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.

Theoretical Modeling of the Doping Process in Polypyrrole by Calculating UV/Vis Absorption Spectra of Neutral and Charged Oligomers

“…However, it has been shown convincingly that this is due not to problems with DFT but to basis set insufficiency at the MP2 level. 64,91,92 Inclusion of zero point vibrational corrections flattens the potential energy surfaces further. 64 Thus, geometries of pyrrole oligomers are quite flexible and planarization energies are small.…”

The elusive excited states of bithiophene: a CASPT2 detective story

“…The corresponding energy barrier is only 0.01-0.04 eV. [23,24,[30][31][32][33] By contrast, in the lowest excited state, 2T has been found to be trans-planar with a deep, steep single minimum on the potential energy surface (PES) [23,24]. One should thus expect that if the 2T molecule assumed the planar conformation, the lowest vertical absorption energy would be considerably lower than the gas-phase value, whereas the adiabatic energy would be reduced only marginally.…”

Computational reinvestigation of the bithiophene torsion potential

“…The following section is devoted to the description of our computational methods. Note that some of our calculations with moderately sized basis sets reproduce previously published data [5][6][7][8] and are included mainly for the readerÕs convenience, to facilitate comparison with the more accurate results. Afterwards, we shall illustrate our numerical results and discuss their dependence on the electron correlation method and on the quality of the basis set.…”

“…Eventually, they were unable to claim convergence of the MP2 potential with respect to the basis set. Finally, Duarte et al [8] studied the effects of several density functionals as well as CCSD (coupled-cluster with single and double excitations) and MP4(SDQ) (fourth-order Møller-Plesset, neglecting triple excitations). Interestingly, these authors observed a fair agreement between the MP4(SDQ)/cc-pVDZ and B3LYP/6-311G** curves, thus reducing the ÔconflictÕ between wavefunction-based and density-functional theories.…”

“…The main features of the torsion energy profile of bithiophene have been known for at least a decade [5][6][7][8][9][10][11][12][13]. The absolute minimum has a transdistorted (TD, also known as anti-gauche) conformation, with an S-C-C-S torsion angle / ffi AE150°.…”