Revisiting the gauche oxygen effect in –[OCH2–CHRO]– (R = H/CH3): experimental reassessment and the origin of conformational stability

Abstract

The conformational preference around the C–C bond in alkylene diether systems expressed as OCH₂–CHRO (R = H or CH₃) is central to the interpretation of poly(alkylene oxide) chain statistics and NMR conformational analyses. However, NMR studies of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) model systems have led to interpretations suggesting a gauche preference (the “gauche oxygen effect”), whereas theoretical calculations consistently favor trans conformations. In this work, we reexamine the experimental basis of this discrepancy by reassessing the key assumption underlying earlier NMR analyses. Using 1,2-dimethoxyethane and 1,2-dimethoxypropane as model compounds, individual vicinal coupling constants are calculated to test the trans/gauche parameters used in prior studies. The revised analysis yields conformational populations consistent with theoretical predictions, resolving the apparent inconsistency between experiment and calculation. Natural energy decomposition analysis (NEDA) shows that although certain gauche conformations exhibit stabilizing orbital delocalization, this effect is insufficient to overcome the associated deformation penalty, which favors the trans conformation around the C–C bond. In addition, intramolecular 1,5 CH₃⋯O interactions, once attributed to the gauche stability around the C–C bond, are quantitatively characterized using NEDA and second-order perturbation analysis. The results reveal that although these interactions contribute to stabilization, they alone do not account for the observed conformational preference. These results demonstrate that conformational stability in alkylene diether systems is governed by the interplay between stabilizing interactions and deformation penalties, rather than by a simple “gauche oxygen effect.”