Matrix isolation study of the products of the interaction of electrons and metastable argon atoms with HCCl2F

Two groups of infrared absorptions common to experiments in which samples of HCCl₂ isolated in an argon matrix at 14 K are exposed to vacuum ultraviolet radiation or to electrons produced by ultraviolet irradiation of an alkali metal, as well as to experiments in which the Ar:HCCl₂F sample is codeposited with a beam of argon atoms excited in a microwave discharge, have been assigned to anions produced upon associative and dissociative electron capture by HCCI₂F. Detailed isotopic substitution studies suggest that these anions are (Cl₂C)H?F^?, representing a unique type of hydrogen bonding, and HCCIF^?. The HCClF^? anion photodecomposes in the 345–250nm spectral region, but the products of its photodecomposition have not been identified. Both CCl₂ and Cl₂CF are also produced in the discharge experiments, but there is no evidence for the production of HCF. Mechanisms for the formation of ion products by electron capture and by exposure of HCCl₂F to radiation or to excited argon atoms of energy equal to or less than 11.8 eV are considered.     

Matrix isolation study of the interaction of excited argon atoms with methyl cyanide,vibrational and electronic spectra of ketenimine

When methyl cyanide mixed with an excess of argon is codeposited at 14 K with a beam of argon atoms that has been excited by a low power microwave discharge, infrared and ultraviolet absorptions of several previously unidentified products appear. The most prominent set of absorptions is assigned to ketenimine, previously tentatively identified as the product of the reaction of NH with C₂H₂ in an argon matrix. Using a molecular geometry resulting from a recent ab initio calculation and a valence force field with four interaction constants, it has been possible to obtain a satisfactory least-squares force constant fit to the infrared data for seven isotopic species of ketenimine. Two electronic band systems are also reported for ketenimine, which is photodissociated by 2537-A radiation. The mechanism by which ketenimine is formed may involve an initial electron transfer from excited argon to methyl cyanide. Spectrospic data are also considered for the other products, one of them tentatively identified as CH₂CH, which differ in their behavior on mercury-are photolysis of the sample.     

Plasma diagnostics of barrier-torch discharge in argon flow in a capillary by cross-correlation spectroscopy

The radiation kinetics of the plasma of barrier-torch disrcharge in argon flow in a capillary has been studied by cross-correlation spectroscopy. It was established that the discharge emission spectrum consists of peaks of electronically excited states of argon, bands of hydroxyl radicals, and a second positive system of nitrogen. An analysis of the spatio-temporal distributions of emission intensity for the selected spectral indicators showed that the causes of the torch are ionization waves that extend through the capillary from the electrode system with a speed of 10⁵ m/s and project up to 3–4 mm. It was established that the formation of electronically excited molecules of nitrogen N₂(C ³Π _u ) in the torch of discharge occurs mainly on the reaction between metastable electronically excited atoms of argon and molecules of nitrogen in the electronic ground state.     

Matrix isolation study of the products of the interaction between metastable Ar and Kr atoms and chloroform. Photodecomposition of HCCl+2 and HCCl−2

The emission spectrum of a microwave discharge through argon and the infrared and ultraviolet spectra of the products of the interaction of HCCl₃ with the periphery of such a discharge, observed after radpid quenching in an argon matrix at 14 K, indicate that metastable argon atoms play an important role in matrix isolation experiments using such a discharge configuration to produce free radicals and molecular ions. Exceptionally high yields of HCCL⁺₂ and HCCl^?₂ and a significant concentration of CCl⁺₃ were stabilized in these experiments. The observations support the assignment of the 1037 cm^?1 peak to “isolated” CCl⁺₃. Upon exposure of the sample to 370-280 nm radiation, a prominent, slightly perturbed absorption of ClHCl^? appeared at 705 cm^?1. This absorption was destroyed by 280-260 nm radiation. The relative stability of both HCCl⁺₂ and CCl⁺₃ upon exposure of the sample to radiation of wavelength longer than 280 nm is attributed to electron scavenging by the HCCl₃ in the matrix; this species is present in considerably greater concentration than are the ion products. Evidence is presented for the photodecomposition of HCCl⁺₂ at wavelengths shorter than about 280 nm. A marked increase in the concentration of HAr⁺_n when HCCl⁺₂ was photolyzed by radiation of wavelength shorter than 260 nm is consistent with the calculated threshold energy for proton transfer from HCCl⁺₂ to Ar. The results of krypton matrix experiments are also consistent with this mechanism. Unstructured absorption bands near 285 and 250 nm are tentatively attributed to ClHCl^? and HCCl⁺₂, respectively. An increase in the concentration of “nonrotating” H₂O, compared to H₂O molecules free to undergo rotational transitions, when ions are present in the matrix can be attributed to the electric field producted by ionic interactions.     

Radiation from metastable highly-charged ions extracted from a gas target

Few-electron ions of argon have been produced by heavy ion impact in a gas target. The recoil ions have been extracted into a region with high vacuum, and the K radiation of long-lived states has been observed by means of a position sensitive proportional counter oriented along the flight path of the ions. Fast ion beams are frequently used to determine the lifetimes τ of excited levels by time-of-flight techniques [1]. Typical ion velocities are v=1 cm/ns (beam energy 0.5 MeV/amu) or higher. The usual length l of experimental chambers rarely exceeds l=1 m. This decay path corresponds to roughly 100ns, and very few experiments on fast ion beams have been done on longer-lived states. Notable examples are the Berkeley measurements on the Ar¹⁶⁺ 1s2s³S₁ lifetime (calculated around τ=210 ns [2, 3], decay path ～8m) which resulted in a lower lifetime result (172±30ns) than expected and which were later shown to suffer from various systematic errors, the most important being the lack of spectral resolution of radiation from the helium-like or from core-excited lithiumlike ions. An alternative approach, a coincidence measurements of x-rays and projectile ions to associate the detected quanta with ions of a given charge state is difficult for high energy and high current ion beams.     

The production and spectroscopy of molecular ions isolated in solid neon

Studies of the molecular spectra of small polyatomic molecular ions are stlll in therr infancy. The availability of survey spectra would facilitate the search for individual vibrational and vibronic bands of gas-phase ion species using sophisticated, highly sensitive laser techniques. Deeectable concentrations not only of simpee ions such as CO⁺ ₂ and H⁺ ₂ but also of dimer cations and anions such as O⁺ ₄ and O^- ₄ have been stabilized in solid neon when the parett molecule is codeposited with a beam of excited neon atoms. The vibrational fundamentals heretofore observed for the simpee ions isolated in solid neon lie very close to the positions of the corresponding gas-phase band centers. Molecular spectroscopic data are not yet available for the gas-phase dimer ions. Therefore, it is difficutt to estimate how closely the neon-matrix vibrational frequencies here reported for a numbrr of dimer ions correspond to the gas-phase band centers. Some guidance is available from comparison of the argon-matrix spectra of small molecules hydrogen-bonded to HF with the gasphaee spectra of theee hydrogen-bonded species [55]. In theee studies, the effect of the argon matrix is to enhance the apparent hydrogen-bond strength; the HF stretching frequency is lowered by 1 to 5% from the gas-phase value, and the absorptions contributed by the flexing of the HF with respect to the other molecule are raised by 5% or more. Since neon matrices are generally less perturbing than argon matrices, the deviation from the gas-phase frequencies shoudd be somewhat less in neon-matrix studies. In the present experiments, only the high frequency stretching fundamentals have been observed, suggesting that matrix shifss shoudd amoutt to less than 3 or 4%. Therefore, matrix isolation studies such as theee promise to provide a valuable new tool for the detection and spectroscopic characterization of small molecular ions and cluster ions.     

Distribution of metastable argon atoms in the modified Grimm-type electrical discharge

The absorbance by metastable argon atoms of the Ar 696.543 nm line in the modified Grimm-type electrical discharge source was measured at different discharge conditions and at distances varying from 0.25 to 6 mm from the cathode. A uranium/argon hollow cathode lamp was used as primary source, which gave an argon gas temperature of 850 K when run at 12 mA. A maximum absorbance of 0.57 was found 3 mm from the cathode at 600 V, 80 mA. The magnitude of absorbance increases with discharge current while the position of maximum absorbance shifts away from the cathode with increase in discharge voltage. The quenching of metastable atoms by nitrogen is demonstrated.The spatial distribution of the intensity of four different types of spectral lines is shown. The approximate number densities of the different particles are 10¹²cm^?3 for metastable argon atoms, 10¹⁶cm^?3 for neutral argon atoms, 10¹³ cm^?3 for sputtered copper atoms and 10¹⁴cm^?3for electrons.     

An Argon–Oxygen Covalent Bond in the ArOH+ Molecular Ion

The OH⁺ cation is a well‐known diatomic for which the triplet (³Σ^?) ground state is 50.5 kcal mol^?1 more stable than its corresponding singlet (¹Δ) excited state. However, the singlet forms a strong donor–acceptor bond to argon with a bond energy of 66.4 kcal mol^?1 at the CCSDT(Q)/CBS level, making the singlet ArOH⁺ cation 3.9 kcal mol^?1 more stable than the lowest energy triplet complex. Both singlet and triplet isomers of this molecular ion were prepared in a cold molecular beam using different ion sources. Infrared photodissociation spectroscopy in combination with messenger atom tagging shows that the two spin isomers exhibit completely different spectral signatures. The ground state of ArOH⁺ is the predicted singlet with a covalent Ar?O bond.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part VI. Dimethylglyoxime electroreduction from weakly acidic aqueous solutions in the presence of I− and SCN−

An investigation of the _d-effect in the absence of ionic specific adsorption shows that the overall charge of the reactants in dimethylglyoxime (DMG) electroreduction on mercury from aqueous solutions of pH?2 equals+1, in agreement with a protonation reaction either preceding (DMG + H⁺DMGH^++e → products) or else simultaneous with (DMG + H⁺ + e → products) the charge transfer. In the presence of the specifically adsorbed supporting anions I^? and SCN^? the logarithm Φ of the rate constant for DMG electroreduction corrected for diffuse-layer effects decreases linearly with an increase in the absolute value |q_i| of the charge density at the inner Helmholtz plane due to the adsorbed anions. This behaviour is explained semiquantitatively by assuming that protonation takes place simultaneously with charge transfer, with the DMG molecule in the adsorbed state and the H₃O⁺ ion in the non-adsorbed state. In this way the H₃O⁺ ion, due to the screening effect of the surrounding diffuse-layer ions, escapes the accelerating attractive effect exerted by the adsorbed I^? or SCN^? ions; on the other hand these latter anions exert a retarding repulsive effect upon the electron transfer from the metal to the adsorbed DMG molecule.     

Polypyrrole Bilayers: Charge Trapping and Application for Amperometric Ions Sensing

Bilayers composed of polypyrrole: doped by perchlorate ions (PPy(ClO₄) – anion exchanging inner layer) and by dodecyl sulfate ions (PPy(DS) – cation exchanging outer layer) are very effective charge trapping systems that are usually not observed for other bilayers comprised of polypyrrole. Chronopotentiometric experiments carried out for oxidation and reduction showed that the trapping effect in the inner layer resulted from different ion exchange properties of the component polymers, leading to a low permeability of the reduced outer layer towards anions. Estimated diffusion coefficients of Cl^? anions in the oxidized and reduced PPy(DS) are in the range of 10^?9 and lower than 10^?10 cm² s^?1, respectively. The presence of the outer layer limiting the ion transfer was found to be beneficial to improve the signal resolution in amperometric mode of ion sensing within wide KCl concentration range, from 10^?5 M up to 3 M. The influence of experimental conditions (film thickness, response time) on optimization of this novel kind of polymeric bilayer ion sensors was studied.     

Chaotropic Monovalent Anion‐Induced Rectification Inversion at Nanopipettes Modified by Polyimidazolium Brushes

A nonintuitive observation of monovalent anion‐induced ion current rectification inversion at polyimidazolium brush (PimB)‐modified nanopipettes is presented. The rectification inversion degree is strongly dependent on the concentration and species of monovalent anions. For chaotropic anions (for example, ClO₄^?), the rectification inversion is easily observed at a low concentration (5 mm ), while there is no rectification inversion observed for kosmotropic anions (Cl^?) even at a high concentration (1 m ). Moreover, at the specific concentration (for example, 10 mm ), the variation of rectification ratio on the type of anions is ranged by Hofmeister series (Cl^?≥NO₃^?>BF₄^?>ClO₄^?>PF₆^?>Tf₂N^?). Estimation of the electrokinetic charge density (σ^ek) demonstrates that rectification inversion originates from the charge inversion owing to the over‐adsorption of chaotropic monovalent anion. To qualitatively understand this phenomenon, a concentration‐dependent adsorption mechanism is proposed.     

Structure de la couche double sur les électrodes d'or: Détermination des composants de charge

The components of the charge q_±^Au at the interface polycrystalline gold electrode—NaF, KCl or KBr solutions and the charge due to specifically adsorbed Cl^? or Br^? anions have been determined by thermodynamical analysis of differential capacity—potential curves, using the two sets of variables q^M, μ (Grahame and Soderberg's method) and E_?, μ (Esin—Markov effect). In the absence of specific adsorption (NaF), variations of charges q_±^Au with potential are in good agreement with those provided by the diffuse layer theory in the negative charge region of the metal. With specific adsorption of Cl^? or Br^? anions, both q_±^Au(q^Au), (q_?¹)^Au(q^Au) curves obtained by the two methods fit well. Determination of components of charge was made in the whole negative charge region and in part of the positive charge region of the electrode.     

Double collision experiments

A variety of double collision experiments, whereby fast species undergo collisional interactions in two distinct regions of a mass spectrometer, are described. These include two-stage charge reversal of negative ions, two-stage double electron transfer from targets to cations, neutralization-reionization experiments as well as delayed analysis of organic cations formed in a one-step charge reversal of anions. Experiments have been performed on a number of systems of current interest in gas-phase ion chemistry. It is concluded that autoelectron detachment of benzyl anions leads to benzyl radicals, whereas the collisionally induced electron detachment produces a mixture of benzyl and tropyl radicals. By contrast, electron detachment from [H₃CNH]^? is not a metastable process and occurs only after excitation to produce H₃CNH˙ radicals, which do not rearrange into the thermodynamically more stable H₂CNH₂˙. It is shown that in the double electron transfer reactions H⁺ + Xe→H˙ + Xe⁺˙ and H˙ + Xe→H^? + Xe⁺˙, excited states are produced. From double collision experiments on methyl formate ions, it is concluded that the non-decomposing ions have undergone rearrangement on the time-scale of 10 μs into the distonic isomer, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm H} - \mathop {\rm C}\limits^ + ({\rm OH}){\rm O}\mathop {\rm C}\limits^. {\rm H}_2 $\end{document}. Finally, it is shown that short-lived (<0.2 μs) [H₂, C, N]⁺ ions generated by charge-reversal of [H₂CN]^? have the [H₂CN]⁺ structure, whereas most of the long-lived (10 μs) ions have the [HCNH]⁺ structure.     

Experimental and Computational Probes of the Nature of Halogen Bonding: Complexes of Bromine‐Containing Molecules with Bromide Anions

The nature of halogen bonding is examined via experimental and computational characterizations of a series of associates between electrophilic bromocarbons R? Br (R? Br=CBr₃F, CBr₃NO₂, CBr₃COCBr₃, CBr₃CONH₂, CBr₃CN, etc.) and bromide anions. The [R? Br, Br^?] complexes show intense absorption bands in the 200–350 nm range which follow the same Mulliken correlation as those observed for the charge‐transfer associates of bromide anions with common organic π‐acceptors. For a wide range of the associates, intermolecular R? Br???Br^? separations decrease and intramolecular C? Br bond lengths increase proportionally to the Br^?→R? Br charge transfer; and the energies of R? Br???Br^? bonds are correlated with the linear combination of orbital (charge‐transfer) and electrostatic interactions. On the whole, spectral, structural and thermodynamic characteristics of the [R? Br, Br^?] complexes indicate that besides electrostatics, the orbital (charge‐transfer) interactions play a vital role in the R? Br???Br^? halogen bonding. This indicates that in addition to controlling the geometries of supramolecular assemblies, halogen bonding leads to electronic coupling between interacting species, and thus affects reactivity of halogenated molecules, as well as conducting and magnetic properties of their solid‐state materials.     

Free radicals formed by ion beam neutralization of [DCO]+, [DOCO] +, [CD3OD2]+ and [(CD3)2COD] + on metal targets

A series of radicals formed when a fast beam of deuterated ions, [DCO] ⁺, [DOCO] ⁺, [CD₃OD₂]⁺ and [(CD) ₂COD] ⁺, is neutralized by electron transfer from K. Na and Zn atoms has been studied by a combination of charge stripping and beam scattering techniques For all systems studied, some fraction of The radicals from charge exchange are formed in excited dissociative states. Radical decomposition products have been identified and fragmentation energies have been measured. A metastable slate of CD3GD2 is observed with a lifetime greater than 0.5 μs. Stable radicals of DCO and (CD₃)₂ COD are formed by ion–Zn charge exchange. Indirect evidence indicates that [DCO] ⁺–K charge exchang; leads to an excited radiative state of the radical.     

Comparative study of the gas-phase bond strengths of CO2 and N2O with the halide ions

Thermodynamic data, ΔH _{n-1, n} ^o and ΔS _{n-1, n} ^o, for clustering reactions of halide ions X^?(X = F, Cl, Br, and I) with N₂Owere measured with a pulsed electron beam high-pressure mass spectrometer. In contrast to the fact that CO₂ forms a covalent bond with the fluoride ion to yield the fluoroformate ion, FCO₂ ^?, the interaction between F^? and N₂O is mainly electrostatic. It was found that the cluster ions F^? (N₂O)_n complete the first shell at n = 6, thus forming an octahedral structure. The difference between F—CO₂ ^? and F^? ... N₂O is discussed in terms of Coulombic, exchange, and charge-transfer interactions. The X^? (N₂O)₂ clusters (X = Cl, Br and I) are found to be of C_2h symmetry, while F^? (N₂O)₂ is of a twisted form and is slightly asymmetric due to a slight participation of covalency (charge transfer) in the core ion F^? ... N₂O.     

Fragmentation mechanisms of oxofatty acids via high-energy collisional activation

Upon high-energy collisional activation, oxofatty-acid ions undergo fragmentations to produce a unique pattern of product ions by which the position of the ketone is revealed. The reactions occurring in the vicinity of the ketone, which are the subject of this article, produce a spectral pattern that is not symmetrical. Although cleavages of α, β, and γ C-C bonds occur on the side proximal to the charge site, giving α, β, and γ ions, respectively, there is only a γ′ ion formed on the side distal to the charge site. The resulting lack of symmetry seemingly contradicts the concept that the reactions are independent of the charge (i.e., that they are charge remote). To eliminate any interaction between the charge and the reaction site, oxofatty acids were linked to glycyrrhetic acid, a steroid with a rigid polycyclic system. The fragmentation pattern remains the same, indicating that the effect does not depend on charge but rather on the ketone. Isotopic labeling and MS/MS/MS studies confirm that the fragmentations of C-C bonds in the vicinity of the ketone are complex, charge-remote processes. Formation of [M-H-H₂O]^? and [M-H-CO₂]^? anions and the ion that is formed by homolytic cleavage of the β bond at the side distal to the charge, however, are charge directed.     

Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP _{H 2}/P _Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar⁺ and H₂ which lead to ArH⁺ and H ₂ ⁺ ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H⁺, H ₂ ⁺ , and H ₃ ⁺ ) or excited species H(n=2) with H₂ producing H atoms.     

Effect of Enhanced UV-B Radiation and Low-Energy N+ Ion Beam Radiation on the Response of Photosynthesis, Antioxidant Enzymes, and Lipid Peroxidation in Rice (Oryza sativa) Seedlings

To understand the effect of enhanced UV-B radiation and low-energy N⁺ ion beam radiation on the response of photosynthesis, antioxidant enzymes, and lipid peroxidation in rice seedlings, Oryza sativa was exposed to three different doses of low-energy N⁺ ion beam and enhanced UV-B alone and in combination. Enhanced UV-B caused a marked decline in some photosynthetic parameters (net photosynthetic rate, transpiration rate, and stomatal conductance) and photosynthetic pigments, whereas it induced an increase in hydrogen peroxide (H₂O₂) accumulation, the rate of superoxide radical production, and the content of malondialdehyde (MDA). Enhanced UV-B also induced an increase in the activity of antioxidant enzymes (superoxide dismutase [SOD], peroxidase (POD), and catalase [CAT]) and some nonenzymatic antioxidants such as proline. Under the combined treatment of enhanced UV-B and low-energy N⁺ ion beam at the dose of 3.0?×?10¹⁷ N⁺ cm^?2, the activity of antioxidant compounds (SOD, POD, CAT, proline, and glutathione), photosynthetic pigments, and some photosynthetic parameters (net photosynthetic rate, transpiration rate, and stomatal conductance) increased significantly; however, the MDA content, H₂O₂ accumulation, and rate of superoxide radical production showed a remarkable decrease compared with the enhanced UV-B treatment alone. These results implied that the appropriate dose of low-energy N⁺ ion beam treatment may alleviate the damage caused by the enhanced UV-B radiation on rice.     

A preliminary investigation of ionic adsorption at the mercury—hexamethyl phosphoric triamide interface

A reference electrode is described that is reversible and reproducible in hexamethyl phosphoric triamide; it was used to study the adsorption of 1 : 1 electrolytes at the mercury—solution interface. The differential capacity curves show features similar to those found for water and the degree of specific adsorption of the anions follows the same sequence: PF^?₆ < BF^?₄, ClO^?₄ < Br^?. There appears to be no specific adsorption of the alkali metal cations at negative charge values whereas the tetraalkyl ammonium ions are strongly adsorbed.