Uptake of small oxidized organic molecules by hydrated nitric acid clusters investigated by molecular dynamics simulations and molecular beam experiments reveals the role that individual functional groups (hydroxyl and carbonyl) play in the process.

Polycyclic aromatic hydrocarbons (PAHs) are omnipresent in the Earth's atmosphere and detected in many regions of the interstellar space. Large PAH clusters (nanoparticles) can mimic soot and aerosol particles in the Earth's atmosphere as well as PAH ice grains in space. We investigate PAH nanoparticles in a molecular beam experiment and the uptake of different atmospherically and astrochemically relevant molecules by these nanoparticles. In our recent article [J. Phys. Chem. Lett. 2022, 13, 3781-3788], we have established a new method to determine the relative uptake ratios for hydrogen-bonding molecules on benzene nanoparticles of average radius of about 2 nm. Here, we extend the method from benzene nanoparticles to more complex PAH N of naphthalene, anthracene, phenanthrene, and pyrene, including also the five-membered ring containing PAHs indene and fluoranthene. The uptake of small molecules water, methanol, ethanol, ammonia, methane, oxygen, and carbon dioxide is investigated, and uptake ratios are determined to assess differences in the pickup of these molecules by PAH nanoparticles. We discuss the mobility and coagulation of the molecules on these nanoparticles. Investigating the uptake and accommodation of different molecules on PAH nanoparticles is desirable for a molecular-level understanding of the chemistry of soot and aerosol particles in the Earth's atmosphere as well as PAH ice grains in the interstellar space, where they interact with different molecules.

We investigated the dissociation of nitric acid on large water clusters (H2O)N, N̄ ≈ 30-500, i.e., ice nanoparticles with diameters of 1-3 nm, in a molecular beam. The (H2O)N clusters were doped with single HNO3 molecules in a pickup cell and probed by mass spectrometry after a low-energy (1.5-15 eV) electron attachment. The negative ion mass spectra provided direct evidence for HNO3 dissociation with the formation of NO3-⋯H3O+ ion pairs, but over half of the observed cluster ions originated from non-dissociated HNO3 molecules. This behavior is in contrast with the complete dissociation of nitric acid on amorphous ice surfaces above 100 K. Thus, the proton transfer is significantly suppressed on nanometer-sized particles compared to macroscopic ice surfaces. This can have considerable implications for heterogeneous processes on atmospheric ice particles.

Uptake of dimethylamine by mixed nitric acid/water clusters is investigated by molecular-beam experiment accompanied by ab initio calculations. The clusters are probed by mass spectrometry after positive ionization and slow electron attachment.

The uptake of molecules on nanometer-size clusters of polyaromatic hydrocarbons (PAHs) is important for the condensation of water on PAH aerosols in the atmosphere and for ice mantle growth on nanoparticles in the interstellar medium. We generate benzene clusters BzN of mean size N̅ ≈ 300 (radius R̅ ≈ 2.2 Å) as a model system for the PAH nanoparticles. Using molecular beams and mass spectrometry detection, we investigate the uptake of water, methanol, and ethanol by these clusters. All picked up molecules are highly mobile on BzN and generate clusters within <3 ms. The relative uptakes for the different investigated molecules can be directly compared and quantified. Water molecules exhibit the lowest relative pickup probability that is ∼30% lower than those for methanol and ethanol, which are approximately the same.

Metronidazole belongs to the class of nitroimidazole molecules and has been considered as a potential radiosensitizer for radiation therapy. During the irradiation of biological tissue, secondary electrons are released that may interact with molecules of the surrounding environment. Here, we present a study of electron attachment to metronidazole that aims to investigate possible reactions in the molecule upon anion formation. Another purpose is to elucidate the effect of microhydration on electron-induced reactions in metronidazole. We use two crossed electron/molecular beam devices with the mass-spectrometric analysis of formed anions. The experiments are supported by quantum chemical calculations on thermodynamic properties such as electron affinities and thresholds of anion formation. For the single molecule, as well as the microhydrated condition, we observe the parent radical anion as the most abundant product anion upon electron attachment. A variety of fragment anions are observed for the isolated molecule, with NO2− as the most abundant fragment species. NO2− and all other fragment anions except weakly abundant OH− are quenched upon microhydration. The relative abundances suggest the parent radical anion of metronidazole as a biologically relevant species after the physicochemical stage of radiation damage. We also conclude from the present results that metronidazole is highly susceptible to low-energy electrons.

It is well established that an isolated benzene radical anion is not electronically stable. In the present study, we experimentally show that electron attachment to benzene clusters leads to weak albeit unequivocal occurrence of a C6H6 - moiety. We propose here-based on electronic structure calculation-that this moiety actually corresponds to linear structures formed by the opening of the benzene ring via electron attachment. The cluster environment is essential in this process since it quenches the internal energy released upon ring opening, which in the gas phase leads to further dissociation of this anion.

Nanoparticles can serve as an efficient reaction environment for bimolecular reactions as the reactants concentrate either inside the nanoparticle or on the surface of the nanoparticle. The reaction rate is then controlled by the rate of formation of the reaction pairs. We demonstrate this concept on the example of electron-induced reactions in hydrogen peroxide. We consider two types of nanoparticle environments: solid argon particles, only weakly interacting with the hydrogen peroxide reactant, and ice particles with a much stronger interaction. The formation of hydrogen peroxide dimers is investigated via classical molecular dynamics (MD) simulations on a microsecond timescale. With a modified force field for hydrogen peroxide, we found out a fast formation and stabilization of the hydrogen peroxide dimer for argon nanoparticles, while the reaction pair was formed reversibly at a much slower rate on the water nanoparticles. We have further investigated the electron-induced reactions using non-adiabatic ab initio MD simulations, identifying the possible reaction products upon the ionization or electron attachment. The major reaction path in all cases corresponded to a proton transfer. The computational findings are supported by mass spectrometry experiments, where large ArM and (H2O)M nanoparticles are generated, and several hydrogen peroxide molecules are embedded on these nanoparticles in a pickup process. Subsequently, the nanoparticles are ionized either positively by 70 eV electrons or negatively by electron attachment at electron energies below 5 eV. The recorded mass spectra demonstrate the efficient coagulation of H2O2 on ArM, while it is quite limited on (H2O)M.

We report an experimental and computational study of the electron-induced chemistry of methanesulfonic acid (MSA, MeSO3H) in clusters. We combine the mass spectra after the 70 eV electron ionization with the negative ion spectra after electron attachment (EA) at low electron energies of 0-15 eV of the MSA molecule, small MSA clusters, and microhydrated MSA clusters to reveal the solvation effects. The MSA/He coexpansion only generates small MSA clusters with up to four molecules, but adding water substantially hydrates the MSA clusters, resulting in clusters composed of 1-2 MSA molecules accompanied by quite a few water molecules. The clustering strongly suppresses the fragmentation of the MSA molecules upon both the positive ionization and EA. The electron-energy-dependent ion yield for different negative ions is measured. For the MSA molecule and pure MSA clusters, EA leads to an H-abstraction yielding MeSO3-. It proceeds efficiently at low electron energies below 2 eV with a shoulder at 3-4 eV and a broad, almost 2 orders of magnitude weaker, peak around 8 eV. The hydrated (H2O)nMeSO3- ions with n ≤ 3 exhibit only a broad peak around 7 eV similar to EA of pure water clusters. Thus, for the small clusters, the electron attachment and hydrogen abstraction from water occur. On the other hand, the larger clusters with n > 4 display a peak below 2 eV, which quickly dominates the spectrum with increasing n. This peak is related to the formation of the H3O+·MeSO3- ion pair upon hydration and subsequent dipole-supported electron attachment followed by the hydronium neutralization and H3O• radical dissociation. The size-resolved experimental data indicate that the ionic dissociation of MSA starts to occur in the neutral MeSO3H(H2O)N clusters with about four water molecules.

We investigate electron attachment to large ammonia clusters doped with a single benzene (Bz) molecule (NH3)N·Bz, N̄ ≈ 320. Negatively charged clusters are probed by mass spectrometry, and the energy-dependent ion yields are derived from mass spectra measured at different electron energies. The ion efficiency curves of pure ammonia clusters exhibit two maxima. At around 6 eV, (NH3)n-1NH2- ions are produced via dissociative electron attachment (DEA) to NH3 molecules. (NH3)n- ions produced at this energy are formed by DEA followed by fragment caging. At low energies around 1.3 eV, only (NH3)n- ions are formed for cluster sizes n ≥ 35 that correspond to solvated electrons in ammonia clusters. The doped (NH3)n·Bz- cluster ions exhibit essentially the same energy dependence. The (NH3)n·Bz- ions are metastable and evaporate NH3 molecule(s), while pure (NH3)n- ions are stable. The lifetime for NH3 molecule evaporation from the Bz-doped clusters was estimated as τ ≈ 18 μs. We interpret the metastability of the doped clusters by the charge localization on a Bz- ion solvated in the ammonia, which is accompanied by an energy release leading to the evaporation of NH3 molecule(s).

Heterogeneous chemistry on the surfaces of atmospheric particles has a wide impact on the properties and composition of the Earth's atmosphere. In laboratory studies, clusters can represent proxies to atmospheric aerosols and help to discern the individual steps in reactions on or in aerosols. We investigate the reactivity of Cl and CCl3 radicals with methane on argon clusters using the pickup method. For radical generation, we built a new pyrolysis source partially adapting the design of radical sources that utilize the supersonic expansion into a heated silicon carbide tube. Large ArN, N̅ ≈ 110, clusters were generated in a supersonic expansion, and CH4 molecules were embedded in the clusters via a pickup process followed by the uptake of the radicals produced in the pyrolysis source. The analysis of the mass spectra recorded under different experimental conditions (i.e., with the pyrolysis ON and OFF and with only one or both reactants) allowed us to identify various products of the radical reactions on ArN. We propose a sequence of reactions based on the reaction energetics. It starts with the hydrogen abstraction from CH4 by a Cl radical resulting in HCl and CH3 followed by a halogenation step where CCl4 molecules react with the available CH3 radicals, yielding CH3Cl. By analogy, the CH3Cl enters another hydrogen abstraction by Cl, producing HCl and the CH2Cl radical, which again undergoes a halogenation step with CCl4, generating CH2Cl2. Further reaction of CH2Cl2 with Cl terminates the sequence by the production of HCl and CHCl2.

Focused electron beam deposition often requires the use of purification techniques to increase the metal content of the respective deposit. One of the promising methods is adding H2O vapor as a reactive agent during the electron irradiation. However, various contrary effects of such addition have been reported depending on the experimental condition. We probe the elementary electron-induced processes that are operative in a heterogeneous system consisting of iron pentacarbonyl as an organometallic precursor and water. We use an electron beam of controlled energy that interacts with free mixed Fe(CO)5/H2O clusters. These mimic the heterogeneous system and, at the same time, allow direct mass spectrometric analysis of the reaction products. The anionic decomposition pathways are initiated by dissociative electron attachment (DEA), either to Fe(CO)5 or to H2O. The former one proceeds mainly at low electron energies (<3 eV). Comparison of nonhydrated and hydrated conditions reveals that the presence of water actually stabilizes the ligands against dissociation. The latter one proceeds at higher electron energies (>6 eV), where the DEA to H2O forms OH- in the first reaction step. This intermediate reacts with Fe(CO)5, leading to enhanced decomposition, with the desorption of up to three CO ligands. The present results demonstrate that the water action on Fe(CO)5 decomposition is sensitive to the involved electron energy range and depends on the hydration degree.

Pyruvic acid represents a key molecule in prebiotic chemistry and it has recently been proposed to be synthesized on interstellar ices. In order to probe the stability of pyruvic acid in the interstellar medium with respect to decomposition by slow electrons, we investigate the electron attachment to its homomolecular and heteromolecular clusters. Using mass spectrometry, we follow the changes in the fragmentation pattern and its dependence on the electron energy for various cluster sizes of pure and microhydrated pyruvic acid. The assignment of fragmentation reaction pathways is supported by ab initio calculations. The fragmentation degree dramatically decreases upon clustering. This decrease is even stronger in the heteromolecular clusters of pyruvic acid with water, where the non-dissociative attachment is by far the strongest channel. In the homomolecular clusters, the dissociative channel leading to dehydrogenation is active over a larger electron energy range than in the isolated molecules. To probe the role of the self-scavenging effects, we explore the excited states of pyruvic acid. This has been done both experimentally, by using electron energy loss spectroscopy, and theoretically, by photochemical calculations. Data on both optically-allowed and forbidden states allow for the explanation of processes emerging upon clustering.

Hydrogen peroxide is a highly reactive and relatively unstable molecule, therefore pure (H2O2)(N) clusters are difficult to generate. In order to investigate the solvent effects on the H2O2 reactivity, experiments with H2O2 clusters are desirable. We have generated (H2O2)(N )clusters on free nanoparticles in molecular beams using a pickup technique. The individual H2O2 molecules were evaporated from the stable complex of urea with hydrogen peroxide (UHP) and they were picked up by Ar-M and (H2O)(M) nanoparticles in a molecular beam prepared by supersonic expansion. The clusters were probed by the time-of-flight mass spectrometry after electron ionization. The mass spectra showed that several H2O2 molecules could be deposited on both Ar-M and (H2O)(M) nanoparticles. On Ar-M, they coagulate and generate (H2O2)(N) clusters with N >= 20, while on ice nanoparticles the coagulation is inhibited on the timescale of our experiment (similar to 0.7 ms). The mass spectra suggest that there is no significant contamination of the (H2O2)(N) clusters with oxygen and water, which would be the products of H2O2 thermal decomposition. The proposed method represents a way of producing (H2O2)(N) clusters for future experiments. (C) 2020 Elsevier B.V. All rights reserved.

In this perspective, we review experiments with molecules picked up on large clusters in molecular beams with the focus on the processes in atmospheric and interstellar chemistry. First, we concentrate on the pickup itself, and we discuss the pickup cross sections. We measure the uptake of different atmospheric molecules on mixed nitric acid-water clusters and determine the accommodation coefficients relevant for aerosol formation in the Earth's atmosphere. Then the coagulation of the adsorbed molecules on the clusters is investigated. In the second part of this perspective, we review examples of different processes triggered by UV-photons or electrons in the clusters with embedded molecules. We start with the photodissociation of hydrogen halides and Freon CF2Cl2 on ice nanoparticles in connection with the polar stratospheric ozone depletion. Next, we mention reactions following the excitation and ionization of the molecules adsorbed on clusters. The first ionization-triggered reaction observed between two different molecules picked up on the cluster was the proton transfer between methanol and formic acid deposited on large argon clusters. Finally, negative ion reactions after slow electron attachment are illustrated by two examples: mixed nitric acid-water clusters, and hydrogen peroxide deposited on large ArN and (H2O)N clusters. The selected examples are discussed from the perspective of the atmospheric and interstellar chemistry, and several future directions are proposed.

We investigate the uptake of the most prominent biogenic volatile organic compounds (VOCs)-isoprene, α-pinene, and their selected oxidation products-by hydrated acid clusters in a molecular beam experiment and by DFT calculations. Our experiments provide a unique and direct way of determination of the surface accommodation coefficient (αS) on the proxies of ultrafine aerosol particles. Since we are able to determine unambiguously the fraction of the clusters to which the molecules stick upon collisions, our αS is a purely kinetic parameter disentangling the molecule pickup from its evaporation. Oxidation increases the αS of VOCs by more than an order of magnitude, because oxidized compounds form hydrogen bonds with the clusters, whereas the interactions of the parent VOCs are weaker and nonspecific. This work provides molecular-level insight into the condensation of single molecules into atmospheric particles, which has important implications for aerosol nucleation and growth.

We investigate the ionization induced chemistry of hydrogen peroxide in (H2O2)N clusters generated after the pickup of individual H2O2 molecules on large free ArM, M[combining macron]≈ 160, nanoparticles in molecular beams. Positive and negative ion mass spectra are recorded after an electron ionization of the clusters at energies 5-70 eV and after a slow electron attachment (below 4 eV), respectively. The spectra demonstrate that (H2O2)N clusters with N≥ 20 are formed on argon nanoparticles. This is the first experimental report on hydrogen peroxide clusters in molecular beams. The major negative cluster ion series (H2O2)nO2- indicates O2- ion formation. The dissociative electron attachment to H2O2 molecules in the gas phase yielded only OH- and O- (Nandi et al., Chem. Phys. Lett., 2003, 373, 454). These ions and the series containing them are much less abundant in the clusters. We propose a sequence of ion-molecule and radical reactions to explain the formation of O2-, HO2- and other ions observed in the negatively charged cluster ion series. Since hydrogen peroxide plays an important role in many areas of chemistry from the Earth's atmosphere to biological tissues, our study opens new horizons for experimental investigations of hydrogen peroxide chemistry in complex environments.

We have investigated reactivity of double bond in 2-methyl-2-butene (also trimethylethylene or amylene) in the excited and ionized states. In a combined experimental and theoretical study, we focused on both the intermolecular and intramolecular reactions. In a molecular beam experiment, we have sequentially picked up several amylene molecules on the surface of argon nanoparticles ArM, M̅ ≈ 90, acting as a cold support. Ionization with 70 eV electrons yields mass spectra strongly dominated by amylene cluster ions Am(Am)n+. Interestingly, upon multiphoton ionization with 193 nm (6.4 eV) photons, a new strong fragment series appears in the spectra, nominally corresponding to an addition of two carbon atoms, i.e., (Am)nC2+. This difference between electron and photoionization suggests a reaction in an excited state of amylene with a neighboring amylene molecule. We used techniques of nonadiabatic molecular dynamics to study the reactivity of amylene molecules both in the excited and in ionized states. Possible reaction pathways are proposed, substantiating the observed differences between the electron ionization and photoionization mass spectra.

We study the reactivity of misonidazole with low-energy electrons in a water environment combining experiment and theoretical modelling. The environment is modelled by sequential hydration of misonidazole clusters in vacuum. The well-defined experimental conditions enable computational modeling of the observed reactions. While the NO2- dissociative electron attachment channel is suppressed, as also observed previously for other molecules, the OH- channel remains open. Such behavior is enabled by the high hydration energy of OH- and ring formation in the neutral radical co-fragment. These observations help to understand the mechanism of bio-reductive drug action. Electron-induced formation of covalent bonds is then important not only for biological processes but may find applications also in technology.

The hexafluoropropylene oxide, C3F6O(HFPO), is commonly used in technological processes involving low-temperature plasmas. In relevance to such processes, we investigate the electron ionization and attachment to HFPO molecules in various cluster environments. Either pure (HFPO)(N) clusters or the clusters deposited on larger argon nanoparticles (HFPO)(N) center dot Ar-M are generated in molecular beams in vacuum. The cluster beam is intercepted with a beam of either slow (0-15 eV) or faster (70 eV) electrons, yielding negative or positive ions, respectively, and their mass spectra are recorded. Qualitatively, the spectra of free and Ar-deposited clusters are similar, exhibiting the same ion fragments. The cluster environment suppresses the fragmentation after ionization for both positively and negatively charged clusters, namely, the parent ions (HFPO)n(+/-) are present in the spectra. The fragmentation is more suppressed in the Ar-deposited clusters. In both the positive ionization and attachment, new fragmentation channels or ion-molecule reactions in the clusters lead to production of new fragments not observed in the gas phase. Mechanisms of these reactions and ionization processes are discussed. (C) 2018 Elsevier B.V. All rights reserved.