Theoretical study of the anomeric effect in CH₂CIOH

The anomeric effect has been analyzed theoretically by adopting a simple model molecule, CH₂ClOH. The total energy has been found at its minimum when the Cl-C-O-H dihedral angle is 66° (gauche-staggered conformation). By representing the wave function of the molecule in terms of the MOs of two fragments, we demonstrate that the oxygen p-type lone-pair orbital participates in electron donation to the C-Cl σ* orbital in a wide range of the Cl-C-O-H dihedral angle. In contrast, a hybrid of s and p atomic orbitals of the oxygen interacts with the C-Cl σ* orbital in the antiperiplanar (or anti-staggered) conformation. Electron delocalization is depressed in the anti conformation. The orbital interaction interprets consistently the shortened C-O bond and the elongated C-Cl bond in CH₂ClOH in the gauche-staggered conformation. The magnitude of the stabilization due to the effect has been estimated by using the fragment interaction orbitals, supporting the significance of electron delocalization in explaining the anomeric effect. The anomeric effect in the SN₂ reaction has also been studied. © 2000 American Chemical Society.