Orbital Interactions in Selenomethyl-Substituted Pyridinium Ions and Carbenium Ions with Higher Electron Demand

Computational, solution phase, and crystal structure analysis of 2- and 4-organoselenylmethyl-substituted pyridinium ions (10a−c and 11a−c) provides strong evidence for C−Se hyperconjugation (σC−Se−π*) between the C−Se σ-bond and the π-deficient aromatic ring and a through-space interaction (nSe−π*) between the selenium p-type lone pair and the π-deficient aromatic ring. There is also a weak anomeric-type interaction (nSe−σ*CC) involving the selenium p-type lone pair electrons and the polarized CH2−C(Ar) σ-bond. NBO analysis of calculated cations with varying electron demand (B3LYP/6-311++G**) show that C−Se hyperconjugation (σC−Se−π*) is the predominant mode of stabilization in the weakly electron-demanding pyridinium ions (10d, 11d, 14, and 15); however, the through-space (nSe−π*) interaction becomes more important as the electron demand of the β-Se-substituted carbocation increases. The anomeric interaction (nSe−σ*CC) is relatively weak in all ions.