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Mechanistic photodissociation of small molecules explored by electronic structure calculation and dynamics simulation
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , vi, 57 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2011:17
National Category
Theoretical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-33585ISBN: 978-91-7415-981-3OAI: oai:DiVA.org:kth-33585DiVA: diva2:416185
Public defence
2011-06-13, FA32, AlbaNova, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110520Available from: 2011-05-20 Created: 2011-05-10 Last updated: 2011-05-20Bibliographically approved
List of papers
1. Photoisomerization mechanism of 4-methylpyridine explored by electronic structure calculations and nonadiabatic dynamics simulations
Open this publication in new window or tab >>Photoisomerization mechanism of 4-methylpyridine explored by electronic structure calculations and nonadiabatic dynamics simulations
2011 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 134, no 4, 044307- p.Article in journal (Refereed) Published
Abstract [en]

In the present paper, different electronic structure methods have been used to determine stationary and intersection structures on the ground (S-0) and (1)pi pi* (S-2) states of 4-methylpyridine, which is followed by adiabatic and nonadiabatic dynamics simulations to explore the mechanistic photoisomerization of 4-methylpyridine. Photoisomerization starts from the S-2((1)pi pi*) state and overcomes a small barrier, leading to formation of the prefulvene isomer in the S-0 state via a S-2/S-0 conical intersection. The ultrafast S-2 -> S-0 nonradiative decay and low quantum yield for the photoisomerization reaction were well reproduced by the combined electronic structure calculation and dynamics simulation. The prefulvene isomer was assigned as a long-lived intermediate and suggested to isomerize to 4-methylpyridine directly in the previous study, which is not supported by the present calculation. The nonadiabatic dynamics simulation and electronic structure calculation reveal that the prefulvene isomer is a short-lived intermediate and isomerizes to benzvalene form very easily. The benzvalene form was predicted as the stable isomer in the present study and is probably the long-lived intermediate observed experimentally. A consecutive light and thermal isomerization cycle via Dewar isomer was determined and this cycle mechanism is different from that reported in the previous study. It should be pointed out that formation of Dewar isomer from the S-2((1)pi pi*) state is not in competition with the isomerization to the prefulvene form. The Dewar structure observed experimentally may originate from other excited states.

Keyword
EXCITATION-ENERGY DEPENDENCE, VAPOR-PHASE PHOTOCHEMISTRY, THEORETICAL CHARACTERIZATION, RADIATIONLESS TRANSITIONS, CONICAL INTERSECTIONS, ULTRAFAST DIFFRACTION, FEMTOSECOND DYNAMICS, EXCITED-STATE, PYRIDINE, MOLECULES
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-31334 (URN)10.1063/1.3547207 (DOI)000286897600050 ()2-s2.0-79551581806 (ScopusID)
Note
QC 20110317Available from: 2011-03-17 Created: 2011-03-14 Last updated: 2011-05-20Bibliographically approved
2. The Conical Intersection Dominates the Generation of Tropospheric Hydroxyl Radicals from NO2 and H2O
Open this publication in new window or tab >>The Conical Intersection Dominates the Generation of Tropospheric Hydroxyl Radicals from NO2 and H2O
Show others...
2010 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 114, no 13, 4601-4608 p.Article in journal (Refereed) Published
Abstract [en]

In the present work, we report a quantitative understanding on how to generate hydroxyl radicals from NO2 and H2O in the troposphere upon photoexcitation at 410 nm by using multiconfigurational perturbation theory and density functional theory. The conical intersections dominate the nonadiabatic relaxation processes after NO2 irradiated at similar to 410 nm in the troposphere and further control the generation of OH radical by means of hydrogen abstraction. In agreement with two-component fluorescence observed by laser techniques, there are two different photophysical relaxation channels along decreasing and increasing O-N-O angle of NO2. In the former case, the conical intersection between (B) over tilde B-2(1) and (A) over tilde B-2(2) (CI (B-2(2)/B-2(1)) first funnels NO2 out of the Franck-Condon region of (B) over tilde B-2(1) and relaxes to the (A) over tilde B-2(2) surface. Following the primary relaxation, the conical intersection between (A) over tilde B-2(2) and (X) over tilde (2)A(1) (CI(B-2(2)/(2)A(1))) drives NO2 to decay into highly vibrationally excited (X) over tilde (2)A(1) state that is more than 20 000 cm(-1) above zeroth-order vertical bar n(1),n(2),n(3) = 0 > vibrational level. In the latter case, increasing the O-N-O angle leads NO2 to relax to a minimum of (B) over tilde B-2(1) with a linear O-N-O arrangement. This minimum point is also funnel region between (B) over tilde B-2(1) and (X) over tilde (2)A(1) (CI(B-2(1)/(2)A(1))) and leads NO2 to relax into a highly vibrationally excited (X) over tilde (2)A(1) state. The high energetic level of vibrationally excited state has enough energy to overcome the barrier of hydrogen abstraction (40-50 kcal/mol) from water vapor, producing OH ((2)Pi(3/2)) radicals. The collision between NO2 and H2O molecules not only is a precondition of hydrogen abstraction but induces the faster internal conversion (CIIC) via conical intersections. The faster internal conversion favors more energy transfer from electronically excited states into highly vibrationally excited (X) over tilde (2)A(1) states. The collision (i.e., the heat motion of molecules) functions as the trigger and accelerator in the generation of OH radicals from NO2 and H2O in the troposphere.

National Category
Physical Chemistry Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-28369 (URN)10.1021/jp911455r (DOI)000276096800017 ()2-s2.0-77950469693 (ScopusID)
Note
QC 20110119Available from: 2011-01-19 Created: 2011-01-14 Last updated: 2012-03-20Bibliographically approved
3. Wavelength-Dependent Photodissociation of Benzoic Acid Monomer in alpha C-O Fission
Open this publication in new window or tab >>Wavelength-Dependent Photodissociation of Benzoic Acid Monomer in alpha C-O Fission
2010 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 114, no 1, 680-684 p.Article in journal (Refereed) Published
Abstract [en]

In concert with the latest laser-induced fluorescence (LIF) experiment [Wei et al. J. Phys. Client. A 2008, 112, 4727], we investigated the photodissociation mechanics of the benzoic acid monomer (BAM) with alpha C-O fission by means of state-of-the-art ab initio calculations. Complete active space self-consistent-field (CASSCF) and multi reference CASSCF second-order perturbation theory (MSCASPT2) calculations were performed on the ground and a number of low-lying excited states of BAM. Our calculations indicated that alpha C-O fission from the S, state is in competition with the fission from the T-2 state upon the 266-284 nm wavelength photon. This differs from the conclusion of the previous theoretical investigation and clarified the vague experimental conclusion made earlier. According to Our calculations, alpha C-O fission mainly occurs at the T, state upon photoexcitation at 284-294 nm, and the photon with a wavelength longer than 294 nm is unable to present the alpha C-O fission. This conclusion agrees with the LIF experimental observation.

Keyword
193 NM, PERTURBATION-THEORY, ACETIC-ACID, DYNAMICS, DECARBOXYLATION, PRODUCT, DISSOCIATION, SPECTROSCOPY, MECHANISMS, PHOTOLYSIS
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-29076 (URN)10.1021/jp908567m (DOI)000273268900081 ()2-s2.0-75249092144 (ScopusID)
Note
QC 20110125Available from: 2011-01-25 Created: 2011-01-25 Last updated: 2011-05-20Bibliographically approved
4. Synchronous Concerted Four-Body Photodissociation of Oxalyl Chloride Explored by ab initio based Molecular Dynamics Simulations
Open this publication in new window or tab >>Synchronous Concerted Four-Body Photodissociation of Oxalyl Chloride Explored by ab initio based Molecular Dynamics Simulations
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-33584 (URN)
Available from: 2011-05-20 Created: 2011-05-10 Last updated: 2011-05-20Bibliographically approved

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