Single N-C Bond Becomes Shorter than a Formally Double N=C Bond in a Thiazete-1,1-dioxide Crystal: An Experimental and Theoretical Study of Strong Crystal Field Effects

3-Diethylamino-4-(4-methoxyphenyl)-1,1-dioxo- 4H-1?6,2-thiazete-4-carbonitrile (DTC) is a synthetic compound that exhibits a significant similarity with ?-sultamic drugs. Its core chemical moiety is an uncommon four-membered thiazete-1,1- dioxide 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-SO2- 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-SO2- 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