Molecular structure and conformational preferences of chloro(dimethyldithio)phosphite, ClP(SCH3)2, as studied by gas electron diffraction and quantum-chemical calculations

Free ClP(SCH3)2 molecule has been studied by gas electron diffraction (GED), B3PW91/6-311þG* (DFT) and MP2/6-31þG* calculations. Each conformer is characterised by two dihedral angles t(CSPlp) where lp denotes the direction of the electron lone pair on the P atom; assumed to coincide with normal to the plane passing through three points lying at the unit distances along each of three P bonds. DFT calculations indicate that the most stable conformer is a gaucheþ, gauche2 ðg þ g2Þ conformer of Cs symmetry characterised by the angles ta;b ¼ ðCSPlpÞ ^ 51:58: It is followed by an anti, gaucheþ conformer ðagþÞ at DE ¼ 0:16 kJ mol21; an anti, gauche2 conformer (ag2) at DE ¼ 1:3 kJ mol21; a g2g2 conformer at DE ¼ 5:1 kJ mol21 and an aa conformer at DE ¼ 13:0 kJ mol21: Whereas MP2 calculated results revealed that relative energies to be as follows: 3.6, 0.0, 1.5, 6.6, 13.0 kJ mol21, respectively. That is agþ conformer is the most stable one. The calculated standard free energies at 298.15 K indicate that the mole fractions in the gas phase at this temperature are (DFT/MP2): xðg þ g2Þ ¼ 55=36%; xðagþÞ ¼ 24=34%; xðag2Þ ¼ 12=23% and xðg 2 g2Þ ¼ 8=7%: Experimental GED data agree well with agþ model alone and do not confirm any presence of g þ g2 conformer, the presence of up to 10% ag2 conformer cannot be completely excluded. Natural bond orbital (NBO) analysis of the wavefunctions suggest that while bond distances and valence angles are determined by anomeric effects, the relative stabilities of the five conformers are not determined by such effects alone. Structure optimisation of F2PSMe molecule by both theoretical methods indicates that the most stable conformation is anti, while the energy of a gauche conformer is about 6.1/10.5 kJ mol21 higher. NBO analysis of the wavefunctions indicates that the relative stabilities of the two conformers as well as the differences between bond distances and valence angles may be determined by anomeric effects. q 2004 Elsevier B.V. All rights reserved.