Solvation structures of sodium halides in dimethyl sulfoxide (DMSO)–methanol (MeOH) mixtures

A constrained molecular dynamics technique has been used to study the structures and dynamics of the solvation shells of three sodium halides, namely sodium chloride (Na⁺–Cl⁻), sodium bromide (Na⁺–Br⁻) and sodium iodide (Na⁺–I⁻) in DMSO–MeOH mixtures. In the case of Na⁺–Cl⁻ and Na⁺–Br⁻, Na⁺ is preferentially solvated by DMSO and Cl⁻ and Br⁻ are preferentially solvated by methanol in the contact ion pair (CIP) state. In the solvent-assisted ion pair (SAIP) configuration, Na⁺ ions of Na⁺–Cl⁻ and Na⁺–Br⁻ are preferentially solvated by methanol and Cl⁻ and Br⁻ also show preferential solvation by methanol over DMSO. In the case of Na⁺–I⁻, the only preferential solvation is in the SAIP state for I⁻ ion by methanol. These observations are supported by the calculated excess coordination numbers and spatial density maps. The heights of the transition states barriers for CIPs and SAIPs/solvent-shared ion pairs (SSHIPs) are significantly affected when the mole fraction of methanol (x_MeOH) changes from 0.0 to 0.25 because of a significant increase in the methanol density around halides. From the analysis of angular distribution functions of DMSO and methanol around the cations and anions, it is seen that DMSO and methanol molecules are present in parallel dipolar orientations (with respect to cation–solvent vector) in the first coordination shell of these three ion pairs at the CIP and SAIP states. Methanol molecules are nearly in an antiparallel (with respect to ion–solvent vector) orientation around the three halide ions.

本研究采用约束分子动力学（constrained molecular dynamics）技术，系统探究了氯化钠（Na⁺–Cl⁻）、溴化钠（Na⁺–Br⁻）与碘化钠（Na⁺–I⁻）三种卤化钠在二甲基亚砜（DMSO）-甲醇（MeOH）混合溶剂中的溶剂化层结构与动力学特性。针对Na⁺–Cl⁻与Na⁺–Br⁻体系，在接触离子对（contact ion pair，CIP）构型下，Na⁺优先被DMSO溶剂化，而Cl⁻与Br⁻则优先与甲醇结合实现溶剂化。在溶剂辅助离子对（solvent-assisted ion pair，SAIP）构型中，Na⁺–Cl⁻与Na⁺–Br⁻体系的Na⁺优先被甲醇溶剂化，且Cl⁻与Br⁻同样表现出相较于DMSO更偏好与甲醇结合的溶剂化倾向。对于Na⁺–I⁻体系，仅在SAIP构型下，I⁻离子存在被甲醇优先溶剂化的现象。上述观测结果均通过计算得到的超额配位数与空间密度分布图得到了验证。当甲醇摩尔分数（x_MeOH）从0.0变化至0.25时，由于卤离子周围甲醇的局域密度显著提升，接触离子对与溶剂辅助离子对/溶剂共享离子对（solvent-shared ion pair，SSHIPs）的过渡态势垒高度会受到显著影响。通过分析阴阳离子周围DMSO与甲醇的角分布函数可知，在CIP与SAIP状态下，三种离子对的第一配位层中，DMSO与甲醇分子均呈现出相对于阳离子-溶剂矢量的平行偶极取向；而甲醇分子在三种卤离子周围则几乎呈现出相对于离子-溶剂矢量的反平行取向。