CONFORMATIONS AND STRUCTURES OF THIOPHOSPHANES CONTAINING ONE, 2 AND 3 P-S BONDS - A GAS ELECTRON-DIFFRACTION AND AB-INITIO STUDY

The conformations and geometric structures of thiophosphanes which con tain one P-S bond. X2PSY, two P-S bonds, XP(SY)2 and three P-S bonds, P(SY)3 are studied by ab initio calculations and gas electron diffract ion (GED). For compounds of the type X2PSY (X = H or F and Y = H, CH3 or F) the calculations predict two possible conformations with the SY bond either synperiplanar to the phosphorus lone pair (THETA (1p-P-S-Y ) = 0-degrees) or antiperiplanar (THETA = 180-degrees). The synperipla nar structure is preferred in compounds with equivalent substituents ( H2PSH and F2PSF), whereas the antiperiplanar form is preferred for mix ed substituents (F2PSH, F2PSCH3 and H2PSF). A GED study of (CF3)2PSCF3 results in a synperiplanar structure (THETA = 0-degrees), confirming this observation. Calculations for HP(SH)2 result in three stable stru ctures, syn-syn (C(s) symmetry), syn anti (C1 symmetry) and anti--anti (C(s) symmetry). The relative energies are predicted to be 0, 0.46 an d 1.26 kcal mol-1 (HF/3-21G). In the case of HP(SCF3)2 the calculated energy of the syn anti form relative to the syn-syn form depends on t he basis set; -1.32 kcal mol-1 for HF/3-21G and 0.23 kcal mol-1 for H F/6-31G*. According to the GED analysis, the prevailing conformer of this thiophosphane is syn-syn with a possible small contribution of th e syn anti form. The parent thiophosphane with three P-S bonds, P(SH)3 , possesses five possible conformations, two of which have C3 symmetry , two C, symmetry and one C1 symmetry. For P(SCF3)3 two of these five forms possess unreasonably short F ... F contacts between neighboring CF3 groups and do not form stable structures. According to the GED ana lysis the main conformer of P(SCF3)3 possesses C3 SYMMetry with all CF 3 groups syn to the phosphorus lone pair (THETA1 = THETA2 = THETA3 = 2 9(4)-degrees). Small contributions ( < 20%) of Cs and/or C1 structures cannot be excluded.