Gas-Phase Protonation of alpha,omega-Diphenylalkanes

The gas-phase protonation of alfa,omega-diphenylalkanes ranging from diphenylmethane to 1,6-diphenylhexane and of l-phenyl-2-(m-tolyl)ethane has been examined by FT-ICR mass spectrometry. Their gas-phase basicities (GB) at 300 K are consistently higher than that of toluene, as expected from their higher polarizability, and depend on the length of the methylene chain joining the two phenyl groups. This noticeable structure dependence is ascribed to an intramolecular solvation effect exerted by the second phenyl ring on the protonated one, allowed by a proper conformation of the linking chain. This effect is confirmed by a ca. 2 kcal/mol decrease in basicity when the protonated and the neutral phenyl rings are separated by the same number of C-C bonds in a rigid trans-cyclohexane-l,4-diyl unit versus a flexible chain.