The structure of hydrated complexes of o-hydroxybenzoic acid in water-modified supercritical carbon dioxide: The Car-Parrinello molecular dynamics simulation

The study of the o-hydroxybenzoic acid (o-HBA)–water molecular structures formed in supercritical carbon dioxide (T = 318 K, ρ = 0.7 g/cm3) by Car-Parrinello molecular dynamics method has been performed. Atom–atom radial distribution functions, coordination numbers, average hydrogen bond (HB) numbers, and vibrational densities of states have been calculated from molecular trajectories saved during simulation procedure. It has been shown that, despite of the high co-solvent polarity, the hydroxyl group of the o-HBA preferably forms an intramolecular HB, whereas o-HBA–water hydrogen bonding involves only acid carboxyl group. The formed o-HBA–(H2O)2 complex and remaining water molecules compose a labile hydrogen-bonded cluster. The average HB number per water molecule is equal to 1.93, 26.4% out of the total amount of HBs formed by water molecules being water–o-HBA HBs, and the rest being water–water HBs. Evolution of hydrogen-bonded clusters has been analyzed, using instantaneous structures saved during the simulation. It was shown that water–water HBs are less stable than water–o-HBA ones.