SHAPES AND THERMODYNAMIC STABILITIES OF E(CH3)(4) COMPOUNDS (E=S, SE,TE)

We report a systematic ab initio computational study, using -a careful ly graded range of basis sets with full geometry optimization at both SCF and MP2 levels of theory, of the structures, isomerism, binding en ergies and vibrational frequencies of E(CH3)(4) compounds (E = S, Se, Te). Some higher-level calculations were also undertaken at MP2 geomet ries. The related dimethyl compounds have also been studied to indicat e the accuracy which should be achieved in our calculations for the te tramethyl compounds, which are as yet unknown for E = S or Se. The equ zilibrium molecular structure of each tetramethyl compound is based on a trigonal bipyramid, with an equatorial lone pair (C-upsilon symmetr y). Inter-methyl repulsions have significant structural effects for th e S derivative, but these are progressively less important for the Se and Te species. All E(CH3)(4) compounds are fluxional, especially the Te system; barriers to Berry pseudorotation are estimated to be about 20, 15, and 3 kJ/mol for the S, Se, and Te tetramethyl. The low-freque ncy vibrational spectra for Te(CH3)(4) have been satisfactorily assign ed and provide no evidence for the coexistence of C-2 upsilon and C-4 upsilon isomers. All E(CH3)(4) compounds are thermodynamically unstabl e with respect to (E(CH3)(2) + ethane), but the instability decreases with the size of the central atom; estimated binding energies are abou t -350, -300, or -225 kJ/mol for E = S, Se, or Te. The influence which d-type orbitals on E have on the stability of the tetramethyls is dis cussed. As the Te compound has recently been prepared, the S and Se de rivatives seem possible but difficult synthetic targets.