Threshold Photoelectron Spectrum of the Phenoxy Radical Fernholz, Christin; Bodi, Andras; Hemberger, Patrick
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
2022-Dec-08, Volume:
126, Issue:
48
Journal Article
Peer reviewed
The high-resolution photoion mass-selected threshold photoelectron spectrum (ms-TPES) of the phenoxy radical (C6H5O•), produced by pyrolysis of anisole, was investigated at the VUV beamline of the ...Swiss Light Source. Adiabatic ionization energies have been determined to be 8.56, 9.42, 9.76, and 9.94 eV to the X̃+ 1A1, ã+ 3A2, Ã+ 1A2, and b̃+ 3B2 cation states, respectively, supported by DFT, WFT, and composite-method calculations. A ring deformation mode was found to be active upon ionization by Franck–Condon analysis and responsible for the vibrational structure of the TPES in all four ion states. While the X̃+ 1A1 and ã+ 3A2 states’ assignment agrees with the literature, we revise the energetic order of the Ã+ 1A2 and b̃+ 3B2 cation states in the ms-TPES, based on a pronounced lifetime broadening of the excited triplet state. This is rationalized by strong coupling between the triplet states as confirmed by EOM-EE-CCSD calculations indicating a conical intersection with a low-lying seam to the ã+ 3A2 state. Our study provides a well-resolved spectrum, to be used for the isomer-selective assignment of reactive species in combustion and catalysis and also serves as benchmark to evaluate theoretical methods to address closed- and open-shell singlet and triplet cation intermediates.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Rate coefficients (
k
4
) for the reaction of hydroxyl radicals (OH) with methyl nitrate (CH
3
ONO
2
) were measured over the temperature range 232-343 K using pulsed laser photolysis to generate OH ...and pulsed laser-induced fluorescence to detect it in real-time and under pseudo-first-order conditions. In order to optimize the accuracy of the rate coefficients obtained, the concentration of CH
3
ONO
2
(the reactant in excess) was measured on-line by absorption spectroscopy at 213.86 nm for which the absorption cross-section was also measured (
σ
213.86
= 1.65 ± 0.09 × 10
−18
cm
2
molecule
−1
). The temperature-dependent rate coefficient is described by
k
4
(
T
) = 7.5 × 10
−13
exp(−1034 ± 40)/
T
cm
3
molecule
−1
s
−1
with a room temperature rate coefficient of
k
4
(296 ± 2 K) = (2.32 ± 0.12) × 10
−14
cm
3
molecule
−1
s
−1
where the uncertainty includes the statistical error of 2
σ
and an estimation of the potential systematic bias of 5%. This new dataset helps to consolidate the database for this rate coefficient and to reduce uncertainty in the atmospheric lifetime of CH
3
ONO
2
. As part of this study, an approximate rate coefficient for the reaction of H-atoms with CH
3
ONO
2
(
k
9
) was also derived at room temperature:
k
9
(298 K) = (1.68 ± 0.45) × 10
−13
cm
3
molecule
−1
s
−1
.
Temperature dependent rate coefficients (
k
4
) for the reaction of OH with CH
3
ONO
2
.
Understanding how isomerism influences photoelectron spectra helps in the assignment and analysis of reactive mixtures, especially for heterocycles with numerous isomers. Threshold photoelectron ...spectra of lutidyl radical isomers, i. e., benzyl derivatives with a nitrogen heteroatom and a methyl substituent, are recorded using vacuum ultraviolet synchrotron radiation. The radicals are produced by flash pyrolysis from aminomethyl methylpyridine precursors. Experimental ionization energies are determined to be 7.54, 7.50, and 7.45 eV for 2,4-, 2,6- and 3,5-lutidyl, respectively, in excellent agreement with composite method calculations. Franck-Condon simulations aid the TPES assignment but are also shown to exhibit artifacts if large-amplitude motions, notably the methyl internal rotation are assumed to be active in the double harmonic approximation. Based on calculated adiabatic ionization energies (AIE) of benzyl, picolyl, and xylyl radicals, the N and CH
substituent effects are found to be additive, position-dependent and decrease in the para>ortho≳meta order in magnitude with the nitrogen heteroatom increasing and the methyl substituent decreasing the AIE. These effects are discussed in light of the charge distribution upon ionization. The additivity of the substituent effects also helps predict the influence of substituents on the binding energy of the unpaired electron in analogous radicals.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The Cover Feature illustrates how the ionization energies of benzyl‐type radicals are influenced by N‐ and CH3‐ substitution, a legacy inherited from their parent compounds. More information can be ...found in the Research Article by Patrick Hemberger and co‐workers.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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