GAS-PHASE PROTONATION OF ALPHA,OMEGA-DIPHENYLALKANES

The gas-phase protonation of alpha,omega-diphenylalkanes ranging from diphenylmethane to 1,6-diphenylhexane and of 1-phenyl-2-(m-tolyl)ethan e has been examined by FT-ICR mass spectrometry. Their gas-phase basic ities (GB) at 300 K are consistently higher than that of toluene, as e xpected from their higher polarizability, and depend on the length of the methylene chain joining the two phenyl groups. This noticeable str ucture dependence is ascribed to an intramolecular solvation effect ex erted by the second phenyl ring on the protonated one, allowed by a pr oper conformation of the linking chain. This effect is confirmed by a ca. 2 kcal mol(-1) decrease in basicity when the protonated and the ne utral phenyl rings are separated by the same number of C-C bonds in a rigid trans-cyclohexane-1,4-diyl unit versus a flexible chain.