Quantum Chemistry of Gauche-Oxygen Effect Observed in Polyethers

Abstract

Gauche conformation about the central C—C bond in the O—C—C—O bond sequence is sometimes more stable than the trans conformation. This phenomenon is anomalous in conventional stereochemistry in terms of electrostatic interaction between partially negative oxygens and of the bulkiness of the substituents on the C—C bond, but is widely observed in polyethers such as poly (oxyethylene)¹⁾ and poly [oxy(l-alkylethylene)]s.^{2, 3)} Abe and Mark called this gauche stabilization a “gauche oxygen” effect and defined its magnitude as the difference between the experimental energy for the gauche conformation relative to that for the trans conformation and the corresponding energy calculated by the molecular force field (MFF) method.^{1, 4)} This definition implies that the gauche-oxygen effect is the least understandable phenomenon within the conventional MFF framework. Thus, a quantum chemical explanation may be desirable.