Intramolecular H···H Interactions for the Crystal Structures of [4-((E)-But-1-enyl)-2,6-dimethoxyphenyl]pyridine-3-carboxylate and [4-((E)-Pent-1-enyl)-2,6-dimethoxyphenyl]pyridine-3-carboxylate; DFT Calculations on Modeled Styrene Derivatives

Experimental and Theoretical Electron Density Study of a Highly Twisted Polycyclic Aromatic Hydrocarbon: 4-Methyl-[4]helicene

Helicenes are molecules of considerable interest in view of their aromaticity which persists despite a marked departure from planarity and because of the extreme potency of some of their metabolites as tumor and mutation promoters. In this study, the electron density of 4-methyl-[4]helicene (or 4-methylbenzo[c]phenanthrene) is studied topologically with an emphasis on the fjord region since this region is where metabolic activation is initiated. The molecule consists of four fused aromatic rings that assume a twisted geometry. This geometry brings two hydrogen atoms into close proximity in the fjord region of the molecule accompanied by the appearance of an intramolecular C-Hdelta+...delta+H-C bond path (an interaction termed hydrogen-hydrogen or H- H bonding to distinguish it from dihydrogen bonding from which it is qualitatively distinct). In addition to the intramolecular H-H interaction, a number of intermolecular interactions are shown to be involved in the packing of this molecule in the crystalline state. The effect of the nonplanarity of the molecule on the local aromaticity of each ring is also discussed.

How Short Can the H···H Intermolecular Contact Be? New Findings that Reveal the Covalent Nature of Extremely Strong Interactions

Ab initio calculations at the MP2/6-311++G(d,p) and MP2/aug-cc-pVDZ//MP2/aug-cc-pVTZ levels have been performed for the following complexes: H2OH+...HBeH, H2OH+...HBeBeH, H2OH+...HBeF, HClOH+...HBeH, Cl2OH+...HBeH, and Cl2OH+...HBeF. For all dimers considered, extremely short H...H intermolecular contacts (1.0-1.3 A) were obtained. These are the shortest intermolecular distances which have ever been reported, with binding energies within the range of 13.7-24.3 kcal/mol (MP2/aug-cc-pVDZ//MP2/aug-cc-pVTZ level). The interaction energies of the complexes analyzed were also extrapolated to the complete basis set (CBS) limit. To explain the nature of such strong interactions, the Bader theory was applied, and the characteristics of the bond critical points (BCPs) were analyzed. It was pointed out that for the major part of the H...H contacts considered here the Laplacian of the electron density at H...H BCP is negative indicating the partly covalent nature of such a connection. The term "covalent character of the hydrogen bond" used sometimes in recent studies is discussed. An analysis of the interaction energy components for dihydrogen bonded systems considered indicates that in contrast to conventional hydrogen bonded systems the attractive electrostatic term is outweighed by the repulsive exchange energy term and that the higher order delocalization energy term is the most important attractive term.

Characterization of a Closed-Shell Fluorine−Fluorine Bonding Interaction in Aromatic Compounds on the Basis of the Electron Density

A bond path linking two saturated fluorine atoms is found to be ubiquitous in crowded difluorinated aromatic compounds. The bond path is shown to persist for a range of internuclear distances (2.3-2.8 A) and a range of relative orientations of the two C-F internuclear axes. The F. . .F bonding is shown to exhibit all the hallmarks of a closed-shell weak interaction. The presence of such a bond path can impart as much as 14 kcal/mol of local stabilization to the molecule in which it exists, a stabilization that can be offset or even overwhelmed by destabilization of other regions in the molecule. Several other weak closed-shell interactions were also found and characterized including F. . .C, F. . .O, and C. . .C interactions, hydrogen bonding, dihydrogen bonding, and hydrogen-hydrogen bonding. This study represents another example of the usefulness and richness of the bond path concept and of the theory of atoms in molecules in general.