title: Role of the 3(ππ*) state in photolysis of lumisantonin: insight from ab initio studies.
abstract: 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 (1)(nπ*) 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)(ππ*) state through intersystem crossing in the region of minimum surface crossings with a notable spin-orbital coupling constant. The (3)(ππ*) state, derived from the promotion of an electron from the π-type orbital mixed with the σ orbital localized on the C-C bond in the three-membered alkyl ring to the π* 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. Accordingly, path B is relatively facile for the pyrolytic reaction. The present results provide a basis to interpret the experimental observations.
authors.affiliation: Department of Chemistry and State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 360015, China.
authors.forename: Xing
authors.initials: X
authors.lastname: Chen
authors.forename: Zilvinas
authors.initials: Z
authors.lastname: Rinkevicius
authors.forename: Yi
authors.initials: Y
authors.lastname: Luo
authors.forename: Hans
authors.initials: H
authors.lastname: Ågren
authors.forename: Zexing
authors.initials: Z
authors.lastname: Cao
mesh_terms: D008958:Models, Molecular; D010777:Photochemistry; D012500:Santonin / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D012500:Santonin; C054379:lumisantonin