A theoretical survey on strength and characteristics of F•••F, Br•••O and Br•••Br interactions in solid phase

A quantum chemical investigation was carried out to study the properties of intermolecular F•••F, Br•••Br and Br•••O interactions in crystalline 1-bromo-2,3,5,6-tetrafluoro-nitrobenzene (BFNB). This system was selected to mimic the halogen-halogen as well as halogen bonding interactions found within crystal structures as well as within biological systems. We found that fluorine atoms have weak positive electrostatic potentials (VS,max≈+1 kcal/mol) and these could be responsible for their weak electrophilic behavior. According to quantum theory of atoms in molecules, the values of electron density at the F•••F critical points are calculated to be in a range of 0.004−0.006 au, whereas the values of ∇2ρBCP are all positive, ranging from 0.023−0.031 au. This indicates that all F•••F interactions in crystalline BFNB, are weak and basically electrostatic in nature. The nature of intermolecular interactions is analyzed using energy decomposition analysis (EDA). Our results indicate that, for those nuclei participating in intermolecular interactions, nuclear magnetic resonance parameters exhibit considerable changes on going from the isolated gas phase molecule model to crystalline BFNB. Of course, the magnitude of these changes at each nucleus depends directly on its amount of contribution to the interactions.