Product Detection of the CH(X²Π) Radical Reaction with Cyclopentadiene: A Novel Route to Benzene

Abstract

The reaction of the methylidyne radical (CH(X 2 Π)) with cyclopentadiene (c-C 5 H 6 ) is studied in the gas phase at 4 Torr and 373 K using a multiplexed photoionization mass spectrometer. Under multiple collision conditions, the dominant product channel observed is the formation of C 6 H 6 + H. Fitting the photoionization spectrum using reference spectra allows for isomeric resolution of C 6 H 6 isomers, where benzene is the largest contributor with a relative branching fraction of 90 (±5)%. Several other C 6 H 6 isomers are found to have smaller contributions, including fulvene with a branching fraction of 8 (±5)%. Master Equation calculations for four different entrance channels on the C 6 H 7 potential energy surface are performed to explore the competition between CH cycloaddition to a CC bond vs CH insertion into C−H bonds of cyclopentadiene. Previous studies on CH addition to unsaturated hydrocarbons show little evidence for the C−H insertion pathway. The present computed branching fractions support benzene as the sole cyclic product from CH cycloaddition, whereas fulvene is the dominant product from two of the three pathways for CH insertion into the C−H bonds of cyclopentadiene. The combination of experiment with Master Equation calculations implies that insertion must account for ∼10 (±5)% of the overall CH + cyclopentadiene mechanism.

A bonding evolution theory study of the reaction between methylidyne radical, CH(X2Π), and cyclopentadiene, C5H6

“…As Caster et al concluded in their work [14], the CH cycloaddition, driven by strong attractive potential between the reactants, is likely to dominate over insertion or abstraction mechanisms at room temperature. In addition, recent product detection experiments coupled with computational studies [18] suggest that cycloaddition is likely to account for up to 90% of the overall mechanism. Therefore, we have chosen as a test processes on which to show the applicability of our methodology on open‐shell systems, only the cycloaddition reaction between the CH (X 2 Π) radical and cyclopentadiene.…”

Formation of a Resonance-Stabilized Radical Intermediate by Hydroxyl Radical Addition to Cyclopentadiene

“…Multiple data sets using a 248 nm photolysis laser with cyclopentadiene and without OH radical precursors were collected at 373 K in a previous study. 47 Argon or krypton (for hν < 7.9 eV) are used in a gas filter to absorb harmonics of the undulator radiation. For data recorded below 8.0 eV with argon in the gas filter, a MgF 2 window is placed in the path of the ionizing radiation to suppress harmonics of the undulator fundamental photon energy.…”

“…The methodology has been described in detail by Glowacki et al 61 and has been employed to investigate OH radical reactions with unsaturated hydrocarbons 60 as well as CH radical reaction with ammonia 62 and cyclopentadiene. 47 Isomerization and decomposition unimolecular rates of the reaction intermediates along the PES were calculated using Rice−Ramsperger−Kassel−Marcus (RRKM) theory. Energy transfer processes were modeled using a constant average energy transferred, ΔE down .…”

“…Reactions of primary products with remaining radicals or other photoproducts in the flow may lead to the formation of secondary products over the experimental reaction time with a slower formation. 47,65 It is therefore possible to discriminate between primary and secondary reactions by integrating mass spectra and photoion spectra over the first few milliseconds of reaction time. Products showing a slow initial formation will have a smaller contribution to the spectra.…”

PAH Growth in Flames and Space: Formation of the Phenalenyl Radical

“…50 Additionally, expansion of a five-membered ring to a sixmembered ring has been demonstrated through the reaction of pyrrole with the methylidyne radical (CH) to form pyridine 51 and the reaction of cyclopentadiene with CH to form benzene (Figure 1c). 52,53 CH in its ground state has been detected in combustion environments, 54−56 in the interstellar medium, 57,58 and under plasma conditions. 59 Reactions between CH and small unsaturated hydrocarbons and carbonyls can result in CH insertion to a π-bond through a cyclic intermediate (cycloaddition) followed by ring-opening.…”