Irradiation of Isobutene with 8.4-, 10-, and 11.6–11.8-eV Photons. Decomposition of Neutral Excited Isobutene and Reactions of C4H8+

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Abstract
Isobutene has been irradiated with 8.4‐, 10‐, and 11.6–11.8‐eV photons in the presence and absence of scavengers such as O2, NO, and H2S. Over the entire energy range, 80% of the neutral excited iso‐C4H8 dissociates as follows: iso‐C4H8*→C3H4+CH3+H. Minor primary fragmentations include: iso‐C4H8*→C3H6+CH2 and iso‐C4H8*→CH4+C3H4. Isotopic analysis of the products formed in the photolysis of (CD3)2CCH2 and iso‐C4H8–iso‐C4D8 mixtures throws some light on the mechanistic details. At 10 and 11.6–11.8 eV, iso‐C4H8+ ions are formed, with photoionization quantum yields of 0.26 and 0.31, respectively. Thirty percent of these ions react to yield isobutane as a product. The mechanism seems to involve internally excited ions which take part in a proton‐transfer reaction: C4H8++C4H8→C4H9++C4H7. When iso‐C4D8 is irradiated in the presence of alkanes (AH2), the following H2‐transfer reaction is shown to occur: C4D8++AH2→(CD3)2CHCD2H+A+. Experiments performed with a partially deuterium‐labeled isopentane demonstrate that the latter reaction is highly stereospecific.