Steric effects and steric inhibition of resonance in benzene derivatives:: 2,3-dimethylbenzoic acid

Structure of 2,3-dimethylbenzoic acid was determined both by singlc-crystal X-ray diffraction and by ab initio calculation at an RHF/6-31+g** level. Comparing with a previous X-ray analysis of another crystal modification of the same compound, it was possible to estimate the effect of crystal forces on the conformation. The isolated molecule is not planar as deduced previously, mainly from IR spectra, but the carboxyl group is twisted out of the ring plane by a torsion angle phi = 12 degrees. In the crystal, the molecule is more flat and phi is reduced to 7 and 1 degrees in the two modifications, respectively. Further significant differences between the modifications were not detected. The flattening in the crystal, structure is accompanied by additional small changes in the geometry, connected with greater steric crowding in the less twisted molecule: the example shows well the limits in,which molecular structure can be deduced from the solid-state structure. Previous division of methylated benzoic acids into two subgroups, planar and nonplanar, is to be formulated with more precision: the former group includes even acids with nearly planar conformation (phi up to 15 degrees).