Studies of protonated and anionic artemisinin in the gas-phase by infrared multi-photon dissociation and by negative ion photoelectron spectroscopies

Protonated and anionic artemisinin in the gas phase have respectively been studied by infrared multi-photon dissociation (IRMPD) spectroscopy and by anion photoelectron spectroscopy. Comparison of the measured IRMPD spectrum with calculated spectra of various conformations showed that the two lowest-energy protonated structures, both corresponding to protonation at the CO14 carbonyl site, were observed experimentally. The calculations also indicated that the peroxide bridge in artemisinin is only slightly modified by protonation. Additionally, stable, intact (parent) artemisinin radical anions have been obtained for the first time in the gas phase and the photoelectron spectrum supports the computational finding that the excess electron is mainly localized on the σ∗ orbital of the peroxide bond. The vertical detachment energy and adiabatic electron affinity, calculated at the MP2/6-31+G∗ level, are in good agreement with the experimental data and the O–O distance is calculated to be stretched by more than 50% in the anion.