In situ flame chemistry tracing by imaging photoelectron photoion coincidence spectroscopy

Formation of polycyclic aromatic hydrocarbons from bimolecular reactions of phenyl radicals at high temperatures

The self-reaction of the phenyl radical is one of the key reactions in combustion chemistry.

Dissociative photoionization and threshold photoelectron spectra of polycyclic aromatic hydrocarbon fragments: an imaging photoelectron photoion coincidence (iPEPICO) study of four substituted benzene radical cations

Four molecules were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy: 1-propynylbenzene, indene, ethynylbenzene, and benzocyclobutene. Their threshold photoelectron spectrum was obtained and electronic transitions were assigned by OVGF (outer valence Green's function) calculations. Vibrational progressions observed in the electronic ground and excited states were simulated by calculating Franck-Condon factors based on the neutral as well as the cation ground and excited state geometries. iPEPICO was used to obtain ion dissociation data in threshold photoionization as a function of photon energy, which were modeled with RRKM theory to extract kinetic parameters for the reactions C9H8(+•) (1-propynylbezene) → C9H7(+) + H (R1); C9H8(+•) (indene) → C9H7(+) + H (R2); C8H8(+•) (benzocyclobutene) → C8H7(+) + H (R3); C8H8(+•) (benzocyclobutene) → C6H6(+) + C2H2 (R4); C8H6(+•) (1-ethynylbenzene, aka phenylacetylene) → C6H4(+) + C2H2 (R5). These results were compared to G3 level calculations. In addition, the enthalpy of formation of the indenyl radical was estimated to be ΔfH°0K = 249 ± 50 kJ mol(-1) based on a previously measured IE and a cation ΔfH°0K = 976 kJ mol(-1), determined herein.

Decomposition of diazomeldrum's acid: a threshold photoelectron spectroscopy study

Derivatives of meldrum's acid are known precursors for a number of reactive intermediates. Therefore, we investigate diazomeldrum's acid (DMA) and its pyrolysis products by photoionization using vacuum ultraviolet (VUV) synchrotron radiation. The threshold photoelectron spectrum of DMA yields an ionization energy (IE) of 9.68 eV. Several channels for dissociative photoionization are observed. The first one is associated with loss of CH3, leading to a daughter ion with m/z = 155. Its appearance energy AE0K was determined to be 10.65 eV by fitting the experimental data using statistical theory. A second parallel channel leads to m/z = 69, corresponding to N2CHCO, with an AE0K of 10.72 eV. Several other channels open up at higher energy, among them the formation of acetone cation, a channel expected to be the result of a Wolff-rearrangement (WR) in the cation. When diazomeldrum's acid is heated in a pyrolysis reactor, three thermal decomposition pathways are observed. The major one is well-known and yields acetone, N2 and CO as consequence of the WR. However, two further channels were identified: The formation of 2-diazoethenone, NNCCO, together with acetone and CO2 as the second channel and E-formylketene (OCCHCHCO), propyne, N2 and O2 as a third one. 2-Diazoethenone and E-formylketene were identified based on their threshold photoelectron spectra and accurate ionization energies could be determined. Ionization energies for several isomers of both molecules were also computed. One of the key findings of this study is that acetone is observed upon decomposition of DMA in the neutral as well as in the ion and both point to a Wolff rearrangement to occur. However, the ion is subject to other decomposition channels favored at lower internal energies.