Molecular structure and conformational preferences of gaseous methylthiodichlorophosphite, Br2PSCH3, studied by gas electron diffraction and quantum-chemical calculations

Abstract

Molecule Br2PSCH3 has been studied by gas electron diffraction method with the use of additional data of quantum-chemical data at the MP2/SDB-AUG-cc-PVTZ and B3PW91/6-311+G* (DFT) levels of theory. The structure optimization of Br2PSMe molecule by both theoretical methods (DFT/MP2) indicate that the most stable conformer is an anti conformer of Cs symmetry, while the energy of the second gauche conformer is by about 1.47/1.05 kcal mol−1 higher. Each conformer is characterized by dihedral angle τ(CSPlp) where lp denotes the direction of the lone electron pair on the P atom. The calculated standard free energies at 298.15 K indicate that the mole fractions in the gas phase at this temperature are: μ(anti) = 75/62%, μ(gauche) = 25/38%. Experimental ED data agree well with joint presence of both conformers in the ratio μ(anti) = 65(6)% and μ(gauche) = 35(6)%. Natural bond orbital analysis (NBO) testifies that anti-conformer is an example of stabilization of the form that is extremely unfavorable from the point of view of steric and electrostatic interactions. It is shown that relative stabilities of two conformers, differences in geometrical parameters, and several NBO descriptors are defined by effects of hyperconjugation, the most important of which is the πlpS→σ*(P-Br) interaction, namely, the delocalization of lone electron pair of π-type of the S atom into the antibonding orbital of P-Br bond.