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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Reaction mechanism of monoethanolamine with CO_2 in aqueous solution from molecular modeling
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Reaction mechanism of monoethanolamine with CO_2 in aqueous solution from molecular modeling

机译：分子模拟法研究单乙醇胺与水溶液中CO_2的反应机理

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We present a theoretical study of the reaction mechanism of monoethanolamine (MEA) with CO_2 in an aqueous solution. We have used molecular orbital reaction pathway calculations to compute reaction free energy landscapes for the reaction steps involved in the formation of carbamic acids and carbamates. We have used the conductor-like polarizable continuum model to calculate reactant, product, and transition state geometries and vibrational frequencies within density functional theory (DFT). We have also computed single point energies for all stationary structures using a coupled cluster approach with singles, doubles, and perturbational triple excitations using the DFT geometries. Our calculations indicate that a two-step reaction mechanism that proceeds via a zwitterion intermediate to form carbamate is the most favorable reaction channel. The first step, leading to formation of the zwitterion, is found to be rate-determining, and the activation free energies are 12.0 (10.2) and 11.3 (9.6) kcal/mol using Pauling (Bondi) radii within the CPCM model at the CCSD(T)/6-311++G(d,p) and CCSD(T)/6-311++G(2df,2p) levels of theory, respectively, using geometries and vibrational frequencies obtained at the B3LYP/6-311++G(d,p) level of theory. These results are in reasonable agreement with the experimental value of about 12 kcal/mol. The second step is an acid-base reaction between a zwitterion and MEA. We have developed a microkinetic model to estimate the effective reaction order at intermediate concentrations. Our model predicts an equilibrium concentration for the zwitterion on the order of 10~(-11) mol/L, which explains why the existence of the zwitterion intermediate has never been detected experimentally. The effective reaction order from our model is close to unity, also in agreement with experiments. Complementary ab initio QM/MM molecular dynamics simulations with umbrella sampling have been carried out to determine the free energy profiles of zwitterion formation and proton transfer in solution; the results confirm that the formation of the zwitterion is rate-determining.
机译：我们提出了在水溶液中单乙醇胺（MEA）与CO_2反应机理的理论研究。我们已经使用分子轨道反应途径计算来计算涉及氨基甲酸和氨基甲酸酯形成的反应步骤的反应自由能态。我们已经使用了类似导体的可极化连续体模型来计算反应物，产物和过渡态的几何形状以及在密度泛函理论（DFT）中的振动频率。我们还使用DFT几何使用单，双和微扰三重激发的耦合簇方法，为所有固定结构计算了单点能量。我们的计算表明，通过两性离子中间体进行形成氨基甲酸酯的两步反应机理是最有利的反应通道。第一步是导致两性离子的形成，这是决定速率的，使用CCSD的CPCM模型中的Pauling（Bondi）半径，活化自由能为12.0（10.2）和11.3（9.6）kcal / mol。 （T）/ 6-311 ++ G（d，p）和CCSD（T）/ 6-311 ++ G（2df，2p）的理论水平，分别使用在B3LYP / 6-处获得的几何形状和振动频率311 ++ G（d，p）的理论水平。这些结果与约12kcal / mol的实验值合理地吻合。第二步是两性离子与MEA之间的酸碱反应。我们已经开发了一种微动力学模型来估计中间浓度下的有效反应顺序。我们的模型预测两性离子的平衡浓度约为10〜（-11）mol / L，这解释了为什么从未通过实验检测到两性离子中间体的存在的原因。我们模型中的有效反应阶数接近统一，也与实验一致。已经进行了从头开始的互补的从头开始的QM / MM分子动力学模拟，并确定了两性离子形成和溶液中质子转移的自由能曲线;结果证实两性离子的形成是速率决定的。

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