The Threshold Photoelectron Spectrum of Fulvenone: A Reactive Ketene Derivative in Lignin Valorization

Abstract

Unveiling reaction mechanisms by isomer-selective detection of reactive intermediates requires advanced spectroscopic knowledge. We study the photoionization of fulvenone (c-C 5 H 4 = C=O), a reactive ketene species relevant in catalytic pyrolysis of lignin, which was generated by pyrolysis of 2-methoxy acetophenone. The high-resolution threshold photoelectron spectrum (TPES) with vacuum ultraviolet synchrotron radiation revealed well-resolved vibrational transitions, assigned to ring deformation modes of the cyclopentadiene moiety. The adiabatic ionization energy was determined to be 8.25 � 0.01 eV and is assigned to theX þ 2 A 2 ! ~X 1 A 1 transition. A broad and featureless band arising at 9 eV is associated with theà þ 2 B 1 ! ~X 1 A 1 excitation. A conical intersection is responsible for the ultrafast relaxation of the fulvenone cation from theà þ into thẽ X þ state resulting in a featureless and lifetime broadened band. These insights will increase the detection capabilities for fulvenone and thereby help to elucidate reaction mechanisms in lignin catalytic pyrolysis.

Isomer-dependent catalytic pyrolysis mechanism of the lignin model compounds catechol, resorcinol and hydroquinone

“…In the 8.5–10.0 eV photon energy range, the fulvenone TPES is dominated by transitions into the strongly coupled ground and first electronically excited state of the cation. 35 The m / z 92 ms-TPES barely rises above the background in the resorcinol and hydroquinone experiments, which precludes assignment (Fig. S4 † ).…”

On the absolute photoionization cross section and threshold photoelectron spectrum of two reactive ketenes in lignin valorization: fulvenone and 2-carbonyl cyclohexadienone

“…A similar pattern was also found in fulvenone 2 ionization. 15 In general, the bond angles are only slightly affected upon ionization into the ground state cation. ms-TPE spectroscopy is a unique tool to detect reactive intermediates and to selectively identify isomers in harsh environments.…”

“…6,12,14 We have measured the ms-TPES and photoionization spectrum of fulvenone and simulated transitions from the neutral ground X ˜2A 2 into both the ground X ˜+2 A 2 and the first excited A ˜+2 B 1 cation states in the Franck-Condon approximation. 15 Fulvenone has, thus, been characterized by different techniques, which helps to trace this crucial intermediate in complex reaction pathways to gain an advanced mechanistic understanding of combustion and lignin valorization processes. However, to obtain reliable kinetics and analytical data, quantification is mandatory.…”

Photoelectron Photoion Coincidence Spectroscopy of NCl3 and NCl2

“…[12] This lack of spectroscopic data motivated us to study the photoionization of NCl 3 and NCl 2 utilizing vacuum ultraviolet (VUV) synchrotron radiation, which is an excellent tool to derive ionization (IE) and appearance energies (AE). [13] NCl 3 was introduced as a 20 % solution with dichloromethane, while NCl 2 was produced by photolysis of NCl 3 in a side-sampled flow reactor. [14] Photoelectron spectroscopy of reactive molecules suffers often from signals due to side products that impede assignments.…”

“…We therefore employ photoelectron-photoion coincidence spectroscopy (PEPICO), which permits to record photoion mass-selected threshold photoelectron spectra (ms-TPES) for each species by correlating ions and electrons. While the majority of work used fluorine discharges [15] or pyrolysis [13,16] to generate reactive species, PEPICO has increasingly been combined with photolysis, [17] which expands the range of available species and gives in addition access to kinetics data.…”