Homolytic SS Bond Dissociation of 11 Bis(thiocarbonyl)disulfides R‐C(S)SSC(S)R and Prediction of A Novel Rubber Vulcanization Accelerator

The structures and energetics of eight substituted bis(thiocarbonyl)disulfides (RCS 2 ) 2 , their associated radicals RCS 2 . , and their coordination compounds with a lithium cation have been studied at the G3X(MP2) level of theory for R=H, Me, F, Cl, OMe, SMe, NMe 2 , and PMe 2 . The effects of substituents on the dissociation of (RCS 2 ) 2 to RCS 2 . were analyzed using isodesmic stabilization reactions. Electron‐donating groups with an unshared pair of electrons have a pronounced stabilization effect on both (RCS 2 ) 2 and RCS 2 . . The SS bond dissociation enthalpy of tetramethylthiuram disulfide (TMTD, R=NMe 2 ) is the lowest in the above series (155 kJ mol −1 ), attributed to the particular stability of the formed Me 2 NCS 2 . radical. Both (RCS 2 ) 2 and the fragmented radicals RCS 2 . form stable chelate complexes with a Li + cation. The SS homolytic bond cleavage in (RCS 2 ) 2 is facilitated by the reaction [Li(RCS 2 ) 2 ] + +Li + →2 [Li(RCS 2 )] .+ . Three other substituted bis(thiocarbonyl) disulfides with the unconventional substituents R=OSF 5 , Gu 1 , and Gu 2 have been explored to find suitable alternative rubber vulcanization accelerators. Bis(thiocarbonyl)disulfide with a guanidine‐type substituent, (Gu 1 CS 2 ) 2 , is predicted to be an effective accelerator in sulfur vulcanization of rubber. Compared to TMTD, (Gu 1 CS 2 ) 2 is calculated to have a lower bond dissociation enthalpy and smaller associated barrier for the SS homolysis.