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Elimination of Ethylene from Metastable Isomeric Silylenium Ions in the Gas Phase:  Experiment and Theory

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journal contribution
posted on 07.08.1998, 00:00 by Belinda B. Willard, Susan T. Graul
The slow unimolecular dissociations (k ≈ 105 s-1) of gas-phase silylenium ions, SiCnH2n+3+ ions (n = 2−4) have been studied by mass-analyzed ion kinetic energy spectroscopy. On the microsecond time scale, the SiC2H7+ isomers undergo dissociations corresponding to H2 (72%) and C2H4 loss (28%). The product ion translational energy distributions and product ratios are the same for HSi(CH3)2+ and H2Si(C2H5)+, indicating that these isomers equilibrate prior to dissociation. For the SiC3H9+ isomers, elimination of C2H4 is the dominant reaction pathway, comprising 97% of the products for Si(CH3)3+ and 89% for HSi(CH3)(C2H5)+. The translational energy distributions for elimination of ethylene from these two ions are different, indicating that equilibration of Si(CH3)3+ and HSi(CH3)(C2H5)+ does not occur prior to dissociation. The mechanisms for C2H4 loss from the Si(CH3)3+ and HSi(CH3)(C2H5)+ ions were characterized by ab initio computational methods, and the results were used for statistical phase space modeling of the experimental translational energy distributions. Excellent agreement between theory and experiment was obtained for ethylene loss from the SiC2H7+ isomers and from HSi(CH3)(C2H5)+. The calculated distribution was broader than the experimental distribution for Si(CH3)3+. Possible reasons for this result are discussed. The microsecond unimolecular dissociation of Si(CH3)2(C2H5)+ proceeds exclusively by elimination of C2H4, which arises from the ethyl group and not from the two methyl groups.