An experimental electron density investigation of squarate and croconate dianions

A comparative study of the structure and topography of charge density in the squarate and croconate dianions has been-carried out on the basis of low-temperature X-ray diffraction measurements. Both the dianion rings exhibit a high degree of planarity with uniform C-C bond lengths and angles. The bond critical points in the two dianions carry similar densities as well as Laplacians. Deformation density maps reveal that the squarate ring is more strained than the croconate. Both the rings exhibit (3, +1) ring critical points. The density and the Laplacian associated with the ring critical point of the squarate anion are 0.59 e Angstrom(-3) and 11.8 e Angstrom(-5), respectively, whereas for the larger croconate ring, they are nearly one-half these values. Ring critical point parameters of the dianions have been compared with those of several aromatic as well as nonaromatic cyclic systems.