Mechanistic study of thermal decomposition of isoprene (2-methyl-1,3-butadiene) using flash pyrolysis supersonic jet VUV photoionization mass spectrometry

The thermal decomposition of isoprene up to 1400 K was performed by flash pyrolysis with an approximately 100 mus 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 lambda = 118.2 nm. Products CH(3), C(2)H(4), C(3)H(3), C(3)H(4), C(4)H(4), C(4)H(5), C(5)H(6), C(5)H(7), and C(6)H(6) were directly observed and provide mechanistic insights to the isoprene pyrolysis. At temperatures >or= approximately 1200 K, the molecular elimination of ethene to form C(3)H(4) and sigma bond homolysis producing C(4)H(5) and CH(3) radicals are competitive reaction pathways. The molecular elimination of acetylene to form C(3)H(6) was minimal and direct C(2)-C(3) sigma bond homolysis was not observed. The C(3)H(3) radicals are also observed, as a result of hydrogen loss of C(3)H(4) by pyrolysis or hydrogen abstraction by the CH(3) radical from C(3)H(4). Above approximately 1250 K, production of C(6)H(6) was observed and identified as the combination product of the C(3)H(3) radicals.