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.

Proton Induced Methyl Group Shifts in Gaseous Xylenium Ions. Distinguishing Isomers by Gas-Phase Titration / Buker, H. H.; Grutzmacher, H. F.; Crestoni, Maria Elisa; Ricci, A.. - In: INTERNATIONAL JOURNAL OF MASS SPECTROMETRY. - ISSN 1387-3806. - STAMPA. - 160:(1997), pp. 167-181.

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

CRESTONI, Maria Elisa;
1997

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 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.
File allegati a questo prodotto
Non ci sono file associati a questo prodotto.

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11573/71609
 Attenzione

Attenzione! I dati visualizzati non sono stati sottoposti a validazione da parte dell'ateneo

Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus 24
  • ???jsp.display-item.citation.isi??? 25
social impact