Abstract The formation of H2CO3 and HCO3− from CO2 is common in aqueous environmental systems and occurs via the CO2 hydration and hydroxylation reactions. The reactions are especially important in the context of CaCO3 mineral precipitation. The reactions carry with them equilibrium and kinetic isotope fractionations that impact paleoclimate proxy records. Herein, quantum mechanical calculations of dissolved inorganic carbon species embedded in H2O clusters are analyzed under a transition state theory framework to predict kinetic isotope fractionations associated with the reactions. Experimental measurements of reaction energetics and equilibrium isotope fractionations are well-reproduced by the models which suggests the computational methods are justified. The CO2 hydration reaction discriminates against 13C by 19–23‰ and against 18O by 14–15‰ at 25 °C. The CO2 hydroxylation reaction discriminates against 13C by 26–31‰ and against 18O by 27–30‰ at 25 °C. Our analyses of the model isotopic fractionations lead to the conclusion that hydrogen-bonds and C(CO2)-O(H2O or OH−) distances influence the calculated values. We infer that when the model H-bond patterns more realistically represent the aqueous species, the accuracy and precision of equilibrium fractionation factor prediction is improved. Implications for coral carbonate paleothermometry are discussed. Results indicate that hydration and hydroxylation δ13C-δ18O slopes bound the values of slopes form corals and abiotic precipitation experiments, albeit with key uncertainties related to precipitation fractionations. The process of regressing molecular model fractionation results against H-bond patterns as presented here is a promising way to improve isotope fractionation calculations in aqueous systems.

中文翻译：

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CO2 水合和羟基化反应中的平衡和动力学同位素分馏：通过量子力学计算分析氢键的作用

摘要 CO2 生成 H2CO3 和 HCO3− 在水环境系统中很常见，通过 CO2 水合和羟基化反应发生。在 CaCO3 矿物沉淀的背景下，这些反应尤其重要。这些反应伴随着影响古气候代理记录的平衡和动力学同位素分馏。在此，在过渡态理论框架下分析了嵌入 H2O 簇中的溶解无机碳物种的量子力学计算，以预测与反应相关的动力学同位素分馏。这些模型很好地再现了反应能量学和平衡同位素分馏的实验测量结果，这表明计算方法是合理的。在 25 °C 下，CO2 水合反应对 13C 的区分为 19-23‰，对 18O 的区分为 14-15‰。在 25 °C 下，CO2 羟基化反应对 13C 的区分率为 26-31‰，对 18O 的区分率为 27-30‰。我们对模型同位素分馏的分析得出的结论是，氢键和 C(CO2)-O(H2O 或 OH-) 距离会影响计算值。我们推断，当模型 H 键模式更真实地代表水性物质时，平衡分馏因子预测的准确性和精度会得到提高。讨论了珊瑚碳酸盐古温度测量法的意义。结果表明，水化和羟基化 δ13C-δ18O 斜率限制了珊瑚和非生物降水实验的斜率值，尽管与降水分馏相关的关键不确定性。