Lowering large 1,2-H shift barriers by proton-transport catalysis: the challenging case of the pyridine radical cation

Full-text article (96 kB)
(subscribers only)

Eur. J. Mass Spectrom. 5, 431 - 439 (1999)

Lowering large 1,2-H shift barriers by proton-transport catalysis: the challenging case of the pyridine radical cation

Moschoula A. Trikoupis, David J. Lavorato and Johan K. Terlouw^*
Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4M1, Canada. E-mail:
Paul J.A. Ruttink
Theoretical Chemistry Group, Department of Chemistry, University of Utrecht, Padualaan 14, 3584 CH Utrecht, The Netherlands
Peter C. Burgers
Hercules European Research Center, Hercules BV, PO Box 252, 3770 AG, Barneveld, The Netherlands

ABSTRACT:

Under conditions of chemical ionization, the pyridine radical cation rearranges to its more stable a-ylide isomer by an ion-molecule reaction with a suitable reagent, such as 2-cyanopyridine. The initially formed [pyridine^"+"""2-cyanopyridine] adduct isomerizes, by way of proton-transport catalysis, to a stable complex of 2-cyanopyridine with the a-ylide ion which may dissociate. Multiple collision experiments on deuterium labelled pyridines indicate that a further isomerization may occur: about half of the (metastable) complex ions undergo a cyano-transfer leading to a very stable distonic ion. A subsequent 1,4-hydrogen shift may lead to the di-2-pyridyl ketimine ion which could account for the observed survivor signal in the neutralization-reionization mass spectrum of the complex. Ab initio calculations at the RHF/6-31G(d) level of theory fully support these findings and provide a rationale for the observed cyano-transfer reaction.

Keywords: proton-transport catalysis, tandem mass spectrometry, pyridine, dimer radical cation, chemical ionization, ab initio calculations

You can now buy this paper on-line in PDF format; it costs only £11.75. Just click on the BUY on-line button. You can pay on-line through a secure server and get access immediately.