AB-INITIO STUDY OF SILYLOXONIUM IONS

The geometries and proton and silylenium cation (H3Si+) affinities as well as ring strain energies of several hydrogen-substituted cyclic si loxanes, cyclotrisiloxane (H2SiO)(3) (5a), cyclotetrasiloxane (H2SiO)( 4) (6a), and 1,3-oxadisilacyclopentane (7a), were calculated by ab ini tio quantum-mechanical methods using the polarized 6-31 basis set. Pr otonation and silylenium cation addition to siloxanes lead to secondar y and tertiary silyloxonium ions, respectively. The calculated strain energies follow the order: 7a > 5a > 6a approximate to 0. Upon protona tion or silylation, the strain in the five-membered ring of 7b and 7c is significantly reduced, while in the cyclotrisiloxane silyloxonium i ons 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 ex ocyclic bonds and are therefore predicted to be cleaved more readily b y nucleophiles, resulting in a ring opening rather than in splitting o f the exocyclic SiH3 group. 7a is by ca. 10 kcal/mol more basic than t he other siloxanes due to the angular strain in the five-membered ring . Its basicity is comparable to that of dialkyl ethers and alkoxysilan es. A linear correlation was found between the gas-phase proton and H3 Si+ affinities. On the basis of SCRF calculations interaction with sol vent (cyclohexane or CH2Cl2) has only a moderate effect on the energie s of protonation and ring-opening reactions. The role of silyloxonium ions as possible active centers in the cationic ring-opening polymeriz ation of cyclosiloxanes is discussed in light-of the calculated basici ties and ring strain energies. The calculations suggest that the polym erizations of the cyclic monomers 5a, 6a, and 7a should reveal differe nt kinetic and thermodynamic behavior. 7a is predicted to be the most reactive monomer, and its polymerization is the most favored thermodyn amically.