Article Mechanistic Study of Thermal Decomposition of Isoprene (2-Methyl-1,3-Butadiene) Using Flash Pyrolysis Supersonic Jet VUV PhotoionizationMass Spectrometry
Kevin H. Weber and Jingsong Zhang*
Department of Chemistry, University of California, Riverside, California 92521 J. Phys. Chem. A, 2007, 111 (45), pp 11487–11492 DOI: 10.1021/jp075689+
Abstract The thermal decomposition of isoprene up to 1400 K was performed by flash pyrolysis with an 100 μs time scale. This pyrolysis was followed by supersonic expansion to isolate the reactive intermediates and initial products, and detection was accomplished by vacuum ultraviolet single photon ionization time-of-flight mass spectrometry (VUV−SPI−TOFMS) at λ = 118.2 nm. Products CH3, C2H4, C3H3, C3H4, C4H4, C4H5, C5H6, C5H7, and C6H6 were directly observed and provide mechanistic insights to the isoprene pyrolysis. At temperatures ≥ 1200 K, the molecular elimination of ethene to form C3H4 and σ bond homolysis producing C4H5 and CH3 radicals are competitive reaction pathways. The molecular elimination of acetylene to form C3H6 was minimal and direct C2−C3 σ bond homolysis was not observed. The C3H3 radicals are also observed, as a result of hydrogen loss of C3H4 by pyrolysis or hydrogen abstraction by the CH3 radical from C3H4. Above 1250 K, production of C6H6 was observed and identified as the combination product of the C3H3 radicals.
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