Effect of Molecular Symmetry and Intermolecular Halogen-Halogen Interactions on the Crystal Structures of Halogen-Substituted Benzoic Acids. X-ray Crystal Structure of m-Iodobenzoic Acid

The crystal structure of m-iodobenzoic acid is reported. The crystals are monoclinic, P2₁/c, with a = 6.220(3) Å, b = 4.689(2) Å, c = 26.67(1) Å, β = 101.80(3)°, Z = 4, C₇H₅IO₂. The structure has been determined by direct methods and refined to R 0.026 for 1288 reflections recorded with an automatic single crystal diffractometer. The structure is composed of essentially planar hydrogen-bonded dimer units with significant intermolecular iodine-iodine contacts. The configuration of the dimer unit is transoid. Since this is the penultimate member of the family of mono-halogen-substituted benzoic acids its structure is compared with those previously reported. In particular, the effects of differences in molecular geometry of the o-, m-, and p-isomers and the strengths of the intermolecular halogen-halogen interactions are surveyed. The geometry of the carboxyl group of m-iodobenzoic acid in the solid state and the carbonyl absorption in the Ftir spectrum of the crystalline acid at room temperature suggest the presence of dynamic proton isomerism as has been recently found by other investigators in crystalline benzoic acid and some of its derivatives.