Proton Shuttling and Reaction Paths in Dissociative Electron Attachment to o- and p-Tetrafluorohydroquinone, an Experimental and Theoretical Study.

Here we present a combined experimental and theoretical study on the fragmentation of o- and p-tetrafluorohydroquinone upon low energy electron attachment. Despite an identical ring-skeleton and identical functional groups in these constitutional isomers, they show distinctly different fragmentation patterns, a phenomenon that cannot be explained by distinct resonances or different thermochemistry. Using high-level quantum chemical calculations with the computationally affordable domain localized pair natural orbital approach, DLPNO-CCSD(T), we are able to provide a complete and accurate description of the respective reaction dynamics, revealing proton shuttling and transition states for competing channels as the explanation for the different behavior of these isomers. The results represent a "schoolbook example" of how the combination of experiment and modern high-level theory may today provide a thorough understanding of complex reaction dynamics by computationally affordable means.

NCO(-), a key fragment upon dissociative electron attachment and electron transfer to pyrimidine bases: site selectivity for a slow decay process.

We report gas phase studies on NCO(-) fragment formation from the nucleobases thymine and uracil and their N-site methylated derivatives upon dissociative electron attachment (DEA) and through electron transfer in potassium collisions. For comparison, the NCO(-) production in metastable decay of the nucleobases after deprotonation in matrix assisted laser desorption/ionization (MALDI) is also reported. We show that the delayed fragmentation of the dehydrogenated closed-shell anion into NCO(-) upon DEA proceeds few microseconds after the electron attachment process, indicating a rather slow unimolecular decomposition. Utilizing partially methylated thymine, we demonstrate that the remarkable site selectivity of the initial hydrogen loss as a function of the electron energy is preserved in the prompt as well as the metastable NCO(-) formation in DEA. Site selectivity in the NCO(-) yield is also pronounced after deprotonation in MALDI, though distinctly different from that observed in DEA. This is discussed in terms of the different electronic states subjected to metastable decay in these experiments. In potassium collisions with 1- and 3-methylthymine and 1- and 3-methyluracil, the dominant fragment is the NCO(-) ion and the branching ratios as a function of the collision energy show evidence of extraordinary site-selectivity in the reactions yielding its formation.

Stabilization, fragmentation and rearrangement reactions in low-energy electron interaction with tetrafluoro-para-benzoquinone: a combined theoretical and experimental study.

Quinones are a class of organic compounds whose extraordinary electron transfer properties are fundamental in ubiquitous processes such as the ATP production and the photosynthesis. Here, we report a combined theoretical and experimental study to shed some light on low-energy electron interaction with tetrafluoro-para-benzoquinone (TFQ) and para-benzoquinone (p-BQ). Similar to it's native counterpart; p-BQ, TFQ forms a metastable molecular anion at unusually high incident energies in electron attachment under single collision conditions. Transition state calculations are used for both p-BQ and TFQ to explore possible stabilization of the molecular anion through isomerization reactions, and stabilization through intramolecular vibrational energy redistribution (IVR) is discussed in relation to the electronic structure of these compounds. We also report exceptionally extensive and complex rearrangement reactions in dissociative electron attachment to TFQ and discuss possible reaction paths based on thermochemical threshold calculations. The observed fragmentation reactions can at large be described by two dissociation series, i.e., the formation of [TFQ - CO -nF](-) (n = 0, 1, 2 or 3) and [TFQ - 2CO -nF](-) (n = 0, 1, 2, 3 or 4). In the latter series we observe, at incident electron energy as low as ~0.5 eV, the excision of two CO molecules followed by recombination of the remaining moiety to form an anionic carbon chain with two terminal CF2 groups. To our knowledge such excision of two non-adjacent carbon atoms and recombination of the remaining moiety have not been observed before in low energy electron attachment.

Dissociative electron attachment to hexafluoroacetylacetone and its bidentate metal complexes M(hfac)2; M = Cu, Pd.

Beta-diketones are a versatile class of compounds that can complex almost any metal in the periodic table of elements. Their metal complexes are found to be fairly stable and generally have sufficient vapor pressure for deposition techniques requiring volatile metal sources. Motivated by the potential role of low energy electrons in focused electron beam induced deposition, we have carried out a crossed electron∕molecular beam study on the dissociative electron attachment and non-dissociative electron attachment (NDEA) to hexafluoroacetylacetone (HFAc) and its bidentate metal complexes: bis-hexafluoroacetylacetonate copper(II), Cu(hfac)2 and bis-hexafluoroacetylacetonate palladium(II), Pd(hfac)2. The relative ion yield curves for the native precursor to the ligand as well as its stable, 16 valence electron Pd(II) complex and open shell, 17 valence electron Cu(II) complex, are presented and compared. For HFAc, the loss of HF leads to the dominant anion observed, and while NDEA is only weakly pronounced for Pd(hfac)2 and loss of hfac(-) is the main dissociation channel, [Cu(hfac)2](-) formation from Cu(hfac)2 dominates. A comparison of the ion yield curves and the associated resonances gives insight into the role of the ligand in the attachment process and highlights the influence of the central metal atom.

Molecular rearrangement reactions in the gas phase triggered by electron attachment.

Bond formation and rearrangement reactions in gas phase electron attachment were studied through dissociative electron attachment (DEA) to pentafluorotoluene (PFT), pentafluoroaniline (PFA) and pentafluorophenol (PFP) in the energy range 0-14 eV. In the case of PFA and PFP, the dominant processes involve formation of [M - HF](-) through the loss of neutral HF. This fragmentation channel is most efficient at low incident electron energy and for PFP it is accompanied by a substantial conformational change of the anionic fragment. At higher energy, HF loss is also observed as well as a number of other fragmentation processes. Thermochemical threshold energies have been computed for all the observed fragments and classical trajectories of the electron attachment process were calculated to elucidate the fragmentation mechanisms. For the dominant reaction channel leading to the loss of HF from PFP, the minimum energy path was calculated using the nudged elastic band method.