| The slow unimolecular reactions of organosilicon ions have been investigated with MIKES spectroscopy, electronic structure calculations, and phase space theory. The lowest energy dissociation pathways of silylenium ions have been found to involve eliminations of a stable neutral molecule. Many of these dissociations involve group migrations to the silylenium ion center resulting in the formation of carbenium ions. These carbenium ions are higher in energy than the silylenium ion isomers and in some cases have been found to be transition states on the potential energy surface. A prominent dissociation pathway for a number of silylenium ions is loss of ethylene. Ions containing ethyl groups, H(R1)Si(C2H5)+, with R 1 = H, Cl, CH3 and C2H5 along with Si(CH3)(C2H5)2+ lose ethylene from the ethyl moiety. The mechanism for these dissociations is either a beta-hydrogen migration pathway or sequential 1,2-hydrogen migrations. The dimethyl substituted ions, R2Si(CH3)2 +, R2 = H, Cl and CH3 also undergo ethylene loss. The loss of ethylene can occur from the two methyl groups by either of two mechanisms. The first involves simultaneous transfer of hydrogens from the methyl groups to the silicon center, forming the ion-molecule complex, R2SiH2+ &cdots; C2H4. The second involves the formation of the alpha-silylcarbenium ions, (R2)HSi(CH3)(CH2+), which are transition states on the potential energy surface connecting the R2Si(CH3)2+ and H(R 2)Si(C2H5)+ ions. The experimental results indicate that the SiC2H7+ isomers, HSi(CH3)2+ and H2Si(C 2H5)+, interconvert prior to dissociation while the SiC3H9+ isomers, Si(CH 3)3+ and HSi(CH3)(C2H 5)+, and the SiC2H6Cl+ isomers, ClSi(CH3)2+ and H(Cl)Si(C 2H5)+ do not. Silylenium ions containing a single methyl group, H(R4)Si(CH3)2 + (R4 = H and Cl) undergo elimination of HR 4. The KERDs for these dissociations are bimodal, indicating that two different processes are occurring. The narrow component of the KERD is due to 1,2-eliminations and the large component is due to 1,1-eliminations. Five trimethylsubstituted ions have been studied, (CH3)3Si-Si(CH 3)2+ and (CH3)3Si-X-Si(CH 3)2+ (X = O, CH2, NH and C≡C). All ions studied exhibit loss of CH4, which has not been observed for any of the monosilicon containing ions. The mechanism for this dissociation is believed to occur across silicon centers and to result in the formation of disilacyclobutyl ions. |
