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Role of the 3(ππ*) state in photolysis of lumisantonin: insight from ab initio studies
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
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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2011 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 115, no 26, 7815-7822 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2011. Vol. 115, no 26, 7815-7822 p.
National Category
Biochemistry and Molecular Biology
URN: urn:nbn:se:kth:diva-37160DOI: 10.1021/jp203369bISI: 000292281300047ScopusID: 2-s2.0-79959795779OAI: diva2:432389
Swedish e‐Science Research Center
QC 20110803Available from: 2011-08-03 Created: 2011-08-02 Last updated: 2012-05-23Bibliographically 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.
Trita-BIO-Report, ISSN 1654-2312 ; 2012:2
National Category
Theoretical Chemistry
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)
QC 20111220Available from: 2011-12-20 Created: 2011-12-19 Last updated: 2012-01-20Bibliographically approved

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