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 us ed for a semi-quantitative mixture analysis. For this ''gas-phase titr ation'' 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 reac tions with appropriate bases within the FT-ICR cell. This method is de monstrated by the analysis of a mixture of three isomeric alkylpyridin es differing in GB by 4 kJ mol(-1). ''Gas-phase titration'' is employe d 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 shi fts around the aromatic ring. Strongly exoenergetic protonation of par a-xylene X-p in the external ion source by chemical ionization (CI) (m ethane) and gas-phase titration of the resulting XH+ after transfer in to the FT-ICR cell reveals isomerization of XpH+ into a mixture of les s than or equal to 15 mol% XpH+, 25 +/- 3 mol% XoH+, and 60 +/- 4 mol% XmH+. The degree of isomerization depends clearly on the exoenergicit y of the initial protonation and is significantly reduced for XH+ ions generated by CI (dimethyl ether). This effect is confirmed by an inve stigation of the controlled protonation of X-p by selected proton dono rs AH(+) (A = C2H5CN, CH3OH, C6H6, C3H6, H2O, C2H4) in the FT-ICR cell . This study shows that the chemical nature of A is also important for the degree of isomerization. The results are explained convincingly b y 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 X-p. The rearrangement is driven by t he excess energy of the complex as a result of exothermic proton trans fer and electrostatic activation of the complex, and competes with the dissociation of the excited complex. (C) 1997 Elsevier Science B.V.