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Unexpected results in gas‐phase tautomerism of differently 1‐nitrogen‐ and 2‐aryl‐substituted imidazolidines on electron ionization
Author(s) -
Joutsiniemi Karoliina,
Vainiotalo Pirjo,
Fülöp Ferenc,
Lázár László
Publication year - 1998
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/(sici)1097-0231(19980715)12:13<876::aid-rcm247>3.0.co;2-0
Subject(s) - chemistry , substituent , tautomer , electron ionization , ring (chemistry) , ion , polar effect , mass spectrum , aryl , ionization , polyatomic ion , fragmentation (computing) , crystallography , gas phase , medicinal chemistry , organic chemistry , alkyl , computer science , operating system
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 [C 2 H 6 N] + 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 increased 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. © 1998 John Wiley & Sons, Ltd.