The source function for the spin density s(r) is introduced, allowing the H and O influence on s(r) to be disentangled.

3-Diethylamino-4-(4-methoxyphenyl)-1,1-dioxo-4H-1λ 6 ,2-thiazete-4-carbonitrile (DTC) is a synthetic compound that exhibits a significant similarity with β-sultamic drugs. Its core chemical moiety is an uncommon four-membered thiazete-1,1dioxide heterocycle. Former crystallographic investigations carried out at room temperature on different DTC polymorphs and a chemically related compound showed a very unusual structural feature: within the conjugated −N−CN−SO 2 − system, the formally single N−C bond is, on average, 0.018 Å shorter than the formally double NC bond. In this work, the charge density distribution of DTC has been explored by both single-crystal X-ray diffraction at T = 100(2) K and quantum mechanical simulations, with the aim of gaining insights into the subtle interplay between structure, electron delocalization, and crystal field polarization effects. To this end, both local and nonlocal topological descriptors provided by the Quantum Theory of Atoms in Molecules have been employed. Topological and structural changes of crystalline and in vacuo DTC have been related to the smaller or larger importance of resonance forms in the −N−CN−SO 2 − moiety. A rationale for the mentioned C−N/CN bond length inversion is found in terms of the large DTC dipole moment enhancement occurring in the crystal, which stabilizes highly polar resonant forms that exploit more favorable electrostatic interactions with neighboring molecules. In turn, this causes a significant electronic rearrangement within the molecule that results in an unusual and counterintuitive bond length alternation pattern. Possible implications from the viewpoint of the accurate in silico modeling of crystal structures are discussed.

A new monoclinic form of the title compound, C14H17N3O3S, has been found upon slow crystallization from water. Another monoclinic form of the compound was obtained previously from a mixture of dichloro­methane and diethyl ether [Clerici et al. (2002 ▶). Tetra­hedron, 58, 5173–5178]. Both phases crystallize in space group P21/n with one mol­ecule in the asymmetric unit. The formally single exocyclic C—N bond that connects the –NEt2 unit with the thia­zete ring is considerably shorter than the adjacent, formally double, endocyclic C=N bond. This is likely to be due to the extended conjugated system between the electron-donor diethyl­ammine fragment and the electron-withdrawing sulfonyl group. In the newly discovered polymorph, the meth­oxy group is rotated by almost 180° around the phen­yl–OCH3 bond, resulting in a different mol­ecular conformation.

A small conformational change in the asymmetric unit has a significant effect on how non-covalent interactions determine (i) the crystal packing and (ii) the effect of T on the relative balance of electrostatics and dispersion–repulsions.