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Does Hydride Ion Transfer from Silanes to Carbenium Ions Proceed Via a Rate-Determining Formation of a Silicenium Ion or Via a Rate-Determining Electron Transfer? An Ab Initio Quantum Mechanical Study and a Curve-Crossing Analysis |
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| Authors: | Yitzhak Apeloig Osnat Merin-Aharoni David Danovich Alexander Ioffe Sason Shaik |
| Affiliation: | 1. Department of Chemistry, Technion—Israel Institute of Technology, 32000 Haifa, Israel
Osnat Merin-Aharoni: received her M.Sc. at the Technion in 1990 and is now a research associate at Tel Aviv Univ.;2. Department of Organic Chemistry and The Fritz Haber Center for Molecular Dynamics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel David Danovich: graduated in physics from Irkutsk State Univ. in Russia in 1982 and obtained his Ph.D. in quantum chemistry there in 1989. He was a postdoctoral fellow with Y. Apeloig at Technion, 1990–1992. In 1992 he moved to Hebrew Univ., where he is now Senior Scientific Programmer at the Fritz Haber Center. His current interests are design and implementation of codes for quantum mechanical computations and development of theoretical models for chemical systems.;3. Department of Organic Chemistry and The Fritz Haber Center for Molecular Dynamics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel Alexander Ioffe: graduated in chemistry from Leningrad State Univ. in 1971. He obtained his Ph.D. in 1975 from the N.D. Zelynsky Inst. of Organic Chemistry, USSR Academy of Sciences (Moscow) and was a postdoctoral fellow there with O. Nefedov. In 1991 he moved to Ben-Gurion Univ., where he is now a Scientific Researcher in the Dept. of Chemistry. He is also associated with the Inst. of Chemistry at Hebrew Univ. His current interests are methodology of quantum chemical computations and development of theoretical models for chemical systems. |
| Abstract: | The hydride transfer reactions from simple silanes to carbenium ions are studied by ab initio calculations. The simplest reaction, H4Si + CH3+ → H3Si+ + CH4, is also studied with inclusion of the solvent effect (with the SCRF method) in the ab initio scheme. Under all conditions the preferred mechanism is the synchronous hydride transfer (SHT), which is barrierless in the gas phase but possesses small barriers in solution. The mechanistic alternative involving a rate-determining single electron transfer (SET) step followed by H-atom abstraction is found to be of very high energy. Modelling of the primary isotope effect for the SHT process of H3SiH(D) + CH3* → H3Si+ + H3CH(D) shows that the primary isotope effect is small, between ca. 1.1 and 2.7, for the entire relevant range of Si—H(D) distances (1.5–2.3 Å). Furthermore, the pattern of the computed primary isotope effect shows it to be an insensitive probe of the SHT mechanism. The curve-crossing method is used to model the mechanistic dichotomy. It is shown that the reaction profiles for both SHT and SET arise from an avoided crossing between the ground state and a charge transfer state of the R3SiH//R′3C+ reactant pair. Thus, in the SHT mechanism a single electron switches sites in synchronicity with bond reorganization, while in SET the electron switch precedes the bond coupling. This avoided bond coupling is the foremost disadvantage of the SET mechanism. The common origin of the avoided crossing elucidates the reason why SHT exhibits characteristics of an electron transfer process without actually being a SET process. |
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