Revisiting the Gauche Oxygen Effect in -[OCH 2 -CHRO]-(R=H/CH 3 ): Experimental Reassessment and Origin of Conformational Stability

Abstract

The conformational preference around the C–C bond in ether-substituted alkanes, including poly(alkylene oxide) repeat units, has been extensively investigated because of its central importance to the interpretation of polymer chain statistics and NMR-derived conformational analyses. Despite decades of experimental and theoretical work, a long-standing discrepancy persists between NMR conformational analyses of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) model systems and high-level ab initio predictions. In particular, NMR studies have reported a gauche preference—commonly referred to as the so-called gauche oxygen effect—whereas ab initio calculations have consistently favored trans conformations. In this work, we reexamine the experimental basis of this discrepancy by critically reassessing the key assumption underlying earlier NMR conformational analyses. Using 1,2-dimethoxyethane and 1,2-dimethoxypropane as molecular models for PEO and PPO, their individual vicinal NMR coupling constants are explicitly calculated and used to evaluate the validity of the trans/gauche coupling constants employed in prior polymer and small-molecule studies. The revised analysis yields conformational populations consistent with ab initio predictions. Building on this resolution, we analyze the energetic origin of conformational preferences using natural energy decomposition analysis, showing that although gauche conformations benefit from orbital delocalization, these stabilizing contributions are insufficient to overcome the associated deformation penalty. In addition, the intramolecular 1,5 CH₃···O interactions previously invoked in polymer conformational models are explicitly characterized using second-order perturbation analysis. Together, these results reconcile experimental and theoretical views of polyether conformational behaviors and clarify the physical origin of conformational preferences in poly(alkylene oxide) systems.