EXCHANGE PROCESSES IN DISELENIUM AND SELENIUM-SULFUR DIHALIDES, SE2X2 AND SESX2 (X = BR, CL), A SE-77 2D-EXSY STUDY

The application of two-dimensional NMR exchange spectroscopy (2D-EXSY) to the study of Se-77 exchange kinetics in mixtures Of S2Cl2 and Se2Br2, which contain disulfur, diselenium, and selenium sulfur dihalides, shows that Se-77 magnetization transfer takes place predominantly by an exchange process, in which only SeX bonds are broken. Direct exchange via mechanisms, involving ChX radicals, four-chalcogen center transition states and mono- and trichalcogen dihalide species are not consistent with the cross peaks observed in the 2D spectra. Magnetization transfer by means of a bimolecular halogen-bridged intermediate [GRAPHICS] or by an ionic process, involving Se-X bonds, are both consistent with the pattern of cross peaks observed. However, the addition of tetraethylammonium chloride to the S2Cl2/Se2Br2 MiXtUre causes the eight-signal spectrum to collapse to three broad signals, which favors an interpretation in terms of an ionic exchange mechanism. The three broad signals are assigned to BrSSeX, ClSSeX, and SeSe dihalides each undergoing rapid ionic exchange, involving SeX bonds.