DFT and MP2 ring puckering potential functions, vibrational analysis and comparison with experiment for 3-chloro-1,3-thiaphosphetane 3-oxide, 3-sulfide and 1,3-dithietane 1,1-dioxide

The ring puckering in the four-membered 3-chloro-1,3-thiaphosphetane 3-oxide, its 3-sulfide derivative and 1,3-dithietane 1,1-dioxide hetero rings were investigated by DFT and ab initio MP2 calculations using a 6-311+G^** basis set. From the calculations at both levels of theory the asymmetric ring puckering potential energy curves in the two phosphetanes were consistent with an almost single minimum that corresponds to a pseudo-axial configuration (P{single bond}Cl bond is in axial position) with a puckering angle of about 15-20°. In the case of the dithietane dioxide the symmetric ring puckering potential was consistent at the B3LYP level with a flat minimum that corresponds to a planar ring but at the MP2 level with a double minimum with a very low barrier of about 185 cal/mol to ring planarity. The potential functions that describe the ring puckering in the three molecules were derived at the B3LYP/6-311+G^** and the MP2/6-311+G^** levels of calculations. The vibrational wavenumbers were calculated and the potential energy distributions PED among the symmetry coordinates of the normal modes were computed for the stable conformers of each of the molecules. Complete vibrational assignments were provided on the basis of the calculated PED values. The experimental infrared and Raman spectra of the two phosphetane molecules were simulated and compared to the calculated ones.