Unimolecular Dissociation of CH3SiH2(1+) and CH3Si(Cl)H(1+) in the Gas Phase

Article abstract of DOI:10.1021/j100151a014

The mechanismus for the lowest energy barrier pathways for unimolecular dissociation of CH3SiH2(1+) and CH3Si(Cl)H(1+) were examined in the gas phase by using Fourier transform mass spectrome (FTMS).Collision activated dissociation (CAD) by using sustained off-resonance irradiation (SORI) was used to determine the lowest energy pathways for dissociation.The lowest energy pathway for decomposition of CH3SiH2(1+) is dehydrogenation.The mechanism for this dehydrogenation process was investigated by studying the decomposition of CH3SiD2(1+).Unfortunately, isotopic scrambling by reversible 1,2-hydrogen migrations precede dehydrogenation.Hence, no mechanistic information is obtained from this isotopic labeling experiment.SORI-CAD of CH3SiD2(1+) yields dehydrogenation as H2 (0.67) and HD (0.33) with no D2 loss.The lowest energy pathway for dissociation of CH3Si(Cl)H(1+) is elimination of HCl.In contrast to CH3SiD2(1+), CH3Si(Cl)D(1+) does not undergo isotopic scrambling upon CAD.SORI-CAD of CH3Si(Cl)D(1+) yields exclusive elimination of HCl (1,2-elimination) to yield CH2SiD(1+).Hence, the lowest energy pathway for dissociation of CH3Si(Cl)H(1+) is 1,2-elimination of HCl. 1,1-Elimination of DCl from CH3Si(Cl)D(1+) to yield CH3Si(1+) is 38 kcal/mol more favorable than the 1,2-elimination process.Consequently, there must be a prohibitive barrier for the energetically more favorable 1,1-elimination process.