Conformational stability and molecular structure of dimethyl (methylthio) phosphine from electron diffraction studies and ab initio calculations

Abstract The molecular structure of dimethyl(methylthio)phosphine, (CH 3 ) 2 PSCH 3 , has been determined from a combined gas-phase electron diffraction study at 20°C and ab initio calculations utilizing both the 3–21G * and 6–31G * basis sets. Two distinct conformers, trans and cis or near-cis (the cis conformer has the thiomethyl group eclipsing the non-bonded electron pair on the phosphorous atom and the trans conformer has this methyl group staggered with respect to the two methyl groups attached to the phosphorous atom) were identified with the isomeric composition of 78±5% of the cis form. The differences between the geometrical parameters were fixed at the values obtained from the ab initio calculations. The determined heavy atom structural parameters for the cis conformer are r (PS)=2.111±0.002, r (PC)=1.862±0.002, r (SC)=1.769±0.006, ∠ PSC=102.3±2.6, ∠ CPC=96.7±0.8 and ∠ CPS=96.8±1.5 where the distances are in A and the angles in degrees. The parameters which differ for the trans conformer are r (PS) =2.103±0.002, ǒ PSC=108.5±2.5 and ∠ CPS=99.9±1.5. The rms amplitudes of vibration with their associated uncertainties have been determined for some of the distances. In addition, the geometry has been calculated by ab initio Hartree-Fock gradient calculations with geometry optimization at the 3-21G * and 6-31G * levels, and the computed structural parameters are in reasonable agreement with those obtained from the electron diffraction data. Features of the structure are compared with those of similar molecules.