Solving the crystal structure of heterocyclic Poly(arylene benzimidazole) via model compound single crystal and theoretical calculations

Abstract

Heterocyclic aramid containing benzimidazole units has demonstrated to be one of the strongest fibers. Due to positional isomerism of asymmetric benzimidazole and conformational isomerism of phenylamide, solving the crystal structure of poly(arylene benzimidazole) (PABI) aramid was a prominent challenge. Herein, we first reported the crystal structure of PABI aramid. First, the model compound: N-(4-(5-benzamido-1H-benzo[d] imidazole-2-yl) phenyl) benzamide (PBA) was cultured into single crystal: the molecules took a perfect ross-tenon-mortise interlocking conformation, in which the hydrogen bonding made a giant three-dimensional network. The PABI crystals with PBA crystal stacking feature were subsequently constructed and optimized through theoretical quantum calculation, disclosing a monoclinic unit cell with the parameters (a = 8.56 Å; b = 5.05 Å; and c = 36.78 Å; and β = 91.4°) and Pn space group. Their plausibility and validity were verified, since the simulated XRD and 2D-WAXD patterns reproduced the experimental XRD patterns. Our strategy of linking model compound PBA single crystals to obtain higher PABI crystals, paved the way to solving asymmetrical heterocyclic macromolecules crystals composed of easily isomerized chemical structures.