ReaxFF reactive force field development and application for molecular dynamics simulations of heptafluoroisobutyronitrile thermal decomposition

Abstract

Heptafluoroisobutyronitrile (i-C3F7CN, C4) has obtained widely attention accounts for its good dielectric property and low GWP as a potential alternative gas. To investigate the thermal decomposition characteristics of C4, new reactive force field (ReaxFF) parameters were developed by training against QM-derived calculations with Monte Carlo Annealing (MC) method. The reliability of new parameters was verified not only by the deviations between QM-based calculations and ReaxFF predictions but also by good agreement between molecular dynamics (MD) simulations and experimental results. Hence, various NVT-MD simulations were employed to determine the reaction mechanisms, rate constants and product distributions. Our simulation results showed that the C4 decay was fitted with first-order kinetic expression, and the apparent activation energy was predicted to be 33.9 ± 1.67 kcal/mol. The main products of C4 decomposition were CF4, C2F6, C3F6, FCN, CF3CN, C2F5CN, which were in accordance with experimental and theoretical results. As for the initial reactions, we noted that C-CF3 bond was revealed to be the weakest bond and followed by C-F and C-CN bonds in C4 as reported in previous work. Similar MD simulation was conducted with C4/CO2 mixture system, which suggested that CO2 could consume F and CF3 radicals, resulting in significant impacts for CF4 and FCN. Noteworthy, new products (CO, COF2) were also seen as observed experimentally. To investigate the C4/CO2 system under the moisture condition, C4/CO2 with 1% H2O content was performed and new toxic gas HF was observed as a characteristic product, indicating an unnegligible impact by water content.