Structure determination through powder X-ray diffraction, Hirshfeld surface analysis, and DFT studies of 2- and 4-(methylthio)benzoic acid

Abstract 2-(methylthio)benzoic acid (1) is an ortho-substituted benzoic acid derivative, whereas 4-(methylthio)benzoic acid (2) is a para-substituted benzoic acid derivative. The structural analysis of both compounds was carried out using PXRD data. 2-(methylthio)benzoic acid shows a triclinic system with the P 1 ‾ $P\overline{1}$ space group, whereas 4-(methylthio)benzoic acid shows a monoclinic system and crystallizes in the P21/a space group. The strength, as well as relative contributions of intermolecular hydrogen bonds, have been examined through Hirshfeld surfaces as well as 2D fingerprint plots. A weak intramolecular hydrogen bond was found only in the case of ortho-substituted 2-(methylthio)benzoic acid. Supramolecular frameworks for 1 are formed by the interplay of intramolecular and intermolecular interactions, whereas for 2, intermolecular contacts form supramolecular assemblies. Intermolecular O–H⋯O interactions involving carboxyl groups form the R22(8) graph-set motif for both compounds. Theoretical DFT calculations using the B3LYP correlation functional reveal that the energy gap of HOMO–LUMO orbitals in compound 1, with the methylthio moiety in the ortho position relative to the carboxyl group, is lower than that of compound 2, with the methylthio moiety in the para position. Vertical and adiabatic ionization energies are also calculated for both compounds.