Solution and Gas-Phase Investigations of Trimethylsilylpropyl-Substituted Pyridinium Ions. Manifestation of the Silicon δ Effect

Computational studies on the N-methyl-2-trimethyl-M-propylpyridinium ions 15a (M = Si), 15b (M = Ge), 15c (M = Sn), and 15d (M = Pb) and N-methyl-4-trimethyl-M-propylpyridinium ions 16a (M = Si), 16b (M = Ge), 16c (M = Sn), and 16d (M = Pb) provide evidence for a significant through-bond (double hyperconjugative) interaction between the M–CH2 bond and the low-lying π* orbital of the pyridinium ion. The strength of this interaction increases in the order Si 13C and 29Si NMR studies; however, the effect is small. The collision-induced dissociation fragmentation reactions of 15a and 16a are strongly influenced by the through-bond interaction, with the major fragmentation pathway proceeding via extrusion of ethylene to yield the trimethylsilylmethyl-substituted pyridinium ions 1a and 2a.

针对N-甲基-2-三甲基-M-丙基吡啶鎓离子（15a，M=硅(Si)、15b，M=锗(Ge)、15c，M=锡(Sn)、15d，M=铅(Pb)）与N-甲基-4-三甲基-M-丙基吡啶鎓离子（16a，M=硅(Si)、16b，M=锗(Ge)、16c，M=锡(Sn)、16d，M=铅(Pb)）开展的计算研究，为M-CH₂键与吡啶鎓离子低能级π*轨道之间显著的键介导（双重超共轭型）相互作用提供了实验证据。该相互作用的强度按Si < Ge < Sn < Pb的顺序递增，这一趋势通过碳-13(¹³C)与硅-29(²⁹Si)核磁共振(NMR)光谱研究得到了验证，但该效应的幅度较小。15a与16a的碰撞诱导解离(collision-induced dissociation, CID)碎裂反应会受到该键介导相互作用的显著影响，其主要碎裂路径为消除乙烯，生成三甲基硅甲基取代的吡啶鎓离子1a与2a。