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SH2 reaction vs. hydrogen abstraction/expulsion in methyl radical-methylsilane reactions: Effects of prereactive complex formation
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Quantum Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Quantum Chemistry.
2008 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 112, no 6, 1330-1338 p.Article in journal (Refereed) Published
Abstract [en]

A quantum chemical study has been undertaken to elucidate the cause of the recently observed S(H)2 reaction between the deuterated methyl radical ((CD3)-C-center dot) and methylsilane (SiD3CH3) following the photolysis of CD3I. [Komaguchi, K.; Norberg, D.; Nakazawa, N.; Shiotani, M.; Persson, P.; Lunell, S. Chem. Phys. Lett. 2005, 410, 1-5.] It is found that the backside SH2 mechanism may proceed favorably for C-Si-C angles deviating with up to 40 degrees from linearity. The competitive hydrogen abstraction reaction is predicted to be active in the range of 90 degrees <= C-Si-C <= 135 degrees. For steeper attack angles, the frontside S(H)2 mechanism is activated. However, high barriers along the corresponding reaction paths probably make the frontside mechanism less important for the present S(H)2 reaction. A number of bound SiH3CH3/CH3I complexes have been located with the MP2 method. At the CCSD(T) level, a complex corresponding to the collinear arrangement where the methyl moiety of methyl iodide points toward the silicon, which is the most favorable conformation for the subsequent S(H)2 reaction with the backside mechanism, is found to be the most stable linear conformer. A complex with similar energy is found where the methyl moiety of methyl iodide points approximately toward an Si-H bond. However, because C-Si-C = 69.4 degrees in this complex, subsequent photolysis of methyl iodide would probably not lead to hydrogen abstraction with full efficiency. These findings could provide an explanation for the observed S(H)2 reaction.

Place, publisher, year, edition, pages
2008. Vol. 112, no 6, 1330-1338 p.
National Category
Chemical Sciences
URN: urn:nbn:se:uu:diva-96121DOI: 10.1021/jp076591yISI: 000252967900031OAI: oai:DiVA.org:uu-96121DiVA: diva2:170590
Available from: 2007-09-04 Created: 2007-09-04 Last updated: 2013-04-04Bibliographically 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.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 330
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
urn:nbn:se:uu:diva-8178 (URN)978-91-554-6949-8 (ISBN)
Public defence
2007-09-26, Polhemsalen, Ångströmlaboratoriet, Uppsala, 10:15
Available from: 2007-09-04 Created: 2007-09-04 Last updated: 2011-04-08Bibliographically approved

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