Structure and torsional potential of p-phenylthiophene: a theoretical comparative study
Abstract
Quantum
chemical calculations employing Hartree–Fock, MP2 and density functional (using distinct functionals)
approaches were carried out for the p-phenylthiophene dimer. The fully optimized stationary points located
on the potential energy surface were characterized as minima or transition state (TS) structures according
to harmonic frequency analysis. A mixture of syn–gauche and anti–gauche conformers was predicted with a relative
percentage of ca. 60% and 40%, respectively. A TS structure connecting the syn–gauche and anti–gauche
minima was also determined, with the MP2 energy barrier being ca. 10 kJ mol−1. A six-term truncated Fourier
series representation of the potential energy for internal rotation was obtained using a fitting procedure
to the calculated HF/6-31G* and B3LYP/6-31G* partially optimized points. Additional fittings
were performed with the MP2/6-31G*//HF/6-31G*, MP2/6-31G*//B3LYP/6-31G*, B3LYP/6-31G*//HF/6-31G*, BLYP/6-31G*//HF/6-31G*, B3P86/6-31G*//HF/6-31G* and SVWN/6-31G*//HF/6-31G* single energy points.
The energy barriers obtained from the fitted curve were compared to the ones calculated from the energy differences between fully optimized minima and TS structures. The fitted Fourier potential is found
to be adequate for the description of the internal rotation in the p-phenylthiophene dimer. The B3LYP/6-31G*//HF/6-31G*
level of calculation seems sufficient for studying this class of compounds. The inclusion of the