Theoretical structures and experimental vibrational spectra of isomeric benzofused thieno [3,2-b] furan compounds

A theoretical structural and experimental vibrational study for the benzothieno [3,2-b] furan and thieno [3,2-b] benzofuran compounds is presented. The density functional theory (DFT) has been used to study its structures and vibrational properties. The geometries were fully optimised at the B3LYP/6-31G∗ and B3LYP/6-311++G∗∗ levels of theory and the harmonic vibrational frequencies were evaluated at the same levels. The calculated harmonic vibrational frequencies for the compound are consistent with the experimental IR spectrum in chloroform solution. These calculations gave us precise knowledge of the normal modes of vibration of these compounds. A complete assignment of all the observed bands in the infrared and Raman spectra for both benzothieno [3,2-b] furans compound was performed. The nature of the different rings bonds in both compounds was quantitatively investigated by means of Natural Bond Orbital (NBO) analysis. The topological properties of electronic charge density are analysed by employing Bader’s Atoms in the Molecules theory (AIM).