Computational chemistry and molecular simulations of phosphoric acid

Quantum mechanics (QM) calculations, molecular dynamics (MD) simulations using the condensed‐phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field, and the atom‐centered density matrix propagation (ADMP) approach have been used to investigate properties of phosphoric acid (PA). QM using B3LYP/6‐31++G(d,p) density functional theory were used to calculate gas‐phase proton affinities and interaction energies of PA and its derivatives. Detailed single coordinate driving, followed by quadratic synchronous transit optimization was used to determine energy barriers for different proton transfer (PT) pathways. Determined energy barrier heights in ascending order are (unit: kJ/mol): H₃O⁺→H₃PO₄ (0); H₄P₂O₇→H₃PO₄ (2.61); H₃PO₄→H₂PO (5.31); H₄PO→H₃PO₄ (～7.33); H₃PO₄→H₄P₂O₇/H₃PO₄→H₃PO₄ (15.99); H₄P₂O₇→H₂O (28.61); H₃PO₄→H₂O (47.14). The COMPASS force field was used to study condensed‐phase properties of PA. Good agreement between experimental data and MD results including density, radial distribution functions, and self‐diffusion coefficient at different temperatures provides validation of the COMPASS force field for PA. Finally, preliminary ADMP studies on a cluster of three PA molecules shows that the ADMP approach can reasonably describe the PT and self‐dissociation processes in PA. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011