Cyclopentene and cyclopentadiene formation in isoprene pyrolysis.

Photoion mass-selected threshold photoelectron spectroscopy (ms-TPES) was used to identify the isoprene pyrolysis products in a SiC microreactor at 1400 °C with the help of literature and Franck-Condon simulated reference spectra for molecular species at the detected m / z ratios. The key observation is the presence of equimolar amounts of isoprene and cyclopentene at the pyrolysis temperature based on the m / z 68 ms-TPES, indicating kinetically allowed isoprene isomerization concurrently with fragmentation reactions. This isomerization was computationally explored and was found to take place via a short-lived vinylcyclopropane intermediate, which was previously proposed to isomerize into isoprene and cyclopentene, with the latter product being dominant. Cyclopentene then decomposes by loss of H 2 to form m / z 66, cyclopentadiene (also observed). Previously postulated products of dimethylallene, methylallene, and allene were not observed. Of the possible C2-C4-products, the extracted ms-TPES confirmed only 1,3-butadiene and 2-butyne ( m / z 54), 1-buten-3-yne ( m / z 52), propene ( m / z 42), propyne ( m / z 40), propargyl radical ( m / z 39), as well as C 2 H 4 , C 2 H 2 , CH 4 , and CH 3 . A trace amount of benzene was also observed at m / z 78, indicative of bimolecular chemistry. The results draw into question a number of the suggested unimolecular reaction products in the recent literature and thus the kinetic models for isoprene pyrolysis.