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The ortho-benzyne diradical, o-C6H4 has been produced with a supersonic nozzle and its subsequent thermal decomposition has been studied. As the temperature of the nozzle is increased, the benzyne molecule fragments: o-C6H4+Δ→ products. The thermal dissociation products were identified by three experimental methods: (i) time-of-flight photoionization mass spectrometry, (ii) matrix-isolation Fourier transform infrared absorption spectroscopy, and (iii) chemical ionization mass spectrometry. At the threshold dissociation temperature, o-benzyne cleanly decomposes into acetylene and diacetylene via an apparent retro-Diels-Alder process: o-C6H4+Δ→HC=CH+HC=C–C=CH. The experimental ΔrxnH298(o-C6H4→HC=CH+HC=C–C=CH) is found to be 57±3 kcal mol-1. Further experiments with the substituted benzyne, 3,6-(CH3)2-o-C6H2, are consistent with a retro-Diels-Alder fragmentation. But at higher nozzle temperatures, the cracking pattern becomes more complicated. To interpret these experiments, the retro-Diels-Alder fragmentation of o-benzyne has been investigated by rigorous ab initio electronic structure computations. These calculations used basis sets as large as [C(7s6p5d4f3g2h1i)/H(6s5p4d3f2g1h)] (cc-pV6Z) and electron correlation treatments as extensive as full coupled cluster through triple excitations (CCSDT), in cases with a perturbative term for connected quadruples [CCSDT(Q)]. Focal point extrapolations of the computational data yield a 0 K barrier for the concerted, C2v-symmetric decomposition of o-benzyne, Eb(o-C6H4→HC=CH+HC=C–C=CH)=88.0±0.5 kcal mol-1. A barrier of this magnitude is consistent with the experimental results. A careful assessment of the thermochemistry for the high temperature fragmentation of benzene is presented: C6H6→H+[C6H5]→H+[o-C6H4]→HC=CH+HC=C–C=CH. Benzyne may be an important intermediate in the thermal decomposition of many alkylbenzenes (arenes). High engine temperatures above 1500 K may crack these alkylbenzenes to a mixture of alkyl radicals and phenyl radicals. The phenyl radicals will then dissociate first to benzyne and then to acetylene and diacetylene. [ABSTRACT FROM AUTHOR]

We have developed an effusive laser photodissociation radical source, aiming for the production of vibrationally relaxed radicals. Employing this radical source, we have measured the vacuum ultraviolet (VUV) photoionization efficiency (PIE) spectrum of the propargyl radical (C3H3) formed by the 193 nm excimer laser photodissociation of propargyl chloride in the energy range of 8.5-9.9 eV using high-resolution (energy bandwidth=1 meV) multibunch synchrotron radiation. The VUV-PIE spectrum of C3H3 thus obtained is found to exhibit pronounced autoionization features, which are tentatively assigned as members of two vibrational progressions of C3H3 in excited autoionizing Rydberg states. The ionization energy (IE=8.674±0.001 eV) of C3H3 determined by a small steplike feature resolved at the photoionization onset of the VUV-PIE spectrum is in excellent agreement with the IE value reported in a previous pulsed field ionization-photoelectron study. We have also calculated the Franck-Condon factors (FCFs) for the photoionization transitions C3H3+(...;vi, 1 - 12) ← C3H3(...). The comparison between the pattern of FCFs and the autoionization peaks resolved in the VUV-PIE spectrum of C3H3 points to the conclusion that the resonance-enhanced autoionization mechanism is most likely responsible for the observation of pronounced autoionization features. We also present here the VUV-PIE spectra for the mass 39 ions observed in the VUV synchrotron-based photoionization mass spectrometric sampling of several premixed flames. The excellent agreement of the IE value and the pattern of autoionizing features of the VUV-PIE spectra observed in the photodissociation and flames studies has provided an unambiguous identification of the propargyl radical as an important intermediate in the premixed combustion flames. The discrepancy found between the PIE spectra obtained in flames and photodissociation at energies above the IE(C3H3) suggests that the PIE spectra obtained in flames might have contributions from the photoionization of vibrationally excited C3H3 and/or the dissociative photoionization processes involving larger hydrocarbon species formed in flames. [ABSTRACT FROM AUTHOR]

ActiveThermochemical Tables (ATcT) thermochemistry for the sequentialbond dissociations of methane, ethane, and methanol systems were obtainedby analyzing and solving a very large thermochemical network (TN).Values for all possible C–H, C–C, C–O, and O–Hbond dissociation enthalpies at 298.15 K (BDE298) and bonddissociation energies at 0K (D0) are presented. The correspondingATcT standard gas-phase enthalpies of formation of the resulting CHn, n= 4–0 species(methane, methyl, methylene, methylidyne, and carbon atom), C2Hn, n= 6–0species (ethane, ethyl, ethylene, ethylidene, vinyl, ethylidyne, acetylene,vinylidene, ethynyl, and ethynylene), and COHn, n= 4–0 species (methanol, hydroxymethyl,methoxy, formaldehyde, hydroxymethylene, formyl, isoformyl, and carbonmonoxide) are also presented. The ATcT thermochemistry of carbon dioxide,water, hydroxyl, and carbon, oxygen, and hydrogen atoms is also included,together with the sequential BDEs of CO2and H2O. The provenances of the ATcT enthalpies of formation, which arequite distributed and involve a large number of relevant determinations,are analyzed by variance decomposition and discussed in terms of principalcontributions. The underlying reasons for periodic appearances ofremarkably low and/or unusually high BDEs, alternating along the dissociationsequences, are analyzed and quantitatively rationalized. The presentATcT results are the most accurate thermochemical values currentlyavailable for these species. [ABSTRACT FROM AUTHOR]

The current Active Thermochemical Tables (ATcT) results for the bond dissociation energies of the homonuclear diatomics H, C, N, O, and F are reported and discussed. The role and origin of the distributed provenance of ATcT values is analyzed. Ramifications in terms of the enthalpies of formation of H, C, N, O, and F atoms, which are fundamental thermochemical quantities, are presented. In addition, the current ATcT bond dissociation energies and enthalpies of formation of HF, CH, CO, CN, NO, OH, CO, HO, and triplet and singlet CH are also reported. [ABSTRACT FROM AUTHOR]

Thethermal decomposition of nitromethane provides a classic exampleof the competition between roaming mediated isomerization and simplebond fission. A recent theoretical analysis suggests that as the pressureis increased from 2 to 200 Torr the product distribution undergoesa sharp transition from roaming dominated to bond-fission dominated.Laser schlieren densitometry is used to explore the variation in theeffect of roaming on the density gradients for CH3NO2decomposition in a shock tube for pressures of 30, 60, and120 Torr at temperatures ranging from 1200 to 1860 K. A complementarytheoretical analysis provides a novel exploration of the effects ofroaming on the thermal decomposition kinetics. The analysis focuseson the roaming dynamics in a reduced dimensional space consistingof the rigid-body motions of the CH3and NO2radicals. A high-level reduced-dimensionality potential energy surfaceis developed from fits to large-scale multireference ab initio calculations.Rigid body trajectory simulations coupled with master equation kineticscalculations provide high-level a priori predictions for the thermalbranching between roaming and dissociation. A statistical model providesa qualitative/semiquantitative interpretation of the results. Modelingefforts explore the relation between the predicted roaming branchingand the observed gradients. Overall, the experiments are found tobe fairly consistent with the theoretically proposed branching ratio,but they are also consistent with a no-roaming scenario and the underlyingreasons are discussed. The theoretical predictions are also comparedwith prior theoretical predictions, with a related statistical model,and with the extant experimental data for the decomposition of CH3NO2, and for the reaction of CH3withNO2. [ABSTRACT FROM AUTHOR]