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.