MOLECULAR-STRUCTURE AND CHARGE-DENSITY ANALYSIS OF P-METHOXYBENZOIC ACID (ANISIC ACID)

A concerted X-ray and ab initio SCF-MO study of the structure and char ge density of p-methoxybenzoic acid (anisic acid) is reported. An exte nsive X-ray data set (7401 reflections) was measured on a single cryst al using Mo K alpha radiation and the structure refined with 2121 uniq ue reflections, leading to a final R(F)-factor of 0.047 calculated for reflections with I > 2 sigma. The molecular geometry of crystalline a nisic acid, where the molecules dimerize via a moderately strong C=O-H ... O hydrogen bond, is compared with that of the isolated molecule, resulting from SCF-MO ab initio calculations. A topological analysis o f the molecular charge density was performed using Bader's method to g ain insight into the dominant intra-and intermolecular interactions in this compound, In particular, the effects of the substituents on the observed distortions of the benzene ring were investigated as well as the internal rotation of the methyl group. (C) 1997 Elsevier Science B .V.