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

Büker H-H, Grützmacher H-F, Crestoni ME, Ricci A (1997)
INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES 160(1-3): 167-181.

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Zeitschriftenaufsatz | Veröffentlicht | Englisch
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Abstract / Bemerkung
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 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 XpH+ into a mixture of less than or equal to 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 X-p by selected proton donors 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 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 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. (C) 1997 Elsevier Science B.V.
Erscheinungsjahr
Zeitschriftentitel
INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES
Band
160
Zeitschriftennummer
1-3
Seite
167-181
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Büker H-H, Grützmacher H-F, Crestoni ME, Ricci A. Proton induced methyl group shifts in gaseous xylenium ions. Distinguishing isomers by gas-phase titration. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES. 1997;160(1-3):167-181.
Büker, H. - H., Grützmacher, H. - F., Crestoni, M. E., & Ricci, A. (1997). Proton induced methyl group shifts in gaseous xylenium ions. Distinguishing isomers by gas-phase titration. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES, 160(1-3), 167-181. doi:10.1016/S0168-1176(96)04518-1
Büker, H. - H., Grützmacher, H. - F., Crestoni, M. E., and Ricci, A. (1997). Proton induced methyl group shifts in gaseous xylenium ions. Distinguishing isomers by gas-phase titration. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES 160, 167-181.
Büker, H.-H., et al., 1997. Proton induced methyl group shifts in gaseous xylenium ions. Distinguishing isomers by gas-phase titration. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES, 160(1-3), p 167-181.
H.-H. Büker, et al., “Proton induced methyl group shifts in gaseous xylenium ions. Distinguishing isomers by gas-phase titration”, INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES, vol. 160, 1997, pp. 167-181.
Büker, H.-H., Grützmacher, H.-F., Crestoni, M.E., Ricci, A.: Proton induced methyl group shifts in gaseous xylenium ions. Distinguishing isomers by gas-phase titration. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES. 160, 167-181 (1997).
Büker, Heinz-Hermann, Grützmacher, Hans-Friedrich, Crestoni, Maria Elisa, and Ricci, Andreina. “Proton induced methyl group shifts in gaseous xylenium ions. Distinguishing isomers by gas-phase titration”. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY AND ION PROCESSES 160.1-3 (1997): 167-181.