First-Principles Crystal Engineering of Nonlinear Optical Materials. II. Effect of Halogen Bonds on the Structure and Properties of Triiodobenzenes

Recently, we proposed a computational design strategy for organic nonlinear optical materials, based on the global minimization of lattice energy to predict the crystal packing from the first principles. Here, we validate this strategy on triiodobenzenes, which include CH center dot center dot center dot I hydrogen and halogen bonding as the structure-determining components of their intermolecular interactions. To refine the van der Waals (vdW) parameters for an I atom, the ab initio potential surfaces for the model dimers were calculated at the CCSD(T)/cc-pVTZ + CP theory level. The hydrogen bond C-H center dot center dot center dot I was found to have an interaction energy of -0.5 kcal/mol. The I center dot center dot center dot I contact of type I (140 degrees-140 degrees) was found to be attractive with a well depth of -0.4 kcal/mol at a 4.6 angstrom distance, whereas type II contact (180 degrees-90 degrees) was found to be nearly twice more attractive. Its potential well depth reaches -0.7 kcal/mol at an I center dot center dot center dot I distance of 4.4 angstrom. These binding energies are therefore weaker than that of the typical hydrogen bonds. The AMOEBA force-field vdW parameters were fit to describe these interactions and used to predict the crystal structures. Our structure prediction, followed by density functional theory-many- body dispersion ranking established the noncentrosymmetric crystal packing to be the global minimum, in agreement with the experimental data. The coupled perturbed Kohn-Sham approach was used to estimate nonlinear susceptibility, and the predicted values were compared to that of the urea standard. The statistical analysis of the angular distribution for the I center dot center dot center dot I contacts in the predicted virtual polymorphs was compared to that found among the experimental crystal structures of iodoaromatic compounds. In both cases, symmetric (type I) contacts dominate for shorter and longer I center dot center dot center dot I distances, whereas L-shaped (type II) contacts are preferred for intermediate distances.