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采用密度泛函B3LYP/6-311+G**方法,对5-氯尿嘧啶分子内质子转移及水助催化质子转移引起的互变异构反应机理进行了计算研究,获得了互变异构过程的反应焓、活化能、活化吉布斯自由能和质子转移反应的速率常数等参数。计算结果表明,5-氯尿嘧啶无论是孤立分子还是一水合物,其双酮式CU1是最稳定异构体,由双酮式向烯醇式异构化找到3条通道(P1,P2,P3),各通道速控步骤的活化能分别为177.85、177.05和197.58kJ/mol。当水分子参与反应以双质子转移机理异构化时,活化能显著降低,各通道速控步骤的活化能依次降为66.24、69.36和77.85kJ/mol,有利于双酮式向烯醇式或酮醇式转变。计算结果还表明,氢键作用在增大5-氯尿嘧啶一水复合物稳定性、降低质子转移异构化反应活化能等方面起着重要作用。
The mechanism of tautomerism induced by intramolecular proton transfer of 5-chlorouracil and water-assisted proton transfer was studied by using density functional theory B3LYP / 6-311 + G **. The tautomerism Reaction enthalpy of reaction, activation energy, activation Gibbs free energy and rate constant of proton transfer reaction. The results show that 5-chloro-uracil, whether isolated or mono-hydrate, is the most stable isomer of di-keto-CU1. Three channels (P1, P2, P3). The activation energies of the quick control steps for each channel were 177.85, 177.05 and 197.58 kJ / mol, respectively. When the water molecules participate in the isomerization process by the double proton transfer mechanism, the activation energy is significantly reduced, and the activation energy of the quick control step of each channel is reduced to 66.24, 69.36 and 77.85 kJ / mol in turn, which is beneficial to the conversion of the diketone type enolase or Keto alcohol conversion. The calculated results also show that the hydrogen bonding plays an important role in increasing the stability of 5-chlorouracil-water complex and reducing the activation energy of proton transfer isomerization reaction.