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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Influence of water content on the acidities in acetonitrile. Quantifying charge delocalization in anions
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Influence of water content on the acidities in acetonitrile. Quantifying charge delocalization in anions

机译：水分含量对乙腈酸度的影响。定量阴离子中的电荷离域

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摘要

The effect of traces of water on the relative strengths of acids (ΔpK_a values) in acetonitrile was quantitatively evaluated experimentally and computationally (COSMO-RS). Water affects first of all the anions by selective solvation. Expectedly, the more localized is the charge in acid anions the higher is the effect of water. The energetic effect of increasing water content from 0 to ca. 10000 ppm on solvation enthalpies of anions ranged from 0.2-0.4 kcal mol~(-1) (anions with delocalized charges) to 15 kcal mol~(-1) in the case of the highly charge-localized acetate ion. In the case of ΔpK_a values the change ranges from 0.01 to ca. 1.7 pK_a units (acid pair involving acetic acid). The COSMO-RS method was found to satisfactorily describe the trends in ΔpK_a values. To quantify the extent of charge localization/delocalization in anions a parameter, weighted average positive σ (WAPS), was introduced, which can be conveniently computed using the COSMO approach. WAPS characterizes the distribution of charge density across the molecular surface and was found to correlate well with the extent of water influence on the dissociation of the respective acid.
机译：痕量水对乙腈中酸的相对强度（ΔpK_a值）的影响通过实验和计算方式进行了定量评估（COSMO-RS）。水首先通过选择性溶剂化作用影响阴离子。可以预期，酸性阴离子中的电荷越局部化，水的作用就越高。将水含量从0增加到大约的能量效应。阴离子的溶剂化焓为10000 ppm，范围从0.2-0.4 kcal mol〜（-1）（带离域电荷的阴离子）到15 kcal mol〜（-1）（在高度带电荷的乙酸根离子的情况下）。在ΔpK_a值的情况下，变化范围为0.01到ca。 1.7 pK_a单位（涉及乙酸的酸对）。发现COSMO-RS方法可以令人满意地描述ΔpK_a值的趋势。为了量化阴离子中电荷的局部化/离域化程度，引入了一个加权平均正σ（WAPS）参数，可以使用COSMO方法方便地计算该参数。 WAPS表征了整个分子表面上电荷密度的分布，并且发现与水对各个酸的离解的影响程度密切相关。

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