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.
|Number of pages||7|
|Journal||Rapid Communications in Mass Spectrometry|
|Publication status||Published - 1998|
ASJC Scopus subject areas
- Analytical Chemistry
- Organic Chemistry