Crystallographic Studies and Physicochemical Properties of π-Electron Systems, Part 7 Refinement of the Crystal and Molecular Structure of p-N,N-Dimethylaminobenzoic Acid, Full ab initio STO-3G Optimization of its Molecular Geometry, and Empirical Analysis of Substituent Effects on the Geometry of Benzene Rings in p-Substituted Benzoic Acids

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Abstract
The crystal and molecular structure of N,N -dimethylaminobenzoic acid (DABA ) was redetermined by means of X-ray diffraction. Crystal data are: Mr = 165.2, triclinic, P1̄, a = 6.651(2) Å , b = 8.045(2) Å , c = 8.609(2) Å , α - 88.97(3)°, β = 94.72(3)°, γ = 113.76(3)°, V = 420.13 Å3, Z = 2, Dm = 1.28 g cm-1, Dx = 1.306 g cm-1; CuKα: λ = 1.54178 Å, μ = 6.74 cm -1, F(000) = 176.00, T - 293 K; R = 0.0684, Rw = 0.0780 for 1185 observed reflections. The DABA molecules are almost planar and form cyclic dimers exhibiting no orientational disorder with separation of oxygen atoms RO...O - 2.627(2) Å . The pattern of molecular geometry in DABA suggests a relatively strong through-resonance effect between the NMe2 and COOH groups and shows non-additivity of substituent effects on valence angles and bond lengths in the ring. Mesomeric equalization of CO bond lengths in the carboxylic groups of 10 well solved p-substituted benzoic acids is found to depend linearly on the RO...O distance in H-bridged dimers. Full abinitio STO -3G optimizations of molecular geometries for monomers of DABA , p-aminobenzoic acid, benzoic acid, and formic acid as well as for aniline and N,N -dimethylaniline show resonable agreement with experimental data. Changes of molecular geometries on formation of planar H-bridged dimers have been obtained from calculations for benzoic acid dimers, benzoic - formic acid complex, and formic acid dimers. The latter was also optimized using the 6 -31 G basis set.

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