Thermodynamic Measures of Binding Strength in Pnicogen, Chalcogen, Halogen, and Hydrogen Bonds (pp. 281-291)
Authors: Jared B. Moss, Upendra, Adhikari, and Steve Scheiner
Abstract: Thermodynamic quantities ∆Eelec, ∆H, and ∆G are computed by various different levels of ab initio quantum theory. The values obtained are searched for any reversals in order of stability between different modes of binding. Complexes contain NH3 as common electron donor, paired with H2XP with various substituents X, and with HSX. The best reproduction of CCSD(T)/aug-cc-pVQZ energetics arises from MP2/aug-cc-pVTZ. Due to cancellation between vibrational and other factors, values of ∆Eelec are quite similar to ∆H. Although ∆G is positive, opposite in sign to ∆E and ∆H, the trends remain essentially uniform amongst all three quantities. In cases where binding energies are very similar, there are occasions where the order of stability calculated for ∆Eelec can be reversed in the context of ∆H or ∆G. For the HSX∙∙∙NH3 series, the H-bonded structure is preferred for X = Cl or Br, while the F substituent favors the chalcogen bond. The halogen bond is similar in energy to the chalcogen bond for X = Br, less stable for X = Cl, and does not correspond to a minimum for X = F.