Exploring the Helical Structure of Ethylene Oxides: Beyond Steric and Related Effects

Dimethoxyethane (DME) is the monomer of polyethylene oxide, a polymer widely used in materials science, with its conformation affecting properties such as host-guest interactions with ions. This quantum-chemical study reveals that DME undergoes rotational isomerization among nine rotamers, favoring a zigzag (all-trans) conformation, followed by a conformer with the OCCO fragment in a gauche arrangement. This preference strengthens with increasing chain length but transitions to a gauche configuration resembling a helical structure in polar solvents like DMSO. Unlike expectations based on steric effects or earlier assumptions of a 1,5 CH₃/O nonbonding attraction, the helical structure arises from reduced dipolar repulsion/dipole stabilization and hyperconjugative interactions that surpass Lewis-type interactions. The helical structure of a 21-heavy-atom oligomer further compresses to encapsulate a potassium cation, forming a highly stable complex.