Proton induced methyl group shifts in gaseous xylenium ions. Distinguishing isomers by gas-phase titration

Abstract

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 X pH + into the more stable ortho and meta isomers X oH + and X mH + by 1,2-methyl shifts around the aromatic ring. Strongly exoenergetic protonation of para-xylene X p 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 X pH + into a mixture of ≤ 15 mol% X pH +, 25 ± 3 mol% X oH +, and 60 ± 4 mol% X mH +. 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 X p by selected proton donors AH + (A = C 2H 5CN, CH 3OH, C 6H 6, C 3H 6, H 2O, C 2H 4) 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 X pH + ions through multiple 1,2-methyl shifts within a long lived ion/molecule complex [A⋯H +X p], formed by proton transfer from AH + to X p. 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.