Conformation and dynamics of atactic poly(acrylonitrile). 1. Trans/gauche ratio from double-quantum solid-state C-13 NMR of the methylene groups

The trans/gauche ratio of backbone bonds in atactic poly(acrylonitrile) (aP AN) was characterized by several two-dimensional solid-state NMR experiment s, using 15% (CH2)-C-13-carbon labeled aPAN. An estimate of the conformatio nal ratio was obtained from a double-quantum filtered NMR experiment, which correlates the C-13-C-13 internuclear vector of two adjacent (CH2)-C-13 gr oups with their C-13 chemical shift anisotropies. The orientation of the ch emical shift tensor was determined in auxiliary experiments. The backbone c onformation of two successive bonds was characterized by double-quantum NMR spectroscopy, which correlates the orientation-dependent C-13 chemical shi fts of two adjacent (CH2)-C-13 segments. The experiments were performed wit h and without C-13-C-13 dipolar decoupling. From the three two-dimensional NMR spectra that were obtained, the trans/gauche ratio of aPAN was determin ed as 90%:10% (+/-10%). This value is compatible with the previously report ed experimental density and wide-angle X-ray diffraction patterns of aPAN. The high trans content is also consistent with the single CH2 resonance lin e observed under magic-angle spinning.