Density Functional Theoretical Study On Reaction Mechanism Of Hexachlorocyclotriphosphazene

Hexachlorocyclotriphosphazene was one of the most important compounds of polyphosphazenes. It played a pivotal role in the development of polyphosphazenes. At present, the researches were mainly focused on the raw materials, the choice of catalysts and how to separate the product efficiently. The report has not been seen of theoretical study on reaction mechanism of this kind of reaction to this day.The mechanism of hexachlorocyclotriphosphazene which was synthesized by phosphorus pentachloride and ammonium chloride was investigated by using the density functional theory (DFT) of quantum chemistry. All of the possible reaction pathways were discussed. At the GGA–HCTH/DND level, the geometry optimizations of reactants, intermediates, the transition states and the products were performed. All of the transition states were verified by the frequency analysis method. Reaction energy barriers of all elementary processes were also calculated.Firstly, the chain initiation reaction mechanism of hexachlorocyclotri- phosphazene was studied. Results showed that the chain initiation reaction was a very complex procedure so there were various intermediates and transition states along reaction paths and the product was [Cl₃P---N---PCl₃]⁺PCl₆^– which was agreed well with experiment results. This reaction was an endothermal process, and it only took place when heated. For this reaction pathway, IM2→TS2→IM3+HCl had the lowest energy barrier, only 12.18 kJ/mol; while the fifth procedure (IM5+PCl₅→TS5→IM6+HCl) was the rate-controlling step, had the highest energy barrier 174.83 kJ/mol.Secondly, the chain propagation reaction mechanism of hexachlorocyclotri- phosphazene was investigated. Results showed that the chain propagation reaction was a very complex procedure,there were four different reaction pathways and three different products were obtained. The product obtained by pathway three and four was [Cl₃P---N---PCl₂---N--- PCl₂---N---PCl₃]⁺Cl^–, which was agreed well with experiment results. Pathway four was the main reaction channel for its lower reaction barriers of each step. For reaction pathway four, IM16→TS17→IM17 had the lowest energy barrier 39.02 kJ/mol; while the last procedure (IM17+Cl₃P=NH→TS(P3)→P3+HCl) had the highest energy barrier 154.79 kJ/mol, was the rate-controlling step.Finally, the chain termination reaction mechanism of hexachlorocyclotri- phosphazene was considered. Results showed that there were two steps of elementary processes and the product obtained by theory was N₃P₃Cl₆ which was agreed well with experimental result. Each elementary process had a higher energy barrier and this reaction was an endothermal process, and it only took place when heated.