Modeling nitric oxide and its dimer: force field development and thermodynamics of dimerization

Nitric oxide, NO, is a free radical that forms dimers, (NO)₂, at its vapor-liquid coexisting temperatures. In this work, we developed an all-atom force field for NO and (NO)₂. To assess the performance of this force field, we computed the vapor-liquid equilibrium (VLE) properties of the reactive NO-(NO)₂ system, as well as those of pure NO and pure (NO)₂, using Continuous Fractional Component Monte Carlo (CFCMC) simulations. We then compared the results with the available experimental data and predictions from two previously developed force fields. For the reactive NO-(NO)₂ system, we performed CFCMC simulations in the reactive Gibbs ensemble in which the formation of NO dimers, 2NO ⇌ (NO)₂, is considered. The predicted coexistence vapor-liquid densities, dimer mole fractions in the liquid phase, saturated vapor pressures, and heats of vaporization using our force field in the temperature range 120 K to 170 K are in excellent agreement with experimental values. In addition, we conducted a systematic parameter study to analyze the sensitivity of the new force field parameters and the isolated molecule partition functions of (NO)₂ on the VLE properties of the reactive NO-(NO)₂ system. The results indicate that the VLE properties of the reactive NO-(NO)₂ system are affected by both the force field parameters of the involved species as well as the isolated molecule partition functions of (NO)₂.