Sigma-hole

The Lewis dot formalism shows any halogen in a molecule surrounded by three electron lone pairs. An unfortunate consequence of this perspective is that it is natural to assume that these electrons are equivalent and symmetrically distributed (i.e., that the iodine is sp3 hybridized). Even simple quantum mechanical calculations, however, show that this is not the case [148]. Consider the diiodine molecule in the gas phase (Fig. 3). There is a region directly opposite the I-I sigma bond where the nucleus is poorly shielded by the atoms electron cloud. Allen described this as polar flattening , where the effective atomic radius is shorter at this point than it is perpendicular to the I-I bond [149]. Politzer and coworkers simply call it a sigma hole [150,151]. This area of positive electrostatic potential also coincides with the LUMO of the molecule (Fig. 4). 

The sigma hole is the key to understanding halogen bonding. Based upon the crystallographic and spectral data presented in the literature and the computational studies outlined below, the following generalizations may be made ... 

Politzer P, Murray JS, Clark T (2013) Halogen bonding and other sigma-hole interactions a perspective. Phys Chem Chem Phys 15(27) 11178-11189. doi 10.1039/c3cp00054k... 

Li QZ, Guo X, Yang X, Li WZ, Cheng JB, Li HB (2014) A sigma-hole interaction with radical species as electron donors does single-electron tetrel bonding exist Phys Chem Chem Phys 16(23) 11617-11625. doi 10.1039/c4cp01209g... 

The multipolar components coming from ab initio calculations were obtained using Stone s analysis and the explicit inclusion of them provides the anisotropic property to the polarizable model required for a good description of a sigma-hole driven interaction between halobenzenes and water and Mg(ll) [27]. 

El Hage K, Piquemal JP, Hobaika Z, Maroun RG, Gresh N (2013) Could an anisotropic molecular mechanics/d5mamics potential account for sigma hole effects in the complexes of halogenated compounds J Comput Chem 34(13) 1125-1135. doi 10.1002/jcc.23242... 

O Hair RAJ, Williams CM, Clark T (2010) Neighboring group stabilization by sigma-holes. J Mol Model 16 559-565... 

Fig. 7.8 Top Energy surfaces of electron deficient halides showing the sigma hole. Bottom Modeled halogen bonding between y-picoline and perfluoropropyl iodide in the capsule...

Sapanification. S xxiifications were carried out in a Readco steam-jaoiketed sigma double-arm mixer with 1 quart working Cc city (Teledyne Raadoo, York, PA). There were holes in the lid to accommodate a reflux condenser and to permit addition of reagents. 

Grozema FC, Siebbeles LDA, Warman JM, Seki S, Tagawa S, Scherf U. (2002) Hole conduction along molecular wires Sigma-bonded silicon versus pi-bond-conjugated carbon. Adv Mater 14 228-231. 

The spatial distributions of thermal and epithermal neutron flux in the 2.94-kg pile, the 3.95-kg pile, and in the various experimental holes of the mock-up assembly, as indicated in the listing below, were measured. These measurements were made with indium foils, used alternately bare and cadmium covered, as described previously. A calibration. of foils in the standard (sigma)reactor indicated that the absolute flux (nv) is obtained from the measured saturated activities, shown in the attached figures by the following relations, ... 

In a recent contribution by Girones et al., MQSSM were calculated from domain-averaged Fermi holes. The correlation between Hammett sigma constants and these MQSSM values for a set of meta- and para-substituted benzene molecules was investigated, and a good linear correlation between both entities was found. The predictive power of the relationship was examined for a test set of molecules, and it was found that the predictive capacity is competitive with the approach described by Sullivan et al., but having the advantage that only one parameter is used instead of five as in the latter study. 

---