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Theoretical Studies of Ground and Excited State Reactivity
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström. (Theoretical Chemistry programme)
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

To exemplify how theoretical chemistry can be applied to understand ground and excited state reactivity, four different chemical reactions have been modeled. The ground state chemical reactions are the simplest models in chemistry. To begin, a route to break down halomethanes through reactions with ground state cyano radical has been selected. Efficient explorations of the potential energy surfaces for these reactions have been carried out using the artificial force induced reaction algorithm. The large number of feasible pathways for reactions of this type, up to eleven, shows that these seemingly simple reactions can be quite complex. This exploration is followed by accurate quantum dynamics with reduced dimensionality for the reaction between Cland PH2Cl. The dynamics indicate that increasing the dimensionality of the model to at least two dimensions is a crucial step for an accurate calculation of the rate constant. After considering multiple pathways on a single potential energy surface, various feasible pathways on multiple surfaces have been investigated. As a prototype of these reactions, the thermal decomposition of a four-membered ring peroxide compound, called 1,2-dioxetane, which is the simplest model of chemi- and bioluminescence, has been studied. A detailed description of this model at the molecular level can give rise to a unified understanding of more complex chemiluminescence mechanisms. The results provide further details on the mechanisms and allow to rationalize the high ratio of triplet to singlet dissociation products. Finally, a thermal decomposition of another dioxetane-like compound, called Dewar dioxetane, has been investigated. This study allows to understand the effect of conjugated double bonds adjacent to the dioxetane moiety in the chemiluminescence mechanism of dioxetane. Our studies illustrate that no matter how complex a system is, theoretical chemistry can give a level of insight into chemical processes that cannot be obtained from other methods.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 86 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1179
Keyword [en]
Chemical Reactivity, Computational Chemistry, Dynamics, Ground and Excited States, Chemiluminescence, Atmospheric Chemistry
National Category
Theoretical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-232219ISBN: 978-91-554-9036-2 (print)OAI: oai:DiVA.org:uu-232219DiVA: diva2:747036
Public defence
2014-10-30, Häggsalen, Ångström laboratory, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2014-10-09 Created: 2014-09-15 Last updated: 2015-01-23Bibliographically approved
List of papers
1. Mechanisms for the Breakdown of Halomethanes through Reactions with Ground-State Cyano Radicals
Open this publication in new window or tab >>Mechanisms for the Breakdown of Halomethanes through Reactions with Ground-State Cyano Radicals
2015 (English)In: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 16, no 1, 181-190 p.Article in journal (Refereed) Published
Abstract [en]

One route to break down halomethanes is through reactions with radical species. The capability of the artificial force-induced reaction algorithm to efficiently explore a large number of radical reaction pathways has been illustrated for reactions between haloalkanes (CX3Y; X=H, F; Y=Cl, Br) and ground-state (2Σ+) cyano radicals (CN). For CH3Cl+CN, 71 stationary points in eight different pathways have been located and, in agreement with experiment, the highest rate constant (108 s−1 M−1 at 298 K) is obtained for hydrogen abstraction. For CH3Br, the rate constants for hydrogen and halogen abstraction are similar (109 s−1 M−1), whereas replacing hydrogen with fluorine eliminates the hydrogen-abstraction route and decreases the rate constants for halogen abstraction by 2–3 orders of magnitude. The detailed mapping of stationary points allows accurate calculations of product distributions, and the encouraging rate constants should motivate future studies with other radicals.

Keyword
gas-phase reactions; quantum chemistry; radical reactions; reaction mechanisms; transition states
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-228448 (URN)10.1002/cphc.201402601 (DOI)000347239200018 ()25263486 (PubMedID)
Funder
Marcus and Amalia Wallenberg FoundationSwedish Research CouncilSwedish National Infrastructure for Computing (SNIC), p2011004
Available from: 2014-07-15 Created: 2014-07-15 Last updated: 2017-12-05Bibliographically approved
2. Ab initio quantum mechanical calculation of the reaction probability for the Cl- + PH2Cl -> ClPH2 + Cl- reaction
Open this publication in new window or tab >>Ab initio quantum mechanical calculation of the reaction probability for the Cl- + PH2Cl -> ClPH2 + Cl- reaction
2013 (English)In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 425, 134-140 p.Article in journal (Refereed) Published
Abstract [en]

The SN2 substitution reactions at phosphorus play a key role in organic and biological processes. Quantum molecular dynamics simulations have been performed to study the prototype reaction Cl-+PH2ClClPH2+Cl-, using one and two-dimensional models. A potential energy surface, showing an energy well for a transition complex, was generated using ab initio electronic structure calculations. The one-dimensional model is essentially reflection free, whereas the more realistic two-dimensional model displays involved resonance structures in the reaction probability. The reaction rate is almost two orders of magnitude smaller for the two-dimensional compared to the one-dimensional model. Energetic errors in the potential energy surface is estimated to affect the rate by only a factor of two. This shows that for these types of reactions it is more important to increase the dimensionality of the modeling than to increase the accuracy of the electronic structure calculation.

Place, publisher, year, edition, pages
Amsterdam: Elsevier, 2013
Keyword
Nucleophilic substitution (S(N)2); Reaction probability; Quantum dynamics
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-210368 (URN)10.1016/j.chemphys.2013.08.011 (DOI)000327443700016 ()
Funder
Marcus and Amalia Wallenberg FoundationSwedish National Infrastructure for Computing (SNIC), p2011004
Available from: 2013-11-06 Created: 2013-11-06 Last updated: 2017-12-06Bibliographically approved
3. Revisiting the Nonadiabatic Process in 1,2-Dioxetane
Open this publication in new window or tab >>Revisiting the Nonadiabatic Process in 1,2-Dioxetane
2013 (English)In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 9, no 12, 5404-5411 p.Article in journal (Refereed) Published
Abstract [en]

Determining the ground and excited-state decomposition mechanisms of 1,2-dioxetane is essential to understand the chemiluminescence and bioluminescence phenomena. Several experimental and theoretical studies has been performed in the past without reaching a converged description. The reason is in part associated with the complex nonadiabatic process taking place along the reaction. The present study is an extension of a previous work (De Vico, L.; Liu, Y.-J.; Krogh, J. W.; Lindh, R. J. Phys. Chem. A 2007, 111, 8013-8019) in which a two-step mechanism was established for the chemiluminescence involving asynchronous O-O' and C-C' bond dissociations. New high-level multistate multi configurational reference second-order perturbation theory calculations and ab initio molecular dynamics simulations at constant temperature are performed in the present study, which provide further details on the mechanisms and allow to rationalize further experimental observations. In particular, the new results explain the high ratio of triplet to singlet dissociation products.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-215286 (URN)10.1021/ct4007844 (DOI)000328437500021 ()
Available from: 2014-01-13 Created: 2014-01-13 Last updated: 2017-12-06Bibliographically approved
4. Theoretical Study of the Chemiluminescence Mechanism of Dewar Dioxetane
Open this publication in new window or tab >>Theoretical Study of the Chemiluminescence Mechanism of Dewar Dioxetane
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Light emission from the heating of Dewar benzene was first reported by McCapra (McCapra, PureAppl. Chem. 1970, 24, 611–629). Since most of the chemiluminescence reactions occur through anO-O cleavage, light observed through the thermal decomposition of Dewar benzene was suggested to becaused by the chemiluminescence mechanism of a dioxetane-based intermediate produced after oxidation.However, no prove of this proposal has been suggested so far and the details of the mechanism are thusunknown. In this paper, thermally activated decomposition mechanism of Dewar dioxetane has been studiedby the multiconfigurational CASPT2//CASSCF approach, and accurate reaction path descriptions havebeen provided for a two-step biradical mechanism consisting of O1-O’1and C2-C’2ruptures based onminimum energy path and intrinsic reaction coordinate computations. A radiationless decay path has beendetermined for the molecule along the excited triplet state, while in the excited singlet state the systemevolves toward an equilibrium structure that might be responsible of the light emission. This findingsprovide clues for rationalizing the observed light emission and point to higher efficiency of fluorescencethan triplet emission. Furthermore, the study allows to understand the effect of conjugated double bondsadjacent to the dioxetane moiety in the chemiluminescence mechanism of dioxetane.

National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-232217 (URN)
Projects
Chemiluminescence, Multiconfigurational Methods, Dewar Dioxetane
Available from: 2014-09-15 Created: 2014-09-15 Last updated: 2014-10-22

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