Polar flattening effect

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). 

Nyburg SC (1979) Polar flattening non-spherical effective shapes of atoms in crystals. Acta CrystA35 641-645... 

Application of an electric field normal to the plates (typically the plates are coated with thin films of conducting glass such as indium-tin oxide) unwinds the helix if there is one, and also may cause the polar axis to orient normal to the plates (along the field), or even flatten the chevrons. It should be stressed that any added orientation of molecular dipoles along the field direction should be a weak secondary effect — the polar order occurring in the FLC phase is a thermodynamic property of the phase and not dependent upon applied fields. 

---