The crystal structure of the N,N,N',N'-tetramethylethylenediammonium dithiocyanate salt has been examined by experimental charge density studies from high-resolution X-ray diffraction data. The corresponding results are compared with multipole refinements, using theoretical structure factors obtained from a periodic density functional theory calculation at the B3LYP level with a 6-31G(**) basis set. The salt crystallizes in space group P and contains only a single ion pair with an inversion center in the cation. The salt has thus one unique classical N+-H...(NCS)(-) hydrogen bond but also has six other weaker interactions: four C-H...S, one C-H...N, and one C-H...C(pi). The nature of all these interactions has been examined topologically using Bader's quantum theory of "atoms in molecules" and all eight of the Koch-Popelier criteria. The experimental and theoretical approaches agree well and both show that the inter-ion interactions, even in this simplest of systems, play an integrated and complex role in the packing of the ions in the crystal. Electrostatic potential maps are derived from experimental charge densities. This is the first time such a system has been examined in detail by these methods.

The experimental charge densities for a series of sym-N-methyl-substituted ethylenediammonium dithiocyanate salts have been investigated based on low-temperature and high-resolution X-ray diffraction data. This series of organic dications provides both strong and weak hydrogen bonding networks that vary depending on the N-H : SCN(-) (donor/acceptor) ratios. The number of N-HN hydrogen bonds connected to each cation increases (linear to bifurcated) as the number of N-H donor groups increases. The bifurcated thiocyanate anions also form a less energetic N-HS hydrogen bond. The presence of more than one hydrogen bond acceptor on each thiocyanate anion results in a competition between the sulfur and nitrogen atoms in forming both strong and weak hydrogen bonds. The formation of a significant number of weak hydrogen bonds is shown to play a crucial role in stabilizing these organic ionic crystals. The progression of these organic dications (smaller to larger N-H : SCN(-) ratios) results in the weaker hydrogen bonds playing a smaller role in stabilizing the crystalline structures. In addition, the electron density along the saddle point has been shown to vary significantly from weak hydrogen bonds to van der Waals interactions. This has led to a better understanding of the progression of hydrogen bonding in the crystalline states of sym-N-methyl substituted ethylenediammonium dithiocyanate salts and provides insight into the relationship between strong and weak hydrogen bonds in organic ionic crystals.

The crystal structures of uncharged tetrahedral dithiocyanato zinc complexes with N‐methylated ethylenediamines have been determined with a view to a study of intermolecular hydrogen‐bonding interactions in these compounds. It is found that the H(N) hydrogen atoms are exhaustively engaged in N–H(N) … S bonds. The majority of these bonds are branched (bifurcated or trifurcated), and the hydrogen‐bond systems they form all contain one of the two characteristic primitive core motifs: either a discrete centrosymmetric […S…H…]2 dimer or an infinite […S…H…]∞  helix about a 21 or pseudo‐21 axis. The hydrogen bonding is analyzed in detail, with particular attention to the existence of correlations between the N–H(N)–S angles and the H(N) … S distances as well as between the corresponding N–H(N)–S/H(N)…S pairs in the bifurcated N–H(N)…2 S bonds.

Structural analyses for a series of tetramethylalkyldiammonium dithiocyanate salts have been carried out on lowtemperature and high-resolution X-ray diffraction data. This series of organic salts contains both strong and weak hydrogen bonds which vary depending on the number of methylene groups bridging the two ammonium portions of the molecules. The S-N • • • H angles associated with the strong N-H • • • N hydrogen bonds in this series progressively decrease as the length of the alkyl bridge increases (ethylene > propylene > butylene). This results in less linear interaction between the N-H donor groups and the lone pair of the N acceptor atoms. The N,N,N′,N′-tetramethylpropylenediammonium and N,N,N′,N′-tetramethylbutylenediammonium dithiocyanate salts also produce weak C-Hbonds in the propylene salt appears to experience a cooperative effect. A comparison of the topological properties of the electron density has shown a number of similarities and differences in the electronic nature of the strong and weak interactions in all three salts. This has all led to a better understanding of the electronic nature of these interactions and how they logically distribute themselves in the crystalline state.