Similarity of Near-Threshold Energy Dependence of Positronium Formation and Photoionization in Molecules.

High-resolution measurements of the positronium formation cross sections for positron energies from threshold to 10 eV are presented for aniline (C_{6}H_{5}NH_{2}), pyridine (C_{5}H_{5}N), and cyclopentane (C_{5}H_{10}). The data reveal that the measured energy dependence of the cross sections on the excess energy in the near-threshold region (1-2 eV) is nearly identical to that of the corresponding photoionization cross sections. This similarity occurs despite the difference between the basic threshold laws for processes without and with a Coulomb interaction between the final-state particles. It is proposed here that the near-threshold behavior of these two different ionization processes is governed by the vibrational dynamics of the final-state cation. This is supported by comparison of the data with the calculated spectrum of vibronic intensities for the pyridine cation [Trofimov et al., J. Chem. Phys. 153, 164307 (2020)JCPSA60021-960610.1063/5.0024446].

Formation of resonances and anionic fragments upon electron attachment to benzaldehyde.

Benzaldehyde is a simple aromatic aldehyde and has a wide range of applications in the food, pharmaceutical, and chemical industries. The positive electron affinity of this compound suggests that low-energy electrons can be easily trapped by neutral benzaldehyde. In the present study, we investigated the formation of negative ions following electron attachment to benzaldehyde in the gas-phase. Calculations on elastic electron scattering from benzaldehyde indicate a π* valence bound state of the anion at -0.48 eV and three π* shape resonances (0.78, 2.48 and 5.51 eV). The excited state spectrum of the neutral benzaldehyde is also reported to complement our findings. Using mass spectrometry, we observed the formation of the intact anionic benzaldehyde at ∼0 eV. We ascribe the detection of the benzaldehyde anion to stabilization of the π* valence bound state upon dissociative electron attachment to a benzaldehyde dimer. In addition, we report the cross sections for nine fragment anions formed through electron attachment to benzaldehyde. Investigations carried out with partially deuterated benzaldehyde show that the hydrogen loss is site-selective with respect to the incident electron energy. In addition, we propose several dissociation pathways, backed up by quantum chemical calculations on their thermodynamic thresholds. The threshold calculations also support that the resonances formed at higher energies lead to fragment anions observable by mass spectrometry, whereas the resonances at low electron energies decay only by electron autodetachment.

Low-energy electron-induced decomposition of 5-trifluoromethanesulfonyl-uracil: A potential radiosensitizer.

5-trifluoromethanesulfonyl-uracil (OTfU), a recently proposed radiosensitizer, is decomposed in the gas-phase by attachment of low-energy electrons. OTfU is a derivative of uracil with a triflate (OTf) group at the C5-position, which substantially increases its ability to undergo effective electron-induced dissociation. We report a rich assortment of fragments formed upon dissociative electron attachment (DEA), mostly by simple bond cleavages (e.g., dehydrogenation or formation of OTf-). The most favorable DEA channel corresponds to the formation of the triflate anion alongside with the reactive uracil-5-yl radical through the cleavage of the O-C5 bond, particularly at about 0 eV. Unlike for halouracils, the parent anion was not detected in our experiments. The experimental findings are accounted by a comprehensive theoretical study carried out at the M06-2X/aug-cc-pVTZ level. The latter comprises the thermodynamic thresholds for the formation of the observed anions calculated under the experimental conditions (383.15 K and 3 × 10-11 atm). The energy-resolved ion yield of the dehydrogenated parent anion, (OTfU-H)-, is discussed in terms of vibrational Feshbach resonances arising from the coupling between the dipole bound state and vibrational levels of the transient negative ion. We also report the mass spectrum of the cations obtained through ionization of OTfU by electrons with a kinetic energy of 70 eV. The current study endorses OTfU as a potential radiosensitizer agent with possible applications in radio-chemotherapy.

Parent anion radical formation in coenzyme Q0: Breaking ubiquinone family rules.

We report electron attachment (EA) measurements for the parent anion radical formation from coenzyme Q0 (CoQ0) at low electron energies (<2 eV) along with quantum chemical calculations. CoQ0 may be considered a prototype for the electron withdrawing properties of the larger CoQ n molecules, in particular ubiquinone (CoQ10), an electron carrier in aerobic cell respiration. Herein, we show that the mechanisms for the parent anion radical formation of CoQ0 and CoQ n (n = 1,2,4) are remarkably distinct. Reported EA data for CoQ1, CoQ2, CoQ4 and para-benzoquinone indicated stabilization of the parent anion radicals around 1.2-1.4 eV. In contrast, we observe for the yield of the parent anion radical of CoQ0 a sharp peak at âˆ¼ 0 eV, a shoulder at 0.07 eV and a peak around 0.49 eV. Although the mechanisms for the latter feature remain unclear, our calculations suggest that a dipole bound state (DBS) would account for the lower energy signals. Additionally, the isoprenoid side chains in CoQ n (n = 1,2,4) molecules seem to influence the DBS formation for these compounds. In contrast, the side chains enhance the parent anion radical stabilization around 1.4 eV. The absence of parent anion radical formation around 1.4 eV for CoQ0 can be attributed to the short auto-ionization lifetimes. The present results shed light on the underappreciated role played by the side chains in the stabilization of the parent anion radical. The isoprenoid tails should be viewed as co-responsible for the electron-accepting properties of ubiquinone, not mere spectators of electron transfer reactions.