or 6-coordinated. To explain this, we suggest the “cluster stability rules”, a set of coordination-number-dependent hydrogen-bond (HB) strengths that can be obtained using a single BSSE correction. Assuming additivity and transferability, these reproduce the relative stability of most of our computed isomers. These rules enable us to elucidate the trends in HB strengths, outlining the major determinants of cluster stability. For n = 4 and 5, we have also performed anharmonic vibrational calculations (VPT2) to compare with available photodissociation infrared spectra of these gas-phase clusters. The comparison suggests that the experiments actually monitor a mixture of predominantly 3-coordinated isomers, which is quite remote from the computed Boltzmann distribution, particularly at low temperatures. Surprisingly, for these experiments, water evaporation pathways can rationalize the non-equilibrium isomer distribution. The equilibrium isomer distribution is, in turn, rationalized by the entropy of internal rotations of “dangling” water molecules.

更新日期：2016-03-21

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