An ab initio study on dilithiosulfoximines

The gas-phase structures of the dilithiosalts of (N-methyl)dimethylsulfoxim ine 1 were calculated by ab initio methods employing different levels of th eory (HF, MP2, B3LYP and CBS). Three low-energy structures were found, with the 1,3-dilithiosulfoximine 2, a metalla spiro structure, as the most stab le isomer. The second minimum 3 contains a alpha,alpha dilithio motif formi ng two four-membered rings with the heteroatoms on sulfur, whereas isomer 4 shows one lithium coordinating to the two anionic carbon atoms and the oth er to the two heteroatoms bound to the sulfur. The values of the relative e nergies of the compounds 3 and 4 are 1-5 kcal mol (1) higher in energy than that of 2 and fairly close to one another depending on the applied level o f theory. A comparison between DFT and CBS-Q calculations is made and revea ls no significant discrepancies in the structures. The monolithium and lith ium-free dianions 2a,b(-) -4a,b(-), 2(2-) -4(2-) and their complexation ene rgies to the lithium cations were also calculated. NBO analyses were carrie d out to reveal donor-acceptor interactions in these dilithio salts. (C) 19 99 Elsevier Science S.A. All rights reserved.