A NEXAFS study of thymine tautomers using coupled cluster methods

Abstract

The gas phase geometries of the thirteen different tautomers of thymine were optimized at the level of coupled cluster singles, doubles and perturbative triples, CCSD(T), using an aug-cc-pVDZ basis set. The total energy of the ground state of thymine was calculated to -453.103 a.u. The core excitation energies and intensities for geometry optimized thymine were obtained by coupled cluster singles and doubles linear response calculations, using the basis set aug-cc-pCVTZ for the atom whose core was excited, and aug-cc-pVDZ for the rest. The core excitation energies and intensities of each atom, were combined, forming near-edge X-ray absorption fine structure, NEXAFS, spectra for different tautomers of thymine. The calculated NEXAFS spectra of the ground state was found to correspond very well with experimental spectra of carbon and oxygen, and fairly well with that of nitrogen. The core excitation energies were overestimated by approximately 2.2 eV for oxygen, 1.3 eV for carbon, and 2.3 eV for nitrogen. Electron localization effects seem to provide a reasonable explanation for the shifts taking place between the different spectra of the tautomers.