Structural data from X-ray crystallographic investigations on aromatic amides and esters are examined for the purpose of deducing bond lengths and bond angles appropriate for poly (p-benzamide), poly(p-phenyleneterephthalamide), and the corresponding polyesters. Conformational energies have been calculated for acetanilide (AA), N-methylbenzamide (NMB), phenyl acetate (PA), and methyl benzoate (MB) as functions of torsion angles about the phenylene axes. An empirical force field (6-exp type) supplemented by terms for frame distortion and electron delocalization was used for this purpose. Bond angles and bond lengths were adjusted to values that minimize the total energy at each value of the torsion angle. Delocalization energies that optimize agreement of the torsional minima with torsion angles observed in the crystals of the four compounds and their analogues are 7 kcal moT1 for AA and NMB and ca. 3 kcal mol-1 for PA, Torsional energies calculated for these molecules exhibit maxima at coplanarity of the amide or ester group with phenyl, owing to steric repulsions involving ortho hydrogens; they exhibit minima near ±30°, ±30°, and ±55°, respectively. Steric interactions are small in MB and the torsional energy is minimal in the coplanar form for any value of the delocalization energy greater than 3 kcal mol'1. Crystallographic results confirm the coplanar form as the stable conformation. The empirical torsional energies are compared with ab initio molecular orbital calculations. © 1980, American Chemical Society. All rights reserved.