Search Results (55)

Ion interactions with molecular structures give insights into physicochemical processes in the cosmos, radiation damage, plasma, combustion, and biomass conversion reactions. At the atomic scale, these interactions lead to excitation, ionization, and dissociation of the molecular components of structures found across all these environments. Furan, cyclic aromatic ether (C₄H₄O), serves as a gas-phase deoxyribose analog and is crucial for understanding key pathways in renewable biomass conversion, as its derivatives are versatile molecules from lignocellulosic biomass degradation. Therefore, collisions of H₃⁺ and C⁺ ions with gas-phase furan molecules were investigated in the 50–1000 eV energy range, exploiting collision-induced emission spectroscopy. High-resolution fragmentation spectra measured at 1000 eV for both cations reveal similar structures, with C⁺ collisions resulting in more significant furan fragmentation. Relative cross-sections for product formation were measured for H₃⁺ + C₄H₄O collisions. Possible collisional processes and fragmentation pathways in furan are discussed. These results are compared with those for tetrahydrofuran and pyridine to illustrate how the type and charge of the projectile influence neutral fragmentation in heterocyclic molecules. Full article

Cannabinoid molecules are the family of molecules that bind to the cannabinoid receptors (CB1 and CB2) of the human body and cause changes in numerous biological functions including motor coordination, emotion, and pain reception. Cannabinoids occur either naturally in the Cannabis Sativa plant or can be produced synthetically in the laboratory. The need for accurate analytical methods for analyzing cannabinoid molecules is of considerable current importance due to demands for detecting illegal cannabinoids and for monitoring the manufacture of popular, non-illegal cannabinoid products. Mass spectrometry has been shown to be an optimum technique for identifying cannabinoids. In this work, we perform Higher Collisional Dissociation (HCD) mass spectrometric measurements on an Orbitrap Fusion Tribrid Mass Spectrometer to measure the collision-energy-dependent molecular fragmentation pathways of a group of key cannabinoids and their metabolites (cannabidiol, Δ9-Tetrahydrocannabinol, 11-Hydroxy-Δ9-tetrahydrocannabinol, 11-nor-9-Carboxy-Δ9-tetrahydrocannabinol, cannabidiolic acid, tetrahydrocannabinolic acid), along with two synthetic cannabinoids (JWH-018 and MDMB-FUBINACA). This is the first time that cannabinoid molecules have been studied using energy-resolved HCD methods. We identified a number of common, primary fragmentation pathways, including loss of water, loss of other small neutral molecule units (e.g., butene), and rupture of the central C-C bond that links the aromatic and alkyl ring groups. Quantum chemical calculations are presented to provide insights into preferred protonation sites and to characterize isomerization of protonated open-ring cannabinoids (e.g., [CBDA + H]⁺) into closed-ring analogues (e.g., [THCA + H]⁺). A key result to emerge from our study is that energy-resolved HCD measurements are particularly valuable in identifying isomerization, since the isobaric pairs of molecular ions studied here (e.g., [CBDA + H]⁺ and [THCA + H]⁺) are associated with identical HCD profiles indicating that isomerization of one structure into the other has occurred during the electrospray–mass spectrometry process. This is an important result as it will have general applicability to other tautomeric ions and thus demonstrates the application of energy-resolved HCD as a tool for identifying tautomerization proclivity. Full article

Single and double electron capture cross-sections for collisions of ¹¹⁸Sn⁴⁺ with molecular hydrogen have been measured in an energy range of 1 keV to 16 keV using a crossed-beam setup. The cross-sections are determined from measurements of charge-state-resolved ion currents obtained through a retarding field analyser. Remarkably, the single electron capture cross-sections for Sn⁴⁺ are more than a factor 3 smaller than the previously determined single electron capture cross-sections for Sn³+–H₂ collisions and the double electron capture cross-sections are only about 20% smaller than the single electron capture cross-sections. These results are understood on the basis of potential energy curve crossings. The first active curve crossings for the Sn⁴⁺–H₂ system happen at a relatively small internuclear distance of about 5.5 a.u., which should be compared to 8 a.u. for Sn³+ ions. Multi-channel Landau–Zener calculations have been performed for single electron capture and confirm these low cross-sections. The curve crossing for double electron capture by Sn⁴⁺ lies very close to the one for single electron capture, which may explain the single and double electron capture cross-sections being of similar magnitude. Full article

Solvents play a crucial role in ion–molecule reactions and have been used to control the outcome effectively. However, little is known about how solvent molecules affect atomic-level mechanisms. Therefore, we executed direct dynamics simulations of the HO⁻(H₂O_w) + CH₃CH₂Br system to elucidate the dynamics behavior of chemical reactions in a microsolvated environment and compared them with previous gas-phase data. Our results show that the presence of a single water solvent molecule significantly suppresses the direct mechanism, reducing its ratio from 0.62 to 0.18, thereby promoting the indirect mechanism. Spatial effects and prolonged ion–molecule collisions combine to drive this mechanism shift. Among them, water molecules impede the reactive collisions of HO⁻ and CH₃CH₂Br, while at the same time, the attractive interaction of hydrogen bonds between ions and molecules produces long-lived intermediates that favor the indirect mechanism. On the other hand, microsolvation also affects the reaction preference of the S_N2 and E2 channels, which is more conducive to stabilizing the transition state of the S_N2 channel due to the difference in solute–solvent interactions, thus increasing the competitiveness of this pathway. These results emphasize the profound influence of solvent molecules in regulating reaction selectivity and underlying microscopic mechanisms in more complex systems. Full article

Ab initio calculations of cross sections for electron capture by protons in collisions with CO₂ are carried out at energies between 100 eV/u and 50 keV/u, employing a semiclassical method within the Franck–Condon framework. The scattering wave function is expanded in a set of ab initio electronic wave functions of the HCO₂⁺ supermolecule. The calculation is performed on several trajectory orientations to obtain orientation-averaged total cross sections. A two-state model with an exponential interaction between the entrance and the lowest charge transfer channel is proposed to describe the main aspects of the charge transfer process and to estimate the precision of the molecular expansion. The symmetry of the HOMO ${\pi }_g$ of CO₂ is relevant to choose the signs of the molecular functions and to set up the orientation average of the cross sections. Very good agreement is found with the experimental charge transfer cross sections. Full article

Mass spectral identification (in particular, in metabolomics) can be refined by comparing the observed and predicted properties of molecules, such as chromatographic retention. Significant advancements have been made in predicting these values using machine learning and deep learning. Usually, model predictions do not contain any indication of the possible error (uncertainty) or only one criterion is used for this purpose. The spread of predictions of several models included in the ensemble, and the molecular similarity of the considered molecule and the most “similar” molecule from the training set, are values that allow us to estimate the uncertainty. The Euclidean distance between vectors, calculated based on real-valued molecular descriptors, can be used for the assessment of molecular similarity. Another factor indicating uncertainty is the molecule’s belonging to one of the clusters (data set clustering). Together, all three factors can be used as features for the uncertainty assessment model. Classification models that predict whether a prediction belongs to the worst 15% were obtained. The area under the receiver operating curve value is in the range of 0.73–0.82 for the considered tasks: the prediction of retention indices in gas chromatography, retention times in liquid chromatography, and collision cross-sections in ion mobility spectroscopy. Full article

Chemical reactions between Be⁺ ions and H₂ molecules have significance in the fields of ultracold chemistry and astrophysics, but the corresponding dynamics studies on the ground-state reaction have not been reported because of the lack of a global potential energy surface (PES). Herein, a globally accurate ground-state BeH₂⁺ PES is constructed using the neural network model based on 18,657 ab initio points calculated by the multi-reference configuration interaction method with the aug-cc-PVQZ basis set. On the newly constructed PES, the state-to-state quantum dynamics calculations of the Be⁺(²S) + H₂(v₀ = 0; j₀ = 0) and Be⁺(²S) + D₂(v₀ = 0; j₀ = 0) reactions are performed using the time-dependent wave packet method. The calculated results suggest that the two reactions are dominated by the complex-forming mechanism and the direct abstraction process at relatively low and high collision energies, respectively, and the isotope substitution has little effect on the reaction dynamics characteristics. The new PES can be used to further study the reaction dynamics of the BeH₂⁺ system, such as the effects of rovibrational excitations and alignment of reactant molecules, and the present dynamics data could provide an important reference for further experimental studies at a finer level. Full article

In the current highly industrialized living environment, air quality has become an increasing public health concern. Natural environments like forests have excellent air quality due to high concentrations of negative oxygen ions originating from low-voltage ionization, without harmful ozone. Traditional negative oxygen ion generators require high voltage for corona discharge to produce ions. However, high voltage can increase electron collisions and excitations, leading to more dissociation and recombination of oxygen molecules and consequently higher ozone production. To address the challenge of generating negative oxygen ions without accompanying ozone production, this study designed and constructed a low-voltage negative oxygen ion generator based on nanometer-tip carbon fiber electrodes. The advantage of this device lies in the high curvature radius of carbon fibers, which provides high local electric field strength. This allows for efficient production of negative oxygen ions at low operating voltages without generating ozone. Experiments demonstrated that the device can efficiently generate negative oxygen ions at a working voltage as low as 2.16 kV, 28% lower than the lowest voltage reported in similar studies. The purification device manufactured in this study had a total decay constant for PM_2.5 purification of 0.8967 min⁻¹ within five minutes, compared to a natural decay constant of only 0.0438 min⁻¹, resulting in a calculated Clean Air Delivery Rate (CADR) of 0.1535 m³/min. Within half an hour, concentrations of PM_2.5, PM₁, PM₁₀, formaldehyde, and TVOC were reduced by 99.09%, 99.40%, 99.37%, 94.39%, and 99.35%, respectively, demonstrating good decay constants and CADR. These findings confirm its effectiveness in improving indoor air quality, highlighting its significant application value in air purification. Full article

The toxic nature of bacterial endotoxins is affected by the structural details of lipid A, including the variety and position of acyl chains and phosphate group(s) on its diglucosamine backbone. Negative-ion mode tandem mass spectrometry is a primary method for the structure elucidation of lipid A, used independently or in combination with separation techniques. However, it is challenging to accurately characterize constitutional isomers of lipid A extracts by direct mass spectrometry, as the elemental composition and molecular mass of these molecules are identical. Thus, their simultaneous fragmentation leads to a composite, so-called chimera mass spectrum. The present study focuses on the phosphopositional isomers of the classical monophosphorylated, hexaacylated Escherichia coli-type lipid A. Collision-induced dissociation (CID) was performed in an HPLC-ESI-QTOF system. Energy-resolved mass spectrometry (ERMS) was applied to uncover the distinct fragmentation profiles of the phosphorylation isomers. A fragmentation strategy applying multi-levels of collision energy has been proposed and applied to reveal sample complexity, whether it contains only a 4′-phosphorylated species or a mixture of 1- and 4′-phosphorylated variants. This comparative fragmentation study of isomeric lipid A species demonstrates the high potential of ERMS-derived information for the successful discrimination of co-ionized phosphorylation isomers of hexaacylated lipid A. Full article

The breakdown of CF₃I gas at low pressure is of significant importance for applications in fields such as aerospace and microelectronics. However, the DC low-pressure breakdown characteristics of CF₃I remain underexplored. In this work, we utilize a one-dimensional implicit particle-in-cell/Monte Carlo collision (PIC/MCC) algorithm to investigate the complete DC breakdown process of low-pressure CF₃I. Our model accounts for ion–molecule collisions, recombination reactions, and external circuit influences. The breakdown process is delineated into three stages: before breakdown, breakdown, and after breakdown. In the before-breakdown stage, both the density and energy of particles are low. In the breakdown stage, the rapid increase in electron density and energy accelerates ionization reactions, leading to successful breakdown. The circuit behavior transitions from capacitive to resistive, sharing voltage with the external resistance. In the after-breakdown stage, continued positive ion growth leads to the formation of a thin anode sheath and a negative plasma potential. Energy production, including heating power and secondary electron emission (SEE) power, balances with energy loss through collision and boundary absorption. Specifically, 62% of the total heating power comes from positive ions, 1.5% from negative ions, and approximately 85% of electron energy is lost via boundary absorption. Finally, we compare the Paschen curves of CF₃I with those of SF₆, providing insights that are beneficial for the application of CF₃I as an SF₆ alternative. Full article

We present Python Quasi-classical atom–molecule scattering (PyQCAMS v0.1.0), a new Python package for atom–diatom scattering within the quasi-classical trajectory approach. The input consists of the mass, collision energy, impact parameter, and pair-wise/three-body interactions. As the output, the code provides the vibrational quenching, dissociation, and reactive cross sections along with the rovibrational energy distribution of the reaction products. We benchmark the program for a reaction involving a molecular ion in a high-density ultracold gas, RbBa⁺ + Rb. Furthermore, we treat H₂ + Ca → CaH + H reactions as a prototypical example to illustrate the properties and performance of the software. Finally, we study the parallelization performance of the code by looking into the speedup of the program as a function of the number of CPUs used. Full article

The gas-phase reaction between the ethyl cation (C₂H₅⁺) and ethyne (C₂H₂) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C₂H₅Br using tuneable VUV radiation in the range 10.5–14.0 eV is employed to generate C₂H₅⁺, which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C₄H₇]⁺ PES and elucidate the underlying reaction mechanisms. Two main product channels have been identified at a centre-of-mass collision energy of $\sim 0.1$ eV: (a) C₃H₃⁺+CH₄, with BR = 0.76$\pm$0.05 and (b) C₄H₅⁺+H₂, with BR = 0.22$\pm$0.02. A third channel giving C₂H₃⁺ in association with C₂H₄ is shown to emerge at both high internal excitation of C₂H₅⁺ and high collision energies. From CS measurements, energy-dependent total rate constants in the range $4.3\times$${10}^{-11}$−$5.2\times$${10}^{-10}$ ${\mathrm{cm}}^3·$${\mathrm{molecule}}^{-1}·$${\mathrm{s}}^{-1}$ have been obtained. Theoretical calculations indicate that both channels stem from a common covalently bound intermediate, CH₃CH₂CHCH⁺, from which barrierless and exothermic pathways exist for the production of both cyclic c−C₃H₃⁺ and linear H₂CCCH⁺ isomers of the main product channel. For the minor C₄H₅⁺ product, two isomers are energetically accessible: the three-member cyclic isomer c−C₃H₂(CH₃)⁺ and the higher energy linear structure CH₂CHCCH₂⁺, but their formation requires multiple isomerization steps and passages via transition states lying only 0.11 eV below the reagents’ energy, thus explaining the smaller BR. Results have implications for the modeling of hydrocarbon chemistry in the interstellar medium and the atmospheres of planets and satellites as well as in laboratory plasmas (e.g., plasma-enhanced chemical vapor deposition of carbon nanotubes and diamond-like carbon films). Full article

Over a wide and partly overlapping energy range, the single-electron capture cross-sections for collisions of metastable ${\mathrm{Sn}}^{2+}(5s5p \mathrm{P}_o^3)$ (${\mathrm{Sn}}^{2+\ast }$) ions with ${\mathrm{H}}_2$ molecules were measured (0.1–10 keV) and calculated (0.3–1000 keV). The semi-classical calculations use a close-coupling method on a basis of electronic wavefunctions of the (SnH₂)²⁺ system. The experimental cross-sections were extracted from double collisions in a crossed-beam experiment of ${\mathrm{Sn}}^{3+}$ with ${\mathrm{H}}_2$. The measured capture cross-sections for ${\mathrm{Sn}}^{2+\ast }$ show good agreement with the calculations between 2 and 10 keV, but increase toward lower energies, whereas the calculations decrease. Additional Landau–Zener calculations were performed and show that the inclusion of spin-orbit splitting cannot explain the large cross-sections at the lowest energies which we now assume to be likely due to vibrational effects in the molecular hydrogen target. Full article

The collision cross sections (CCS), momentum transfer cross sections (MTCS), or scattering cross sections (SCS) of an electron–neutral pair are important components for computing the electric conductivity of a plasma gas. Larger collision cross sections for electrons moving freely within neutral particles (molecules or atoms) cause more scattering of these electrons by the neutral particles, which leads to degraded electron mobility, and thus reduced electric conductivity of the plasma gas that consists of electrons, neutral particles, and ions. The present work aimed to identify the level of disagreement between four different methods for describing how electron–neutral collision cross sections vary when they are treated as a function of electron temperature alone. These four methods are based on data or models previously reported in the literature. The analysis covered six selected gaseous species that are relevant to combustion plasma, which are as follows: carbon monoxide (CO), carbon dioxide (CO₂), molecular hydrogen (H₂), water vapor (H₂O), potassium vapor (K), and molecular oxygen (O₂). The temperature dependence of the collision cross sections for these species was investigated in the range from 2000 K to 3000 K, which is suitable for both conventional air–fuel combustion and elevated-temperature oxygen–fuel (oxy-fuel) combustion. The findings of the present study suggest that linear functions are enough to describe the variations in the collision cross sections of the considered species in the temperature range of interest for combustion plasma. Also, the values of the coefficient of variation (defined as the sample standard deviation divided by the mean) in the collision cross sections using the four methods were approximately 27% for CO, 42% for CO₂, 13% for H₂, 39% for H₂O, 44% for K, and 19% for O₂. The information provided herein can assist in simulating magnetohydrodynamic (MHD) power generators using computational fluid dynamics (CFD) models for combustion plasma flows. Full article

We extend the two-centre wave-packet convergent close-coupling approach to doubly differential ionisation in proton collisions with H₂ to intermediate projectile energies. The results for the doubly differential cross section at projectile energies from 48 to 200 keV are presented as a function of the energy and angle of emitted electrons. We consider a wide range of emission angles from 10 to ${160}^{\circ }$, and compare our results to experimental data, where available. Excellent agreement between the presented results and the experimental data was found, especially for emission angles less than ${130}^{\circ }$. For very large backward emission angles our calculations tended to slightly overestimate the experimental data when energetic electrons are ejected and the doubly differential cross section is very small. This discrepancy may be due to the large uncertainties in the experimental data in this region and the model target description. Overall, the present results show significant improvement upon currently available theoretical results and provide a consistently accurate description of this process across a wide range of incident energies. Full article