Low Energy (< 10 eV) Electron Collision with Benzonitrile-CCl4 Admixture: A Combined Theoretical and Experimental Study.

Benzonitrile (BZN) and carbon tetrachloride (CCl4) are versatile solvents used as a precursor for the synthesis of many products. As multi-usage molecules, these compounds may be involved in sustainable chemistry processes such as the cold plasma techniques for which the generated electrons are known to be responsible for reactions. Therefore, it is desirable to explore the interaction of low energy electrons with the co-compounds in the gas phase. The production of chlorine and cyanine anions, initiated by the electron collision with CCl4 and BZN, respectively, undergo nucleophilic substitution SN2 reaction with the precursors molecules for the synthesis of chlorobenzene and tricholoacetonitrile. The mechanism of fragmentation of benzonitrile and the synthesis reactions are rationalized by DFT calculations.  The yield of the cyanine anion produced from the ion reaction increases with the temperature of the admixture gas, probed in the 25 °C-100 °C temperature range. The present work may contribute to a potential process for the production of chlorobenzene for instance via (cold) plasma techniques.

Novel Approach for Predicting Vertical Electron Attachment Energies in Bulk-Solvated Molecules.

When low-energy electrons interact with molecules, they can give rise to transient anion states commonly known as resonances. These states are formed through vertical electron attachment processes and have the potential to induce various forms of DNA lesions, including base damage, single- and double-strand breaks, cross-links, and clustered lesions that are challenging to repair. So far, most experimental and theoretical studies have investigated the formation of resonances of (bio)molecules in the gas phase or in microsolvated environments. Since cellular environments are mainly composed of water molecules, it is crucial to understand how bulk water affects the resonances of (bio)molecules. Given the existing gap in studies on resonances of bulk-solvated molecules, we propose a novel theoretical-computational approach to address this void. Our approach combines the multibasis-set (time-dependent-)density functional theory and self-consistent sequential quantum mechanics/molecular mechanics polarizable electrostatic embedding methods. We apply this combined methodology to predict the vertical electron attachment energies of 1-methyl-5-nitroimidazole (1M5NI), a well-known radiosensitizer model, in bulk water. In addition, we analyze the rapid mutual polarization between the resonances (both shape- and core-excited) of 1M5NI and the surrounding bulk water environment. For comparison, we also studied the isolated and microsolvated 1M5NI. Overall, while the polarization of the environment is clearly sensitive to the solute charge, causing a significant impact on the vertical electron affinity and consequently on the attachment electron energies, it does not have a significant impact on the excitation energies of the anion.

Size Effect in Correlation-Driven Charge Migration in Correlation Bands of Alkyne Chains.

Correlation-driven charge migration initiated by inner-valence ionization leading to the population of the correlation bands of alkyne chains containing between 4 and 12 carbon atoms is explored through ab initio simulations. Scaling laws are observed, both for the time scale of the charge migration and for the slope of the density of states of the correlation bands. These can be used for predicting the relaxation time scale in much larger systems from the same molecular family and for finding promising candidates for the development of an attochemistry scheme taking advantages of the specificity of the dynamics in the correlation bands of molecules.

How Accurately Can DFT Describe Non-valence Anions?

Weakly bound non-valence anions are molecular systems where the excess electron stabilizes in a very diffuse orbital whose size, shape, and binding energy (∼1-100 meV) are governed by the long-range electrostatic potential of the molecule. Its binding energy comes mainly from charge-dipole or charge-multipole interactions or dispersion forces. While highly correlated methods, like coupled cluster methods, are considered to be the state of the art for describing anionic systems, especially when the electron lies in a very diffuse orbital, we consider here the possibility to use DFT-based calculations. In such molecular anions, the outer electron experiences long-range exchange and correlation interactions. We show that DFT can describe long-range bound states provided that a correct asymptotic exchange and correlation potential is used, namely, that from a range-separated hybrid functional. This opens an alternative to the computationally demanding highly correlated method calculations. It is also suggested that the study of weakly bound anions could help in the construction of new DFT potentials to study systems where nonlocal effects are significant.

Experimental and Theoretical Investigations of the Fragmentation of Ethylenediamine Induced by Low-Energy (<10 eV) Electrons.

Ethylenediamine is industrially used as an intermediate for the fabrication of many products. The development of new methodologies for synthesis compatible with the environment and sustainability, such as cold plasma processes, implicates reactions induced by nonthermal electrons. In this contribution, we study the interaction of low-energy (<10 eV) electrons with ethylenediamine. We show that electrons induce the fragmentation of the molecule into various anion fragments and associated neutral counterparts via dissociative electron attachment. The fragmentation mechanisms and energetics are discussed in the frame of DFT calculations. The fragmentation processes are quantified by the estimation of the cross sections and the branching ratios for competitive accessible dissociation routes.

Coordination of Ethylamine on Small Silver Clusters: Structural and Topological (ELF, QTAIM) Analyses.

Amine ligands are expected to drive the organization of metallic centers as well as the chemical reactivity of silver clusters early growing during the very first steps of the synthesis of silver nanoparticles via an organometallic route. Density functional theory (DFT) computational studies have been performed to characterize the structure, the atomic charge distribution, and the planar two-dimensional (2D)/three-dimensional (3D) relative stability of small-size silver clusters (Agn, 2 ≤ n ≤ 7), with or without an ethylamine (EA) ligand coordinated to the Ag clusters. The transition from 2D to 3D structures is shifted from n = 7 to 6 in the presence of one EA coordinating ligand, and it is explained from the analysis of the Ag-N and Ag-Ag bond energies. For fully EA saturated silver clusters (Agn-EAn), the effect on the 2D/3D transition is even more pronounced with a shift between n = 4 and 5. Subsequent electron localization function (ELF) and quantum theory of atoms in molecules (QTAIM) topological analyses allow for the fine characterization of the dative Ag-N and metallic Ag-Ag bonds, both in nature and in strength. Electron transfer from ethylamine to the coordinated silver atoms induces an increase of the polarization of the metallic core.

Multi-Basis-Set (TD-)DFT Methods for Predicting Electron Attachment Energies.

The interaction of low-energy electron collisions with molecules may lead to temporary anions via resonant processes. While experimental measurements, e.g., electron transmission spectroscopy or dissociation electron attachment spectroscopy, are efficient to characterize the temporary anions, simulating the electron attachment is still very challenging. Here, we propose a methodology to calculate the resonance energies of the electron attachment using ab initio (TD)-DFT calculations together with two different basis sets: a large basis set with diffuse functions to compute the vertical electron affinity and a smaller one to calculate the excitation energy of the anion. To demonstrate the capabilities and the reliability of this computational approach, 53 resonance energies from 18 molecules are calculated and compared to experimental data.

Experimental and Theoretical Studies of Dissociative Electron Attachment to Metabolites Oxaloacetic and Citric Acids.

In this contribution the dissociative electron attachment to metabolites found in aerobic organisms, namely oxaloacetic and citric acids, was studied both experimentally by means of a crossed-beam setup and theoretically through density functional theory calculations. Prominent negative ion resonances from both compounds are observed peaking below 0.5 eV resulting in intense formation of fragment anions associated with a decomposition of the carboxyl groups. In addition, resonances at higher energies (3-9 eV) are observed exclusively from the decomposition of the oxaloacetic acid. These fragments are generated with considerably smaller intensities. The striking findings of our calculations indicate the different mechanism by which the near 0 eV electron is trapped by the precursor molecule to form the transitory negative ion prior to dissociation. For the oxaloacetic acid, the transitory anion arises from the capture of the electron directly into some valence states, while, for the citric acid, dipole- or multipole-bound states mediate the transition into the valence states. What is also of high importance is that both compounds while undergoing DEA reactions generate highly reactive neutral species that can lead to severe cell damage in a biological environment.

Fragmentation of Nickel(II) and Cobalt(II) Bis(acetylacetonate) Complexes Induced by Slow (<10 eV) Electrons.

Metal acetylacetonate complexes have high potentiality in nanoscale fabrication processes (e.g., focus electron beam-induced deposition) thanks to the versatile character and ease of preparation compounds. In this work, we study and compare the physics and the physicochemistry induced by the interaction of low-energy (<10 eV) electrons with nickel(II) and cobalt(II) bis(acetylacetonate) complexes. The slow particles decompose the molecules via dissociative electron attachment. The nickel(II) and cobalt(II) bis(acetylacetonate) anions and the acetylacetonate negative fragments are the most dominant detected species. The experimental data are completed with density functional theory calculations to provide information on the electronic states of the molecules and the energetics for fragmentation. Finally, it is found that the interaction of low-energy electrons resulting in the decomposition of organometallic complexes in the gas phase is more efficient with the nickel(II) than with the cobalt(II) bis(acetylacetonate) complex. These results are found to be in a relative agreement with the surface experiments.

Energy-Selective Decomposition of Organometallic Compounds by Slow Electrons: The Case of Chloro(dimethyl sulfide)gold(I).

Gold-containing compounds offer many applications in nanoscale materials science, and electron-beam methods are versatile for shaping nanostructures. In this study, we report the energy-selective fragmentation of chloro(dimethyl sulfide)gold(I) (ClAuS(CH3)2) induced by slow electrons. We observe the resonant formation of four fragment anions, namely [Cl]-, [S]-, [CH2S]-, and [ClAuH···SH]-, which are generated in the energy range of 0-9 eV. The predominant fragment anion is formed below 1 eV from the cleavage of a single Au-Cl bond to produce the [Cl]- anion. The resonant states and the energetics of the fragmentation are investigated by DFT methods. These findings may contribute to future strategies in the elaboration of specific nanomaterials or for selective chemistry using electron-beam techniques.

Decomposition of Bis(acetylacetonate)zinc(II) by Slow Electrons.

The production of zinc-containing nanostructures has a large variety of applications. Using electron beam techniques to degrade organometallic molecules for that purpose is perhaps one of the most versatile methods. In this work, we investigate the scattering of low-energy (<12 eV) electrons with bis(acetylacetonate)zinc(II) molecules. We show that core excited and high-lying shape resonances are mainly responsible for the production of the precursor anions as well as the ligand negative fragments, which are observed exclusively at electron energies of >3 eV. The mechanisms for electron capture and then molecular dissociation are discussed in terms of density functional theory studies.

Interaction of Slow Electrons with Thermally Evaporated Manganese(II) Acetylacetonate Complexes.

Complexes of metal acetylacetonate are used as general precursors for the synthesis of metal oxide nanomaterials. In the present work, we study the interaction of low-energy (<10 eV) electrons, produced abundantly as secondary electrons during the bombardment of the substrate by the primary particles, with thermally evaporated manganese(II) acetylacetonate complexes. We found that the acetylacetonate anion ([acac]-) is the major anionic species produced, while the second most abundant is the parent anion [Mn(II)(acac)2]-. This observation differs from those reported from electron attachment to Cu(acac)2, for which [Cu(II)(acac)2]- is the predominant anion [Kopyra et al. Phys. Chem. Chem. Phys. 2018, 20, 7746]. The experimental data are supported by theory to provide information on the physical-chemistry processes initiated by slow electrons to the organometallic precursor and to interpret the different behavior of Mn(acac)2 compared to Cu(acac)2.

Optical properties of size selected neutral Ag clusters: electronic shell structures and the surface plasmon resonance.

We present optical absorption spectra from the ultraviolet to the visible for size selected neutral Agn clusters (n = 5-120) embedded in solid Ne. We compare the spectra to time-dependent density functional calculations (TDDFT) that address the influence of the Ne matrix. With increasing size, several highly correlated electron excitations gradually develop into a single surface plasmon. Its energy is situated between 3.9 and 4.1 eV and varies with size according to the spherical electronic shell model. The plasmon energy is highest for clusters with atom numbers fully filling states with the lowest radial quantum number (e.g. 1s, 1p, 1d,...). TDDFT calculations for clusters with several candidate geometrical structures embedded in Ne show excellent agreement with the experimental data, demonstrating that the absorption bands depend only weakly on the exact structure of the cluster.

Interaction of gas phase copper(ii) acetylacetonate with slow electrons.

Understanding the fundamental processes underlying the interaction of organometallic compounds with low energy electrons is desirable for optimizing methodologies for nanoscale applications. In this work, we couple experimental measurements with theories to investigate the interaction of gas phase copper(ii) acetylacetonate, Cu(acac)2, with low energy (<12 eV) electrons. Near 0 eV, a multipole-bound anion is likely to act as the doorway for the formation of a transitory molecular anion which then undergoes stabilization via a 90°-rotation of one of the acac units. The production of the parent anion competes with the dissociation processes, generating preferentially the acetylacetonate negative ion. Moreover, at incident electron energies above 3.5 eV, the electron driven fragmentation of Cu(acac)2 is likely to produce atomic Cu. These results can suggest some potential strategies for the deposition of pure copper using an appropriate electron irradiation technique.

Growth Behavior and Electronic Structure of Noble Metal-Doped Germanium Clusters.

Structures, energetics, and electronic properties of noble metal-doped germanium (MGen with M = Cu, Ag, Au; n = 1-19) clusters are systematically investigated by using the density functional theory (DFT) approach. The endohedral structures in which the metal atom is encapsulated inside of a germanium cage appear at n = 10 when the dopant is Cu and n = 12 for M = Ag and Au. While Cu doping enhances the stability of the corresponding germanium frame, the binding energies of AgGen and AuGen are always lower than those of pure germanium clusters. Our results highlight the great stability of the CuGe10 cluster in a D4d structure and, to a lesser extent, that of AgGe15 and AuGe15, which exhibits a hollow cage-like geometry. The sphere-type geometries obtained for n = 10-15 present a peculiar electronic structure in which the valence electrons of the noble metal and Ge atoms are delocalized and exhibit a shell structure associated with the quasi-spherical geometry. It is found that the coinage metal is able to give both s- and d-type electrons to be reorganized together with the valence electrons of Ge atoms through a pooling of electrons. The cluster size dependence of the stability, the frontier orbital energy gap, the vertical ionization potentials, and electron affinities are given.

Oxidation-Induced Surface Roughening of Aluminum Nanoparticles Formed in an Ablation Plume.

Nanoparticles formed within an ablation plume produced by the impact of a nanosecond laser pulse on the surface of an aluminum target have been directly measured using small-angle x-ray scattering. The target was immersed in an oxygen-nitrogen gas mixture at atmospheric pressure with the O_{2}/N_{2} ratio being precisely controlled. The results for an increasing oxygen content reveal remarkable effects on the morphology of the generated particles, which include a decrease in the particle volume but a marked increase in its surface ruggedness. Molecular dynamics simulations using a reactive potential and performed under similar conditions as the experiment reproduce the experimental trends and show in detail how the shape and surface structure of the nanoparticles evolve with increasing oxygen content. This good agreement between in situ observations in the plume and atomistic simulations emphasizes the key role of chemical reactivity together with thermodynamic conditions on the morphology of the particles thus produced.

Description of plasmon-like band in silver clusters: the importance of the long-range Hartree-Fock exchange in time-dependent density-functional theory simulations.

Absorption spectra of Ag20 and Ag55(q) (q = +1, -3) nanoclusters are investigated in the framework of the time-dependent density functional theory in order to analyse the role of the d electrons in plasmon-like band of silver clusters. The description of the plasmon-like band from calculations using density functionals containing an amount of Hartree-Fock exchange at long range, namely, hybrid and range-separated hybrid (RSH) density functionals, is in good agreement with the classical interpretation of the plasmon-like structure as a collective excitation of valence s-electrons. In contrast, using local or semi-local exchange functionals (generalized gradient approximations (GGAs) or meta-GGAs) leads to a strong overestimation of the role of d electrons in the plasmon-like band. The semi-local asymptotically corrected model potentials also describe the plasmon as mainly associated to d electrons, though calculated spectra are in fairly good agreement with those calculated using the RSH scheme. Our analysis shows that a portion of non-local exchange modifies the description of the plasmon-like band.

Absorption spectra of alkali-C60 nanoclusters.

We investigate the absorption spectra of alkali-doped C60 nanoclusters, namely C60Nan, C60Kn, and C60Lin, with n = 1, 2, 6, 12, in the framework of the time-dependent density-functional theory (TDDFT). We study the dependence of the absorption spectra on the nature of the alkali. We show that in few cases the absorption spectra depend on the arrangement of the alkali atoms over the fullerene, though sometimes the absorption spectra do not allow us to distinguish between different configurations. When only one or two alkali atoms are adsorbed on the fullerene, the optical response of alkali-doped C60 is similar to that of the anion C60(-) with a strong response in the UV domain. In contrast, for higher concentration of alkali, a strong optical response is predicted in the visible range, particularly when metal-metal bonds are formed. The weak optical response of the I(h)-symmetry C60Li12 is proposed to be used as a signature of its structure.

Assessment of the performance of long-range-corrected density functionals for calculating the absorption spectra of silver clusters.

We assess the accuracy of several long-range-corrected (LC-) density functionals for the prediction of absorption spectra of silver clusters by time-dependent density functional theory. Several types of LC-functionals, with the exact Hartree-Fock exchange at long-range, are used: those applying the long-range correction to a standard GGA-type functional (LC-BP86, LC-ωPBE) or to a local meta-GGA functional (LC-M06L) and two global hybrid functionals (CAM-B3LYP and ωB97x). The spectra calculated with those density functionals are in good agreement with the recent accurate experimental measurements. We show that CAM-B3LYP and ωB97x are some of the most accurate functionals for evaluating the electronic excitation energies, while LC-M06L is more effective in reducing the occurrence of spurious states. The long-range correction appears to be essential in describing the absorption spectra of large clusters. The description of the plasmon-like band with LC functionals as transitions associated with excitations from s orbitals to s + p orbitals is in fairly good agreement with the classical interpretation as a collective excitation of valence s electrons.

Structure and electronic properties of alkali-C60 nanoclusters.

I investigated the structural and electronic properties of both Na(n)C(60) and Li(n)C(60) (n < or = 12) clusters in the framework of density-functional theory. The behavior of the alkali atoms over the C(60) surface is analyzed. The hypotheses for either an homogeneous coating of the C(60) surface by the alkali atoms or the growth of an alkali droplet not wetting the fullerene surface are discussed. Lithium atoms are found to coat homogeneously the fullerene on the C(60) surface via pentagonal sites, contrary to sodium atoms, which prefer to form 4-atom islands on the surface. The charge transfer, the energetics, and the dipole moments are discussed in relation with available results. The adsorption of alkali atoms on the C(60) surface considerably enhances the capacity of C(60) to accept electrons. The arrangement of lithium atoms seems more favorable for the hydrogen storage than that of sodium atoms.