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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Reaction mechanism and tautomeric equilibrium of 2-mercaptopyrimidine in the gas phase and in aqueous solution: A combined Monte Carlo and quantum mechanics study
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Reaction mechanism and tautomeric equilibrium of 2-mercaptopyrimidine in the gas phase and in aqueous solution: A combined Monte Carlo and quantum mechanics study

机译：2-巯基嘧啶在气相和水溶液中的反应机理和互变异构平衡：蒙特卡罗和量子力学的结合

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摘要

A combined Monte Carlo and quantum mechanical study was carried out to analyze the tautomeric equilibrium of 2-mercaptopyrimidine in the gas phase and in aqueous solution. Second- and fourth-order Moller-Plesset perturbation theory calculations indicate that in the gas phase thiol (Pym-SH) is more stable than the thione (Pym-NH) by ca. 8 kcal/mol. In aqueous solution, thermodynamic perturbation theory implemented on a Monte Carlo NpT simulation indicates that both the differential enthalpy and Gibbs free energy favor the thione form. The calculated differential enthalpy is Delta H-SH -> NH(solv) = -1.7 kcal/mol and the differential Gibbs free energy is Delta G(SH)->(NH)(solv) = -1.9 kcal/mol. Analysis is made of the contribution of the solute-solvent hydrogen bonds and it is noted that the SH group in the thiol and NH group in the thione tautomers act exclusively as a hydrogen bond donor in aqueous solution. The proton transfer reaction between the tautomeric forms was also investigated in the gas phase and in aqueous solution. Two distinct mechanisms were considered: a direct intramolecular transfer and a water-assisted mechanism. In the gas phase, the intramolecular transfer leads to a large energy barrier of 34.4 kcal/mol, passing through a three-center transition state. The proton transfer with the assistance of one water molecule decreases the energy barrier to 17.2 kcal/mol. In solution, these calculated activation barriers are, respectively, 32.0 and 14.8 kcal/mol. The solvent effect is found to be sizable but it is considerably more important as a participant in the water-assisted mechanism than the solvent field of the solute-solvent interaction. Finally, the calculated total Gibbs free energy is used to estimate the equilibrium constant.
机译：进行了蒙特卡罗和量子力学的组合研究，以分析2-巯基嘧啶在气相和水溶液中的互变异构平衡。二阶和四阶Moller-Plesset微扰理论计算表明，在气相中，硫醇（Pym-SH）的稳定性比硫酮（Pym-NH）大约高。 8大卡/摩尔在水溶液中，基于蒙特卡洛NpT模拟的热力学摄动理论表明，微分焓和吉布斯自由能均倾向于硫酮形式。计算出的微分焓为ΔH-SH-> NH（solv）= -1.7 kcal / mol，微分吉布斯自由能为Delta G（SH）->（NH）（solv）= -1.9 kcal / mol。分析了溶质-溶剂氢键的作用，注意到硫醇互变异构体中硫醇中的SH基和巯基互变异构体中的NH基仅在水溶液中充当氢键供体。还研究了在气相和水溶液中互变异构形式之间的质子转移反应。考虑了两种不同的机制：分子内直接转移和水辅助机制。在气相中，分子内转移导致通过三中心过渡态的34.4 kcal / mol的大能垒。借助于一个水分子的质子转移将能垒降低到17.2 kcal / mol。在溶液中，这些计算的活化势垒分别为32.0和14.8 kcal / mol。发现溶剂作用相当大，但是作为水辅助机理的参与者，其作用比溶质-溶剂相互作用的溶剂领域重要得多。最后，计算出的总吉布斯自由能用于估计平衡常数。

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