Protonated benzene:: A case for structural memory effects?

Photoionization tandem-mass spectrometry of selectively deuterium labeled precursor molecules is used to probe the structure of protonated benzene which had been questioned in a provocative publication by Mason and co-workers (J. Chem. Soc., Chein. Commun. 1995, 1027). Specifically, we address the intriguing aspect of a postulated delayed hydrogen ring-walk by modulating the internal energy content of mass-selected C6H7-nDn+ ions. To this end, ionization of selectively deuterated precursors by tunable synchrotron photons is combined with chemical monitoring of H+/D+ transfer from C6H7-nDn+ to a strong base B. The resulting BH+/BD+ ratios monotonically decrease with increasing internal energy content and, due to the virtue of the regioselective deuterium label incorporations, thereby disprove a delayed hydrogen ring-walk or any other structural memory effect in C6H7-D-n(n)+ ions. As a consequence, the experimental studies of Mason et al. were reconsidered using sector-field mass spectrometry. These extensive studies suggest that the previous observations might have been obscured by a combination of isobaric impurities, metastable-ion contributions, and artifact signals.