Kinetics of the Self Reaction of Cyclopentadienyl Radicals

Abstract

The kinetics of the self-reaction of cyclopentadienyl radicals (c-C5H5) was studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 304-600 K and at bath gas densities of (3.00-12.0) × 10(16) molecules cm(-3). The room-temperature value of the rate constant, (3.98 ± 0.41) × 10(-10) cm(3) molecule(-1) s(-1), is significantly higher than the rate constants for most hydrocarbon radical-radical reactions and coincides with the estimated collision rate. The observed overall c-C5H5 + c-C5H5 rate constant demonstrates an unprecedented strong negative temperature dependence: k1 = 2.9 × 10(-12) exp(+1489 K/T) cm(3) molecule(-1) s(-1), with estimated uncertainty increasing with temperature, from 13% at 304 to 32% at 600 K. Formation of C10H10 as the primary product of cyclopentadienyl self-reaction was observed. In additional experiments performed at the temperature of 800 K, formation of C10H10, C10H9, and C10H8 was observed. Final product analysis by gas chromatography/mass spectrometry detected two isomers of C10H8 at 800 K: naphthalene (major) and azulene (minor).

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

“…Another prominent peak at m / z 64 suggests the formation of C 5 H 4 species, ethynylallene, methyldiacetylene, and pentatetraene, which are identified by fitting their absolute reference spectra to the experimental photoion spectrum (Figure S3). The m / z 64 products are likely to be the result of H 2 loss from the cyclopentadiene reactant and have been previously reported in CPD photolysis studies. , The shape of the photoionization spectrum of C 5 H 4 ( m / z 64) in Figure S3 is also noticeable in the experimental m / z 65 spectrum, supporting the assignment of a portion of the m / z 65 signal to the 13 CC 4 H 4 isotopologue of m / z 64, formed from CPD photolysis. The resonantly stabilized radical propargyl (C 3 H 3 ) is identified at m / z 39 by its ionization energy onset at ∼8.65 eV and through comparison of the recorded photoionization spectrum with reference spectra .…”

“…The corresponding kinetic time trace in Figure a displays a fast increase and a slower decay that is consistent with the formation of a resonantly stabilized radical. The vinyl radical (C 2 H 3 ) is also observed at m / z 27 as a propargyl radical coproduct . The vinyl radical is expected to be much more reactive than the resonantly stabilized propargyl radical and thus accumulate in lower concentrations than those reported for m / z 39.…”

Pressure dependent kinetic analysis of pathways to naphthalene from cyclopentadienyl recombination

“…For the CPDyl + CPDyl association rate, the high pressure limit rate expression from Knyazev and Popov is used. This value was determined experimentally for a range of 300-600K [36]. Sensitivities for these assumed rates are included in the supporting information.…”

“…More recently, in 2015, Knyazev and Popov experimentally investigated the total self-reaction rate of CPDyl, finding it to show significantly faster kinetics when compared with other similar radical selfreactions. Naphthalene and Azulene were detected as final products with the former being the major product [36].…”

Molecular mass growth through ring expansion in polycyclic aromatic hydrocarbons via radical–radical reactions

“…However, the elucidation of the fundamental reaction pathways to polycyclic aromatic hydrocarbons (PAHs)—organic molecules composed of fused benzene rings with naphthalene (C 10 H 8 ) denoting the simplest representative—in combustion environments 1 and in the interstellar medium (ISM) 2 has posed a long-standing challenge. Sophisticated combustion 3 and astrochemical reaction networks 4 along with flame sampling studies 5 and astronomical surveys 2 reveal that PAHs represent the critical link between resonantly stabilized free radicals (RSFRs) like propargyl (C 3 H 3 • ), allyl (C 3 H 5 • ), and cyclopentadienyl (C 5 H 5 • ) 6,7 and carbonaceous nanoparticles universally referred to as soot and interstellar grains on Earth and in extraterrestrial environments, respectively 2,8,9 . Whereas on Earth, PAHs represent undesirable toxic, often carcinogenic byproducts released in incomplete combustion 10 , in the interstellar medium, PAHs embody up to 20% of the galactic carbon budget 11 and are—along with carbonaceous grains—suggested to play a central role in the formation of vital precursors to molecular building blocks of life such as amino acids 8 .…”