Ab initio and density functional study on singlet and triplet states of artemisinin

Calculations on the singlet (S₀) and triplet (T₁) states of artimisinin were carried out to study its singlet-triplet excitations using several levels of theory. Geometries of the singlet and the triplet states were at first optimized at Hartree-Fock (HF) level using 3-21g basis. An additional calculation was performed for the triplet state using the ground-state geometry. The adiabatic transition (ΔE_ad) of -4.62 kcal/mol was obtained from the energy difference of S₀ and T₁ at their optimized geometries. The vertical transition (ΔE_V) of 90.96 kcal/mol was computed from the energy difference of S₀ and T₁ at the singlet optimized geometry. The result suggests that artimisinin would readily become diradical under normal condition which is in contrast to the experiments. Thus, B3LYP density functional theory calculations using 6-31g* basis were performed to confirm the HF results. In contrast to the HF calculations, the ΔE_ad and ΔE_V of 37.42 and 96.53 kcal/mol were obtained, respectively. Geometry optimizations at B3LYP level using 6-31g* basis were also performed on both singlet and triplet. Using BSLYP's geometries, the Møller-Plesset second order perturbation theory (MP2) were applied to the two states and in respective order ΔE_ad and ΔE_V of 50.81 and 117.48 kcal/mol were reported. In summary, HF theory is not sufficient to estimate the adiabatic transition of this endoperoxide compound, and correlated calculations would be required.