Charge Density Analysis of 2-Pyridineformamide N(4)-Methylthiosemicarbazone (Z' = 4): Role of an Enhanced N-H···S Thioureido Dimer.

Charge density distribution in 2-pyridineformamide N(4)-methylthiosemicarbazone (TSC4) has been determined using high-resolution X-ray diffraction data (100 K) and Hansen-Coppens multipole formalism. The results are interpreted in terms of Bader's quantum theory of atoms in molecules (QTAIM), electrostatic potential analysis, and theoretical calculations (CRYSTAL09). The study highlights the molecular dissimilarity at the electronic level. The N-H···S interactions have a dominant role in stabilization of the TSC4 crystal structure. As each of the four S acceptors simultaneously engages in several interactions, the focus of study is on the lone pairs' electron density of these acceptors which is particularly able to adjust to the various spatial distributions of the interacting donor groups. The main structural feature of TSC4 is the formation of specific N-H···S bonded dimers between two pairs of independent molecules. These dimers are the main building block in the crystal structure, and with the two additional N-H···S contacts they represent a significant enhancement of a typical R²2(8) dimer synthon which dominates among the thioureido-based crystal structures. Two TSC4 dimers are structurally very similar and exhibit considerable cohesive energy (-20.8 and -21.4 kcal/mol). This type of dimer is the only structural motif that is common for S acceptors of all four independent molecules. [ABSTRACT FROM AUTHOR]