The electron-impact-induced ionization and fragmentation of six CsHlo structural isomers have been studied in order to determine the effect of isomerism upon their mass spectrometric behavior. The 70 eV mass spectra, metastable transitions and appearance potentials of the principal 299 * R. E. Winters and J. H. Collins, unpublished results.

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Downloaded 26 Sep 2012 to 136.159.235.223. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions ION-MOLECULE REACTIONS OF METItANE 1931 methane. The reactions C2H2++CH.-+C2Ha++CHa (0.47) (24) -+CaHa++H2+H (0.03) (25) -+C a H4++H2 (0.09) (26) -+C a H 5 ++H·, (0.41) (27) C2Ha++CH.-+CaH5++H2 (0.77) (28) -+CaHs++H·, (0.23)were observed with percentages of the reaction paths as noted in parentheses. These percentages were the same for C2H2+ from ethylene or acetylene and for C2Ha+ from ethylene or ethane. We further agree with the findings of Munson and Field l5 that there is a large difference in the reaction rates for C2Ha+ and C2H2+' They find that the C 2 H 2 + species reacts about five times faster than C2Ha+, and we observe an approximately 30-fold difference in reactivity. We attempted to follow the ionic polymerization chain one step further by impacting C 3 ions on methane. For C3H5+ from propylene and cyclopropane and C3H4+ and CaHa+ from propylene we did not observe any reaction products, from which we conclude that chain-lengthening ion-molecule reactions of these ions with methane have cross sections at least two orders of magnitude smaller than those of the methane primary ions.An energy-distribution difference method for determining ionization potentials and for detecting fine structure, i.e., breaks, in ionization-efficiency curves is described which eliminates the difficulties arising from the thermal-energy spread of the ionizing electrons. Although the technique gives essentially the same results as monoenergetic methods, it is much easier experimentally. Ionization-efficiency curves are given for the inert-gas ions Ar+, Kr+, and Xe+ as well as the molecular ions CO+, N 2 +, and O2+, The "breaks" in the efficiency curves are presented and the results are compared to previously reported monoenergetic electron-impact data.

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The mass spectra, metastable transitions, and the appearance potentials of several ions have been obtained for four monocyclic and two bicyclic GHI2 isomers. The molecular ionization potentials determined by the energy distribution difference (EDD) technique are: 8.67 + 0.02, 8.94 f 0.03, 8.92 f 0.02, 9.04 f 0.03, 9.93 =t 0.02, and 9.03 f 0.02 eV for 1-methylcyclohexene, 3-methylcyclohexene, 4-methylcyclohexene, methylenecyclohexane, bicyclo[2.2.l]heptane, and bicyclo[4.1 .O]heptane, respectively. It is shown that these ionization potentials are dependent upon structural features of the isomeric molecules, e.g., double-bond substitution and ring strain. The differences between the calculated heats of formation for the gaseous parent molecule ions are discussed in terms of the structural differences between the original molecules. Heats of formation for fragment ions indicate that ionization-fragmentation usually produces the same ionic species independent of the original molecules.he 70-eV mass spectra of many molecules, especially d a Calculated from reported heat of combustion data [A.