Thermodynamic, structural, and dynamic study of the N-H•••O=C hydrogen bond association in aqueous solution

A molecular dynamics simulation of an infinitely dilute aqueous solution of the formamide-formaldehyde complex was carried out using an ab initio 12-6-1 potential to describe the solute-solvent interaction. The solute-solvent interaction energy is used to evaluate the enthalpies and free energies associated with the solvation process. The conclusion is that the surrounding aqueous medium hinders the formation of the complex compared with the same process in gas phase, with a value of Delta G(as) = -3.59 kJ mol(-1) and K-as = 3.8 x 10(-2) M-1 being found for the association energy and equilibrium constant, respectively. The potential of the mean force created with the two-solute approximation presents the double minimum, which is typical of these molecular associations that contain the contact solutes and solvent-separated solutes configurations. The structure of water surrounding the complex shows a minor coordination of solvent molecules when a hydrogen bond is formed, and a slight increase of the R-H ... O inter-molecular distance is produced by the aqueous medium. Finally, the dynamics of the complex in solution was studied via the mean square displacement and time correlation functions, which allow us to deduce the diffusion coefficient, reorientation time, and frequencies associated with translational and rotational movements of the bonding hydrogen. (C) 2000 Elsevier Science B.V. All rights reserved.