PROPERTIES OF R(3)SIX COMPOUNDS AND R(3)SI(+) IONS - DO SILYLIUM IONS EXIST IN SOLUTION

More than 40 Si compounds comprising R(3)SiX (R = H or CH3; X = H, CH3, CN, OH, Cl, OClO3), R(3)SiX(S) (S = NH3, H2O, HCl), and R(3)Si(S)(n)(+) (S = NH3, HCN, CH3CN, H2O, (CH3)(2)O, HCl, CH3Cl for n = 1; HCN, NH3, H2O for n = 2; H2O for n = 3, 4, 5) have been investigated at the Hartree-Fock level with both the 6-31G(d) and the 6-311G(d,p) basis sets. IGLO/[7s6p2d/5s4p1d/3s1p] NMR chemical shift calculations have been carried out at optimized HF/6-31G(d) and HF/6-311G(d,p) geometries. Solvent effects on calculated chemical shifts have been determined with the PISA continuum model. In addition, the nature of SiX or SiS interactions has been investigated on the basis of calculated electron density and energy density distributions. R(3)Si(+) ions (R = Me, Et) possess in the gas phase delta Si-29 values at 400 ppm, in noncoordinating solvents between 370 and 400 ppm, and in very weakly coordinating solvents between 200 and 370 ppm. In weak or normal nucleophilic solvents, silylium cations react with one or more solvent molecules to form tetra- or pentacoordinated covalently bonded Si compounds with complexation energies that can be as high as 100 kcal/mol and delta Si-29 values between -50 and 190 ppm (SiR(3)-(S)(+)) or -30 and 210 ppm (SiR(3)(S)(2)(+)). Any silylium cation character is lost in these compounds. There are chances of generating silylium cations in solution, but silyl, perchlorates are not good starting compounds for this purpose. Calculations indicate that carbocations differ from silylium ions in so far as their positive charge is largely delocalized due to hyperconjugative and inductive effects, and therefore, they interact much weaker-with solvent molecules than silylium cations.