A DFT investigation of structure and spectroscopic properties of pyrazolo[4,3-e][1,2,4]triazine natural products

The controversial structural issue of a set of pyrazolo[4,3-e][1,2,4]triazine natural products with high antibiotic activity has been tackled with density functional theory calculations. The electronic structure of these nitrogen-rich drugs has been characterized by analyzing the 13C NMR chemical shifts and UV-visible absorption spectra that are most sensitive to stereoelectronic factors. In particular, the remarkably changeable pattern of the π → π∗ excitations accounts for the palette of bright colours exhibited by these compounds.