Conformation of poly(ethylene oxide)-hydroxybenzene molecular complexes studied by solid-state NMR

The conformation of poly(ethylene oxide), PEG, in molecular complexes with resorcinol (RES) and p-nitrophenol (PNP) was investigated by solid-state NMR. The two-dimensional double-quantum NMR spectrum shows that all OC-CO bonds in the PEO/RES molecular complex are gauche with psi = 74 +/- 9 degrees and a torsion angle distribution with a width sigma < 10 degrees. The spectra of the PEO/PNP complex, however, show that 33 +/- 8% of the OC-CO bonds are trans with sigma < 7 degrees. The fraction of trans bonds was also confirmed by C-13 CODEX experiments. The gauche torsion angle of 70 +/- 9 degrees differs significantly from the value of 60 degrees assumed in some models. Magic-angle-spinning C-13 INADEQUATE NMR, the CODEX data, and H-2-C-13 REDOR NMR allowed assignment of the two observed PEO C-13 NMR peaks. Both carbons on the trans bond, characterized by minimal dipolar broadening in the INADEQUATE spectrum, have chemical shifts of 69.2 ppm. The downfield gauche carbon at 71.4 ppm is 3.2 +/- 0.2 A from the d-PNP OD deuteron, while the upfield 69.8 ppm gauche carbon is estimated to be at a 1.13 +/- 0.06 times larger distance, or 3.6 +/- 0.3 Angstrom, from the deuteron.