We performed molecular dynamics simulations of carboxylate/methylated ammonium ion pairs solvated in bulk water and of carboxylate/methylated ammonium salt solutions at ambient conditions using an ab initio-based polarizable force field whose parameters are assigned to reproduce only high end quantum computations, at the Møller-Plesset second-order perturbation theory/complete basis set limit level, regarding single ions and ion pairs as isolated and micro-hydrated in gas phase. Our results agree with the available experimental results regarding carboxylate/ammonium salt solutions. For instance, our force field approach predicts the percentage of acetate associated with ammonium ions in ${CH}_3{COO}^{-}/{CH}_3{NH}_3^{+}$ solutions at the 0.2–0.8M concentration scale to range from 14% to 35%, in line with the estimates computed from the experimental ion association constant in liquid water. Moreover our simulations predict the number of water molecules released from the ion first hydration shell to the bulk upon ion association to be about 2.0 $\pm$ 0.6 molecules for acetate/protonated amine ion pairs, 3.1 $\pm$ 1.5 molecules for the ${HCOO}^{-}/{NH}_4^{+}$ pair and 3.3 $\pm$ 1.2 molecules for the CH₃COO⁻/(CH₃)₄N⁺ pair. For protonated amine-based ion pairs, these values are in line with experiment for alkali/halide pairs solvated in bulk water. All these results demonstrate the promising feature of ab initio-based force fields, i.e., their capacity in accurately modeling chemical systems that cannot be readily investigated using available experimental techniques.

中文翻译：

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水相和从头算起的力场中的有机离子缔合：羧酸盐/铵盐的情况

我们使用基于从头算的极化力场进行了在环境条件下散装在水中的羧酸盐/甲基化铵离子对和羧酸盐/甲基化铵盐溶液的分子动力学模拟，其参数被指定为仅重现高端量子计算。 Møller-Plesset二阶微扰理论/完整基集极限水平，涉及在气相中分离并微水合的单个离子和离子对。我们的结果与关于羧酸盐/铵盐溶液的可用实验结果一致。例如，我们的力场方法可以预测醋酸铵中与铵离子相关的百分比${CH}_3{CØØ}^{-}/{CH}_3{ñH}_3^{+}$浓度范围为0.2–0.8M的溶液浓度范围为14％至35％，这与从液态水中的实验离子缔合常数计算得出的估计值一致。此外，我们的模拟预测，离子缔合后从离子优先水合壳释放到主体的水分子数量约为2.0$\pm$ 乙酸酯/质子化胺离子对为0.6分子，3.1 $\pm$ 1.5个分子 ${HCØØ}^{-}/{ñH}_4^{+}$ 对和3.3 $\pm$CH ₃ COO ^- /（CH ₃）₄ N ⁺对的分子为1.2 。对于质子化胺基离子对，这些值与在散装水中溶解的碱金属/卤化物对的实验一致。所有这些结果证明了从头算起的力场的有前途的特征，即它们在精确建模化学系统中的能力，这些能力无法使用现有的实验技术轻易地进行研究。