Abstract
We report the absolute photoionization cross section (PICS) of fulvenone and 2-carbonyl cyclohexadienone, two crucial ketene intermediates in lignin pyrolysis, combustion and organic synthesis. Both species were generated in situ by pyrolyzing salicylamide and dectected via imaging photoelectron photoion coincidence spectroscopy. In a deamination reaction, salicylamide loses ammonia yielding 2-carbonyl cyclohexadienone, a ketoketene, which further decarbonylates at higher pyrolysis temperatures to form fulvenone. We recorded the threshold photoelectron spectrum of the ketoketene and assigned the ground state (X̃+2A′′ ← X̃1A′) and excited state (Ã+2A′ ← X̃1A′) bands with the help of Franck–Condon simulations. Adiabatic ionization energies are 8.35 ± 0.01 and 9.19 ± 0.01 eV. In a minor reaction channel, the ketoketene isomerizes to benzpropiolactone, which decomposes subsequently to benzyne by CO2 loss. Potential energy surface and RRKM rate constant calculations agree with our experimental observations that the decarbonylation to fulvenone outcompetes the decarboxylation to benzyne by almost two orders of magnitude. The absolute PICS of fulvenone at 10.48 eV was determined to be 18.8 ± 3.8 Mb using NH3 as a calibrant. The PICS of 2-carbonyl cyclohexadienone was found to be 21.5 ± 8.6 Mb at 9 eV. Our PICS measument will enable the quantification of reactive ketenes in lignin valorization and combustion processes using photoionization techniques and provide advanced mechanistic and kinetics insights to aid the bottom-up optimization of such processes.
Highlights
Due to its high reactivity, fulvenone evades detection using standard chemical analysis tools, such as GC/MS and NMR, which is the reason why fulvenone was only observed using photoionization mass spectrometry (PIMS), photoelectron spectroscopy (PES), matrix infrared spectroscopy (IR) and PEPICO detection.[7,11,12,13] Most recently, Genossar et al recorded the IR spectrum of fulvenone produced by salicylaldehyde pyrolysis.[13]
We investigated other isomers which may contribute to the msTPES above 9.2 eV, which are depicted in Fig. 3 and Fig. S5 (ESI†)
Fulvenone was produced in salicylamide pyrolysis together with ammonia and CO to determine its photoionization cross section
Summary
Due to its high reactivity, fulvenone evades detection using standard chemical analysis tools, such as GC/MS and NMR, which is the reason why fulvenone was only observed using photoionization mass spectrometry (PIMS), photoelectron spectroscopy (PES), matrix infrared spectroscopy (IR) and PEPICO detection.[7,11,12,13] Most recently, Genossar et al recorded the IR spectrum of fulvenone produced by salicylaldehyde pyrolysis.[13]. Products and intermediates were isomer-selectively assigned based on their photoion massselected threshold photoelectron (ms-TPE) spectrum, as compared with Franck–Condon simulations (Fig. S2, ESI†) and accurate ionization energy calculations (Table S1, ESI†), or with reference spectra. Upon increasing the reactor temperature, the salicylamide 1 (m/z 137) and ketoketene 2 peaks decreased while the fulvenone 3 and NH3 signals increased. By increasing the reactor temperature, the concentration of salicylamide 1 in the molecular beam was lowered and the dissociative ionization contribution to fulvenone was minimized. In 1979, Schulz and Schweig measured the photoelectron spectrum in the 8–18 eV energy range and assigned the ionization bands.[11] Chapman et al measured the IR spectrum of the ketoketene 2 in an Ar matrix at 8 K and identified its characteristic bands at 2139 and 1650 cmÀ1 In conventional PES literature, Schulz and Schweig assigned the first band to the X+2A00’X1A0 transition and reported an ionization energy of
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