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Bicyclopropylidene radical cation: A rehybridization ring opening to tetramethyleneethane
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Quantum Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Quantum Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Quantum Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Quantum Chemistry.
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2004 (English)In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 98, no 5, 473-483 p.Article in journal (Refereed) Published
Abstract [en]

A computational study has been undertaken to elucidate the mechanism of the bicyclopropylidene radical cation (BCP•+) rearrangement into the tetramethyleneethane radical cation (TME•+). A stepwise mechanism is found for the first ring opening, with an activation energy of 7.3 kcal mol−1, while the second ring opening proceeds with no activation energy. Each ring opening is combined with a striking pyramidalization of one carbon atom in the central bond. In a natural bond orbital (NBO) analysis, the dominating reaction coordinate during the ring opening is found to be the olefinic carbon atom rehybridization, which also favors the continued bond breaking. Widely different ESR parameters are computed for the two sets of four protons in BCP•+, in excellent agreement with the observed spectrum, which are interpreted in the NBO analysis in terms of two hyperconjugative effects. Two minimum energy structures are located for TME•+, separated by a cusp on the internal rotation path of this cation, both of which show ESR parameters in good agreement with the observed spectrum for TME•+.

Place, publisher, year, edition, pages
2004. Vol. 98, no 5, 473-483 p.
National Category
Theoretical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-96116DOI: 10.1002/qua.20082OAI: oai:DiVA.org:uu-96116DiVA: diva2:170585
Available from: 2007-09-04 Created: 2007-09-04 Last updated: 2011-04-09Bibliographically approved
In thesis
1. Quantum Chemical Studies of Radical Cation Rearrangement, Radical Carbonylation, and Homolytic Substitution Reactions
Open this publication in new window or tab >>Quantum Chemical Studies of Radical Cation Rearrangement, Radical Carbonylation, and Homolytic Substitution Reactions
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Quantum chemical calculations have been performed to investigate radical cation rearrangement, radical carbonylation, and homolytic substitution reactions of organic molecules.

The rearrangement of the bicyclopropylidiene radical cation to the tetramethyleneethane radical cation is predicted to proceed with stepwise disrotatory opening of the two rings. Each ring opening is found to be combined with a striking pyramidalization of a carbon atom in the central bond.

The isomerization of the norbornadiene radical cation to the cycloheptatriene radical cation (CHT.+), initialized by opening of a bridgehead–methylene bond, is investigated. The most favorable path involves concerted rearrangement to the norcaradiene radical cation followed by ring opening to CHT.+. The barrier of this channel is found to be significantly reduced upon substitution of the methylene group with C(CH3)2.

Stepwise mechanisms are predicted to be favored over concerted isomerization for the McLafferty rearrangement of the radical cations of butanal and 3-fluorobutanal. The barrier for the concerted rearrangement is found to be lowered by 17.2 kcal/mol upon substitution, a result which is rationalized by the calculated dipole moments and atomic charges.

Recent experiments showed that photoinitiated carbonylation of alkyl iodides with [11C]carbon monoxide may be significantly enhanced by using small amounts of ketones that have nπ* character of their excited triplet state. DFT calculations show the feasibility of an atom transfer type mechanism, proposed to explain these observations. Moreover, the computational results rationalize the observed differences in yield when using various alcohol solvents.

Finally, following photolysis of methyliodide, recent electron spin resonance spectroscopy experiments demonstrated that the SH2 reaction CD3 + SiD3CH3 → CD3SiD3 + CH3 proceeds with high selectivity over the energetically more favorable D abstraction. The role of geometrical effects, especially the formation of prereactive complexes between methylsilane and methyliodide is studied, and a plausible explanation for the experimentally observed paradox is presented.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2007. 93 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 330
Keyword
Quantum chemistry, quantum chemistry, coupled-cluster, density functional theory, meta-GGA, reaction mechanism, potential energy surface, isomerization, fragmentation, dissociation, condensation, addition, SH2, hydrogen abstraction, iodine atom transfer, complex, weakly interacting system, hyperfine coupling constant, Kvantkemi
Identifiers
urn:nbn:se:uu:diva-8178 (URN)978-91-554-6949-8 (ISBN)
Public defence
2007-09-26, Polhemsalen, Ångströmlaboratoriet, Uppsala, 10:15
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Supervisors
Available from: 2007-09-04 Created: 2007-09-04 Last updated: 2011-04-08Bibliographically approved

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Salhi-Benachenhou, NessimaLunell, Sten

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