Solvatochromism of heteroaromatic compounds: VII. Bifurcate hydrogen bond in solvatochromic complexes of 2-(1,2,2-tritricyanovinyl)pyrrole

IR and UV spectrosocopy and quantum chemistry were used to reveal a bifurca te (three-center) hydrogen bond in the complexes of 2-(1,2,2-tricyanovinyl) pyrrole with hydrogen-bond acceptors. In the gas phase and aprotic inert so lvents this compound exists predominantly as the sp conformer stabilized by intramolecular hydrogen bond involving an orthogonal pi system of the nitr ile group and the NH hydrogen. The formation of the three-center hydrogen b ond with an aprotic protophilic solvent shifts the conformational equilibri um to the ap conformer. As the strength of the intermolecular hydrogen bond increases in the series nitromethane < acetonitrile < dioxane, the fractio n of the ap conformer increases and becomes prevailing already in the moder ately protophilic THE Solvatochromism of the long-wave absorption band in t he electronic spectra of the sp and ap conformers was studied in detail.