Unexpected results in gas-phase tautomerism of differently 1-nitrogen- and 2-aryl-substituted imidazolidines on electron ionization

Karoliina Joutsiniemi, Pirjo Vainiotalo, Ferenc Fülöp, László Lázár

Research output: Contribution to journalArticle

7 Citations (Scopus)

Abstract

The 70 eV electron ionization mass spectra of 35 differently 2-phenyl- and 1-nitrogen-substituted imidazolidines were studied. Tautomerism was observed between the open-chain and ring forms in the gas phase. The fragment ions detected in the low-energy mass spectra showed that in the gas phase the compounds mostly existed in the ring form; except those having a nitro substituent on the 2-phenyl group, for which only open-chain fragment ions were found. The most important fragment ion relating to the ring form for N-methyl-substituted compounds was the [M-43]+ ion, whereas the [C2H6N)+ ion at m/z 44 and the [M-44]+ ion are due to the open-chain form. The ring-chain ratio was clearly dependent on the electron-donating or withdrawing ability of the substituent X on the 2-phenyl group. As the electron-donating properties of substituent X increased, the intensities of the fragment ions relating to the ring form also increased, whereas electron, withdrawing substituents in ease the intensities of the fragment ions relating to the open-chain form. This kind of behaviour seems to be totally inconsistent with the results observed in solution and also with those on certain O,N-heterocycles in the gas phase, where electron-donating substituents favour the open-chain form. The present results can be explained in terms of the differences in fragmentation efficiency between chain forms with different substituents X on the phenyl group: the ionization site varied with the electron-donating or withdrawing ability of the substituent X on the phenyl group. The easy decomposition of the open-chain form in the case of electron-withdrawing substituents shifts the equilibrium towards the open-chain form to an unusual extent. The unusual substituent effect in electron-withdrawing substituents made it impossible to say anything about the pre-ionization equilibria with compounds having this kind of substituent.

Original languageEnglish
Pages (from-to)876-882
Number of pages7
JournalRapid Communications in Mass Spectrometry
Volume12
Issue number13
Publication statusPublished - Jan 1 1998

ASJC Scopus subject areas

  • Analytical Chemistry
  • Spectroscopy
  • Organic Chemistry

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