Formation of intra- and intermolecular hydrogen bonds in 2-thiophen-3-ylmalonic acid. the precursor of a polythiophene derivative bearing two carboxylic acid groups in the side chain, have been examined by Fourier transform infrared (FTIR) spectroscopy and ab initio quantum mechanical calculations. Interactions found in the FTIR spectra recorded for the melted and solid states are in good agreement with results provided by MP2/6-31+G(d,p) calculations on monomers and dimers, respectively. Specifically, inter- and intramolecular hydrogen bonds were detected in the solid and melted states, respectively. Calculations on dimers stabilized by intermolecular hydrogen bonds exclusively and by both intra- and intermolecular interactions indicated that the former structures are significantly more stable than the latter ones. which is fully consistent with experimental observations. On the other hand, intramolecular interactions in isolated monomers are favored in the melted state, which is dominated by a thermally driven entropic process. [GRAPHICS]