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

A helical structure in ethylene oxides begins to emerge early as the chain length increases in solution. This formation is facilitated by the attenuation of dipolar repulsion and stabilization of dipoles. However, it is primarily driven by hyperconjugative interactions, which often underpin gauche effects and dominate over Lewis-type interactions.

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.

Zum Volltext