The whole range of hydrogen bonds in one crystal structure: neutron diffraction and charge-density studies ofN,N-dimethylbiguanidinium bis(hydrogensquarate)

Abstract

N,N-Dimethylbiguanidinium bis(hydrogensquarate) features an impressive range of hydrogen bonds within the same crystal structure: neighbouring anions aggregate to a dianionic pair through two strong O-H···O interactions; one of these can be classified among the shortest hydrogen bonds ever studied. Cations and anions in this organic salt further interact via conventional N-H···O and nonclassical C-H···O contacts to an extended structure. As all these interactions occur in the same sample, the title compound is particularly suitable to monitor even subtle trends in hydrogen bonds. Neutron and high-resolution X-ray diffraction experiments have enabled us to determine the electron density precisely and to address its properties with an emphasis on the nature of the X-H···O interactions. Sensitive criteria such as the Laplacian of the electron density and energy densities in the bond-critical points reveal the incipient covalent character of the shortest O-H···O bond. These findings are in agreement with the precise geometry from neutron diffraction: the shortest hydrogen bond is also significantly more symmetric than the longer interactions.

Charge Density of Intra‐ and Intermolecular Halogen Contacts

“…In the bond critical point of the H ··· Cl contact, these dominating non‐classical hydrogen bonds are characterized by an electron density of ca. 0.1 e · Å –3 and a Laplacian of ca 1 e · Å –5 ; comparable values have been established for rather weak N‐H···O interactions 27. Interhalogen contacts in 1–7 are either weak or not even associated with a bond critical point at all.…”

“…In Figure 8, the electron density in the bond critical point as published by Guru Row et al,43 Espinosa et al44 and in this work has been drawn as a function of the Cl ··· Cl distance. The shortest among these interactions are associated with similar features for the electron density (Figure 8 and Table 1) and its Laplacian (Table 1) as weak N–H ··· O hydrogen bonds 27…”

Competing protonation sites in sulfadiazine: answers from chemistry and electron density

“…6 and 7, respectively, are associated with the highest electron density and the most positive Laplacian, in agreement with expectation for essentially ionic hydrogen bonds. [35][36][37] The energy densities in the bcps for the H … A interactions (Table 2) follow the expectation for moderately strong and weak classical hydrogen bonds: the kinetic energy density G exceeds the absolute values for the potential energy density V, and the ratio G/r bcp (expressed in atomic units) typically ranges around 1, whereas values of ca. 0.3 are commonly encountered for covalent bonds.…”

The halogen bond made visible: experimental charge density of a very short intermolecular Cl⋯Cl donor–acceptor contact

“…7,9 According to charge density criteria, the short halogen bond in 1 might compete with moderately strong or weak hydrogen bonds. 29 The observed charge accumulations and depletions are more pronounced than in the chemically related compound 2 which shows longer and presumably weaker contacts. One may speculate why the ClÁ Á ÁCl interaction in 1 is so short and particularly well-suited for an experimental verification of the halogen bond: in contrast to earlier work on interhalogen contacts, the halogen atoms in 1 differ clearly with respect to their role in the complex.…”