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

journal contribution

posted on 07.08.1998, 00:00 by Belinda B. Willard, Susan T. GraulThe slow unimolecular dissociations (

*k*≈ 10^{5}s^{-1}) of gas-phase silylenium ions, SiC*H*_{n}_{2}_{n}_{+3}^{+}ions (*n*= 2−4) have been studied by mass-analyzed ion kinetic energy spectroscopy. On the microsecond time scale, the SiC_{2}H_{7}^{+}isomers undergo dissociations corresponding to H_{2}(72%) and C_{2}H_{4}loss (28%). The product ion translational energy distributions and product ratios are the same for HSi(CH_{3})_{2}^{+}and H_{2}Si(C_{2}H_{5})^{+}, indicating that these isomers equilibrate prior to dissociation. For the SiC_{3}H_{9}^{+}isomers, elimination of C_{2}H_{4}is the dominant reaction pathway, comprising 97% of the products for Si(CH_{3})_{3}^{+}and 89% for HSi(CH_{3})(C_{2}H_{5})^{+}. The translational energy distributions for elimination of ethylene from these two ions are different, indicating that equilibration of Si(CH_{3})_{3}^{+}and HSi(CH_{3})(C_{2}H_{5})^{+}does not occur prior to dissociation. The mechanisms for C_{2}H_{4}loss from the Si(CH_{3})_{3}^{+}and HSi(CH_{3})(C_{2}H_{5})^{+}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 SiC_{2}H_{7}^{+}isomers and from HSi(CH_{3})(C_{2}H_{5})^{+}. The calculated distribution was broader than the experimental distribution for Si(CH_{3})_{3}^{+}. Possible reasons for this result are discussed. The microsecond unimolecular dissociation of Si(CH_{3})_{2}(C_{2}H_{5})^{+}proceeds exclusively by elimination of C_{2}H_{4}, which arises from the ethyl group and not from the two methyl groups.#### Read the peer-reviewed publication

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HSienergy spectroscopyisomers equilibrateethylene lossproduct ratiosproduct ion translational energy distributionsmicrosecond time scaleC 2 H 4 lossmicrosecond unimolecular dissociationC 2 H 4C 2 H 5methyl groupsPossible reasonsMetastable Isomeric Silylenium Ionstranslational energy distributionseliminationSiC 2 H 7unimolecular dissociationsSiC 3 H 9ethyl groupExcellent agreementab initiophase space modelingreaction pathway

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#### Read the peer-reviewed publication

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HSienergy spectroscopyisomers equilibrateethylene lossproduct ratiosproduct ion translational energy distributionsmicrosecond time scaleC 2 H 4 lossmicrosecond unimolecular dissociationC 2 H 4C 2 H 5methyl groupsPossible reasonsMetastable Isomeric Silylenium Ionstranslational energy distributionseliminationSiC 2 H 7unimolecular dissociationsSiC 3 H 9ethyl groupExcellent agreementab initiophase space modelingreaction pathway