Gas-phase Protonation of Spiropentane - A Novel Entry Into the C5h9+ Potential-energy Surface

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 7 X 10(-9) s. This local C5H9+ minimum is separated from the 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.