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

• Published in: Journal of mass spectrometry. 2001-07, Vol.36 (7), p.816-824
• Main Authors: Tajima, Susumu, Sekiguchi, Osamu, Watanabe, Yuko, Nakajima, Satoshi, Takahashi, Yutaka
• Format: Article
• Language: English
• Subjects: C2H4 loss; CH2O loss; CH3OCH3 loss; Chemistry; Exact sciences and technology; Mass spectrometry; MIKE spectrometry; Organic chemistry; organosilicon compound; Reactivity and mechanisms
• ISSN: 1076-5174; 1096-9888
• DOI: 10.1002/jms.184

Unimolecular metastable fragmentations of dimethoxydimethylsilane, (CH3)2Si(OCH3)2 (MW 120, 1), and dimethoxymethylsilane, CH3SiH(OCH3)2 (MW 106, 2), upon electron impact ionization have been studied by means of mass‐analyzed ion kinetic energy (MIKE) spectrometry and the D‐labeling technique in conjunction with thermochemistry. The results have been compared with those of the corresponding carbon analogues, 2,2‐dimethoxypropane, (CH3)2C(OCH3)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 formed 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 decompositions are followed by the loss of a formaldehyde molecule (CH2O), as commonly observed in the mass spectra of methoxysilanes. Further, an ethylene (C2H4) or a dimethyl ether (CH3OCH3) molecule loss is observed in the fragmentation 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 these fragmentations are also different than those reported previously. The relative abundance of the ions in many MIKE spectra is explained by the extension of the Stevenson–Audier rule. The reaction, which is in contrast to the rule, however, is rationalized by the energy of the transition state for the reaction, estimated by semi‐empirical molecular orbital calculation. The peak at m/z 59 from 2+· consists only of CH3OSi+ ion, whereas the peak from 1+· consists of two different ions, CH3OSi+ and (CH3)2Si+H. The ions CH3OSi+ from 1+· and 2+· are generated by at least two and three separate routes respectively. Copyright © 2001 John Wiley & Sons, Ltd.