Very low‐pressure pyrolysis (VLPP) of pentynes. III. Pent‐2‐yne. Heat of formation and resonance stabilization energy of the 3‐methylpropargyl radical

Abstract

The thermal unimolecular decomposition of pent-2-yne has been studied over the temperature range of 988-1234 K using the technique of very low-pressure pyrolysis (VLPP). The main reaction pathway is Cd-Ch bond fission producing the resonance-stabilized 3-methylpropargyl radical. There is a concurrent process producing molecular hydrogen and penta-lA4-triene presumably via the intermediate formation of cis-penta-1,3-diene. The 1,4-hydrogen elimination from cis-penta-1,3-diene is the rate-determining step in the molecular pathway. This is supported by an independent VLPP study of cis-and transpenta-1,3-diene. RRKM calculations show that the experimental rate constants for C-C bond fission are consistent with the following high-pressure rate expression a t 1100 K: = 87.4 kcal/mol. The resonance stabilization energy of the 3-methylpropargyl radical is 10.6 f 2.5 kcal/mol, which is consistent with previous results for this and other propargylic radicals.

Photofragment translational spectroscopy of 1,2-butadiene at 193 nm

“…Figure 1 shows the accessible photodissociation channels for 1,2-butadiene at 193 nm ͑148.1 kcal/mol͒. Figure 1 was constructed using previously determined experimental and theoretical heats of formation; [13][14][15][16][17][18][19][20] For m/eϭ39 and 15, all TOF spectra were collected using molecular beams seeded in He, while TOF spectra for m/eϭ53 were obtained for Ne-and He-seeded beams. Figure 2 shows TOF spectra for m/eϭ39(C 3 H 3 ϩ ) at laboratory angles ⌰ LAB ϭ15°and 30°.…”

Aromatic and Polycyclic Aromatic Hydrocarbon Formation in a Premixed Propane Flame

“…This was due to the low predicted levels of HCCHCCH and CH 2CHCHCH in the flame which prevented any significant net benzene production from these pathways. Consideration was given toward adjusting the heat of formation value of H 2CCCCH from the Sandia value of 111.27 kcal/rnol (Kee, 1986) 10.5kcaljmol which is reflective of the propargalic resonance stabilization energy (Nguyen, 1981;Nguyen, 1982) Cyclopentadiene The model well predicted the cyclopentadiene (c-C SH 6 ) profile in the post-reaction zones, as shown in Figure 8a, although the experimental measurements give no indication that a large cyclopentadiene concentration peak exists around 0.25 em downstream of the burner head. The difficulty in sampling the gases with the quartz microprobe close to the burner Downloaded by [University of Tasmania] at 03:47 13 October 2014 surface may be the cause for missing the peak cyclopentadiene concentration.…”

Ab Initio Study of the Most Stable C4H5 Isomers

“…Several C 4 H 5 radical isomers are important intermediates in the pyrolysis − and photochemistry − of unsaturated hydrocarbons, and are candidates for detection in the interstellar medium . Various C 4 H 5 isomers have been under investigation since 1967, when Martin and Sanders measured the kinetics of perester decomposition to radical products and Benson and Haugen included the 1,3-butadienyl radicals in their kinetic model of radical-mediated carbon chain formation .…”

“…Various C 4 H 5 isomers have been under investigation since 1967, when Martin and Sanders measured the kinetics of perester decomposition to radical products and Benson and Haugen included the 1,3-butadienyl radicals in their kinetic model of radical-mediated carbon chain formation . Subsequent kinetics studies have probed the role of these radicals in the photolysis of 2-butyne, the thermal decomposition of pentyne and pentadiene, , the pyrolysis of 1-butyne 3 and 1,2-butadiene, and the radical-mediated chain formation leading to aromatic compounds. − ,,,, Several of these studies have succeeded in predicting formation enthalpies for the more stable C 4 H 5 isomers. There are also many ESR measurements of the magnetic and hyperfine properties of selected C 4 H 5 isomers. − An elegant crossed-beam study by Kaiser and co-workers has recently probed the mechanism for formation of several of these structures from the reaction of propene and atomic carbon …”