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  • 1
    Electronic Resource
    Electronic Resource
    Direct observation of the ionization threshold of triplet methylene by photoionization mass spectrometry (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 6748-6755 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoionization spectrum of the ionization threshold region of methylene has been recorded for the first time. The CH2 radical was produced in situ by successive hydrogen abstractions from methane precursor. The observed steplike onset corresponds to the vibrationless transition CH2+X˜ 2A1←CH2X˜ 3B1 and leads to the adiabatic ionization energy of CH2 of 10.393±0.011 eV. This value is slightly higher than the nominal midrise of the threshold step structure, which is depressed by rotational autoionization effects. In a separate set of experiments, the threshold region of the CH2+ fragment from CH3 was recorded at room temperature. The fragment appearance energy was accurately determined by fitting to be 15.120±0.006 eV at 0 K. The combination of these two measurements provides the best current experimental value for the bond dissociation energy of the methyl radical, D0(H–CH2)=4.727±0.012 eV=109.0±0.3 kcal/mol (corresponding to 110.4±0.3 kcal/mol at 298 K), and yields ΔHf0(minus sign in circle)(CH2,X˜ 3B1)=93.2±0.3 kcal/mol (93.3±0.3 kcal/mol at 298 K) and ΔHf0(minus sign in circle)(CH2,a˜ 1A2)=102.2±0.3 kcal/mol (102.3±0.3 kcal/mol at 298 K). The latter makes the reaction CH2 (a˜ 1A2)+H2O→CH3+OH essentially thermoneutral, ΔHr0(minus sign in circle)=0.0±0.3 kcal/mol. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476090
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  • 2
    Electronic Resource
    Electronic Resource
    Evidence of rotational autoionization in the threshold region of the photoionization spectrum of CH3 (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 9852-9856 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoionization spectrum of the threshold region of CH3 equilibrated at room temperature has been recorded and compared to the zero electron kinetic energy (ZEKE) spectrum of Blush et al. [J. Chem. Phys. 98, 3557 (1993)]. The ionization onset region is at ∼70 cm−1 higher energy than previous high-temperature photoionization work [Chupka and Lifshitz, J. Chem. Phys. 48, 1109 (1967)], but still ∼34 cm−1 lower than that implied by invoking only direct ionization. The residual discrepancy can be accounted for by including fully allowed quadrupole-induced and partially allowed dipole-induced rotational autoionization, thus making the observed onset completely congruous with the ZEKE ionization potential. In addition, the fragment appearance potential of CH3+ from CH4 was redetermined by accurate fitting as AP0(CH3+/CH4)=14.322±0.003 eV. With the very precise ZEKE ionization potential, this yields the best current value for the bond dissociation energy in methane, D0(H–CH3)=4.484±0.003 eV=103.40±0.07 kcal/mol (104.96±0.07 kcal/mol at 298 K). © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475282
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  • 3
    Electronic Resource
    Electronic Resource
    A photoionization study of trifluoromethanol, CF3OH, trifluoromethyl hypofluorite, CF3OF, and trifluoromethyl hypochlorite, CF3OCl (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9111-9121 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: CF3OH, an important and controversial by-product of atmospheric decomposition of CF3CFH2 (HFC-134a) and other hydrofluorocarbons, has been examined by photoionization mass spectrometry. The ionization onset is characterized by a broad Franck–Condon distribution, arising primarily from a substantial elongation of the C–O bond upon ionization. An upper limit to the adiabatic ionization potential (IP) of ≤13.08±0.05 eV has been established. The appearance potentials (APs) of the first two fragments have been accurately determined by fitting with appropriate model functions as AP0(CF2OH+/CF3OH)≤13.830±0.005 eV and AP0(CF3+/CF3OH)≤13.996±0.005 eV. While the exact nature of the lowest-energy fragment (nominally CF2OH+) is not clear, the CF3+ fragment threshold leads unambiguously to ΔHf 298&convolu;(CF3OH)≥−217.2±0.9 kcal/mol and D298(CF3–OH)≤115.2±0.3 kcal/mol. With previously derived ΔHf 298&convolu;(CF3O)=−151.8−1.1+1.7 kcal/mol, this yields D298(CF3O–H)=117.5−1.4+1.9 kcal/mol, very close to, or only slightly weaker than the O–H bond energy in water: D298(CF3O–H)–D298(HO–H)=−1.8−1.4+1.9 kcal/mol(approximate)0 kcal/mol. Similarly, with the recently redetermined value for ΔHf&convolu;(CF2O), this implies a 298 K reaction enthalpy for the 1,2-elimination of HF from CF3OH of 2.8−1.1+1.7 kcal/mol. CF3OF and CF3OCl have also been examined by photoionization. CF3OF produces a very weak parent, with an apparent adiabatic IP(CF3OF)≤12.710±0.007 eV. An analysis of the CF3+ and CF2O+ fragments from CF3OF, when combined with literature data, suggests ΔHf 298&convolu;(CF3OF)=−176.9−1.3+1.8 kcal/mol. The fitted value for the appearance potential of CF3+ from CF3OCl, AP0(CF3+/CF3OCl)≤12.85±0.01 eV, leads to ΔHf 298&convolu;(CF3OCl)≥−175.6±1.0 kcal/mol, D298(CF3–OCl)≤88.4±0.3 kcal/mol, and D298(CF3O–Cl)≤52.8−1.5+2.0 kcal/mol.© 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474017
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