Threshold Photoelectron Spectrum of the Phenoxy Radical

Abstract

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.

Photoion Mass-Selected Threshold Photoelectron Spectroscopy to Detect Reactive Intermediates in Catalysis: From Instrumentation and Examples to Peculiarities and a Database

“…In the absence of active large-amplitude motions, i.e., internal rotations, the double harmonic approximation reproduces the vibrational fine structure generally well. As an example, the phenoxy radical ms-TPES is depicted in Figure c and shows transitions into singlet and triplet ground and excited states, respectively, reproduced by FC spectral modeling . There are, however, instances, when the spectral fine structure is not entirely or not at all reproduced by a double harmonic Franck–Condon simulation.…”

Bonding in Low‐Coordinated Organoarsenic and Organoantimony Compounds: A Threshold Photoelectron Spectroscopic Investigation

“…Due to the insufficient expansion cooling, ms‐TPES recorded from pyrolysis reactors, mostly suffer from broadening by hot and sequence band transitions. A reduction of these can be achieved by integrating only the room temperature background velocity component in the ions’ VMI as was recently shown [31–32] . The resulting mass‐selected threshold photoelectron (ms‐TPE) spectra were corrected for photon flux and the ionisation energy in addition corrected for the Stark shift.…”

“…A reduction of these can be achieved by integrating only the room temperature background velocity component in the ions' VMI as was recently shown. [31][32] The resulting mass-selected threshold photoelectron (ms-TPE) spectra were corrected for photon flux and the ionisation energy in addition corrected for the Stark shift.…”

Combustion in a Sustainable World: From Molecules to Processes

“…Experimental investigations with laser and synchrotron-based techniques have led to an improved understanding of several reaction systems of combustion and astrophysical interest, including the CH + H 2 ⇌ 3 CH 2 + H reaction, and the photodissociation dynamics of propyl derivatives . Such techniques also support the identification of transient reaction intermediates, such as phenoxy radical, which is a key intermediate in benzene oxidation and biomass conversion . The well resolved high-resolution mass-selected threshold photoionization spectrum from Fernholz et al .…”

“…5 Such techniques also support the identification of transient reaction intermediates, such as phenoxy radical, which is a key intermediate in benzene oxidation and biomass conversion. 6 The well resolved high-resolution mass-selected threshold photoionization spectrum from Fernholz et al 6 provides a means for identifying this species in subsequent combustion kinetics studies.…”