Photoionization of C₇H₆ and C₇H₅: Observation of the Fulvenallenyl Radical

Abstract

Species of the composition C 7 H n are known intermediates in combustion processes.[1] One out of five C 7 H 5 isomers, fulvenallenyl, was recently computed to be a resonance-stabilized radical [2] expected to be long-lived and to play an important role in the formation of soot and polycyclic aromatic hydrocarbons.[3] Thus, its structural and thermochemical characterization is relevant for the combustion community. A C 7 H 5 species was previously observed in a toluene flame, [4] but could not be unambiguously assigned to a specific isomer. Herein we report the direct observation of fulvenallenyl in a threshold photoelectron spectrum (TPES) obtained using synchrotron radiation. We also report a TPES of the C 7 H 6 isomer fulvenallene together with an accurate ionization energy (IE) of both species. Such data on reactive species are necessary for their isomer-selective detection in flames. [5] Fulvenallene as well as fulvenallenyl were produced by flash pyrolysis [6] of phthalide according to Scheme 1. This approach has been used before [7] to record conventional photoelectron spectra of fulvenallene, an unstable species despite being a closed-shell molecule. Figure 1 shows mass spectra recorded at a photon energy of 12.0 eV. Without pyrolysis (lower trace) a peak due to the phthalide precursor at m/z = 134 is visible as well as a smaller peak at m/z = 105, which is due to dissociative photoionization via the loss of HCO. When the pyrolysis is turned on (upper trace), the precursor signal decreases and a peak at m/z = 90 appears, corresponding to C 7 H 6 . In addition, a peak is revealed at m/z = 89, which grows in intensity with increasing pyrolysis temperature. The calculated bond dissociation energy of the allenic CÀH bond is 340 kJ mol À1 and is the by far lowest in fulvenallene.[2] The selective loss of this H-atom at high temperatures is thus not surprising. Small fragment peaks at lower masses are also present at high temperatures.We subsequently recorded TPE-spectra of both species using photoelectron-photoion coincidence spectroscopy. [8] This method permits recording a mass-selected photoelectron spectrum for each species, and is well-suited for the investigation of reactive intermediates.[9] Both spectra in Figure 2 are dominated by one large peak at 8.22 eV (C 7 H 6 ) and 8.19 eV (C 7 H 5 ). Additional scans below 8 eV did not reveal any further signal. The dissociative ionization of fulvenallene only sets in above 11 eV and does not perturb the mass-selected TPES of C 7 H 5 . Ionization energies of five different C 7 H 5 isomers were computed by density functional theory (DFT) using the B3LYP exchange correlation functional and the 6-311 + + G** basis. [10] The fulvenallenyl IE (Scheme 1) was computed to be 8.17 eV

Isomer-Specific Product Detection of Gas-Phase Xylyl Radical Rearrangement and Decomposition Using VUV Synchrotron Photoionization

“…[2][3][4][5] In 2009 it was found that a subsequent hydrogen abstraction could lead to fulvenallenyl radical (the global C 7 H 5 minimum), which was unambiguously identified by measuring its mass-selected threshold photoelectron spectrum (ms-TPES). [6][7][8] The products arising from the dissociation of substituted benzyl radicals, however, are less well understood. The decomposition dynamics of xylenes are also relatively unexplored, despite their prevalence in gasoline.…”

Photoionisation of the tropyl radical

“…Ionisation energies have been determined from vibrationally resolved photoelectron spectra for several open-shell species ranging from allyl [22] and propargyl [23] to indenyl (C 9 H 7 ) [24], cyclopropenylidene [25] and fulvenallenyl [26]. In the case of allyl [27–28] and propargyl [29] they are in excellent agreement with high-resolution laser studies.…”

On the Separation of Timescales in Chemically Activated Reactions

“…. Atomic hydrogen can add at the 4 and 6 positions in HCCCCCH 2 CCH with small barriers (1–4 kcal mol −1 ) to produce open‐chain C 7 H 5 radicals that can isomerize to the C 7 H 5 minima fulvenallenyl in a process that is ∼100 kcal mol −1 exothermic with barriers over 20 kcal mol −1 below the energy of the entrance transition states. The vibrationally excited C 7 H 5 adducts can also dissociate to form the propargyl radical plus diacetylene or the resonance‐stabilized radicals i ‐C 5 H 3 or n ‐C 5 H 3 plus acetylene, with barriers more than 10 kcal mol −1 below the entrance transition states.…”