OHO Hydrogen Bond Geometries and NMR Chemical Shifts: From Equilibrium Structures to Geometric H/D Isotope Effects, with Applications for Water, Protonated Water, and Compressed Ice

Hans-Heinrich Limbach; Peter M. Tolstoy; Natalia Perez-Hernandez

摘要

Hydrogen bond geometries and ~1H NMR chemical shifts of OHO hydrogen-bonded systems have been analyzed using an improved valence bond order model. This model predicts that the heavy atom hydrogen bond coordinate q2 = r, + r2 is a function of the proton coordinate q1 = 1/2(r1 - r2). where r, and r, represent the OH and the HO distances. In the first part, it is shown that this correlation reproduces published equilibrium geometries of the Zundel cation H5O_2~+ as well as those of water clusters in the gas phase and embedded in the fullerene C180. Using the example of the water hexamer, it is shown that changing the level of calculation shifts the calculated geometries along the correlation curve, but not away from the curve. In order to take quantum zero-point vibrational effccls (QZPVH) into account, an empirical correction is proposed. It is shown that this correction properly describes the calculated classical and quantum hydrogen bond geometries of compressed ice as well as calculated geometric H/D isotope effects. The improved valence bond order model is used to analyze a large number of OHO hydrogen bond geometries contained in the Cambridge Structural Database. In the second part, a relation between the geometries and the ~1H NMR chemical shieldings of OHO hydrogen bonded systems is established using the valence bond order model. GIAO calculations of the isolated symmetric Zundel cation where H is located in the hydrogen bond center show only a small dependence of the chemical shifts on the O...O distance. This result is rationalized in terms of neighbor group effects and deshielding in the naked proton. The consequence is that the 'H NMR chemical shifts are not much affected by QZPVE. Calculations on water clusters indicate that the influence of the chemical environment of the OHO hydrogen bonds on their ~1H NMR chemical shifts is smaller for the strong hydrogen bond regime but large for the weak hydrogen bond regime. A simple chemical shift vs. q1 relation is then used to calculate the average chemical shifts of water clusters in the regime of fast hydrogen bond exchange between hydrogen bonded and free OH groups. It is shown that average chemical shifts of about 6 ppm are possible as the clusters considered exhibit a broad distribution of stronger and weaker hydrogen bonds. The implications for water in organic solvents and for liquid water arc discussed, based on published data on the ~1H chemical shift distribution in the latter.

机译：使用改进的价键订单模型分析了OHO氢键系统的氢键几何形状和〜1H NMR化学移位。该模型预测，重原子氢键坐标Q2 = R， + R2是质子坐标Q1 = 1/2（R1 -R2）的函数。 r和r，代表oh和ho距离。在第一部分中，这表明这种相关性再现了Zundel阳离子H5O_2〜+的公开平衡几何形状以及气相中的水簇的平衡几何形状，并嵌入富勒烯C180中。以水六聚体的示例，表明改变计算水平会沿着相关曲线移动计算出的几何形状，但不会远离曲线。为了考虑量子零点振动EFFCCL（QZPVH），提出了经验校正。结果表明，这种校正正确描述了压缩冰的经典和量子氢键几何以及计算出的几何H/D同位素效应。改进的价值键顺序模型用于分析剑桥结构数据库中包含的大量OHO氢键几何形状。在第二部分中，使用Valence Bond Order模型建立了OHO氢键系统的几何形状与〜1H NMR化学屏蔽之间的关系。 H氢键中H中H仅显示化学移位对O ... O距离的小依赖性，其中H位于氢键中的H中的围ao计算。该结果是根据邻居群体的效应和裸质质子中的持续效果而合理化的。结果是'H NMR化学位移不受QZPVE的影响很大。水簇上的计算表明，对于强氢键状态，OHO氢键的化学环境对其〜1H NMR化学位移的影响较小，但对于弱氢键状态而言，很大。然后，使用简单的化学位移与Q1关系来计算在氢键和自由OH基团之间快速氢键交换方案中水簇的平均化学移位。结果表明，由于群集认为表现出较强和较弱的氢键的广泛分布，因此平均化学位移可能约为6 ppm。基于有关后者〜1H化学位移分布的公开数据，对有机溶剂和液态水弧的水的影响。

OHO Hydrogen Bond Geometries and NMR Chemical Shifts: From Equilibrium Structures to Geometric H/D Isotope Effects, with Applications for Water, Protonated Water, and Compressed Ice

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