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 / ratios. The key observation is the presence of equimolar amounts of isoprene and cyclopentene at the pyrolysis temperature based on the / 68 ms-TPES, indicating kinetically allowed isoprene isomerization concurrently with fragmentation reactions. This isomerization was computationally explored and was found to take place 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 to form / 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 (/ 54), 1-buten-3-yne (/ 52), propene (/ 42), propyne (/ 40), propargyl radical (/ 39), as well as CH, CH, CH, and CH. A trace amount of benzene was also observed at / 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.