Improved quantum mechanical study of the potential energy surface for the bithiophene molecule

Abstract

The potential energy surface (PES) for the 2,2′-bithiophene molecule was investigated using Hartree–Fock, correlated MP2, MP4(SDQ), CCSD, and density functional theory levels. Distinct basis sets ranging from double-zeta to triple-zeta quality, with polarization functions added on all atoms, were employed as well as the Dunning correlated consistent polarized valence double-zeta (cc-pVDZ) basis set. Single point configuration interaction CISD calculations were also performed using the cc-pVDZ basis set. Harmonic frequency calculations were performed for the unambiguous characterization of the stationary points located on the PES and also to calculate thermal Gibbs free energy corrections. Regarding the structural predictions we found that the B3LYP/6-311G** and MP2/cc-pVDZ fully optimized geometries exhibit the best agreement with the gas phase electron diffraction data. The calculated B3LYP/6-311G**, MP2/cc-pVDZ and experimental torsional angle for the syn-gauche structure are, respectively, 37.4° (B3LYP), 39.9° (MP2), and 36°±5° (expt.) with the corresponding values for the anti-gauche form being, respectively, 150.3° (B3LYP), 146.0° (MP2), and 148°±3° (expt.). The relative energy between the two minima and torsional barriers are sensitive both to the size of the basis set and the level of the quantum mechanical method used. Therefore, larger basis sets are needed to assess the ability of the DFT approach for describing torsional barriers. The MP4(SDQ) and CCSD relative energy results, reported in this work, can be considered as the most reliable torsional potential data available for the 2,2′-bithiophene molecule. Our results indicate that the experimentally estimated relative energy value for the two equilibrium structures present on the PES for the bithiophene molecule, and consequently the relative abundance of the anti-gauche species, is somewhat underestimated. By comparison with MP4(SDQ) and CCSD results we have shown that single point DFT/6-311G** calculations using HF/6-31G* geometries is the most computationally efficient procedure to study bithiophene like systems, with energy barriers agreeing within 2 kJ/mol.