NEXAFS Spectra\nSimulations of Nitrogen-Bearing Heterocycles

Five-membered heterocyclic\ncompounds containing nitrogen\natoms\nare important biomolecule building blocks. In addition to their fundamental\nbiological importance, these molecular structures are used in several\ntechnological applications. Consequently, it is essential to develop\ntechniques that allow the characterization of these fundamental systems.\nWe address this issue by performing simulations of K-edge NEXAFS spectra\nby applying a time-dependent density functional theory (TDDFT) and\nan inner-shell multiconfigurational self-consistent field (IS-MCSCF)\nof selected molecules. Also, vibronic coupling simulations were considered\nfor the TDDFT computations. Surprisingly, molecular orbital binding\nenergies do not reproduce the order of the transition energies obtained\nby IS-MCSCF, indicating a possible breakdown of the orbital picture\nconcerning the NEXAFS spectrum. In general, the TDDFT and IS-MCSCF\nresults are compatible and are in close agreement with experimental\ndata. Moreover, vibronic coupling and vertical transition results\nwere very similar. Finally, it is important to mention that, to the\nbest of our knowledge, this is the first time that the IS-MCSCF method\nhas been applied to molecular systems of this size.