Anomeric effect, ring inversion, structural stability and vibrational assignments of 2-chloro-1,3,2-dioxaphospholane 2-oxide and 2-sulfide, c-C₂H₄O₂PXCl (X is O or S)

The structural stability and ring inversion of 2-chloro-1,3,2-dioxaphospholane 2-oxide and 2-sulfide were investigated by the DFT-B3LYP and the ab initio MP2 calculations with 6-311+G^** basis set. From the calculations the two molecules were predicted at the two levels of theory to exist predominantly in the twist conformation. The effect of the mutual interactions between the two oxygen atoms in the ring and the -PXCl moiety on the inversion of the five-membered hetero-ring of the two molecules and hence on determining their conformational stability was thoroughly explained. The repulsive interaction between the lone-pairs at the two in-ring oxygen atoms was concluded to be superior to the non-bonded overlap between the occupied lone-pair orbitals at the in-ring O atoms with the empty, antibonding orbitals of both the P{single bond}Cl and the P{double bond, long}X (X is O or S) bonds. The vibrational wavenumbers were calculated and the potential energy distributions PED among the symmetry coordinates of the normal modes were computed for the low energy structure 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 molecules were compared to the calculated ones.