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+
Publication Date (Web): October 23, 2007
Copyright © 2007 American Chemical Society
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.