Fluorine-containing molecules have been particularly sought after in recent years due to their relevance in the biological and materials area. [1] In this context, Nuclear Magnetic Resonance (NMR) has stood out as a crucial tool for the characterization of structures and understanding the non-covalent interactions that govern molecular properties.
Therefore, the present work aims to obtain the relationship between the NMR parameters (in particular the scalar couplings), the sign, and the magnitude of interactions involving fluorine atoms to predict the behavior of these molecules simply.
The molecules chosen for this study were a set of benzenes derivatives, as shown in Fig. 1.
Although the intramolecular interactions involving the fluorine atom were successfully characterized by NCI e NBO analysis, they do not seem relevant to the conformational equilibria, and the conformers where the interaction F---H-X could be established were the least populated. These smaller energetic interactions (attractive or repulsive), on the other hand, are capable of remarkably changing the J values when compared to the conformers where the interaction between F and H-X is spatially impossible.
In general, the structures where steric interaction is more energetic than delocalization have negatives values of nJFH. The notable exception occurs when it is possible to characterize a proper hydrogen bond, where hyperconjugation is the main effect, and the coupling is strongly negative.
A generic correlation between the nature of the existing interaction and the sign of the expected coupling cannot yet be reached. However, in some cases, such as comparing compounds 5 with 3, it is possible to infer that the delocalization interactions lead to a positive increase in coupling.