Halogen Bonding with Dihalogens and Interhalogens - Halogen Bonding: Fundamentals and Applications - page 89

3
Computation Studies
3.1
General Observations
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 symmetri-
cally distributed (i.e., that the iodine is sp 3 hybridized). Even simple quantum
mechanical calculations, however, show that this is not the case [148]. Con-
sider 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 ef-
fective 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).
Fig. 3 The electron density map (0.002 au contour) for diiodine, colored by the electro-
static potential. Red is the most negative potential and blue is the most positive
Fig. 4 The lowest unoccupied molecular orbital for diiodine
The sigma hole is the key to understanding halogen bonding. Based upon
the crystallographic and spectral data presented in the literature and the com-
putational studies outlined below, the following generalizations may be made: