On the existence and stability of branched selenium chains: isomers of Me2Se3 and Et2Se3

According to ab initio MO calculations at the G2(MP2) level of theory, branched isomers of dialkyl triselenides, R-Se(=Se)-Se-R (1; R = Me, Et), are less stable by more than 60 kJ mol(-1) than the isomeric unbranched chains R-Se-Se-Se-R (2). Therefore, species 1 cannot be generated in substantial concentrations under equilibrium conditions at moderate temperatures, as has recently been claimed by Meja and Caruso (Inorg. Chem. 2004, 43, 7486). Alternatively, the isomeric CH3-Se-CH2-Se-Se-Et (3) can be considered to explain the reported gas chromatograms and mass spectra previously assigned to Et-Se(=Se)-Se-Et (1b). However, the isomerization 2b --> 3 is also endothermic, by deltaG(o)298 = 63 kJ mol(-1). The isomeric selenols HSe-C2H4-Se-Se-Et (4) and CH3-CH(SeH)-Se-Se-Et (5) are also less stable than 2b (by ca. 56 kJ mol(-1)), but 4 is another candidate to explain the mass spectrum formerly assigned to 1b. The calculated structures of 1-5 are reported.