Modes of neighbouring group participation by the methyl selenyl substituent in β-methylselenylmethyl-substituted 1-phenylethyl carbenium ions.

Selenium substituents which are disposed β to an electron deficient centre, such as a carbocation p-orbital, or the π* orbital of an electron deficient p-system, interact in a stabilising way by a combination of C-Se hyperconjugation (σ(Se-C)-π* interaction), and a through-space homoconjugative n(Se)-π* interaction. The relative importance of these two modes of interaction is dependant on the electron demand of the cation, with hyperconjugation predominating for low electron demand systems, and the n(Se)-π* interaction predominating for high electron demand cations.