1,2-diacylcyclopentadiene (1,2-DACP) compounds (also known as fulvene compounds) belong to a category of γ-dicarbonyl compounds with the capability of forming intramolecular hydrogen bond (IHB) (see Fig.1). In the enol form of these compounds, as a result of IHB, and because of a long π-electronic conjugation between carbonyl and cyclopentadiene double bands, a 7-membered ring-like structure (see Fig.1) is formed which is referred to as chelated or enolone ring (like 6-membered chelated ring in β-dicarbonyls). Since the chelated ring is heptagonal [1], the spatial orientations (distances and angles) for O and H−O segments of molecule are more proper to form much stronger O···H−O hydrogen bond in 1,2-DACPs, in comparison to that of β-dicarbonyls. Also the longer resonance structure of delocalized π-electrons makes the resonance assisted intramolecular hydrogen band, RAHB [2], in this type of γ-dicarbonyls to be stronger than that of β-dicarbonyls. In the present work, 1,2-dithenoylcyclopentadien (DTCP) has been investigated with special attention to its IHB, in order to obtain detailed information on the conformational relative stabilities, structures and electron conjugation in its resonance assisted hydrogen bond (RAHB). It is also important to estimate the barrier height for proton transfer and the strength of the IHB, which is the main factor governing the conformational stability.

Keywords