Proton Induced Methyl Group Shifts in Gaseous Xylenium Ions. Distinguishing Isomers by Gas-Phase Titration

The differences in the gas-phase basicities (GBs) of isomers can be used for a semi-quantitative mixture analysis. For this “gas-phase titration” method, the mixture of isomers is completely protonated in the external ion source of a FT-ion cyclotron resonance spectrometry (ICR) spectrometer and undergoes selective, stepwise deprotonation by reactions with appropriate bases within the FT-ICR cell. This method is demonstrated by the analysis of a mixture of three isomeric alkylpyridines differing in GB by 4 kJ mol−1. “Gas-phase titration” is employed to detect the isomerization of gaseous para-xylenium ions XpH+ into the more stable ortho and meta isomers XoH+ and XmH+ by 1,2-methyl shifts around the aromatic ring. Strongly exoenergetic protonation of para-xylene Xp in the external ion source by chemical ionization (CI) (methane) and gas-phase titration of the resulting XH+ after transfer into the FT-ICR cell reveals isomerization of XpH+ into a mixture of ≤ 15 mol% XpH+, 25 ± 3 mol% XoH+, and 60 ± 4 mol% XmH+. The degree of isomerization depends clearly on the exoenergicity of the initial protonation and is significantly reduced for XH+ ions generated by CI (dimethyl ether). This effect is confirmed by an investigation of the controlled protonation of Xp by selected proton donors AH+ (A = C2H5CN, CH3OH, C6H6, C3H6, H2O, C2H4) in the FTICR cell. This study shows that the chemical nature of A is also important for the degree of isomerization. The results are explained convincingly by assuming isomerization of the XpH+ ions through multiple 1,2-methyl shifts within a long lived ion/molecule complex [A⋯H+Xp], formed by proton transfer from AH+ to Xp. The rearrangement is driven by the excess energy of the complex as a result of exothermic proton transfer and electrostatic activation of the complex, and competes with the dissociation of the excited complex.