A look at bonds and bonding

Structural Chemistry - Even after roughly a century of quantum theory, there is still debate, sometimes rather contentious, as to the nature of the chemical bond—or is it bonds, or is it...     

Halogen Bonding: An Interim Discussion

Halogen bonding is a noncovalent interaction that is receiving rapidly increasing attention because of its significance in biological systems and its importance in the design of new materials in a variety of areas, for example, electronics, nonlinear optical activity, and pharmaceuticals. The interactions can be understood in terms of electrostatics/polarization and dispersion; they involve a region of positive electrostatic potential on a covalently bonded halogen and a negative site, such as the lone pair of a Lewis base. The positive potential, labeled a σ hole, is on the extension of the covalent bond to the halogen, which accounts for the characteristic near‐linearity of halogen bonding. In many instances, the lateral sides of the halogen have negative electrostatic potentials, allowing it to also interact favorably with positive sites. In this discussion, after looking at some of the experimental observations of halogen bonding, we address the origins of σ holes, the factors that govern the magnitudes of their electrostatic potentials, and the properties of the resulting complexes with negative sites. The relationship of halogen and hydrogen bonding is examined. We also point out that σ‐hole interactions are not limited to halogens, but can also involve covalently bonded atoms of Groups IV–VI. Examples of applications in biological/medicinal chemistry and in crystal engineering are mentioned, taking note that halogen bonding can be “tuned” to fit various requirements, that is, strength of interaction, steric factors, and so forth.     

Real-Time Monitoring of the Mechanical Properties of a Soy Protein and Rubber Polymer during its Production Using Transient Infrared Spectroscopy

Soy protein–based polymers offer promising performance properties, but their characteristics are sensitively dependent on production conditions, so on-line monitoring could help provide the needed control during production. Mid-infrared spectroscopy combined with partial least squares offer the needed analysis, but the opacity of many materials in the mid-infrared range limits its conventional application. Transient infrared spectroscopy is a method of acquiring mid-infrared spectra from moving streams in real time that avoids the opacity problem. We apply transient infrared spectroscopy to a polymer of soy protein and polyisoprene-graft-maleic anhydride–modified natural rubber during its compounding extrusion to measure tensile strength and Young's modulus.     

Computational analysis of relative stabilities of polyazine N-oxides

There is considerable interest in polyazine N-oxides as potential frameworks for energetic compounds with relatively high enthalpies of formation and crystal densities. The N⁺→O^? linkages, if appropriately located, may diminish the destabilization associated with nitrogen catenation. We have computationally characterized 40 N-oxides of the isomeric diazines, triazines, and tetrazines in terms of their geometries, relative energies, and (for a representative selection) electrostatic potentials. The presence of N⁺→O^? linkages does partially counteract the destabilizing effects of nitrogen catenation, although the isomers with complete catenation remain the least stable. The stabilizing influence of N⁺→O^? groups, and the accompanying changes in bond lengths, can be understood in terms of resonance charge delocalization to the polyazine rings. The N(O)–N(O) bonds between nitrogens that both bear oxygens tend to be relatively weak. The electrostatic potentials above the polyazine rings become increasingly positive as there are more nitrogens and oxygens; eventually they are positive above all of the carbons and nitrogens and possibly even the oxygens, with negative regions only on the peripheries of the molecules. However, the nitrogens that bear oxygens always have more positive potentials than those that do not.