Molecular structure and spectroscopic properties of polyaromatic heterocycles by first principle calculations: spectroscopic shifts with the adsorption of thiophene on phyllosilicate surface

The molecular and electronic structure and spectroscopic properties of thiophene, benzothiophene, and dibenzothiophene molecules have been studied theoretically, using different levels of quantum-mechanical calculations based on Hartree–Fock and density functional theory (DFT) approximations. In all calculation levels used, the molecular structures, dipole moments, thermodynamic properties, and vibration normal modes agree well with the available experimental data and predict the non-available values. The calculated frequencies agree with experimental values within scale factors in the range of 0.94–1.0 for thiophene. Our calculations have refined the mode assignments previously reported. The adsorption of thiophene on a pyrophyllite surface along the 001 plane was also investigated by means of a quantum-mechanical method based on the DFT approximation in a periodical crystal lattice model. The adsorbed thiophene adopts a parallel orientation with respect to the phyllosilicate surface. The shifts observed experimentally in the IR bands of thiophene upon adsorption on pyrophyllite are well reproduced in the thiophene–phyllosilicate complex, finding explanations of this phenomenon and new effects with our calculations.