The use of ab initio and DFT calculations in the interpretation of ultraviolet photoelectron spectra: the rotational isomerism of anisole and thioanisole as a case study

The rotational isomerism of anisole and thioanisole was investigated by means of several computational methods, in the framework of the Møller–Plesset and Density Functional theories. Total energy curves as a function of the C–O(S)–C–C dihedral angle were calculated. For anisole, all methods employed, with the exception of HF/6-31G*, predicted the planar conformer to be the only stable species, in agreement with experiment. For thioanisole, HF and Møller–Plesset calculations predicted the perpendicular conformer to be the only stable species. Nearly all DFT methods predicted, instead, the existence of two minima, in agreement with experimental findings. Ionisation energies calculated according to the Koopmans’ theorem were compared with the values obtained from UP spectra. DFT ionisation energies, especially those obtained by the B3LYP, B1LYP, mPW1PW and B3PW functionals, were found to be in fair agreement with experimental values. HF ionisation energies showed only a rough agreement with experiment.