Structural Studies of Benzene Derivatives, XII [1]. The Molecular Structure of 3,5-Dimethylbenzoic Acid in the Crystal, and a Comparison with the Gas Phase Structure of Related Molecules

Abstract

The molecular structure of 3,5-dimethylbenzoic acid has been accurately determ ined by X-ray crystallography. The crystals are monoclinic, space group P21/n, with a = 8.165(2), b = 6.992(1), c = 14.422(2) Å , β = 93.21(2)°, Z = 4. The structure has been refined by full-matrix least-squares techniques to R = 0.0415 on 1662 counter intensities. The carbon skeleton of the benzene ring has C2v symmetry within experimental error. The six aromatic C-C bond distances do not differ significantly from each other; some of the internal ring angles, however, deviate from 120° by as much as 2°. After correction for systematic effects, such as the asphericities of the electron density distribution and the libration of the molecule in the crystal, the mean aromatic C-C bond distance is 1.401(1) Å , and the two independent C-CH3 bond distances are 1.509 and 1.511(3) Å. These values are in outstanding agreement with the corresponding rg distances measured by gas electron diffraction in p-xylene (1.400±0.003 Å and 1.512±0.003 Å , respectively) and mesitylene (1.401 ±0.002 A and 1.509±0.002 Å, respectively). The deviations of the ring angles from 120° are interpreted fairly well as arising from the superposition of separate, independent angular distortions from each substituent.