Study of Halogen-Mediated Weak Interactions in a Series of Halogen-Substituted Azobenzenes

The azobenzenes, known for their
various importance in the industry,
have been chosen as model compounds to understand the role of weak
interactions involving the C–X (X = F, Cl, and Br) bond using
single-crystal X-ray diffraction technique, especially in the absence
of other stronger intermolecular forces such as hydrogen bonds. The
fluorinated compounds have been found to pack in the lattice by utilizing
C–H···F hydrogen bonds, whereas the chlorinated
and brominated analogues have been found to prefer C–X···X–C,
C–X···π, and π···π
interactions while packing in the lattice. The stabilization energy
offered by the C–H···F hydrogen bonds and the
C–X···X–C interactions have been estimated
by computational methods using Gaussian 09, and the topological properties
have been determined by using the AIM2000 package. The lattice energy
decomposition has been done using semiclassical density sums (SCDS)
PIXEL method. Our studies indicate that the stabilization energy offered
by each C–H···F hydrogen bond lies in the range
from −0.8 to −1.0 kcal/mol, while that for the C–X···X–C
interaction has been found to be −0.35 kcal/mol for the X =
Cl interaction and −0.73 kcal/mol for the X = Br interaction.
Further, the analysis of these interaction by atoms in molecules (AIM)
theory indicates that the electron densities (ρc)
at the bond critical points (BCP) for C–H···F
and C–X···X–C (X = Cl and Br), calculated
using the AIM2000 package, are small (<0.007 eÅ–3), and the values of Laplacian (∇2ρc) are positive. This indicates that these interactions are of the
hydrogen bond type. A detailed study of these interactions by experimental
and computational methods has been described in the manuscript.