UNEXPECTED RESULTS IN GAS-PHASE TAUTOMERISM OF DIFFERENTLY 1-NITROGEN-SUBSTITUTED AND 2-ARYL-SUBSTITUTED IMIDAZOLIDINES ON ELECTRON IONIZATION

The 70 eV electron ionization mass spectra of 35 differently 2-phenyl- and 1-nitrogen substituted imidazolidines were studied. Tautomerism w as observed between the open chain and ring forms in the gas phase. Th e fragment ions detected in the low-energy mass spectra showed that in the gas phase the compounds mostly existed in the ring form; except t hose having a nitro subtitituent 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](+) i on are due to the open-chain form. The ring-chain ratio was clearly de pendent on the electron-donating or withdrawing ability of the substit uent X on the 2-phenyl group. As the electron-donating properties of s ubstituent X increased, the intensities of the fragment ions relating to the ring form also increased, whereas electron-withdrawing substitu ents increased the intensities of the fragment ions relating to the op en-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 substituent s favour the open chain form. The present results can be explained in terms of the differences in fragmentation efficiency between chain for ms with different substituents X on the phenyl group: the ionization s ite varied with the electron-donating or withdrawing ability of the su bstituent X on the phenyl group. The easy decomposition of the open-ch ain form in the case of electron-withdrawing substituents shifts the e quilibrium towards the open-chain form to an unusual extent. The unusu al substituent effect in electron-withdrawing substituents made it imp ossible to say anything about the pre-ionization equilibria with compo unds having this kind of substituent. (C) 1998 John Wiley & Sons, Ltd.