Gas-Phase Protonation of Spiropentane. A Novel Entry into the C5H9+ Potential Energy Surface

Article abstract of DOI:10.1021/ja00075a058

The structures, stabilities, and isomerization patterns of C5H9⁺ ions arising from the gas-phase protonation of spiropentane have been investigated by nuclear-decay, radiolytic, and FT-ICR techniques combined with ab initio calculations.The experimental and theoretical results are consistent with the initial formation of a corner-protonated spiropentane intermediate 17, whose lifetime in the gas phase exceeds 7E-9 s.This local C5H9⁺ minimum is separated from ca. 30 kcal mol^-1 more stable cyclopentyl cation as well as from dimethylallyl open-chain isomers by significant energy barriers.Persistency of 17 in the gas phase does not find any correspondence in solution.Solvation and ion-pairing effects may explain the failure to detect C5H9⁺ structures retaining the spirobicyclic framework of spiropentane in the condensed phase.