Conformation and dynamics of atactic poly(acrylonitrile).: 2.: Torsion angle distributions in meso dyads from two-dimensional solid-state double-quantum 13C NMR

The average trans torsion angles and their distributions have been investigated for meso dyads in a disordered crystalline polymer, atactic poly(acrylonitrile) (aPAN), by two-dimensional (2D) solid-state C-13 double-quantum NMR spectroscopy (DOQSY). The 2D DOQSY spectra of an aPAN sample with C-13-labeled nitrile side groups show significant deviations from the ideal trans conformations. The C-13 equivalent toN bonds in meso trans-trans dyads make an average angle of 20 degrees with each other, with distributions of 25 degrees standard deviation. This translates into standard deviations of sigma = 20 degrees for the two successive torsion angles in these dyads, The experimental spectrum constrains the average torsion angles to a line between (+160 +/- 10 degrees, +160 +/- 10 degrees) and (+/- 170 +/- 5 degrees, -/+ 170 +/- 5 degrees), with distributions of standard deviation sigma = 20 +/- 5 degrees for both torsion angles. It is also confirmed that the trans:gauche ratio of backbone bonds in aPAN is 90:10 (+/-5%). The high trans content is apparently enabled by the torsion angle deviations from the ideal trans state in meso trans-trans dyads, which alleviate the steric hindrance and the large electric dipole interaction between the CN groups and thus lower the intramolecular conformational energy.