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Ab Initio Study of Silyloxonium Ions Marek Cypryk*, and Yitzhak Apeloig

Summary: Ab Initio Study of Silyloxonium Ions
Marek Cypryk*, and Yitzhak Apeloig
Center of Molecular and Macromolecular Studies of the Polish Academy of Sciences,
90-363 Loīdzī, Sienkiewicza 112, Poland, and Department of Chemistry and the Lisa Meitner
Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of
Technology, 32000 Haifa, Israel
Received February 12, 1997X
The geometries and proton and silylenium cation (H3Si+
) affinities as well as ring strain
energies of several hydrogen-substituted cyclic siloxanes, cyclotrisiloxane (H2SiO)3 (5a),
cyclotetrasiloxane (H2SiO)4 (6a), and 1,3-oxadisilacyclopentane (7a), were calculated by ab
initio quantum-mechanical methods using the polarized 6-31* basis set. Protonation and
silylenium cation addition to siloxanes lead to secondary and tertiary silyloxonium ions,
respectively. The calculated strain energies follow the order: 7a > 5a > 6a 0. Upon
protonation or silylation, the strain in the five-membered ring of 7b and 7c is significantly
reduced, while in the cyclotrisiloxane silyloxonium ions 5b,c the strain is preserved. The
endocyclic Si-O bonds in 7a and 5a are weakened upon protonation or addition of H3Si+
more than the exocyclic bonds and are therefore predicted to be cleaved more readily by
nucleophiles, resulting in a ring opening rather than in splitting of the exocyclic SiH3 group.
7a is by ca. 10 kcal/mol more basic than the other siloxanes due to the angular strain in the


Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology


Collections: Chemistry