Metastable ion study of organosilicon compounds. Part XIII: dimethoxydimethylsilane, (CH3)(2)Si(OCH3)(2), and dimethoxymethylsilane, CH3SiH(OCH3)(2)

Unimolecular metastable fragmentations of dimethoxydimethylsilane, (CH3)(2) Si(OCH3)(2) (MW120, 1), and dimethoxymethylsilane, CH3SiH(OCH3)(2) (MW 106, 2), upon electron impact ionization have been studied by means of mass-ana lyzed ion kinetic energy (MIKE) spectrometry and the D-labeling technique i n conjunction with thermochemistry. The results have been compared with tho se of the corresponding carbon analogues, 2,2-dimethoxypropane, (CH3)(2)C(O CH3)(2) (MW 104, 3) and 1,1-dimethoxyethane, CH3CH(OCH3)(2) (MW 90, 4). In analogy with the cases of 3 and 4, both molecular ions from 1 and 2 are for med at very low abundance at 70 eV, and begin to decompose by the expulsion of the substituents (H, CH3 or OCH3) on the central silicon atom. These de compositions are followed by the loss of a formaldehyde molecule (CH2O), as commonly observed in the mass spectra of methoxysilanes. Further, an ethyl ene (C2H4) or a dimethyl ether (CH3OCH3) molecule loss is observed in the f ragmentation of some intermediate ions generated from 1(+.) and 2(+.), but the mechanisms are different than those in the cases of 3 and 4. Some of th ese fragmentations are also different than those reported previously. The r elative abundance of the ions in many MIKE spectra is explained by the exte nsion of the Stevenson-Audier rule. The reaction, which is in contrast to t he rule, however, is rationalized by the energy of the transition state for the reaction, estimated by semi-empirical molecular orbital calculation. T he peak at m/z 59 from 2(+.) consists only of CH3OSi+ ion, whereas the peak from 1(+.) consists of two different ions, CH3OSi+ and (CH3OSi+ from 1(+.) and 2(+.) are generated by at least two and three separate routes respecti vely The ions CH respectively. Copyright (C) 2001 John Wiley & Sons, Ltd.