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

Abstract

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 variable-temperature X-ray diffraction and theoretical study of conformational polymorphism in a complex organic molecule (DTC)

“…Being DTC neutral, both m P and m CT are origin-independent and retain a physical meaning in their own. 23,70 Parameters used to assess the quality of the integration of the charge density, such as the residual charge of the unit cell and the absolute values of atomic Lagrangian integrals, were all satisfactorily low, being on average as low as 0.0115(8) (0.0108(7)) e and 4.6(4) Â 10 À4 (5.5(3) Â 10 À4 ) eÅ À2 , respectively, for the form A(B). Table 2 reports the modules of the various polarization and charge transfer vectors, as well as the angle between them (g), for the asymmetric unit of both polymorphs as a function of T. As it is oen the case, the two contributions oppose each other as atoms customarily polarize in a direction counter to the local electric eld caused by the interatomic charge transfer; indeed, g was found to be close to 180 deg within 10-25 deg.…”

“…In this work, a multi-temperature study of the two known conformational polymorphs [21][22][23] of 3-diethylamino-4-(4-methoxyphenyl)-1,1-dioxo-4H-1l 6 , 2-thiazete-4-carbonitrile (DTC, Scheme 1, C 14 H 17 N 3 O 3 S) is reported. The main objectives are (i) to study correlations between molecular conformation and interaction energetics in the two forms, providing a rationale for their relative stability, and (ii) to investigate how and why electrostatic interactions and the various NCI patterns depend on temperature.…”

Source function and plane waves: Toward complete bader analysis

“…It was demonstrated for a variety of test systems that SF values of quality comparable to LBS reference calculations can be obtained for PW methods using the YT integration algorithm. The LBS SF is affected by the correlation and basis set effect as described by Gatti et al However, these effects are so small that they have negligible influence on the picture described by the SF calculation. Instead, SF values performed on PW charge densities are subject to influences from three effects: (i) kinetic energy cut‐off, (ii) correlation, (iii) pseudo‐potential.…”

“…Based on the existence of QTAIM atomic basins, the SF describes the contribution of each atom to a single point of space. Initially, the SF was applied only to the charge density at molecules, proving a powerful tool for solving a number of difficult chemical questions, and more recently to periodic systems . The SF is a an indicator that can be employed to reveal subtle electron‐delocalization effects in crystals .…”

“… can be interpreted as the local contribution of point to the density at . This is a particularly useful definition as it provides an immediate characterisation of point : when , then and is a “sink” (depleting the charge density);when, then and is a “source” (enriching the charge density).…”