DFT study on isomerization and decomposition of cuprous dialkyldithiophosphate and its reaction with alkylperoxy radical

The cuprous dialkyldithiophosphate [(RO)(2)PS2Cu, CuDDP] as an antioxidant has been industrially used in lubricating oil. In this paper, for the first time, a computational study has been carried out for CuDDP. The scaled hypersphere search method has been used to explore the isomerization and decomposition pathways of (HO)(2)PS2Cu, a model compound of CuDDP, and its reaction with CH3OO center dot radical. The calculations were performed at the B3LYP level of theory. The results show that the most stable structure of (HO)(2)PS2Cu has pseudo-C-2 nu, symmetry and a four-membered ring constructed by P, Cu, and two S atoms. The bondrearrangement isomerization leading to a (H)O-bridging structure is kinetically feasible. Three dissociation channels have been found for (HO)(2)PS2Cu, which require high energy (> 60 kcal/mol) under the investigated condition. The reaction of (HO)(2)PS2Cu with CH3OO center dot includes bond-rearrangement isomerization and the decomposition of CH3OO center dot moiety. The (HO)(2)PS2Cu-assisted CH3OO center dot decomposition occurs via its O-O bond cleavage or the C-O bond dissociation. The former decomposition manner has been computed to be preferable over the latter at low temperature, but calculations suggested for the latter decomposition manner at higher temperature. Such a decomposition reaction, which is endothermic but possible, may be related to the antioxidation process of CuDDP.