黄嘌呤异构体质子转移异构化反应机理的理论研究

Theoretical Investigation on the Proton Transfer Isomerization of Xanthine

采用量子化学密度泛函B3LYP/6-31G(d,p)和M06-2X/6-311++G(d,p)方法对黄嘌呤两种酮式异构体X(1,3,7)与X(1,3,9)间质子转移引起的互变异构反应机理进行了计算研究,获得了异构化反应过程的反应焓﹑活化吉布斯自由能和质子转移反应的速率常数等参数。水相计算采用极化连续(PCM)模型。结果表明,由于可能的氢迁移顺序差异,分子内由X(1,3,7)向X(1,3,9)异构化可能共有16条反应通道,涉及11个中间体和20个过渡态,其主反应通道速控步骤的活化吉布斯自由能为183.10k J/mol,速率常数为5.17×10-20s-1,其余各通道速控步骤活化吉布斯自由能均较高,而且整体水溶剂效应不利于质子转移的发生。

proton transfer The reaction mechanism has been reaction enthalpies, acti investigated ration free using between X（1,3,7） both B3 LYP/6-31 G （ energies and rate constants and X （ 1 , 3,9 ） tautomers of xanthine resulted d,p） and M06-2X/6-311 ＋ ＋G（d,p） methods of the tautomerization reactions were obtained. PCM model was used for the aqueous calculation. The results showed that 16 reaction pathways from X（ 1,3,7） from The The to X （1,3,9） via intramoleeular proton transfer involving the 11 tautomers and 20 transition states were found. The activation Gibbs free energy needed in the main pathway and its rate constant are 183. 10 kJ/mol and 5. 17 × 10^-20s^-1 , respectively. The solvent effect is unfavorable for proton transfer.