Molecular structure and conformational preferences of methylthiodichlorophosphite, Cl2PSCH3, as studied by gas electron diffraction and quantum-chemical calculations

Free Cl2PSCH3 molecule has been studied by gas electron diffraction (ED), B3PW91/6-311+G∗ (DFT) and MP2/AUG-cc-PVTZ quantum-chemical calculations. Structure optimizations of Cl2PSMe molecule by both theoretical methods (DFT/MP2) indicate that the most stable conformer is an anti conformer of Cs symmetry, while the energy of a gauche conformer is about 1.3/1.6 kcal mol-1 higher. Each conformer is characterized by dihedral angle τ(CSPlp) where lp denotes the direction of the electron lone pair on the P atom; assumed to lay in the plane passing through P-S bond and bisector of the ClPCl bond angle. The calculated standard free energies at 298.15 K indicate that the mole fractions in the gas phase at this temperature are: χ(anti) = 65/79%, χ(gauche) = 35/21%. Experimental ED data agree well with joint presence of both conformers in the ratio χ(anti) = 68(12)% and χ(gauche) = 32(12)%. Natural Bond Orbital (NBO) analysis suggests that the relative stabilities of the two conformers as well as the differences between bond distances, valence angles and different NBO descriptors may be determined by anomeric effects. The most important of which is πlpS → σPCl∗, that is delocalizations of π lone pair of the S atom into antibonding orbital of P-Cl bond.