Low-energy electron scattering from the aza-derivatives of pyrrole, furan, and thiophene

Interaction of low-energy electrons with radiosensitizers

We provide an experimentalist's perspective on the present state-of-the-art in the studies of low-energy electron interactions with common radiosensitizers, including compounds used in combined chemo-radiation therapy and their model systems. Low-energy electrons are important secondary species formed during the interaction of ionizing radiation with matter. Their role in the radiation chemistry of living organisms has become an important topic for more than 20 years. With the increasing number of works and reviews in the field, we would like to focus here on a very narrow area of compounds that have been shown to have radio-sensitizing properties on the one hand, and high reactivity towards low-energy electrons on the other hand. Gas phase experiments studying electron attachment to isolated molecules and environmental effects on reaction dynamics are reviewed for modified DNA components, nitroimidazoles, and organometallics. In the end, we provide a perspective on the future directions that may be important for transferring the fundamental knowledge about the processes induced by low-energy electrons into practice in the field of rational design of agents for concomitant chemo-radiation therapy.

Fragmentation of the DNA Lesion 8-oxo-Guanine by Low-Energy Electrons

8-oxo-Guanine is a mutagenic lesion produced by reactions involving reactive oxygen species and guanine in DNA. Its production induces mispairing between the canonical nucleobases during DNA replication such that various types of cancers are associated with the DNA lesion. Since radiation therapy is used in some cases, the interaction of low-energy electrons with 8-oxo-guanine can in turn produce other reactive species, which in principle could have either a detrimental or protective effect on the organism. Motivated by these facts, we report a comparative experimental study of electron-induced fragmentation of guanine and 8-oxo-guanine, along with a theoretical study of the π* shape resonances and bound anion states, which may trigger those dissociation reactions. The electron-induced fragmentation of 8-oxo-guanine is remarkably distinct from the native form. More complex reactions were observed for the oxidized species, which may produce several anion fragments at very low energies (∼0 eV). The dehydrogenated parent anion, which is already a minor fragment in guanine, was completely suppressed in 8-oxo-guanine. The calculated thermodynamical thresholds also suggest that NH2 elimination in guanine, at sub-excitation energies, proceeds via a complex reaction involving rearrangement steps.

Electron Scattering from 1-Methyl-5-Nitroimidazole: Cross-Sections for Modeling Electron Transport through Potential Radiosensitizers

In this study, we present a complete set of electron scattering cross-sections from 1-Methyl-5-Nitroimidazole (1M5NI) molecules for impact energies ranging from 0.1 to 1000 eV. This information is relevant to evaluate the potential role of 1M5NI as a molecular radiosensitizers. The total electron scattering cross-sections (TCS) that we previously measured with a magnetically confined electron transmission apparatus were considered as the reference values for the present analysis. Elastic scattering cross-sections were calculated by means of two different schemes: The Schwinger multichannel (SMC) method for the lower energies (below 15 eV) and the independent atom model-based screening-corrected additivity rule with interferences (IAM-SCARI) for higher energies (above 15 eV). The latter was also applied to calculate the total ionization cross-sections, which were complemented with experimental values of the induced cationic fragmentation by electron impact. Double differential ionization cross-sections were measured with a reaction microscope multi-particle coincidence spectrometer. Using a momentum imaging spectrometer, direct measurements of the anion fragment yields and kinetic energies by the dissociative electron attachment are also presented. Cross-sections for the other inelastic channels were derived with a self-consistent procedure by sampling their values at a given energy to ensure that the sum of the cross-sections of all the scattering processes available at that energy coincides with the corresponding TCS. This cross-section data set is ready to be used for modelling electron-induced radiation damage at the molecular level to biologically relevant media containing 1M5NI as a potential radiosensitizer. Nonetheless, a proper evaluation of its radiosensitizing effects would require further radiobiological experiments.

Theoretical Investigation of Electron Impact Scattering on Imidazole

This study presents the results of electron scattering calculations on a biologically important molecule, imidazole, using the UK molecular R-matrix method. The R-matrix calculations are performed using SE, SEP, and CC models, and the resonance detected in the present SEP model is found to be in better agreement with available experimental data than previous theoretical data. The study also reports an inelastic scattering cross section, which comprises dissociative electron attachment (DEA), excitation, and ionization cross section, for the first time. The total scattering cross sections are also reported for the first time. We confirm the presence of the two well-known π* shape resonances predicted earlier experimentally. Due to the scarcity of total scattering cross section (TCS) data for imidazole, we have compared the TCS of imidazole with its isoelectronic target isoxazole and drawn important conclusions. A comparison among the resonances of imidazole with those of isoxazole helps us to conclude that electron attachment to π* molecular orbitals is a general feature displayed by these five-membered heterocyclic compounds. The comprehensive electron scattering studies presented in this work are expected to provide a deeper understanding of electron-induced biochemical processes and fill gaps in the available data. Furthermore, this study is anticipated to inspire further investigations on imidazole and other five-membered heterocyclic ring molecules, which have significant applications in medicine.

Low-Energy Electron Interactions with Resveratrol and Resorcinol: Anion States and Likely Dissociation Pathways

We report a computational study of the anion states of the resveratrol (RV) and resorcinol (RS) molecules, also investigating dissociative electron attachment (DEA) pathways. RV has well-known beneficial effects in human health, and its antioxidant activity was previously associated with DEA reactions producing H₂. Our calculations indicate a valence bound state (π1*) and four resonances (π2* to π5*) for that system. While the computed thermodynamic thresholds are compatible with DEA reactions producing H₂ at 0 eV, the well-known mechanism involving vibrational Feshbach resonances built on a dipole bound state should not be present in RV. Our results suggest that the shallow π1* valence bound state is expected to account for H₂ elimination, probably involving π1*/σOH* couplings along the vibration dynamics. The RS molecule is also an oxidant and a subunit of RV. Because two close-lying hydroxyl groups are found in the RS moiety, the H₂-elimination reaction in RV should take place at the RS site. Our calculations point out a correspondence between the anion states of RV and RS and even between the thresholds. Nevertheless, the absence of bound anion states in RS, indicated by our calculations, is expected to suppress the H₂-formation channel at 0 eV. One is led to conclude that the ethene and phenol subunits in RV stabilize the π1* state, thus switching on the DEA mechanism producing H₂.

do N. Varella M, Beyer MK, Ončák M, Ferreira da Silva F, Denifl S. Formation of Temporary Negative Ions and Their Subsequent Fragmentation upon Electron Attachment to CoQ0 and CoQ0 H2

Ubiquinone molecules have a high biological relevance due to their action as electron carriers in the mitochondrial electron transport chain. Here, we studied the dissociative interaction of free electrons with CoQ₀ , the smallest ubiquinone derivative with no isoprenyl units, and its fully reduced form, 2,3-dimethoxy-5-methylhydroquinone (CoQ₀ H₂ ), an ubiquinol derivative. The anionic products produced upon dissociative electron attachment (DEA) were detected by quadrupole mass spectrometry and studied theoretically through quantum chemical and electron scattering calculations. Despite the structural similarity of the two studied molecules, remarkably only a few DEA reactions are present for both compounds, such as abstraction of a neutral hydrogen atom or the release of a negatively charged methyl group. While the loss of a neutral methyl group represents the most abundant reaction observed in DEA to CoQ₀ , this pathway is not observed for CoQ₀ H₂ . Instead, the loss of a neutral OH radical from the CoQ₀ H₂ temporary negative ion is observed as the most abundant reaction channel. Overall, this study gives insights into electron attachment properties of simple derivatives of more complex molecules found in biochemical pathways.

Anion states of halocamphor molecules: insights into chirally sensitive dissociative electron attachment

Recent measurements of spin-polarized electron collisions with halocamphor molecules have observed intriguing trends in their dissociative electron attachment (DEA) chiral asymmetries. While the differences between the DEA asymmetries of 3-bromocamphor (3BrC) and 3-iodocamphor (3IC) were consistent with the larger atomic number of iodine, the even higher chiral asymmetry reported for 10-iodocamphor (10IC) was unexpected. In fact, the helicity densities and the distances from the iodine atoms to the closest chiral centers would suggest smaller asymmetries for 10IC compared to 3IC. To better understand the observed trends, we performed electron scattering and bound state calculations, as well as Born-Oppenheimer molecular dynamics simulations for the three halocamphors. Our results indicate that the DEA signals stem exclusively from halide ions produced by the fast dissociation of low-lying σ* anion states. While we also found dipole bound states and higher-lying shape resonances, we do not expect those states to significantly contribute to the observed yields. Despite the fact that we do not account for the spin-orbit interactions or reaction dynamics, the energies and autoionization lifetimes of the σ* resonances strongly support larger DEA yields for 10IC than 3BrC. The more efficient dissociation could explain the fourfold difference between the maximum DEA chiral asymmetries, since the difference in the atomic numbers of iodine and bromine only accounts for a factor of two. Additionally, our calculations suggest that the twofold difference between the DEA asymmetries of the iodocamphor isomers could be related to the partial suppression of the cross section for electron attachment to 3IC, compared to 10IC.

Selective bond breaking of halothane induced by electron transfer in potassium collisions

We present novel experimental results of negative ion formation of halothane (C₂HBrClF₃) upon electron transfer from hyperthermal neutral potassium atoms (K°) in the collision energy range of 8-1000 eV. The experiments were performed in a crossed molecular beam setup allowing a comprehensive analysis of the time-of-flight (TOF) mass negative ions fragmentation pattern and a detailed knowledge of the collision dynamics in the energy range investigated. Such TOF mass spectra data show that the only negative ions formed are Br^-, Cl^- and F^-, with a strong energy dependence in the low-energy collision region, with the bromine anion being the most abundant and sole fragment at the lowest collision energy probed. In addition, potassium cation (K⁺) energy loss spectra in the forward scattering direction were obtained in a hemispherical energy analyser at different K° impact energies. In order to support our experimental findings, ab initio quantum chemical calculations have been performed to help interpret the role of the electronic structure of halothane. Potential energy curves were obtained along the C-X (X = Br, Cl) coordinate to lend support to the dissociation processes yielding anion formation.

Electron and Positron Scattering by the Formamide Molecule

We report calculated elastic integral, differential, and momentum transfer cross sections for electron and positron collisions with the formamide (HCONH₂) molecule, for impact energies up to 10 eV. We have used the Schwinger multichannel method in the static-exchange and static-exchange plus polarization approximation for collisions of electrons and the static plus polarization approximation for collisions of positrons. The Born-closure procedure was applied to account for the long-range potential due to the permanent dipole moment of formamide. We obtained the well-characterized π* shape resonance located at around 2.38 eV, which belongs to the A″ symmetry of the C_s point group. Our integral and differential cross sections for collisions of electrons were compared with the data available in the literature and showed a good qualitative agreement. To the best of our knowledge, no experimental and theoretical data are currently available for positron-formamide collision, so our present cross sections were compared with the cross sections of formic acid, which is also polar and is isoeletronic to formamide.

Low-energy electron scattering by cyanamide: anion spectra and dissociation pathways

The low-energy anion spectra of cyanamide and its rare tautomer carbodiimide were surveyed with elastic electron scattering calculations. Our assignments differ qualitatively and quantitatively from a previous theoretical report. We support that both tautomers present two π* and two shape resonances, while cyanamide should also display a dipole bound state and a shape resonance. Available dissociative electron attachment measurements have shown several structures for dehydrogenation below 4 eV, but no sharp peaks related to vibrational Feshbach resonances. The absence of these resonances is explained by the lack of a potential barrier for tunneling of the hydrogen atom, despite the coupling between dipole bound and states. We found that the π* resonances initiate the dynamics that lead to hydrogen loss at 1.5, 2.5 and 3 eV. The later two structures arise from the anion states of cyanamide, while carbodiimide should account for the lower-lying one. The rarity of the second tautomer would be offset by its larger dissociative electron attachment cross section, enough to leave a distinct signature in the measured ion yield spectra. Low-energy electrons should thus decompose carbodiimide much more efficiently than cyanamide.

Electron-Induced Reactions in 3-Bromopyruvic Acid

3-Bromopyruvic acid (3BP) is a potential anti-cancer drug, the action of which on cellular metabolism is not yet entirely clear. The presence of a bromine atom suggests that it is also reactive towards low-energy electrons, which are produced in large quantities during tumour radiation therapy. Detailed knowledge of the interaction of 3BP with secondary electrons is a prerequisite to gain a complete picture of the effects of 3BP in different forms of cancer therapy. Herein, dissociative electron attachment (DEA) to 3BP in the gas phase has been studied both experimentally by using a crossed-beam setup and theoretically through scattering and quantum chemical calculations. These results are complemented by a vacuum ultraviolet absorption spectrum. The main fragmentation channel is the formation of Br- close to 0 eV and within several resonant features at 1.9 and 3-8 eV. At low electron energies, Br- formation proceeds through σ* and π* shape resonances, and at higher energies through core-excited resonances. It is found that the electron-capture cross-section is clearly increased compared with that of non-brominated pyruvic acid, but, at the same time, fragmentation reactions through DEA are significantly altered as well. The 3BP transient negative ion is subject to a lower number of fragmentation reactions than those of pyruvic acid, which indicates that 3BP could indeed act by modifying the electron-transport chains within oxidative phosphorylation. It could also act as a radio-sensitiser.

How does methylation suppress the electron-induced decomposition of 1-methyl-nitroimidazoles?

The efficient decomposition of nitroimidazoles (NIs) by low energy electrons is believed to underlie their radiosensitizing properties. Recent dissociative electron attachment (DEA) measurements showed that methylation at the N1 site unexpectedly suppresses the electron-induced reactions in 4(5)-NI. We report theoretical results that provide a clear interpretation of that astounding finding. Around 1.5 eV, DEA reactions into several fragments are initiated by a π^* resonance, not considered in previous studies. The autoionization lifetime of this anion state, which limits the predissociation dynamics, is considerably shorter in the methylated species, thereby suppressing the DEA signals. On the other hand, the lifetime of the π^* resonance located around 3 eV is less affected by methylation, which explains why DEA is still observed at these energies. Our results demonstrate how even a simple methylation can significantly modify the probabilities for DEA reactions, which may be significant for NI-based cancer therapy.

Low-energy electron collisions with proline and pyrrolidine: A comparative study

We present a comparative study on the calculated cross sections obtained for the elastic collisions of low-energy electrons with the amino acid proline (C₅H₉NO₂) and its building block pyrrolidine (C₄H₉N). We employed the Schwinger multichannel method implemented with pseudopotentials to compute integral, differential, and momentum transfer cross sections in the static-exchange plus polarization approximation, for energies up to 15 eV. We report three shape resonances for proline at around 1.7 eV, 6.8 eV, and 10 eV and two shape resonances for pyrrolidine centered at 7 eV and 10.2 eV. The present resonance energies are compared with available experimental data on vertical attachment energies and dissociative electron attachment, where a good agreement is found. From the comparison of the present results with available calculated cross sections for the simplest carboxylic acid, formic acid (HCOOH), and from electronic structure calculations, we found that the first resonance of proline, at 1.7 eV, is due the presence of the carboxylic group, whereas the other two structures, at 6.8 eV and 10 eV, clearly arise from the pyrrolidine ring. A comparison between the differential cross sections for proline and pyrrolidine at some selected energies of the incident electron is also reported in this paper.

Shape resonances, virtual state, and Ramsauer-Townsend minimum in the low-energy electron collisions with benzene

In this work, we revisit the low-energy electron scattering by benzene. We employed the Schwinger multichannel method implemented with pseudopotentials to carry out systematic cross section calculations with different schemes of polarization for the resonant and the totally symmetric irreducible representations within the D_2h symmetry group. We present integral and differential cross sections for incident electron energies up to 12 eV and discuss the shape resonances and the presence of a Ramsauer-Townsend minimum and a virtual state in the former. We also discuss the relation of these physical phenomena with the different schemes of the polarization effects employed in our calculations. Finally, the comparison of our calculated integral and differential cross sections with the available data from the literature suggests improvement in the agreement between theory and experiment.

Positron and electron scattering by glycine and alanine: Shape resonances and methylation effect

We report integral cross sections (ICSs) for both positron and electron scattering by glycine and alanine amino acids. These molecules differ only by a methyl group. We computed the scattering cross sections using the Schwinger multichannel method for both glycine and alanine in different levels of approximation for both projectiles. The alanine ICSs are greater in magnitude than the glycine ICSs for both positron and electron scattering, probably due to the larger size of the molecule. In electron scattering calculations, we found two resonances for each molecule. Glycine presents one at 1.8 eV, and another centered at around 8.5 eV, in the static-exchange plus polarization (SEP) approximation. The ICS for alanine shows one resonance at 2.5 eV and another at around 9.5 eV, also in SEP approximation. The results are in good agreement with most of the data present in the literature. The comparison of the electron scattering ICSs for both molecules indicates that the methylation of glycine destabilizes the resonances, shifting them to higher energies.

Elastic electron scattering from nitrobenzene

We present integral, momentum transfer, and differential cross sections for an elastic scattering of electrons by nitrobenzene. Our calculations employed the Schwinger multichannel method with pseudopotentials and were performed in the static-exchange and static-exchange plus polarization approximations. The cross sections were computed for impact energies up to 10 eV. We observed four resonances in the static-exchange calculations, and three when polarization effects were included. This result indicates that the low-lying resonance in the B₁ symmetry of C_2v group, observed in the static-exchange calculation, became a bound state when polarization was taken into account. Our calculations including polarization effects assigned the low-lying resonance located at 0.92 eV to the A₂ symmetry and the other two resonances, located at 2.07 eV and 6 eV, to the B₁ symmetry. These results compare well with the attachment energies obtained through electron transmission spectroscopy data and with dissociative electron attachment results for the NO₂^- sub-product, suggesting for the latter that the π^* resonances mediate the dissociation process.

Negative ion states of 5-bromouracil and 5-iodouracil

The valence anion states of the potential radiosensitisers 5-bromouracil and 5-iodouracil were investigated through elastic scattering calculations. These compounds have rich spectra of negative ion states that trigger off different mechanisms for dissociative electron attachment. For each molecule, we obtained a bound π* anion, two π* shape resonances and a low lying σ* anion state, in addition to a dipole-bound state (the latter was obtained using bound-state techniques). The σ* anion, formed by electron attachment to an anti-bonding carbon-halogen orbital, was found to have resonant character in 5-bromouracil, and bound-state character in 5-iodouracil. The present calculations place the σCBr* resonance around 0.7 eV, considerably below the energy inferred from the electron transmission data (1.3 eV). The signature of this anion state, not evident in the measurements, would be obscured by the large background arising from the dipolar interaction, not by the strong signature of the π2*, as presumed. Our results support the π2* resonance as a precursor state to dissociative electron attachment around 1.5 eV in both 5-bromouracil and 5-iodouracil, while the interplay among π1*, σ* and dipole-bound states would be expected close to 0 eV. We also discuss the suppression of the hydrogen elimination channels in these species.

Experimental and theoretical cross sections for positron scattering from the pentane isomers

Isomerism is ubiquitous in chemistry, physics, and biology. In atomic and molecular physics, in particular, isomer effects are well known in electron-impact phenomena; however, very little is known for positron collisions. Here we report on a set of experimental and theoretical cross sections for low-energy positron scattering from the three structural isomers of pentane: normal-pentane, isopentane, and neopentane. Total cross sections for positron scattering from normal-pentane and isopentane were measured at the University of Trento at incident energies between 0.1 and 50 eV. Calculations of the total cross sections, integral cross sections for elastic scattering, positronium formation, and electronic excitations plus direct ionization, as well as elastic differential cross sections were computed for all three isomers between 1 and 1000 eV using the independent atom model with screening corrected additivity rule. No definitive evidence of a significant isomer effect in positron scattering from the pentane isomers appears to be present.

Anion states and fragmentation of 2-chloroadenine upon low-energy electron collisions

We report on a joint theoretical and experimental investigation into the electron-induced fragmentation of 2-chloroadenine, for electrons up to 12 eV. Our results suggest that 2-chloroadenine can be considered as potential radiosensitiser.

Collisions of low-energy electrons with cyclohexane

We report calculated cross sections for elastic scattering of low-energy electrons by cyclohexane (c-C6H12). We employed the Schwinger multichannel method implemented with norm-conserving pseudopotentials in the static-exchange and static-exchange plus polarization approximations, for impact energies up to 30 eV. We compare our calculated integral cross section with experimental total cross sections available in the literature. We also compare our calculated differential cross sections (DCSs) with experimental results for benzene and experimental and theoretical results for 1,4-dioxane, in order to investigate the similarities between those molecules under electron collisions. Although benzene is a cyclic six-carbon molecule, as cyclohexane, we found that the differential cross sections of the latter are more similar to those of 1,4-dioxane than those of benzene. These similarities suggest that the geometry may play an important role in the behavior of the DCSs of these molecules. Our integral cross section displays a broad structure at around 8.5 eV, in agreement with the total cross section experimental data of 8 eV and vibrational excitation data of 7.5 eV. The present integral cross section also shows the presence of a Ramsauer-Townsend minimum at around 0.12 eV. In general, our integral cross section shows a qualitative agreement with the experimental total cross section.

Shape resonance spectra of uracil, 5-fluorouracil, and 5-chlorouracil

We report on the shape resonance spectra of uracil, 5-fluorouracil, and 5-chlorouracil, as obtained from fixed-nuclei elastic scattering calculations performed with the Schwinger multichannel method with pseudopotentials. Our results are in good agreement with the available electron transmission spectroscopy data, and support the existence of three π∗ resonances in uracil and 5-fluorouracil. As expected, the anion states are more stable in the substituted molecules than in uracil. Since the stabilization is stronger in 5-chlorouracil, the lowest π∗ resonance in this system becomes a bound anion state. The present results also support the existence of a low-lying σCCl (*) shape resonance in 5-chlorouracil. Exploratory calculations performed at selected C-Cl bond lengths suggest that the σCCl (*) resonance could couple to the two lowest π∗ states, giving rise to a very rich dissociation dynamics. These facts would be compatible with the complex branching of the dissociative electron attachment cross sections, even though we cannot discuss any details of the vibration dynamics based only on the present fixed-nuclei results.

Shape resonances in low-energy-electron collisions with halopyrimidines

We report calculated cross sections for elastic collisions of low-energy electrons with halopyrimidines, namely, 2-chloro, 2-bromo, and 5-bromopyrimidine. We employed the Schwinger multichannel method with pseudopotentials to compute the cross sections in the static-exchange and static-exchange plus polarization levels of approximation for energies up to 10 eV. We found four shape resonances for each molecule: three of π* nature localized on the ring and one of σ* nature localized along the carbon-halogen bond. We compared the calculated positions of the resonances with the electron transmission spectroscopy data measured by Modelli et al. [J. Phys. Chem. A 115, 10775 (2011)]. In general the agreement between theory and experiment is good. In particular, our results show the existence of a π* temporary anion state of A2 symmetry for all three halopyrimidines, in agreement with the dissociative electron attachment spectra also reported by Modelli et al. [J. Phys. Chem. A 115, 10775 (2011)].

Elastic scattering of low-energy electrons by 1,4-dioxane

We report calculated cross sections for elastic collisions of low-energy-electrons with 1,4-dioxane. Our calculations employed the Schwinger multichannel method with pseudopotentials and were carried out in the static-exchange and static-exchange plus polarization approximations for energies up to 30 eV. Our results show the presence of three shape resonances belonging to the Bu, Au, and Bg symmetries and located at 7.0 eV, 8.4 eV, and 9.8 eV, respectively. We also report the presence of a Ramsauer-Townsend minimum located at around 0.05 eV. We compare our calculated cross sections with experimental data and R-matrix and independent atom model along with the additivity rule corrected by using screening coefficients theoretical results for 1,4-dioxane obtained by Palihawadana et al. [J. Chem. Phys. 139, 014308 (2013)]. The agreement between the present and the R-matrix theoretical calculations of Palihawadana et al. is relatively good at energies below 10 eV. Our calculated differential cross sections agree well with the experimental data, showing only some discrepancies at higher energies.

Electron driven reactions in sulphur containing analogues of uracil: the case of 2-thiouracil

Electron induced fragmentation of 2-thiouracil.