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Theoretical Studies on the Photoinduced Rearrangement Mechanism of α‐Santonin
Author(s) -
Chen Xing,
Rinkevicius Zilvinas,
Luo Yi,
Ågren Hans,
Cao Zexing
Publication year - 2012
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201100451
Subject(s) - intersystem crossing , excited state , chemistry , photochemistry , isomerization , singlet state , ground state , stacking , potential energy surface , chemical physics , computational chemistry , atomic physics , molecule , physics , biochemistry , organic chemistry , catalysis
α‐Santonin is the first organic compound observed to feature a photoinduced rearrangement and is now known to undergo a series of photochemical processes under UV irradiation. On the basis of the considerable interest of this system as a prototype, and of the yet limited insights reached for the basic photo mechanisms, we calculate the high‐level electronic structures and explore the potential energy surfaces (PES) of α‐santonin in the ground and lowest‐lying excited states, their couplings, and the possible photoinduced isomerization pathways. The calculations identify the low‐lying singlet excited state 1 (nπ*) accessible under light irradiation, which decays to the low‐energy 3 (ππ*) state through an intersystem crossing in the Franck–Condon region to initiate the photoinduced rearrangement. The initial reaction from the C3C5 bond coupling, which takes place on the 3 (ππ*) state potential energy surface, leads to a three‐membered alkyl‐ring compound intermediate state INT . The following photochemical reactions have the possibility to arise from two distinct CC bond cleavages, C4C5 and C3C4, denoted as path A and path B. Path A is favored both dynamically on the excited‐state PES and thermodynamically on the ground‐state PES in vacuo. Experiments show that it also becomes the dominant photoinduced rearrangement process in the crystal, which can be explained by considering the requirement for less space and the stacking effect under the confined environment. Path B is dynamical advantaged both on the ground‐ and excited‐state PESs in a weak polar solvent, such as dioxane. Once the biradical intermediate B ‐ INT is accessible on the ground‐state PES, the formation of the product B ‐ P is almost barrier free.