摘要
采用密度泛函B3LYP/6-311G**方法,对6种5-氟胞嘧啶异构体孤立分子的稳定性及质子转移引起的酮式-烯醇式、氨基式-亚胺式互变异构反应机理进行了计算研究,获得了零点能、吉布斯自由能及质子转移过程的反应焓、活化能、活化吉布斯自由能和速率常数等参数。计算结果表明,气相中烯醇-氨基式FC4是最稳定的异构体。分子内质子转移设计了FC1→FC2和FC1→FC6两条通道,分别标记为P(1)和P(2),各通道速控步骤的活化能和速率常数分别为155.9 kJ.mol-1,4.70×10-15s-1和173.1 kJ.mo-l1,1.41×10-18s-1。水助催化时,相应通道P(3)和P(4)速控步骤的活化能和速率常数分别为51.0 kJ.mol-1,1.41×103s-1和88.2 kJ.mo-l1,4.53×10-3s-1。可见,水分子的加入极大地降低了质子转移的活化能垒。另外发现,水分子参与形成协同的双质子转移机理比水助单质子转移机理更利于降低活化能垒。
The relative stabilities of six possible tautomers in isolated 5-fluorocytosine and the reaction mechanism of keto-enol and amino-imino obtained from proton transfer were calculated by DFT B3LYP method with 6-311G^＊＊ basis set. And the zero point energies, Gibbs free energies and the reaction enthalpies, activation energies, activation Gibbs free energies and the rate constants of tautomerization were obtained. The results showed that FC4 （enol-amino） was the predominant isomer in the gas phase. The P（ 1 ） and P（2） in intramolecular proton transfer were marked for FC1→FC2 and FC1→FC6 paths,and the activation energies of the rate-determining steps and the rate constants were 155.9 kJ · mol^-1 , 4. 70 ± 10^-15 s^-1 and 173. 1 kJ · mol^-1 , 1.41 × 10^-18 s^-1 , respectively. In water catalysis,the activation energies and the rate constants of the rote-determining steps in P（3） and P（4） pathways were 51.0 kJ·mol^-1,1.41 × 10^3 s^-1 and 88.2 kJ· mol^-1 ,4.53 × 10^-3 s^-1, respectively. And the water lowed the activation energies dramatically. We also found that the concerted double proton transfer mechanism lowered the activation energies more than the forming single proton transfer mechanism.
出处
《化学研究与应用》
CAS
CSCD
北大核心
2009年第8期1129-1135,共7页
Chemical Research and Application
基金
陕西省自然科学基金(2006B12)资助
西安文理学院专项科研基金(Kyc200806)资助
