Denekamp Chagit, Tenetov Elena, Horev Yonit
Department of Chemistry and Institute of Catalysis Science and Technology, Technion-Israel Institute of Technology, Haifa, Israel.
J Am Soc Mass Spectrom. 2003 Jul;14(7):790-801. doi: 10.1016/S1044-0305(03)00324-6.
The favored fragmentation pathway for protonated and alkylated pyridinium cations of the general formula p-XC(6)H(4)CH(2)CH(2)CH=CH Py(+)R (R=H, Me; Py=pyridine) is a C-C homolytic cleavage. The tendency to form radicals is higher for alkylated pyridinium cations than for the protonated ones that can also afford closed-shell products. Theoretical calculations show that the singlet-triplet gap for transient structures with an elongated benzylic C-C bond is very low and the formation of radicals may result from mixing of these states. In addition to the notable substituent effect on the fragmentation efficiency of the cations under study, calculated results show a clear substituent effect on the singlet-triplet transitions. We also observe that triphenylphosphonium cations behave notably different. Thus, the pyridinium system that contains a p-chloro benzyl moiety loses a benzyl radical readily while the analogous triphenylphosphonium cation is very stable under the same conditions.
通式为p-XC(6)H(4)CH(2)CH(2)CH=CH Py(+)R（R = H，Me；Py = 吡啶）的质子化和烷基化吡啶鎓阳离子最有利的碎裂途径是C-C均裂。烷基化吡啶鎓阳离子形成自由基的倾向高于质子化的吡啶鎓阳离子，质子化的吡啶鎓阳离子也能生成闭壳层产物。理论计算表明，具有伸长的苄基C-C键的瞬态结构的单重态-三重态能隙非常低，自由基的形成可能是由于这些态的混合。除了对所研究阳离子的碎裂效率有显著的取代基效应外，计算结果还表明对单重态-三重态跃迁有明显的取代基效应。我们还观察到三苯基鏻阳离子的行为明显不同。因此，含有对氯苄基部分的吡啶鎓体系很容易失去苄基自由基，而类似的三苯基鏻阳离子在相同条件下非常稳定。
