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

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, N H3, 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 se ts. IGLO/[7s6p2d/5s4p1d/3s1p] NMR chemical shift calculations have bee n carried out at optimized HF/6-31G(d) and HF/6-311G(d,p) geometries. Solvent effects on calculated chemical shifts have been determined wit h the PISA continuum model. In addition, the nature of SiX or SiS inte ractions has been investigated on the basis of calculated electron den sity and energy density distributions. R(3)Si(+) ions (R = Me, Et) pos sess in the gas phase delta Si-29 values at 400 ppm, in noncoordinatin g solvents between 370 and 400 ppm, and in very weakly coordinating so lvents between 200 and 370 ppm. In weak or normal nucleophilic solvent s, silylium cations react with one or more solvent molecules to form t etra- or pentacoordinated covalently bonded Si compounds with complexa tion energies that can be as high as 100 kcal/mol and delta Si-29 valu es 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. T here are chances of generating silylium cations in solution, but silyl , perchlorates are not good starting compounds for this purpose. Calcu lations indicate that carbocations differ from silylium ions in so far as their positive charge is largely delocalized due to hyperconjugati ve and inductive effects, and therefore, they interact much weaker-wit h solvent molecules than silylium cations.