A Multiscale Treatment of Angeli's Salt Decomposition

Sodium trioxodinitrate's (Na2N2O3, Angeli's salt) unique cardiovascular effects have been associated with its ability to yield HNO upon dissociation under physiological conditions. Due to its potential applications in new therapies for heart failure, the dissociation of Angeli's salt has recently received increased attention. The decomposition mechanism has been previously studied by quantum mechanical methods using a continuum approximation (PCM) for the solvent effects. In this work we use our recently developed interface of the Amber and Gaussian packages via the PUPIL package to study Angeli's salt dissociation in a hybrid QM/MM scheme where the water solvent molecules are treated explicitly with classical mechanics while the solute is treated with full quantum mechanics (UB3LYP/6-31+G(d) and UMP2/6-31+G(d)) level. Multiple steered molecular dynamics was used with the Jarzynski relationship to extract the free energy profile for the process. We obtain 4.8 kcal mol(-1) and 6.4 kcal mol(-1) free energy barriers for the N-N bond breaking for UB3LYP and UMP2, respectively. The geometries and Mulliken charges for reactant, transition state, and products have been characterized through a number of hybrid QM/MM molecular dynamics runs with the N-N distance restrained to representative values of each species. The results highlight the role of individual solvent molecules for the reaction energetics and provide a comparison point against implicit solvation methods.