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Isomerization pathways from the norbornadiene to the cycloheptatriene radical cation by opening a bridgehead-methylene bond: a theoretical investigation
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.
2006 (English)In: Organic and biomolecular chemistry, ISSN 1477-0520, E-ISSN 1477-0539, Vol. 4, no 22, 4241-4250 p.Article in journal (Refereed) Published
Abstract [en]

Three skeletal rearrangement channels for the norbornadiene (N center dot+) to the 1,3,5-cycloheptatriene (CHT center dot+) radical cation conversion, initialized by opening a bridgehead-methylene bond in N center dot+, are investigated using the quantum chemical B3LYP, MP2 and CCSD(T) methods in conjunction with the 6-311+G(d,p) basis set. Two of the isomerizations proceed through the norcaradiene radical cation (NCD center dot+), either through a concerted path (N center dot+-NCD center dot+), or by a stepwise mechanism via a stable intermediate (N center dot+-I1- NCD center dot+). At the CCSD(T)/6-311+G(d,p)//B3LYP/6-311+G(d,p) level, the lowest activation energy, 28.9 kcal mol(-1), is found for the concerted path whereas the stepwise path is found to be 2.3 kcal mol(-1) higher. On both pathways, NCD center dot+ rearranges further to CHT center dot+ with significantly less activation energy. The third channel proceeds from N center dot+ through I1 and then directly to CHT center dot+, with an activation energy of 37.1 kcal mol(-1). The multi-step channel reported earlier by our group, which proceeds from N center dot+ to CHT center dot+ via the quadricyclane and the bicyclo[2.2.1] hepta-2-ene-5-yl-7-ylium radical cations, is 4.6 kcal mol(-1) lower than the most favorable path of the present study. If the methylene group is substituted with C(CH3)(2), however, the concerted path is estimated to be 5.6 kcal mol(-1) lower than the corresponding substituted multi-step path at the B3LYP/6-311+(d,p) level. This shows that substitution of particular positions can have dramatic effects on altering reaction barriers in the studied rearrangements. We also note that identical energies are computed for CHT center dot+ and NCD center dot+ whereas, in earlier theoretical investigations, the former was reported to be 6-17 kcal mol(-1) more stable than the latter. Finally, a bent geometry is obtained for CHT center dot+ with MP2/6-311+G(d,p) in contradiction with the planar conformation reported for this cation in earlier computational studies.

Place, publisher, year, edition, pages
2006. Vol. 4, no 22, 4241-4250 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-96117DOI: 10.1039/b612791fISI: 000241720500029PubMedID: 17312982OAI: oai:DiVA.org:uu-96117DiVA: diva2:170586
Available from: 2007-09-04 Created: 2007-09-04 Last updated: 2017-12-14Bibliographically 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
Opponent
Supervisors
Available from: 2007-09-04 Created: 2007-09-04 Last updated: 2011-04-08Bibliographically approved

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