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 NM R. The two-dimensional double-quantum NMR spectrum shows that all OC-CO bon ds 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 spect ra 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 conf irmed by C-13 CODEX experiments. The gauche torsion angle of 70 +/- 9 degre es differs significantly from the value of 60 degrees assumed in some model s. Magic-angle-spinning C-13 INADEQUATE NMR, the CODEX data, and H-2-C-13 R EDOR NMR allowed assignment of the two observed PEO C-13 NMR peaks. Both ca rbons on the trans bond, characterized by minimal dipolar broadening in the INADEQUATE spectrum, have chemical shifts of 69.2 ppm. The downfield gauch e 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.