Threshold Photoionization of Fluorenyl, Benzhydryl, Diphenylmethylene, and Their Dimers

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• 作者：Melanie Lang ; Fabian Holzmeier ; Ingo Fischer ; Patrick Hemberger
• 刊名：The Journal of Physical Chemistry A
• 出版年：2013
• 出版时间：June 27, 2013
• 年：2013
• 卷：117
• 期：25
• 页码：5260-5268
• 全文大小：449K
• 年卷期：v.117,no.25(June 27, 2013)
• ISSN：1520-5215

文摘

Two 蟺-conjugated radicals, fluorenyl (C₁₃H₉) and benzhydryl (C₁₃H₁₁), as well as the carbene diphenylmethylene (C₁₃H₁₀) were studied by imaging photoelectron鈥損hotoion coincidence spectroscopy using VUV synchrotron radiation. The reactive intermediates were generated by flash pyrolysis from 9-bromofluorene and 伪-aminodiphenylmethane (adpm), respectively. Adiabatic ionization energies (IE_ad) for all three species were extracted. Values of 7.01 卤 0.02 eV for fluorenyl and 6.7 卤 0.1 eV for benzhydryl are reported. For the triplet diphenylmethylene, an IE_ad of 6.8 卤 0.1 eV is found. The dissociative photoionization of 9-bromofluorene, the precursor for fluorenyl, was also studied and modeled with an SSACM approach, yielding an appearance energy AE_0K(C₁₃H₉⁺/C₁₃H₉Br) of 9.4 eV. All experimental values are in very good agreement with computations. For fluorenyl, the IE_ad agrees well with earlier values, while for the benzhydryl radical, we report a value that is more than 0.6 eV lower than the one previously reported. The geometry change upon ionization is small for all three species. Although individual vibrational bands cannot be resolved, some vibrational transitions in the threshold photoelectron spectrum of fluorenyl are tentatively assigned based on a Franck鈥揅ondon simulation. In addition, the dimerization products of fluorenyl and the benzhydryl radical were detected. Ionization energies of (7.69 卤 0.04) and (8.11 卤 0.04) eV were determined for C₂₆H₁₈ and C₂₆H₂₂, respectively. On the basis of the ionization energies, we identified both molecules to be the direct dimerization products, formed in the pyrolysis without further rearrangement. Both dimers might be expected to play a role in soot formation because the radical monomers do appear in flames.