Ab initio studies on polymers. VII. Polyoxymethylene

The structure of an isolated, infinite polyoxymethylene chain has been investigated with the aid of the ab initio crystal orbital method applying a basis set of double‐zeta quality. Restricting the primitive unit cell to a single CH 2 O group, conformational potential curves as a function of the torsional angle have been evaluated. Only a single minimum closely corresponding to an all‐gauche structure was detected. The all‐trans conformation is a maximum on the energy curve for simultaneous rotation around CO single bonds. Detailed geometry optimization in the vicinity of the all‐gauche conformation led to the following structure: r CO = r OC = 1.425 Å, r CH = 1.072 Å, ∠HCH = 111.7°, ∠OCO = 110.9°, ∠COC = 115.1°, and τ OCOC = 70.75°. The computed torsional angle τ OCOC lies midway between the hexagonal (78.2°) and the orthorhombic (63.5°) modification of solid polyoxymethylene.