Figure 6 Energy diagram of R123 decomposition pathway.
In order to unravel the fire extinguishing mechanism of R123, the reaction pathways of R123 with H and OH radicals and the direct decomposition pathway of R123, were separately explored. the B3LYP/6-311++G(d,p) method was used for structural optimization of transition states, and the CCSD/aug-cc-pVDZ method was used for correlation energy calculations.
The decomposition pathway of the ground state of R123 should be first considered. As revealed in Figure 6, there are three main pyrolysis paths for R123, and path 4 and 5 are the subsequent reactions of the pyrolysis products. In path 1, the break of the C-Cl bond occurred, the products CF3CHCl· and Cl· (P1) are obtained by overcoming an energy barrier of 343.78 kj·mol-1. Path 2 predominantly undergoes the elimination reaction, the path eliminates from R123 to form a HF molecule and CF2=CCl2 by overcoming an energy barrier of 348.68 kj·mol-1. There are two paths for the decomposition of CF2=CCl2. The CF2=CCl2 can consume 656.02 kj·mol-1 energy to produce CF=CHCl2· and F· via the transition state of TSa4 in path 4, or consume 431.56 kj·mol-1 energy to produce CF2=CHCl· and Cl· via the transition state of TSa5 in path 5. In addition, it is found that the above paths are endothermic reactions, indicating that R123 can play a physical fire extinguishing method of cooling in fire extinguishing. In path 3, the products CF3H and CCl2 are obtained by overcoming an energy barrier of 420.5 kj·mol-1. As shown in path 6~8, CF3H has three paths to generate plentiful CF3· free radicals, where CF3H decomposition produces CF3· without transition state generation. It is worth noted that CF3· is one of the free radical groups that will play an important role in the process of fire suppression. According to the literature[34, 35], CF3· free radicals can effectively capture both H· and OH· in combustion, thereby interrupting the combustion chain reaction. R123 is capable of producing enough CF3·, indicating that R123 has the potential to extinguish fire as an alternative extinguishing agent. According to the literature, R125 mainly occurs C-C fracture reactions reaction and elimination reaction, wherein the product contains CF3· and CF2·. Like the pyrolysis products of R123, these pyrolysis products are very easy to react with H· and OH· free radicals, interrupting the combustion chain reaction.