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Theoretical studies on the mechanism of α-santonin photo-induced rearrangement
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0003-2729-0290
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0003-0007-0394
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0002-1763-9383
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2012 (English)In: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 13, no 1, 353-362 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Germany: John Wiley & Sons, 2012. Vol. 13, no 1, 353-362 p.
National Category
Theoretical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-52823DOI: 10.1002/cphc.201100451ISI: 000298913500047PubMedID: 22120926Scopus ID: 2-s2.0-84862958847OAI: oai:DiVA.org:kth-52823DiVA: diva2:467790
Funder
Swedish e‐Science Research Center
Note

QC 20120227

Available from: 2011-12-19 Created: 2011-12-19 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Theoretical Studies on Magnetic and Photochemical Properties of Organic Molecules
Open this publication in new window or tab >>Theoretical Studies on Magnetic and Photochemical Properties of Organic Molecules
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present thesis is concerned with the theoretical studies on magnetic and photochemical properties of organic molecules. The ab initio and first principles theories were employed to investigate the vibrational effects on the isotropic hyperfine coupling constant (HFCC) known as the critical parameter in electron paramagnetic resonance spectrum, the theoretical simulations of the vibronically resolved molecular spectra, the photo-induced reaction mechanism of α-santonin and the spin-forbidden reaction of triplet-state dioxygen with cofactor-free enzyme. The theoretical predictions shed light on the interpretation of experimental observations, the understanding of reaction mechanism, and importantly the guideline and perspective in respect of the popularized applications.

We focused on the vibrational corrections to the isotropic HFCCs of hydrogen and carbon atoms in organic radicals. The calculations indicate that the vibrational contributions induce or enhance the effect of spin polarization. A set of rules were stated to guide experimentalist and theoretician in identification of the contributions from the molecular vibrations to HFCCs. And the coupling of spin density with vibrational modes in the backbone is significant and provides the insight into the spin density transfer mechanism in organic π radicals.

The spectral characters of the intermediates in solid-state photoarrangement of α-santonin were investigated in order to well understand the underlying experimental spectra. The molecular spectra simulated with Franck-Condon principle show that the positions of the absorption and emission bands of photosantonic acid well match with the experimental observations and the absorption spectrum has a vibrationally resolved character.

α-Santonin is the first found organic molecule that has the photoreaction activities. The photorearrangement mechanism is theoretically predicted that the low-lying excited state 1(nπ*) undergoing an intersystem crossing process decays to 3(ππ*) state in the Franck-Condon region. A pathway which is favored in the solid-state reaction requires less space and dynamic advantage on the excited-state potential energy surface (PES). And the other pathway is predominant in the weak polar solvent due to the thermodynamical and dynamical preferences. Lumisantonin is a critical intermediate derived from α-santonin photoreaction. The 3(ππ*) state plays a key role in lumisantonin photolysis. The photolytic pathway is in advantage of dynamics and thermodynamics on the triplet-state PES. In contrast, the other reaction pathway is facile for pyrolysis ascribed to a stable intermediate formed on the ground-state PES.

 The mechanism of the oxidation reaction involving cofactor-free enzyme and triplet-state dioxygen were studied. The theoretical calculations show that the charge-transfer mechanism is not a sole way to make a spin-forbidden oxidation allowed. It is more likely to take place in the reactant consisting of a non-conjugated substrate. The other mechanism involving the surface hopping between the triplet- and singlet-state PESs via a minimum energy crossing point (MECP) without a significant charge migration. The electronic state of MECP exhibits a mixed characteristic of the singlet and triplet states. The enhanced conjugation of the substrate slows down the spin-flip rate, and this step can in fact control the rate of the reaction that a dioxygen attaches to a substrate.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. xvi, 62 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2012:2
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-52818 (URN)978-91-7501-227-8 (ISBN)
Public defence
2012-01-18, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20111220Available from: 2011-12-20 Created: 2011-12-19 Last updated: 2012-01-20Bibliographically approved

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