Face-to-face stacking in sulfonamide based bis-ethylene bridged heteroaromatic dimers

Four sulfonamide based bis-ethylene bridged heteroaromatic dimers were crystallized in offset face-to-face stacked geometry. Further, density functional theory revealed that crystallized structures were the most stable conformers.

Folded conformations due to arene interactions in dissymmetric and symmetric butylidene-linker models based on pyrazolo[3,4-d]pyrimidine, purine and 7-deazapurine

The butylidene-linker models 1-[2-(2,6-dimethylsulfanyl-9H-purin-9-yl)-2-methylidenepropyl]-4,6-bis(methylsulfanyl)-1H-pyrazolo[3,4-d]pyrimidine, C18H20N8S4, (XI), 7,7'-(2-methylidenepropane-1,3-diyl)bis[3-methyl-2-methylsulfanyl-3H-pyrrolo[2,3-d]pyrimidin-4(7H)-one], C20H22N6O2S2, (XIV), and 7-[2-(4,6-dimethylsulfanyl-1H-pyrazolo[3,4-d]pyrimidin-1-yl)-2-methylidenepropyl]-3-methyl-2-methylsulfanyl-3H-pyrrolo[2,3-d]pyrimidin-4(7H)-one, C19H21N7OS3, (XV), show folded conformations in solution, as shown by (1)H NMR analysis. This folding carries over to the crystalline state. Intramolecular π-π interactions are observed in all three compounds, but only (XIV) shows additional intramolecular C-H···π interactions in the solid state. As far as can be established, this is the first report incorporating the pyrrolo[2,3-d]pyrimidine nucleus for such a study. In addition to the π-π interactions, the crystal structures are also stabilized by other weak intermolecular C-H···S/N/O and/or S···N/S interactions.

The CH/π hydrogen bond in chemistry. Conformation, supramolecules, optical resolution and interactions involving carbohydrates

The CH/π hydrogen bond is an attractive molecular force occurring between a soft acid and a soft base. Contribution from the dispersion energy is important in typical cases where aliphatic or aromatic CH groups are involved. Coulombic energy is of minor importance as compared to the other weak hydrogen bonds. The hydrogen bond nature of this force, however, has been confirmed by AIM analyses. The dual characteristic of the CH/π hydrogen bond is the basis for ubiquitous existence of this force in various fields of chemistry. A salient feature is that the CH/π hydrogen bond works cooperatively. Another significant point is that it works in nonpolar as well as polar, protic solvents such as water. The interaction energy depends on the nature of the molecular fragments, CH as well as π-groups: the stronger the proton donating ability of the CH group, the larger the stabilizing effect. This Perspective focuses on the consequence of this molecular force in the conformation of organic compounds and supramolecular chemistry. Implication of the CH/π hydrogen bond extends to the specificity of molecular recognition or selectivity in organic reactions, polymer science, surface phenomena and interactions involving proteins. Many problems, unsettled to date, will become clearer in the light of the CH/π paradigm.