Crystal and Molecular Structures of Heterocyclic Compounds. III. α-Phenazine near 80°K

Abstract

A partial three‐dimensional analysis of the crystal structure of α‐phenazine at the temperature of boiling nitrogen has yielded, after refinement by a weighted least‐squares procedure, carbon and nitrogen coordinates accurate to 0.003 A as well as approximate hydrogen positions. Within the experimental error, the C–C bond lengths are identical with those in anthracene. This agrees with theoretical predictions, for direct molecular‐orbital calculations, with allowance for overlap of adjacent atomic orbitals, indicate a maximum difference between corresponding C–C bond lengths in phenazine and anthracene of about 0.01 A, which is of the order of the error in the latter structure. The results imply that neither inductive effects due to chemical substitution nor crystal forces can have any great influence on bond lengths in an alternant hydrocarbon such as anthracene. The C–H bond lengths average 1.00 A but are not to be compared with direct measurements of the internuclear distance such as are given by electron‐diffraction or magnetic‐resonance data. Deviations from molecular symmetry mmm exceed the experimental error only in the direction normal to the mean molecular plane. These displacements from the plane are of the order of 0.01 A and appear to arise from an electrostatic attraction between overlapping atoms in adjacent parallel molecules. Electrostatic interaction, however, is probably not important in determining the type of crystal packing, for a survey of other aromatic structures indicates that the packing type is, in most instances, approximately independent of electrostatic forces.