Modes and Rates of Reaction of Cyclopentene and Methylcyclopentene Ions with Their Parent Molecules

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Abstract
Cyclopentene and methylcyclopentene ions were generated by irradiating the respective parent compounds with 10.0-eV photons; cyclopentene was also irradiated with 11.6–11.8-eV photons. The ionic products of ion–molecule reactions were observed in the NBS high-pressure photoionization mass spectrometer, while the neutral products of these reactions were determined by chemical analysis of products formed in photolytic experiments in a closed system. The cyclopentene ion, which at 10.0 eV retains its cyclic structure, undergoes an H2 transfer reaction (c-C5H8++c-C6H8→c-C5H10+c-C5H6+, k = 3.3 × 10−10cm3/molecule·sec) and a condensation reaction (c-C5H8++c-C5H8→C10H16, k = 2.7 × 10−10cm3/molecule·sec) with the parent molecule. The same reactions are observed for ions formed at 11.6–11.8 eV, but at the higher energy, approximately 20% of the ions are observed to undergo ring opening to form 1,3-C5H8+ ions; the latter ions undergo an H2 transfer reaction with the cyclopentene molecule (1,3-C5H8++c-C5H8→2-C5H10+C5H6+, k = 1 × 10−10cm3/molecule·sec). The three methylcyclopentene ions also undergo H2 transfer reactions with the corresponding parent molecules [C6H10++C6H10→(CH3)C5H9+C6H8+, k = 4.6, 1.8, and 2.9 × 10−10cm3/molecule·sec for 1-, 3-, and 4-methylcyclopentene, respectively]. Condensation reactions between these ions and their parent molecules are observed to only a minor extent, but a second major mode of reaction, namely H transfer, is observed (C6H10++C6H10→C6H11+C6H9+, k = 0.6, 2.8, and 1.2 × 10−10cm3/molecule·sec for 1-, 3-, and 4-methylcyclopentene, respectively). Structural reasons for these rate variations are discussed.