Thermophysical, crystalline and infrared studies of the 2- and 3-thiophenecarboxylic acids

A thermophysical study by differential scanning calorimetry (DSC) for the 2- and 3-isomers of thiophenecarboxylic acid, in the temperature intervals between T=268 K and their respective melting temperatures has been carried out, and enhancements in the heat capacity curves were observed. The supramolecular structure of both compounds was analysed by X-ray crystallography and IR spectroscopy to determine which structural modifications take place in the temperature range at which the heat absorption occurs. Changes in bands assigned mainly to δ(OH) + ν(CO), δ(CH) + ν(CS) and out of plane CH vibrations point to structural modifications taking place in the temperature range at which the heat absorption occurs. All those groups are involved in the weak CH⋯O or CH⋯S contacts that form the crystal three-dimensional network and so it can be concluded that changes in these H-bond strength, are responsible for the small heat absorptions founded in the heat capacity curves.