Intermolecular potential parameters for transport property modeling of energetic organic molecules

Abstract

Transport properties such as coefficients of diffusion, viscosities and thermal conductivities of chemical species are required for combustion modeling of energetic materials. These can be obtained from intermolecular potential energy surfaces. Hence, we present a theoretical study of intermolecular potential energy surfaces for the interactions of various energetic organic molecules with four bath gases – He, Ne, Ar and N2. High-level ab initio calculations, i.e., counterpoise corrected QCISD(T) with complete basis set (CBS) extrapolations based on aug-cc-pVDZ and aug-cc-pVTZ basis sets, are used to evaluate the accuracy of various computationally feasible yet accurate low-level quantum chemistry methods. Intermolecular potential energy surfaces are then calculated for a training set of six molecules including two nitramines (CH2NNO2 and CH3NHNO2), two nitrate esters (CH3ONO2 and CH3CH2ONO2) and two nitroalkanes (CH3NO2 and CH3CH2NO2). The ab initio data are used to parametrize an analytical pairwise intermolecular potential. The potential is then used to calculate Lennard-Jones parameters for the training set, as well as a separate test set containing nine energetic organic molecules. Lennard-Jones collision rates for molecules in test set, calculated using analytical pairwise potential, are within ∼20% of those obtained via the ab initio calculations. A novel strategy is proposed to calculate pure-gas Lennard-Jones parameters from various combining rules and four bath gases. Gas-phase transport properties of nitromethane – viscosity, thermal conductivity and diffusion coefficient – calculated using pure-gas Lennard-Jones parameters obtained from the proposed method, are in good agreement with the experimental results at various temperatures. The validated analytical potentials and the strategy to calculate pure-gas Lennard-Jones parameters are then applied to develop a transport properties database for nitramines, which is subsequently used in combustion modeling of RDX. Comparative sensitivity analysis indicates that the sensitivity of burn-rate to transport parameters is as significant as the sensitivity to reaction rate parameters.