NMR shieldings as descriptors for halogen bonds in halogen-containing dimers

Looking for another and maybe optimum way to describe the main characteristics of a given halogen-bond (XB), we studied a family of dimers whose monomers are bonded by halogen bonds (XBs): F₁X⋯F₃X (X = F, Cl, Br, I, At). We report some of their main geometric and electronic characteristics calculated at the four-component DFT level of theory and calculations of NMR shieldings performed at the non-relativistic CCSD and SOPPA(CCSD) levels of approximations. Their geometrical parameters, together with their electronic potentials, suggest that the XBs involved are not properly described by their previous classes of XB found in the literature. In addition, we studied the electronic origin of the NMR magnetic shieldings, σ, of the two fluorine atoms that are fixed in the whole series. The dependence of σ(F) on both the strength of the covalent bond of one of the monomers and the strength of the XB in the dimers shows that these spectroscopic parameters are good candidates for being used as descriptors of XB bonding. Relativistic effects are also analyzed by applying the linear response within the elimination of the small component (LRESC) model. The main electronic mechanism that is behind the trend of σ(F₃) is the spin-orbit, SO, and within it the Fermi-contact contribution is not the largest. In fact, we found that the SO-SD (spin-dipolar contribution to the SO mechanism) is the one that defines the behavior of σ(F₃); it is also found that the XB enhances the SO-SD contribution. These findings demonstrate that the NMR magnetic shielding of atoms that are being part of XBs could be used as one of the new descriptors that characterize the XBs.