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

Calculations on the singlet (S-0) and triplet (T-1) 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 3-21 g basis. An additional calculation was performed for the triplet state using the ground-state geometry. The adiabatic transition (Delta E-ad) Of - 4.62 kcal/mol was obtained from the energy difference of So and T1 at their optimized geometries. The vertical transition (Delta E-v) of 90.96 kcal/mol was computed from the energy difference of So 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 Delta E-ad and Delta E-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 tripler. Using B3LYP's geometries, the Moller-Plesset second order perturbation theory (MP2) were applied to the two states and in respective order Delta E-ad and Delta E-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. (C) 1998 Elsevier Science B.V. All rights reserved.