Models for boronic acid receptors: a computational structural, bonding, and thermochemical investigation of the HB(OH)2∙H2O∙NH3 and HB(-O-CH2-CH2-O-)∙NH3∙H2O potential energy surfaces

Results of structural and thermochemical calculations involving boronic acid, HB(OH)2, and the corresponding ethylene glycol ester, HB(-O-CH2-CH2-O-), in the presence of explicit NH3 and/or H2O molecules are reported. Calculations were performed in a polarizable continuum model (PCM) water solution and in the gas phase using density functional theory (DFT) and second-order Moller-Plesset perturbation theory (MP2) with the Dunning-Woon aug-cc-pVTZ basis set. Different classes of local minima on the HB(OH)2·NH3·H2O and HB(-O-CH2-CH2-O-)·NH3·H2O potential energy surfaces (PESs) in PCM water solution have been identified: (1) structures with a N→B dative bond, [H3N→BH(OH)2]·H2O, and [H3N→B(H)(-O-CH2-CH2-O-)]·H2O, where the H2O is involved in hydrogen bonding; (2) water-inserted structures involving either a novel O→B dative bond, H3N·H(H)O→BH(OH)2, and H3N···H(H)O→B(H)(-O-CH2-CH2-O-) where the H2O molecule remains essentially intact or lower-energy zwitterionic arrangements in which a water H atom has been transferred to the ammonia, [H4N]+[HO-BH(OH)2]−, and [H4N]+[BH(OH)(-OCH2-CH2-O-)]−; (3) structures where both the NH3 and H2O molecules are exclusively involved in hydrogen bonding. In these simple model systems, arrangements with N→B dative bonds, and some structures with only O···H and N···H hydrogen bonds, are ca. 5–6 kcal/mol lower in energy than either of the corresponding water-inserted structures.