Role of the (3)(pi pi*) State in Photolysis of Lumisantonin: Insight from ab Initio Studies
Rinkevicius, Zilvinas（Royal Inst Technol, Sch Biotechnol）
Luo, Yi（Royal Inst Technol, Sch Biotechnol）
Agren, Hans（Royal Inst Technol, Sch Biotechnol）
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The CASSCF and CASPT2 methodologies have been used to explore the potential energy surfaces of lumisantonin in the ground and low-lying triplet states along the photoisomerization pathways. Calculations indicate that the (n pi*) state is the accessible low-lying singlet state with a notable oscillator strength under an excitation wavelength of 320 nm and that it can effectively decay to the (3)(pi pi*) state through intersystem crossing in the region of minimum surface crossings with a notable spin-orbital coupling constant. The 3(pi pi*) state, derived from the promotion of an electron from the pi-type orbital mixed with the sigma orbital localized on the C-C bond in the three-membered alkyl ring to the pi* orbital of conjugation carbon atoms, plays a critical role in C-C bond cleavage. Based on the different C-C bond rupture patterns, the reaction pathways can be divided into paths A and B. Photolysis along path A arising from C1-C5 bond rupture is favorable because of the dynamic and thermodynamic preferences on the triplet excited-state PES. Path B is derived from the cleavage of the C5-C6 bond, leading first to a relatively stable species, compared to intermediate A-INT formed on the ground state PES. path B is relatively facile for the pyrolytic reaction. The present results provide a basis to interpret the experimental observations.
CitationJOURNAL OF PHYSICAL CHEMISTRY A，2011,115（26）：7815-7822