Conformation and Dynamics of Atactic Poly(acrylonitrile). 2. Torsion Angle Distributions in Meso Dyads from Two-Dimensional Solid-State Double-Quantum 13C NMR

Abstract

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 13C double-quantum NMR spectroscopy (DOQSY). The 2D DOQSY spectra of an aPAN sample with 13C-labeled nitrile side groups show significant deviations from the ideal trans conformations. The 13C⋮N bonds in meso trans−trans dyads make an average angle of 20° with each other, with distributions of 25° standard deviation. This translates into standard deviations of σ = 20° for the two successive torsion angles in these dyads. The experimental spectrum constrains the average torsion angles to a line between (+160 ± 10°, +160 ± 10°) and (±170 ± 5°, ∓170 ± 5°), with distributions of standard deviation σ = 20 ± 5° 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.