Molecular packing in the new unit cell of poly(p-hydroxybenzoic acid) whisker crystal

A preliminary structure model for the new unit cell (orthorhombic, $\text{a=1.487}\text{nm}\text{;}$ $\text{b=0.572}\text{nm}\text{;}$ $\text{c}\text{(}\text{chain}\text{axis}\text{)=1.258}\text{nm}\text{)}$ of poly(p-hydroxybenzoic acid) whisker crystals was proposed on the basis of energy calculations, assuming the space group to be Pbca as the highest one. First, the torsion angle of phenyl ring and mutual shift of molecular stems along the chain direction were varied, and several models were selected according to the packing energy. Then the models were further optimized by varying other parameters. The most probable model was selected by comparing the simulated powder X-ray diffractograms of the optimized models with the experimental one from literature. According to the calculated X-ray and electron diffraction intensity values of the final model, the model was judged to be reasonable. The final model possesses a molecular conformation similar to those for previous unit cell proposed by other researchers.