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Politzer
Halogen bonding and the design of new materials: organic bromides, chlorides and perhaps even fluorides as donors.Politzer P, Murray JS, Concha MC. J Mol Model. 2007 Mar 15; [Epub ahead of print] [abstract only]
"In some halides RX, the halogen X has a region of positive electrostatic potential on its outermost portion, centered around the extension of the R-X bond. The electrostatic attraction between this positive region and a lone pair of a Lewis base is termed halogen bonding. The existence and magnitudes of such positive potentials on some covalently bonded halogens, and the characteristic directionality of the interaction, can be explained in terms of the degree of sp hybridization and polarizability of X and the electronegativity of R. Halogen bonding increases in strength in the order Cl < Br < I; fluorine is frequently said to not form halogen bonds, although a notable result of the present study is computational evidence that it does have the capability of doing so, if R is sufficiently electron withdrawing. An increasingly important application of halogen bonding is in the design of new materials (e.g., crystal engineering). In this paper, we present the calculated energies of a series of halogen-bonding interactions that could be the basis for forming linear chains, of types X----X----X---- or X----Y----X----Y----. We focus upon chlorides and bromides, and nitrogen bases. The B3PW91/6-311G(3df,2p) and MP2/6-311++G(3df,2p) procedures were used. We show how the computed electrostatic potentials (B3PW91/6-31G**) can provide guidance in selecting appropriate halide/base pairs. Figure Computed electrostatic potential of CH(3)CH(2)Br on the molecular surface defined by the 0.001-au contour of the electronic density. The bromine is facing the reader, and has a small positive (green) region centered around the intersection of the C-Br axis with the surface."
An overview of halogen bonding.Politzer P, Lane P, Concha MC, Ma Y, Murray JS. J Mol Model. 2007 Feb;13(2):305-11. Epub 2006 Sep 30. [abstract only]
"Halogen bonding (XB) is a type of noncovalent interaction between a halogen atom X in one molecule and a negative site in another. X can be chlorine, bromine or iodine. The strength of the interaction increases in the order Cl<Br<I. After a brief review of experimental evidence relating to halogen bonding, we present an explanation for its occurrence in terms of a region of positive electrostatic potential that is present on the outermost portions of some covalently-bonded halogen atoms. The existence and magnitude of this positive region, which we call the sigma-hole, depends upon the relative electron-attracting powers of X and the remainder of its molecule, as well as the degree of sp hybridization of the s unshared electrons of X. The high electronegativity of fluorine and its tendency to undergo significant sp hybridization account for its failure to halogen bond. Some computed XB interaction energies are presented and discussed. Mention is also made of the importance of halogen bonding in biological systems and processes, and in crystal engineering."
Halogen bonding: the sigma-hole: proceedings of "Modeling interactions in biomolecules II", Prague, September 5th-9th, 2005.Clark T, Hennemann M, Murray JS, Politzer P. J Mol Model. 2007 Feb;13(2):291-6. Epub 2006 Aug 23. [abstract only]
"Halogen bonding refers to the non-covalent interactions of halogen atoms X in some molecules, RX, with negative sites on others. It can be explained by the presence of a region of positive electrostatic potential, the sigma-hole, on the outermost portion of the halogen's surface, centered on the R-X axis. We have carried out a natural bond order B3LYP analysis of the molecules CF(3)X, with X = F, Cl, Br and I. It shows that the Cl, Br and I atoms in these molecules closely approximate the [Formula: see text] configuration, where the z-axis is along the R-X bond. The three unshared pairs of electrons produce a belt of negative electrostatic potential around the central part of X, leaving the outermost region positive, the sigma-hole. This is not found in the case of fluorine, for which the combination of its high electronegativity plus significant sp-hybridization causes an influx of electronic charge that neutralizes the sigma-hole. These factors become progressively less important in proceeding to Cl, Br and I, and their effects are also counteracted by the presence of electron-withdrawing substituents in the remainder of the molecule. Thus a sigma-hole is observed for the Cl in CF(3)Cl, but not in CH(3)Cl."
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