New Theoretical Insight into Fluorination and Fluorine-Fluorine Interactions as a Driving Force in Crystal Structures

Fluorination of compounds causes an increase in the proton-donating ability and a decrease in the proton-accepting capacity of groups in their vicinity. The formation of F center dot center dot center dot F interactions is followed by the shift of the electron density in the area of F center dot center dot center dot F contact, which creates a new region with a larger surface area, a higher negative potential, and, hence, a more pronounced accepting ability. The new region also has a greater ability to form multiple (simultaneous) interactions with species from the environment, thus compensating for the reduction of the accepting capacity of the groups in the vicinity. This phenomenon explains not only the abundance of F center dot center dot center dot F interactions in crystal structures, but also a large number of structures with F center dot center dot center dot O interactions. Only C-H center dot center dot center dot F interactions are more numerous than F center dot center dot center dot F interactions in crystal structures, which indicates a high affinity of fluorinated compounds for nonpolar groups.