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

The cuprous dialkyldithiophosphate [(RO)2PS2Cu, 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)2PS2Cu, a model compound of CuDDP, and its reaction with CH3OO* radical. The calculations were performed at the B3LYP level of theory. The results show that the most stable structure of (HO)2PS2Cu has pseudo-C2v symmetry and a four-membered ring constructed by P, Cu, and two S atoms. The bond-rearrangement isomerization leading to a (H)O-bridging structure is kinetically feasible. Three dissociation channels have been found for (HO)2PS2Cu, which require high energy (>60 kcal/mol) under the investigated condition. The reaction of (HO)2PS2Cu with CH3OO* includes bond-rearrangement isomerization and the decomposition of CH3OO* moiety. The (HO)2PS2Cu-assisted CH3OO* 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.