Electron ionization mass spectra of 8-aryl-3,4-dioxo-2H,8H-6,7-dihydroimidazo[2.1-c][1,2,4]triazines. Do they exhibit tautomerism in the gas phase?

The electron ionization mass spectra of the title compounds (1: a R = H, b 2-CH3, c 4-CH3, d 2,3-diMe, e 2-OCH3, f 4-OCH3, g 2-Cl, h 3-Cl, i 4-Cl, j 3,4-diCl) were recorded at 70 eV to determine the effects of substituents and the possible keto-enol tautomerism. The compounds showed several common fragment ions but also fragment ions which divided them into three classes, namely 1a-1d (parent compound and Me-substituted derivatives), 1e and 1f (MeO-substituted derivatives), and 1g-1j (Cl-substituted derivatives). The presence of the HOCN+center dot ion as well as the exponential dependence of its total ion current in the case of p- and also 3-Cl-substituted compounds (1a, c, f, h-j) on the Hammett sigma constants and the loss of CHO or one or two HOCN moieties can be somewhat easier explained by the presence of the enol form but as a whole the results support the predominance of the keto form, in parallel to the situation in solution. Copyright (c) 2006 John Wiley & Sons, Ltd.