Conformational equilibria of peroxynitrous acid in water: A first-principles molecular dynamics study

An aqueous solution of peroxynitrous acid has been studied using first-prin ciples molecular dynamics simulations based on density functional theory. T he relative Helmholtz energies of different conformers have been determined via thermodynamic integration with constraints. At contrast to the gas pha se, only two conformers, a cis and a trans isomer, are present in solution and their relative Helmholtz energy is enhanced with respect to the gas pha se. The average structural properties of the two conformational forms on th e other hand are very close to the respective gas phase values. The interco nversion pathway between the two conformers has: been determined, and the H elmholtz energy profile for the isomerization reaction in solution is prese nted. The rotational barrier is calculated to be substantially higher than in gas phase due to a strong rearrangement of the solvent during the reacti on. The structure of the transition state can only be described correctly w hen the solvent is taken explicitly into account. Our calculations indicate that the cis form is the dominant species in aqueous solution at ambient t emperatures.