Theoretical study of the decomposition mechanism of C ₄ F ₇ N

Abstract

Abstract Investigations into alternative gases to reduce the usage of SF 6 have great benefits on global warming issues and the health of maintenance personnel. C 4 F 7 N is one of the most remarkable replacements for SF 6 owing to its good insulating performance, low global warming potential and non-toxicity. The decomposition mechanism of C 4 F 7 N is important in evaluating the insulation performance but it is still not clear. Therefore, the B3LYP/6-311G(d,p) method in conjunction with transition state theory is used to study the decomposition mechanism of C 4 F 7 N. Sixteen reactions are found in the decomposition pathways of C 4 F 7 N. The optimized configurations and harmonic vibrational frequencies of selected species are very consistent with experimental data to verify the method adopted in this paper. The potential energy surface of these reactions are obtained and the reaction mechanisms are analyzed. The rate constants over 300 K–3500 K relevant to the insulation breakdown temperature are computed based on the above quantum chemistry calculations and dominant reactions in different temperature regions are selected. For example, reaction R 5 (C 4 F 7 N → TS2 → FCN + CF 2 CFCF 3 ) is the most important reaction leading to the dissociation of C 4 F 7 N below 600 K, while reaction R 2 (C 4 F 7 N → C 2 F 4 CN + CF 3 ) takes the place of reaction R 5 over 600 K to 3300 K and reaction R 3 (C 4 F 7 N → TS1 → CF 2 CFCN + CF 4 ) becomes dominant above 700 K; reaction R 15 (CF 2 CFCNCF 3 → CF 2 CFCN + CF 3 ) plays the major role in the generation of CF 3 with the overwhelming contribution rate. The results obtained here are expected to construct a relatively complete C 4 F 7 N decomposition scheme, including the main byproduct formation processes and to lay a theoretical basis for the investigation of its insulation performance.