On the zwitterionic nature of strongly twisted push-pull ethylenes: the case of 3-(1,3-diisopropyl-2-imidazolidinylidene)-2,4-pentanedione monohydrate, studied by x-ray diffraction and ab initio calculations

Abstract

In the crystal structure of the title compound ( 1), a formally double bond 1.475(3) Å long, and a twist angle about it as large as 84.2(2)°, are observed. These geometrical parameters, compared with those reported in the literature for the related molecules 3-(1,3-dimethyl-2-imidazolidinylidene)-2,4-pentanedione trihydrate ( 2), and 3-(2-imidazolidinylidene)-2,4-pentanedione ( 3) indicate a stronger push-pull effect in compound ( 1). Group charge distributions derived from ab initio STO-3G calculations on molecules ( 1) and ( 2), and based on the non-optimized, experimental geometries, reveal the zwitterionic nature of these substituted ethylenes. In molecule ( 1), a negative charge of 0.638 e on the pentanedione fragment, and a positive charge of 0.156 e on the imidazolidine ring are calculated, with the remaining positive charge almost equally distributed on the two isopropyl groups. A formal negative charge of 0.255 e is given to the ethylenic carbon atom belonging to the pentanedione moiety, and a positive charge of 0.363 e to the other. Relevant bond properties derived from a topological analysis of the theoretical electron density ϱ are discussed; both the position of the critical point of the CC bond and the corresponding positive value of the Laplacian of ϱ confirm the highly polar character of the ethylenic fragment.