化学学报 ›› 2014, Vol. 72 ›› Issue (2): 253-256.DOI: 10.6023/A13121206 上一篇    下一篇

研究论文

四嗪与一些环烯烃反应机理的理论研究

方德彩, 陈彦梅   

  1. 北京师范大学化学学院 北京 100875
  • 收稿日期:2013-12-02 出版日期:2014-02-14 发布日期:2014-01-01
  • 通讯作者: 方德彩,E-mail:dcfang@bnu.edu.cn;Tel.:010-58805422;Fax:010-58802075 E-mail:dcfang@bnu.edu.cn
  • 基金资助:

    项目受国家自然科学基金(No. 21373030)资助.

Theoretical Studies on the Mechanism of Cycloaddition Reaction between 1,2,4,5-Tetrazine and Cycloolefines

Fang Decai, Chen Yanmei   

  1. College of Chemistry, Beijing Normal University, Beijing 100875
  • Received:2013-12-02 Online:2014-02-14 Published:2014-01-01
  • Supported by:

    Project supported by the National Natural Science Foundation of China (No. 21373030).

采用多种密度泛函方法并辅以极化连续介质模型研究了四嗪与一些环烯烃发生[4+2]环加成反应的反应机理,得出反应过程按协同机理进行,即反应过程中只有一个过渡态. 主要使用的方法有:CAM-B3LYP,B3LYP,X3LYP,BMK,LC-wPBE,wB97x,wB97xd,M062x和M11. 这种过渡态的稳定性与前线分子轨道的相互作用有关,从过渡态电荷迁移的方向来看,环烯烃作为电子给体,而四嗪作为电子受体. 反应的难易取决于环烯烃的环的大小,环越大反应越难,这与环在形成过渡态时的变形能有关. 通过气相平动熵和溶液平动熵计算得到的活化自由能的数据,比较发现气相平动能得出的活化自由能离实验估计值相差很远. 而对于溶液中活化自由能垒数据对于大多数计算方法所得的结果比较接近,其中BMK,CAM-B3LYP和X3LYP的结果更合理一些. 同时也发现M062x和M11方法计算的反应速度常数与实验值差别较大,说明这些方法不太适合用来研究此类反应;而考虑色散作用的wB97xd也过高估计过渡态时分子之间的作用能,导致低自由能垒,具体产生的原因在文中进行了详细的讨论.

关键词: 四嗪, 环烯烃, [4+2]环加成, 溶液中自由能, 反应速度常数

The reaction mechanism of [4+2] cycloaddition reaction between 1,2,4,5-tetrazine and cycloolefines has been investigated with several density functional theory (DFT) methods, such as CAM-B3LYP, B3LYP, X3LYP, BMK, LC-wPBE, wB97x, wB97xd, M062x and M11, and the obtained results show that all of title reactions are concerted regardless of methods used. The stability of transition state is moderated by the interaction of frontier molecular orbitals (FMO), in which cycloolefine acts as an electron-donating partner and 1,2,4,5-tetrazine acts as an electron-accepter from the charge transfer direction in the transition state. The calculated activation free-energy barrier is dependent on the size of ring in cycloolefines, i.e., the larger the size of ring in cycloolefine, the more difficult the reaction, which is related to the deformation energy when transition state is formed. The activation free energy barriers have been calculated both with gas-phase translational entropy and solution translational entropy, in which those from gas-phase translational entropy (output of Gaussian job) are far away from the experimental estimations. For those generated from solution translational entropy are comparable for most of DFT methods, in which those generated from BMK, CAM-B3LYP and X3LYP methods are more reasonable. However, rate constants calculated from both M062x and M11 methods are largely deviated from experimental observations, indicating that these two methods are not suitable for studying these types of reactions. Moreover, the wB97xd method with classical dispersion overestimates the interaction between two fragments in transition states, and thus leads to low activation free energy barrier, meaning that those transition states, stabilized by FMO interactions, are hardly to form temperate and regional dipoles for the interacting partners. The reason for the failure of these methods has been addressed in this paper.

Key words: 1,2,4,5-tetrazine, cycloolefine, [4+2] cycloaddition, free-energy in solution, rate constant