Primary Reaction Channels Research Articles

Distinguishing the XUV-induced Coulomb explosion dynamics of iodobenzene using covariance analysis

Abstract The primary and secondary fragmentation dynamics of iodobenzene following its ionization at 120 eV were determined using three-dimensional velocity map imaging and covariance analysis. Site-selective iodine 4d ionization was used to populate a range of excited polycationic parent states, which primarily broke apart at the carbon-iodine bond to produce I+ with phenyl or phenyl-like cations (CnHx + or CnHx 2+, with n = 1-6 and x = 1-5). The molecular products were produced with varying degrees of internal excitation and dehydrogenation, leading to stable and unstable outcomes. This further allowed the secondary dynamics of C6Hx 2+ intermediates to be distinguished using native-frame covariance analysis, which isolated these processes in their own centre-of-mass reference frames. The mass resolution of the imaging mass spectrometer used for these measurements enabled the primary and secondary reaction channels to be specified at the level of individual hydrogen atoms, demonstrating the ability of covariance analysis to comprehensively measure the competing fragmentation channels of aryl cations, including those involving intermediate steps.

Theoretical Kinetics of Radical-Radical Reaction NH2ṄH + ṄH2 and Its Implications for Monomethylhydrazine Pyrolysis Mechanism.

Significant discrepancies were observed between the experiments and the simulations for ṄH2 time-histories in monomethylhydrazine pyrolysis with the robust mechanism proposed by Pascal and Catoire. The rate of formation analyses for ṄH2 indicated the significance of the reaction NH2ṄH + ṄH2 = H2NN + NH3, which has not been well-defined. In this study, ab initio calculations were performed for the theoretical description of the NH2ṄH + ṄH2 chemistry. Most stationary points on the potential energy surface were quantified at the CCSD(T)/CBS//M06-2X/aug-cc-pVTZ level, and the multireference methods were employed for barrier-less reaction and some transition states. The temperature- and pressure-dependent rate coefficients were determined using classical and microcanonical variational transition state theories. Four primary reaction channels were identified as competitive: 1) The H atom abstraction reaction yielding N2H2(T) + NH3, dominating at 1350-3000 K across the 0.001-100 atm pressure range. 2) The H atom abstraction reaction forming N2H2(S) + NH3, dominating at 800-1350 K and competing with the processes of chemical activation and collisional stabilization below 800 K. 3) The chemical-activated reaction resulting in H2NN(S) + NH3, dominating below 800 K at 0.001 atm. 4) The collisional-stabilized recombination reaction leading to N3H5, becoming significant as pressure increases and dominating below 600 and 650 K at 1 and 100 atm, respectively. The implications of newly calculated NH2ṄH + ṄH2 kinetics for the monomethylhydrazine pyrolysis mechanism were evaluated, and updates were implemented. Sensitivity analyses indicated the necessity of additional research efforts to comprehend the dynamics of CH3NH2 unimolecular and N2H2 + ṄH2 reaction systems. The rate coefficients presented in this study can be employed to develop the chemical kinetic model of nitryl-containing systems.

A zero-dimensional model for atmospheric non-thermal plasma CO2 hydrogenation: insights into the reaction mechanism

Recently, plasma CO2 hydrogenation to generate valuable products (e.g. CO and CH4, hydrocarbons and oxygenates) has attracted more and more attention. The conversion of carbon dioxide (CO2) and hydrogen (H2) in an atmospheric non-thermal plasma was investigated by a zero-dimensional plasma kinetic model. This paper focuses on the effect of different feed gas composition ratios (H2/CO2 volume ratio) on the plasma CO2 hydrogenation reaction mechanisms. It is found that H2 addition in plasma not only promotes CO2 dissociation but also changes the plasma chemistry, which seems to significantly enhance the electron density and temperature. Besides, larger H2 content in the H2/CO2 mixture is beneficial to obtaining a higher number density of methanol, which is in good agreement with published experimental data. The temporal distributions of abundant radicals, ions and molecule densities are determined under a series of initial hydrogen content. Conversions of inlet H2/CO2 and selections toward CO and CH4 are calculated and presented. The primary reaction channels related to the production and destruction of CO, H, CH2O, CH3OH, CH4 and H2O are determined. Finally, the underlying overall reaction mechanisms regarding the plasma CO2 hydrogenation are analyzed in detail by the chemical reaction flow chart.

Hierarchical numbering-up of modular reactors: A multi-objective optimization approach

Proper upscaling of modular reactors for renewable energy transformation, storage and power-to-fuels/chemicals, including fuel cells, redox flow cells and micro catalytic/electrocatalytic reactors, is crucial for enabling an economically feasible net-zero-emission future. In this work, we eatablish a multi-objective optimization model for hierarchical numbering-up of modular reactors considering flow uniformity, flow resistance and total device volume as the optimization objectives; the hierarchical topology of reaction channel networks and the geometry of reaction channels and multi-level distributors serve as the decision variables. An evolutionary optimization algorithm is developed to solve the formulated mixed integer non-linear programming problem cost-effectively. Appropriate sizing of primary reaction channels for equipment integration is found to be application-specific. Hierarchical channel layouts are shown to be essential for large-scale applications. Increasing the number of plates per reactor-stack and incorporating reactor-stacks into arrays prove to be superior to the common practice of enlarging individual plates. The proposed approach and the revelation of optimal numbering-up modes under different representative scenarios will enlighten development of systematic design methodologies for a variety of modular devices.

PTR-MS studies of the reactions of H3O+ with a number of deuterated volatile organic compounds and the subsequent sequential reactions of the primary product ions with water under normal and humid drift tube conditions: Implications for use of deuterated compounds for breath analysis

Product ion distributions resulting from the primary reactions of H3O+ with nine D-labeled volatile organic compounds and the subsequent sequential reactions with H2O have been determined using a Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-TOF 8000 (IONICON Analytik GmbH)) at various reduced electric field (E/N) values ranging from 80 up to 150 Td and for two different absolute humidity levels of air sample < 0.1% and 5%. The specific D-labeled compounds used in this study are acetone-d6, toluene-d8, benzene-d6, ethanol-d (C2H5OD), ethanol-d2 (CH3CD2OH), ethanol-d6, 2-propanol-d8, 2-propanol-d3 (CD3CH(OH)CH3), and isoprene-d5 (CH2CHC(CD2)CD3). With the exception of the two 2-propanol compounds, non-dissociative proton transfer is the dominant primary reaction pathway. For 2-propanol-d8 and 2-propanol-d3 the major primary reaction channel involved is dissociative proton transfer. However, unlike their undeuterated counterparts, the primary product ions undergo subsequent deuterium/hydrogen isotope exchange reactions with the ever present water in the drift tube, the extent of which of course depends on the humidity within that tube. This exchange leads to the generation of various isotopologue product ions, the product ion branching percentages of which are also dependent on the humidity in the drift tube. This results in complex mass spectra and the distribution of product ions leads to issues of reduced sensitivity and accuracy. However, the effect of D/H exchange considerably varies between the compounds under study. In the case of acetone-d6 it is very weak (< 1%), because the exchange process is not facile when the deuterium is in the methyl functional group. In comparison, the H3O+/ benzene-d6 (C6D6) reaction and sequential reactions with water result in the production of the isotopologue ions C6Dn(H7-n)+ (where n = 0–6). Changing the value of E/N and/or the humidity in the drift tube considerably affects the amount of the isotope exchange reactions and hence the resulting sequential product ion distributions. An important conclusion of the findings from this work is that care must be taken in the choice of an exogenous deuterated compound for use in breath pharmacokinetic studies using proton transfer reaction mass spectrometry; otherwise the resulting D/H exchange processes impose interpretative problems.

Direct Measurement of High-Temperature Rate Constants of the Thermal Decomposition of Dimethoxymethane, a Shock Tube and Modeling Study.

Shock-tube experiments have been performed to investigate the thermal decomposition of the oxygenated hydrocarbon dimethoxymethane (DMM; CH3OCH2OCH3). The primary initial reaction channels of DMM decomposition are considered to be the two bond fissions: CH3OCH2OCH3 → CH3O + CH2OCH3 (1) and CH3OCH2OCH3 → CH3 + OCH2OCH3 (2). In the present work, two shock-tube facilities and three different detection techniques have been combined: Behind reflected shock waves, we have carried out time-resolved measurements of (i) the formation of H atoms using the highly sensitive H-ARAS (Atomic Resonance Absorption Spectrometry) technique and (ii) the depletion of the DMM reactant by high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS). In addition, (iii) the temperature-dependent composition of stable reaction products was measured in single-pulse shock-tube experiments via gas chromatography (GC/MS). The experiments span a temperature range of 1100-1430 K, a pressure range of 1.2-2.5 bar, and initial reactant mole fractions from 0.5 ppm (for H-ARAS experiments) up to 10 000 ppm (for HRR-TOF-MS experiments). Experimental rate constants ktotal, ktotal = k1 + k2, obtained from these three completely different methods were in excellent agreement among each other, i.e., deviations are within ±30-40%, and they can be well represented by the Arrhenius expression ktotal( T) = 1013.28±0.27 exp(-247.90 ± 6.36 kJ mol-1/ RT) s-1 (valid over the 1100-1400 K temperature and the 1.2-2.5 bar pressure range). By replacing the respective ktotal values used in a recently published DMM chemical kinetics combustion mechanism (Vermeire et al. Combust. Flame 2018, 190, 270-283), it was also possible to successfully reproduce measured product distributions.

Momentum dependence of pion-induced ϕ meson production on nuclei near threshold

We study the near-threshold pion-induced production of ϕ mesons off nuclei in the kinematical conditions of the HADES experiment, recently performed at GSI. The calculations have been performed within a collision model based on the nuclear spectral function. The model accounts for both the primary π− meson–proton π−p → ϕn and the secondary pion–nucleon πN → ϕN ϕ production processes as well as the effects of the nuclear ϕ and nucleon mean-field potentials. We find that the primary reaction channel π−p → ϕn dominates in the ϕ production off 12C and 184W target nuclei in the HADES acceptance window at incident pion momentum of 1.7 GeV/c. We calculate the momentum dependence of the absolute and relative (transparency ratio) ϕ meson yields from the above direct channel. The calculations have been performed for this initial pion momentum allowing for different options for the ϕN absorption cross section σϕN and different scenarios for the in-medium mass shifts of the ϕ meson and secondary neutron, produced together with ϕ in this channel. We demonstrate that the transparency ratio for the ϕ mesons has, contrary to the absolute cross sections, an insignificant sensitivity to the ϕ meson and secondary neutron in-medium mass shifts at ϕ momenta studied in the HADES experiment. On the other hand, we show that there are measurable changes in the transparency ratio due to the ϕN absorption cross section, which means that such a relative observable can be useful to help determine this cross section from the data taken in the HADES experiment.

Theoretical Studies on Isomerization and Decomposition Reactions of 2-Methyl-1-butanol Radicals

2-Methyl-1-butanol (2M1B) is a favorable candidate of substitute fuels characterized with high energy density and low hygroscopicity. 2M1B radicals, which are the products of H-abstraction reactions of 2M1B, and their isomerization and decomposition reactions play a cardinal impact on the distribution of combustion products. In this work, the primary isomerization and decomposition reaction channels of 2M1B radicals were investigated by using QCISD(T)/CBS//M062x/cc-pVTZ and CBS-QB3 method, respectively. The accurate phenomenological temperature- and pressure-dependent rate constants covering temperatures of 250–2500 K and pressures from 1 × 10–3 to 1 × 103 bar along with high-pressure limit rate constants for these channels were computed by solving the RRKM/master equation. The calculations revealed that the isomerization reaction of RC2 → RC6 has the highest energy barrier among these reactions, while the decomposition reaction RC6 → CH3CH2CHCH3 + CH2O has the lowest energy barrier. Furthermore, the comp...

Theoretic studies on decomposition mechanism of o-methoxy phenethyl phenyl ether: Primary and secondary reactions

In order to understand the pyrolysis mechanism of lignin and identify the chemical pathways for the formation of main products during pyrolysis, the primary and secondary reactions of o-methoxy phenethyl phenyl (o-CH3O-PPE) pyrolysis are investigated by DFT method at (M06-2X)/6-31G+ (d,p) level. It is found that the o-methoxy group reduces the bond dissociation energies (BDEs) of CβO and C(aromatic)O bonds of the β-O-4 linkage. Kinetic analysis results support the concerted reaction pathway 12 as the major primary reaction channel which leads to the formation of guaiacol and styrene. Salicylaldehyde is arisen from the secondary pyrolysis of guaiacol via bimolecular reaction at higher temperature.

Reaction dynamics of oxygen atoms with unsaturated hydrocarbons from crossed molecular beam studies: primary products, branching ratios and role of intersystem crossing

We review the progress made in the understanding of the dynamics of the reactions of ground state oxygen atoms, O(3P), with unsaturated hydrocarbons (acetylene, ethylene, allene, propyne and propene) which are of great relevance, besides from a fundamental point of view, in combustion chemistry and of interest also in atmosphere- and astro-chemistry. Advances in this area have been made possible by an improved crossed molecular beams (CMBs) instrument with rotating mass spectrometric detection and time-of-flight analysis which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power. This apparatus offers the capability of identifying virtually all primary reaction channels, characterising the dynamics, and determining the branching ratios (BRs) for these polyatomic multichannel nonadiabatic reactions. The reactive scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces (PESs). For the simpler prototypical systems, such as O(3P) + ethylene, detailed comparisons with synergic state-of-the-art quasiclassical trajectory surface-hopping calculations on full dimensional ab initio coupled triplet and singlet PESs have recently been possible. For more complex systems, such as O(3P) + propene, comparisons with the results of synergic statistical calculations of BRs on ab initio coupled triplet and singlet PESs, have also been carried out very recently. The combined experimental/theoretical approach has allowed for a better understanding of the mechanism of these reactions. And overall has deepened considerably our understanding of chemical reactivity; in addition, these studies provide an important bridge between CMBs dynamics and thermal kinetics as well as valuable information for improving combustion and astrochemistry models.

Neutron yield and induced radioactivity: a study of 235-MeV proton and 3-GeV electron accelerators.

This study evaluated the magnitude of potential neutron yield and induced radioactivity of two new accelerators in Taiwan: a 235-MeV proton cyclotron for radiation therapy and a 3-GeV electron synchrotron serving as the injector for the Taiwan Photon Source. From a nuclear interaction point of view, neutron production from targets bombarded with high-energy particles is intrinsically related to the resulting target activation. Two multi-particle interaction and transport codes, FLUKA and MCNPX, were used in this study. To ensure prediction quality, much effort was devoted to the associated benchmark calculations. Comparisons of the accelerators' results for three target materials (copper, stainless steel and tissue) are presented. Although the proton-induced neutron yields were higher than those induced by electrons, the maximal neutron production rates of both accelerators were comparable according to their respective beam outputs during typical operation. Activation products in the targets of the two accelerators were unexpectedly similar because the primary reaction channels for proton- and electron-induced activation are (p,pn) and (γ,n), respectively. The resulting residual activities and remnant dose rates as a function of time were examined and discussed.

Thermal Activation of Ammonia by Transition-Metal Hydroxide Cations.

With the exception of [Cu(OH)]+ , the thermal reactions of the first-row transition-metal hydroxide cations, [Sc(OH)]+ -[Zn(OH)]+ , with ammonia have been studied by means of gas-phase experiments and by computational methods for the whole series. The primary reaction channels involve NH bond activation, forming [M(NH2 )]+ concomitantly with the elimination of water, and adduct formation, leading to [M(OH)(NH3 )]+ . Furthermore, [Ti(OH)]+ and [V(OH)]+ react with ammonia under dehydrogenation conditions, leading to [M,O,N,H2 ]+ (M=Ti, V), and for [Ni(OH)]+ ligand exchange is observed. Computations of the main reaction channels have been performed for the [M(OH)]+ /NH3 couples (M=Sc-Zn) to uncover the underlying reaction mechanism and periodic trends across the first row. For NH bond activation, σ-bond metathesis was found to be the underlying mechanism.

Reactivity of lanthanoid mono-cations with ammonia: A combined inductively coupled plasma mass spectrometry and computational investigation

The behavior of La+, Sm+, Eu+ and Gd+ with NH3(g) and ND3(g) was studied to understand gas phase chemical reactions used for separations in the reaction cell of a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS). For Ln+=La+ and Gd+, the primary reaction channel is the formation of the LnNH+ protonated nitride leading to H2 elimination. The LnNH(NH3)1–5+ ammonia complexes of the Ln protonated nitride are further generated. Sm+ and Eu+ are less reactive: the protonated nitride is not detected, and only small amounts of Ln(NH3)0–6+ are observed. Quantum chemical calculations at the DFT, MP2, CCSD(T) and CASPT2 levels of theory were employed to explore the potential energy surfaces. For the La+ and Gd+ ions of f-block elements, the reaction pathways are composed of three steps: first the formation of LnNH3+, then the isomerization to HLnNH2+, and finally the loss of H2 associated with the formation of an LnN triple bond in the final product LnNH+. On the other hand, the isomerization leading to triple bond formation with H2 loss did not proceed for Sm+ and Eu+ ions.

A search for ϕ meson nucleus bound state using antiproton annihilation on nucleus

The mass shift of the vector mesons in nuclei is known to be a powerful tool for investigating the mechanism of generating hadron mass from the QCD vacuum. The mechanism is known to be the spontaneous breaking of chiral symmetry. In 2007, KEK-PS E325 experiment reported about 3.4 % mass reduction of the ϕ meson in medium-heavy nuclei (Cu). This result is possibly one of the indications of the partial restoration of chiral symmetry in nuclei, however, unfortunately it is hard to make strong conclusions from the data. One of the ways to conclude the strength of the ϕ meson mass shift in nuclei will be by trying to produce only slowly moving ϕ mesons where the maximum nuclear matter effect can be probed. The observed mass reduction of the ϕ meson in the nucleus can be translated as the existence of an attractive force between ϕ meson and nucleus. Thus, one of the extreme conditions that can be achieved in the laboratory is indeed the formation of a ϕ-nucleus bound state, where the ϕ meson is “trapped” in the nucleus. The purpose of the experiment is to search for a ϕ-nucleus bound state and measure the binding energy of the system. We will demonstrate that a completely background-free missing-mass spectrum can be obtained efficiently by \((\bar{p}, \phi)\) spectroscopy together with K + Λ tagging, using the primary reaction channel \(\bar{p} p \rightarrow \phi \phi\). This paper gives an overview of the physics motivation and the detector concept, and explains the direction of the initial research and development effort.

Thermal Ammonia Activation by Cationic Transition-Metal Hydrides of the First Row - Small but Mighty

The thermal reactions of cationic 3d transition-metal hydrides MH(+) (M=Sc-Zn, except V and Cu) with ammonia have been studied by gas-phase experiments and computational methods. There are three primary reaction channels: 1) H(2) elimination by N-H bond activation, 2) ligand exchange under the formation of M(NH(3))(+), and 3) proton transfer to yield NH(4)(+). Computational studies of these three reaction channels have been performed for the couples MH(+)/NH(3) (M=Sc-Zn) to elucidate mechanistic aspects and characteristic reaction patterns of the first row. For N-H activation, σ-bond metathesis was found to be operative.

The nascent OH detection in photodissociation of 2-(bromomethyl)hexafluoro-2-propanol at 193 nm: Laser-induced fluorescence study

Photodissociation of 2-(bromomethyl)hexafluoro-2-propanol (BMHFP) and 3-bromo-1-propanol (BP), involving σ C – Br * ← n Br transition at 193 nm, has been investigated by measuring laser-induced fluorescence spectra of the expected OH product. The OH channel is a minor dissociation pathway with a quantum yield of 0.17 ± 0.05 in BMHFP, whereas it was not observed in BP. Partitioning of the available energy into translation, rotation, and vibration of the photoproducts has been measured by state selective detection of the nascent OH product in BMHFP. OH is produced mostly in the ground vibrational level ( v″ = 0), with a rotational distribution being characterized by a temperature of 465 ± 25 K. But, a significant fraction of the available energy of 30.2 kcal mol −1 is partitioned into translation of OH (14.6 kcal mol −1). The OH( v″ = 0, J″) populations in the spin-orbit states as well as in the Λ-doublet states are statistical. A plausible mechanism of OH formation on excitation of BMHFP at 193 nm is suggested, with the primary reaction channel being elimination of Br atom by direct C–Br bond dissociation from a repulsive surface. The Br radical is detected using (2 + 1) resonance-enhanced multiphoton ionization (REMPI) at ∼234 nm. It is produced in both the ground ( 2P 3/2) and the excited ( 2P 1/2) spin-orbit states with the relative quantum yield of the latter to be 0.36. The co-fragment of Br undergoes secondary C–O bond dissociation to produce OH and F 3C–C( CH 2)–CF 3, with the reaction having a barrier located in the exit channel. In this two-step three-body dissociation process, a major fraction of the available energy is released into translation (〈 f T〉 ∼ 0.75), resulting from an impulsive C–Br bond dissociation in the primary step and presence of an exit barrier in the secondary process. Experimental results combined with theoretical calculations provide a clear picture of the dynamics of OH formation from BMHFP at 193 nm. In addition, the energetics of another channel, competing with OH, have been calculated from the primary product F 3C–C(CH 2)(OH)–CF 3. In contrast to BMHFP, the OH product could not be observed from the photolysis of 3-bromo-1-propanol (another saturated halogenated propanol) at 193 nm under the detection limit of the present experimental condition, although it has a higher absorption cross-section at 193 nm.

Floquet Analysis for Vibronically Modulated Electron Tunneling

Electron tunneling provides the primary reaction channel for electron transfer (ET) in many molecular systems. The analysis of such systems therefore requires the consideration of electronic coherence and interference effects. A model system for which tunneling may be either symmetry forbidden or allowed is considered here in the presence of a driving infrared (IR) field. It was previously shown that inelastic tunneling allows ET in the symmetry forbidden system via vibronic interactions. We show here that explicit considerations of IR interactions with these systems further changes the ET kinetics. Analysis in the framework of Floquet theory reveals that interaction with an IR field may increase the probability of inelastic tunneling and thus enhance the ET rate for a system in which elastic ET is forbidden. It is shown that IR driving of a nuclear oscillator promotes the oscillator into excited states that couple more strongly to the tunneling electron. Furthermore, it is shown that IR driving may suppress the ET rate in this same system, depending on system energetics. In a model where elastic tunneling is symmetry allowed, we examine vibronic modulation of ET in the Floquet framework. ET rates are computed for symmetry allowed and forbidden model systems, and vibronic interactions are found to suppress or enhance ET in both systems. The inelastic ET rate may be enhanced over 4 orders of magnitude in the symmetry disfavored case or suppressed by 15% in the same system. Effects of IR on ET rates in the symmetry-allowed system are weaker with enhancements up to 34% over the undriven rate and suppression of about 3% with IR driving present. This study is the first theoretical and computational exploration of ET rate control by IR irradiation of the bridge.

Thermal Activation of NH Bonds by Transition‐metal Oxide Cations: Does a Hierarchy Exist in the First Row?

The thermal reactions of first-row transition-metal oxide cations [MO](+) (M=Sc-Ni, Zn) with ammonia have been studied by gas-phase experiments and computational methods. The activation of N-H bonds is brought about by the monoxides of the middle and late 3d metals Mn-Ni and Zn. The two primary reaction channels correspond to dehydration, which leads to [M(NH)](+), and hydrogen-atom abstraction to form [M(OH)](+). Oxygen-atom transfer from [MO](+) to NH(3) to produce neutral or ionized hydroxylamine was observed as a minor channel for some of the late transition-metal oxides. The computational analysis of these reactions, which was aimed at elucidating the reaction mechanisms and to uncover possible periodic trends across the first row, have been performed for the couples [MO](+) /NH(3) (M=Sc-Zn). Dehydration is found to be endothermic for the oxides of scandium to vanadium and exothermic for the other systems. Hydrogen-atom abstraction becomes exothermic starting with [MnO](+) and, finally, oxygen-atom transfer is feasible for the cationic oxides of nickel to zinc.

Heating a bowl of single-molecule-soup: structure and desorption energetics of water-encapsulated open-cage [60] fullerenoid anions in the gas-phase

The gas-phase unimolecular decay kinetics of an anionic, open-cage [60] fullerene derivative encapsulating one water molecule is studied by means of black-body IR radiation induced dissociation (BIRD) in the temperature programmable ion trap of a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. The primary reaction channel observed is escape of the water molecule from the fullerenoid bowl. The rate constants for this water loss as a function of temperature are evaluated using the Arrhenius equation to yield an activation energy of 104 ± 4 kJ mol(-1). A complementary ion mobility spectrometry study contrasting the water-encapsulated and the empty fullerene cages finds identical collision cross sections to within experimental error-supporting the structural assignment of this gas-phase anion as an endohedral (i.e. encapsulated) species. Both experiments were compared with quantum-chemical computations which well-describe the transition state for water desorption and the concomitant binding and activation energies.

CRITICAL TEST OF SIMULATIONS OF CHARGE-EXCHANGE-INDUCED X-RAY EMISSION IN THE SOLAR SYSTEM

Experimental and theoretical state-selective X-ray spectra resulting from single-electron capture in charge exchange (CX) collisions of Ne^10+ with He, Ne, and Ar are presented for a collision velocity of 933 km s^-1 (4.54 keV nucleon^-1), comparable to the highest velocity components of the fast solar wind. The experimental spectra were obtained by detecting scattered projectiles, target recoil ions, and X-rays in coincidence; with simultaneous determination of the recoil ion momenta. Use and interpretation of these spectra are free from the complications of non-coincident total X-ray measurements that do not differentiate between the primary reaction channels. The spectra offer the opportunity to test critically the ability of CX theories to describe such interactions at the quantum orbital angular momentum level of the final projectile ion. To this end, new classical trajectory Monte Carlo calculations are compared here with the measurements. The current work demonstrates that modeling of cometary, heliospheric, planetary, and laboratory X-ray emission based on approximate state-selective CX models may result in erroneous conclusions and deductions of relevant parameters.