Two-dimensional solid-state NMR studies of crystalline poly(ethylene oxide): Conformations and chemical shifts

The torsion angles of the OC-CO bonds in crystalline poly(ethylene oxide), PEO, were investigated by solid-state nuclear magnetic resonance (NMR). Two-dimensional double-quantum (DOQSY) spectra indicate that the OC-CO bonds are all gauche with an average torsion angle of ψ=74±4° and a narrow torsion-angle distribution, σψ<8°. This is contradictory to the wider range of gauche torsion angles in the distorted helical structure previously proposed based on X-ray fiber diffraction. The low-temperature magic-angle-spinning (MAS) 13C NMR spectrum of unlabeled PEO contains four maxima and several shoulders, over a range of 3.1 ppm. Deconvolution of this spectrum, together with two-dimensional 13C INADEQUATE NMR and exchange MAS spectra, suggests a possible assignment of chemical shifts to the 14 carbons in the 72 helical repeat unit. The small line widths of the individual peaks indicate that the helical repeat unit is accurately replicated throughout the crystals. The results show that packing effects or small conformational differences can change chemical shifts by amounts that had previously been ascribed only to trans/gauche differences.