Abstract or Keywords
Boronic acids and their conjugate esters have been employed in a variety of biomedical applications. This work reports structural, bonding, and thermochemical calculations for the boronic acids, RB(OH)(2) (R = H, methyl, phenyl, and ortho-methyl-phenyl) and the corresponding ethylene glycol diesters, RB(-OCH2CH2O-), in the presence of explicit NH3 and/or H2O molecules. Calculations were performed in vacuo and in polarizable continuum model (PCM) aqueous media using density functional theory (DFT) and second-order Moller-Plesset perturbation theory (MP2) with the Dunning-Woon aug-cc-pVTZ basis set. These results quantify the relative stability of N -> B dative-bonded, water-inserted O-water -> B (O-w -> B) dative-bonded, Zwitterionic, and hydrogen-bonded conformers on the model RB(OH)(2).NH3.H2O and RB(-OCH2CH2O-).NH3.H2O potential energy surfaces (PESs). In the PCM aqueous media model, conformers containing an H3N -> B dative bond and a water molecule bridging the NH3 and acid/ester moieties were found consistently to be lower in energy than arrangements with either a H3NMIDLINE HORIZONTAL ELLIPSISH(H)O-w -> B linkage or Zwitterionic forms, [H4N](+)[HOw-BR(OH)(2)](-) or [H4N](+)[HOw-BR(-OCH2CH2O-)](-). For the acids, however, conformers with a H3NMIDLINE HORIZONTAL ELLIPSISHOwH moiety hydrogen bonded to the boronate hydroxyl group(s) proved to be lowest in energy.