用从头标方法，通过对单取代味唑与单线态氧反应体系的计称，进一步证实了咪唑环与单线态氧1，4一加成反应的协同反应机理。否定了Wasserman关于该反应经历 极性中间体的推测以及Dewar 基于MINDO/3方法关于类似反应体系的分步机理。计标结果与咪唑衍生物光敏氧化反应不存在溶剂化效应的实验事实是一致的。在位能面上对咪唑环与单线态氧1，2-加成反应的中间体进行了寻找，没有发现两性离子中间体，1,2-加成反应通过桥环过氧化物中间体的分步机理也是因难的。推测为协同机理或经历双自由基中间体过程。对1.4一加成反应的计标发现，咪唑环上取代基对反应起着比较重要的作用。在4位单取代情况下，凡是取代基超共轭效应 常数为负值的，均对反应有利;而为正值时对反应不利。2位或5位单取代情况比较复杂，既要考虑取代基超共轭效应的影响，又要考虑6-诱导效应的影响。

In this paper, we have investigated the reaction mech-anism on photosensitized oxidation of monosubstituted imi-dazoles by means of ab initio SCF MO method and energy gradient technique. The calculated results showed that the 1,4-cycloaddition of imidazole ring with singlet oxygen is proceeded via a concerted mechanism. Wasserman's zwitterion intermediate mechanism and Dewar's two step mechanism based on MINDO/3 scheme were also discussed.We found that the con-certed mechanism proposed by our calculation is more reason-able and in agreement with experimental results. The calculated results of the 1,4-cycloaddition reactions showed that substituent effects are rather important. When the hyperconjugative constant of the substituent located on the 4-place of the imidazole ring is negative, the reactions are promoted. Otherwise the reactions are retarded. If the substituent locats at 2-place or 5-place of the imidazole ring, the situation is complicated. As for the 1,2-cycloaddition of this system, the zwitterion intermediate has also not been found on the potential energy surface.Besides,the stepwise 1，2-cycloaddition reaction which takes place via a perepoxide intermediate proceeds difficultly. Therefore, a concerted mechanism or a diradical intermediate is to be expected.