Complexes of HArF and AuX (X = F, Cl, Br, I). Comparison of H-bonds, halogen bonds, F-shared bonds and covalent bonds

The various sorts of complexes in which HArF and AuX (X = F, Cl, Br, I) can engage are probed by MP2/aug-cc-pVTZ calculations. The most weakly bound are those containing a halogen bond (XB) of the AuXMIDLINE HORIZONTAL ELLIPSISFArH sort, with binding energies less than 8 kcal/mol. H-bonded dimers FArHMIDLINE HORIZONTAL ELLIPSISXAu are a little stronger, held together by some 12 kcal/mol. Being the most strongly bound places the F atom of HArF roughly midway between Ar and Au in an F-shaped structure, bound by some 43-54 kcal/mol. The last sort of product involves atomic rearrangements wherein the H atom migrates from Ar to Au, followed by formation of a covalent Ar-Au bond. The resulting molecular unit is stabilized by 30-40 kcal/mol relative to the original HArF and AuX reactants. The H-bonded dimers are held together by an unusually large polarization component, surpassing electrostatic attraction, while dispersion predominates for the halogen bonds. Perturbations of the geometries and stretching frequencies offer a ready means of distinguishing the different types of complexes by spectroscopic techniques.