NEXAFS Spectra Simulations of Nitrogen-Bearing Heterocycles.

Five-membered heterocyclic compounds containing nitrogen atoms are important biomolecule building blocks. In addition to their fundamental biological importance, these molecular structures are used in several technological applications. Consequently, it is essential to develop techniques that allow the characterization of these fundamental systems. We address this issue by performing simulations of K-edge NEXAFS spectra by applying a time-dependent density functional theory (TDDFT) and an inner-shell multiconfigurational self-consistent field (IS-MCSCF) of selected molecules. Also, vibronic coupling simulations were considered for the TDDFT computations. Surprisingly, molecular orbital binding energies do not reproduce the order of the transition energies obtained by IS-MCSCF, indicating a possible breakdown of the orbital picture concerning the NEXAFS spectrum. In general, the TDDFT and IS-MCSCF results are compatible and are in close agreement with experimental data. Moreover, vibronic coupling and vertical transition results were very similar. Finally, it is important to mention that, to the best of our knowledge, this is the first time that the IS-MCSCF method has been applied to molecular systems of this size.