Energetics of the rotational isomers (rotamers) of 2- and 3-thiophenecarboxaldehydes has been investigated in Ar matrices by means of matrix-isolation infrared spectroscopy in the 700-1800 cm (1) region, combined with UV irradiation as well as DFT and Coupled Cluster calculations. Utilizing photo-induced rotational isomerism, the two rotamers (anti and syn) have been identified and the infrared bands of each rotamer have been assigned. The energy differences between the two rotamers in the ground state have been evaluated based on the infrared data, which are in excellent agreement with those obtained by the Coupled Cluster calculations. (C) 2011 Elsevier B. V. All rights reserved.