A comprehensive theoretical study of the hydrogen bonding interactions and microscopic solvation structures of a pyridyl-urea-based hydrogelator in aqueous solution

Abstract

The solvent effects, intermolecular hydrogen-bonding interactions, and supramolecular structures of N,N′-bis(4-pyridyl) urea, a hydrogelator, in aqueous solution were studied through density functional theory (DFT) calculations and molecular dynamics (MD) simulations. DFT calculations show that N,N′-bis(4-pyridyl) urea is polarized from gas phase to highly polar aqueous solution. Correspondingly, the magnitude of the dipole moments of N,N′-bis(4-pyridyl) urea is enhanced as the polarity of the medium increases, which suggests that significant intermolecular interactions may exist in its aqueous solution. Both DFT calculations and MD simulations demonstrate that explicit O⋯HO, N⋯HO, and NH⋯O hydrogen-bonding interactions coexist in aqueous solution and are evenly distributed around N,N′-bis(4-pyridyl) urea, which explains why N,N′-bis(4-pyridyl) urea is a hydrogelator. The hydrogen-bond strength trend is NH⋯O>N⋯HO>O⋯HO for N,N′-bis(4-pyridyl) urea⋯H2O dimers. The natural bond orbital analysis shows that the intermolecular nY→σXH∗ orbital interaction dominates the XH⋯Y hydrogen-bonding interactions in N,N′-bis(4-pyridyl) urea⋯H2O dimers. In addition, the hydrogen-bonded supramolecular clusters (solute+solvent molecules) are also obtained from MD simulations and DFT optimizations. The hydrogen-bond lengths and angles from DFT optimizations are in good agreement with the values from the experimentally obtained crystal structures of N,N′-bis(4-pyridyl) urea with ethylene glycol and water molecules, whereas MD simulations displayed the wider structural variety characteristic of more realistic solutions.