2016. 57, 8 – UDC 541.6:548.737:543.422 DENSITY FUNCTIONAL THEORY STUDIES OF CONFORMATIONAL STABILITIES AND ROTATIONAL BARRIERS OF 2- AND 3-THIOPHENECARBOXALDEHYDES Y. Umar1, J. Tijani2, S. Abdalla3 1Department of Chemical and Process Engineering Technology, Jubail Industrial College, Jubail Industrial City, Saudi Arabia E-mail: umar_y@jic.edu.sa 2Department of General Studies, Jubail Industrial College, Jubail Industrial City, Saudi Arabia 3Department of Chemistry, Faculty of Science, University of Khartoum, P.O. Box 321, Khartoum, Sudan Received June, 09, 2015 Revised — September, 21, 2015 The molecular structures, conformational stabilities, and infrared vibrational wavenumbers of 2-thiophenecarboxaldehyde and 3-thiophenecarboxaldehyde are computed using Becke-3— Lee—Yang—Parr (B3LYP) with the 6-311++G** basis set. <...> From the computations, cis2-thiophenecarboxaldehyde is found to be more stable than the transfer conformer with an energy difference of 1.22 kcal/mol, while trans-3-thiophenecarboxaldehyde is found to be more stable than the cis conformer by 0.89 kcal/mol. <...> The computed dipole moments, structural parameters, relative stabilities of the conformers and infrared vibrational wavenumbers of the two molecules coherently support the experimental data in the literature. <...> The normal vibrational wavenumbers are characterized in terms of the potential energy distribution using the VEDA4 program. <...> The effect of solvents on the conformational stability of the molecules in nine different solvents is investigated using the polarizable continuum model. <...> DOI: 10.15372/JSC20160808 Keywords: density functional method, rotational barrier, vibrational wavenumber, solvent effect, thiophenecarboxaldehyde, conformational preference, infrared spectra, geometry optimization, polarizable continuum model, potential energy distribution (PED). <...> INTRODUCTION The conformational preference of heterocyclic compounds has recently been the subject of several experimental and theoretical studies [ 1—7]. <...> The molecules of 2-thiophenecarboxaldehyde (hereinafter called 2-TC) and 3-thiophenecarboxaldehyde (hereinafter called 3-TC) are thiophene derivatives that contain an aldehyde (—CHO) group substituted at positions 2 and 3 to the sulfur atom of the thiophene ring. <...> The rotation of the aldehyde group (CHO) on the thiophene rings of these compounds leads to two equilibrium configurations (isomers). <...> Thus, both 2-TC and 3-TC can exist in two asymmetric planar rotational conformers whereby the sulfur and oxygen atoms can be either OS-cis or OStrans. <...> Several experimental and theoretical <...>