A Charge-Charge Flux-Dipole Flux Analysis of Simple Molecular Systems with Halogen Bonds

The presence of halogen bonds (R-X···B; R = substituent group, X = halogen, and B = Lewis base) provides quite amazing molecular systems for electronic structure investigations, presenting unique characteristics of fundamental relevance to supramolecular chemistry among other areas. Here, we use a double-hybrid approach from Density Functional Theory and triple-ζ basis sets augmented with diffuse functions (B2PLYP/def2-TZVPD) to deal with a large group of simple molecular systems containing halogen bonds (XBs), focusing on geometrical structures, binding energies, harmonic vibrational frequencies, and fundamental infrared intensities. Next, the electron densities and their variations on vibrations are carefully studied with the Quantum Theory of Atoms in Molecules (QTAIM) formalism and the charge-charge flux-dipole flux (CCFDF) model. We notice that the R-X stretching mode usually shows vibrational frequency decrements and infrared intensifications during the XB formation. Such features were also observed in hydrogen bonds, although the explanation for the band strengthening is different. Surprisingly, the most important contribution to these intensity increments due to complexation is now the interaction term between the charge flux and dipole flux (CF × DF). Thus, the use of atomic dipoles is mandatory to fully understand this phenomenon. In fact, the huge charge flux contributions to changes in dipole moment derivatives of R-X stretchings on halogen bonding are no longer accompanied by opposite variations of similar magnitudes in polarizations described by atomic dipole fluxes, which provided nearly unaltered values during the XB formation. Thus, the electronic charge flux direction change that takes place in complexes (from B to R) now reinforces dipole moment derivative terms from such atomic polarizations (mainly from the X atom). This intermolecular charge flux seems to be responsible for the unusual features noticed in the R-X stretching mode with the CCDDF/QTAIM model.