Comparison of [C6H6]2+ ion structures: Application of electron capture induced decomposition to the study of isomers

Structures of [C₆H₆]²⁺ ions formed by electron impact from benzene, 2,4-hexadiyne and 1,5-hexadiyne have been investigated by the recently developed electron capture induced decomposition method, by charge-separation reactions and by ion abundances in electron impact mass spectra. Significant differences were found among the isomers indicating the structural integrity of these [C₆H₆]²⁺ ions. The observed differences indicate that the most likely atomic configuration of [C₆H₆]²⁺ ions produced from benzene and 2,4-hexadiyne is the same as in the corresponding neutral species. A new method is suggested by which the structure (atomic configuration) of doubly charged ions may be determined.     

Cation influence on the structure and electron density of water in some Men+·H2O complexes

The cation influence on the water molecule in the Li⁺·H₂O, Be²⁺·H₂O, Mg²⁺·H₂O and A1³⁺·H₂O complexes has been studied by means of quantum-mechanical ab initio calculations. A number of general trends are noted. (1) The calculated equilibrium water O-H distances increase with increasing binding energies, i.e. in the order Li⁺, Mg²⁺, Be²⁺, Al³⁺. The H-O-H angles differ by about ±1 ° from the calculated equilibrium angle for the free H₂O molecule; the variation has no systematic trend. (2) The electron density redistribution accompanying the change in the internal H₂O geometry in these complexes is considerably smaller than the redistribution brought about by the direct influence of the external field. (3) The harmonic O-H stretching force constant decreases with increased cation-water bonding. (4) The qualitative features of the density changes are very similar for the four complexes. The magnitudes of the interactions follow the relation Li⁺ < Mg²⁺ < Be²⁺ Al³⁺. An increased polarization of the H₂O molecule occurs with electron migration from the H atoms towards the O atom and an accumulation of electron charge approximately at the centre of the Meⁿ⁺—O bond, especially in Be²⁺·H₂O and A1³⁺·H₂O. An electron deficiency is found in the lone-pair region.     

Electrochemical evidence of catalysis of oxygen reduction at the polarized liquid–liquid interface by tetraphenylporphyrin monoacid and diacid

Cyclic voltammetry is used to study the role of 5,10,15,20-tetraphenyl-21H,23H-porphine (H₂TPP) in the reduction of molecular oxygen by decamethylferrocene (DMFc) at the polarized water|1,2-dichloroethane (DCE) interface. It is shown that this rather slow reaction proceeds remarkably faster in the presence of tetraphenylporphyrin monoacid (H₃TPP⁺) and diacid (H₄TPP²⁺), which are formed in DCE by the successive transfer of two protons from the acidified aqueous phase. A mechanism is proposed, which includes the formation of adduct between H₃TPP⁺ or H₄TPP²⁺ and O₂ that is followed by electron transfer from DMFc to the adduct leading to the observed production of DMFc⁺ and to the regeneration of H₂TPP or H₃TPP⁺, respectively.     

Ionic organoboranes. Part 7. Molecular orbital study of electron distribution in the [7.10]hemiousenide ion; explanation for the anomalous B NMR spectrum

The ¹¹B NMR spectrum of the [7.10²]hemiousenide ion (C₇H₆B₁₀H₉⁻) demonstrates ground state +T cage to ring electron donation from the region of the two apical borons. The chemically non-equivalent apical borons give an apparent singlet NMR absorption and thus appear to be in very similar electronic environments, even though they are unsymmetrically arranged relative to the cationic ring. Ab initio 3-21G(∗) and semi-empirical PM3 calculations suggest that the π-system of the ring only interacts with cage molecular orbitals that are symmetrical relative to both apical borons in B₁₀H₁₀²− and that the electron density about the two apical borons is essentially identical in the [7.10²]hemiousenide ion. Significant ground state electron donation from cage to ring is observed, consonant with conclusions from previous polarographic studies.     

Exceptionally High Rates of Biological Hydrogen Production by Biomimetic In Vitro Synthetic Enzymatic Pathways

Hydrogen production by water splitting energized by biomass sugars is one of the most promising technologies for distributed green H₂ production. Direct H₂ generation from NADPH, catalysed by an NADPH‐dependent, soluble [NiFe]‐hydrogenase (SH1) is thermodynamically unfavourable, resulting in slow volumetric productivity. We designed the biomimetic electron transport chain from NADPH to H₂ by the introduction of an oxygen‐insensitive electron mediator benzyl viologen (BV) and an enzyme (NADPH rubredoxin oxidoreductase, NROR), catalysing electron transport between NADPH and BV. The H₂ generation rates using this biomimetic chain increased by approximately five‐fold compared to those catalysed only by SH1. The peak volumetric H₂ productivity via the in vitro enzymatic pathway comprised of hyperthermophilic glucose 6‐phosphate dehydrogenase, 6‐phosphogluconolactonase, and 6‐phosphogluconate dehydrogenase, NROR, and SH1 was 310 mmol H₂/L h^?1, the highest rate yet reported. The concept of biomimetic electron transport chains could be applied to both in vitro and in vivo H₂ production biosystems and artificial photosynthesis.     

Quantum chemical approach to electron transfer in molecular systems

A theoretical treatment of electron transfer in and between molecules is presented. The nuclei are assumed to move classically. A time-dependent electron probability density is calculated using general equations given by Nikitin. Variational methods of different kinds may be employed in these equations. In the present paper some examples are studied at the extended Hückel level for the systems H₂-H ₂ ⁺ , H₂-O^2–-H ₂ ⁺ , H₂-S^2–-H ₂ ⁺ and H₂-H₂-H ₂ ⁺ .     

Metastable ions in chemical ionization

The ion [C₃H₅]⁺ generated in a chemical ionization source by a variety of methods, including protonation and charge exchange, exhibits a metastable peak for H₂ loss which is two orders of magnitude weaker than that formed in an electron impact source. The stable [C₃H₅]⁺ ions generated by electron impact and chemical ionization undergo collision-induced dissociation to a comparable extent, both losing H₂ by only one of the two competitive mechanisms observed for metastable ions. In contrast to the behavior of [C₃H₅]⁺, the molecular ions of p-substituted nitrobenzene, generated by charge exchange at high source pressure, yield composite metastable peaks for NO loss which are very similar in shape and intensity to those generated by electron impact. The contrasting behavior of the metastable ions extracted from high pressure ion sources in the two systems may be due to differences in the efficiencies of quenching of the ionic states responsible for fragmentation as metastable ions. It is noteworthy that the NO loss reactions require considerably lower activation energies than does the H₂ loss reaction.     

Correlation effects on energy curves for proton transfer. The cation [H5O2]+

Potential curves for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺ were calculated by both ab initio and semi-empirical LCAO MO SCF CI methods. The energy barrier of the symmetric double minimum potential in [H₅O₂]⁺ is very sensitive to electron correlation. At an OO distance of 2.74 Å it decreases from the HF value of 9.5 kcal/mole to about 7.0 kcal/mole. The results of the semi-empirical calculations agree well with the ab initio data as long as only relative effects are regarded. The partitioning of correlation energy into contributions of individual electron pairs is very similar for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺. In this example the proton transfer appears as a superposition of two “contracted ionic dissociation” processes. An interpretation of the behaviour of correlation during these processes is presented.     

Stereochemical applications of mass spectrometry: 1—The utility of electron impact and chemical ionization mass spectrometry in the differentiation of stereoisomeric benzoin oximes and phenylhydrazones

A study of the electron impact and chemical ionization (H₂, CH₄, and iso-C₄H₁₀) mass spectra of stereoisomeric benzoin oximes and phenylhydrazones indicates that while the former can be distinguished only by their chemical ionization mass spectra the latter are readily distinguishable by both their electron impact and chemical ionization mass spectra. The electron impact mass spectra of the isomeric oximes are practically identical; however, the chemical ionization spectra show that the E isomer forms more stable [MH]⁺ and [MH? H₂O]⁺ ions than the Z isomer for which both the [MH]⁺ and [MH? H₂O]⁺ ions are relatively unstable. In electron impact the Z-phenylhydrazone shows a lower [M]⁺˙ ion abundance and more facile loss of H₂O than does the E isomer. This more facile H₂O loss also is observed for the [MH]⁺ ion of the Z isomer under chemical ionization conditions.     

Density Functional Theory Analysis of Anthraquinone Derivative Hydrogenation over Palladium Catalyst

A density functional theory (DFT) analysis was conducted on the hydrogenation of 2‐alkyl‐anthraquinone (AQ), including 2‐ethyl‐9,10‐anthraquinone (eAQ) and 2‐ethyl‐5,6,7,8‐tetrahydro‐9,10‐anthraquinone (H₄eAQ), to the corresponding anthrahydroquinone (AQH₂) over a Pd₆H₂ cluster. Hydrogenation of H₄eAQ is suggested to be more favorable than that of eAQ owing to a higher adsorption energy of the reactant (H₄eAQ), lower barrier of activation energy, and smaller desorption energy of the target product (2‐ethyl‐5,6,7,8‐tetrahydro‐9,10‐anthrahydroquinone, H₄eAQH₂). For the most probable reaction routes, the energy barrier of the second hydrogenation step of AQ is circa 8 kcal mol^?1 higher than that of the first step. Electron transfer of these processes were systematically investigated. Facile electron transfer from Pd₆H₂ cluster to AQ/AQH intermediate favors the hydrogenation of C=O. The electron delocalization over the boundary aromatic ring of AQ/AQH intermediate and the electron‐withdrawing effect of C=O are responsible for the electron transfer. In addition, a pathway of the electron transfer is proposed for the adsorption and subsequent hydrogenation of AQ on the surface of Pd₆H₂ cluster. The electron transfers from the abstracted H atom (reactive H) to a neighbor Pd atom (Pd_H), and finally goes to the carbonyl group through the C₄ atom of AQ aromatic ring (C₄).     

Widely Controllable Syngas Production by a Dye‐Sensitized TiO2 Hybrid System with ReI and CoIII Catalysts under Visible‐Light Irradiation

Visible‐light irradiation of a ternary hybrid catalyst prepared by grafting a dye, an H₂ evolving Co^III catalyst and a CO‐producing Re^I catalyst on TiO₂ have been found to produce both H₂ and CO (syngas) in CO₂‐saturated N ,N ‐dimethyl formamide (DMF)/water solution containing a 0.1 m sacrificial electron donor. The H₂/CO ratios are effectively controlled by changing either the water content of the solvent or the molar ratio of the Re^I and Co^III catalysts ranging from 1:2 to 15:1. The controlled syngas formation is discussed in terms of competitive electron flow from TiO₂ to each of the CO₂‐reduction and hydrogen‐evolving sites depending on the efficiencies of the two catalytic reaction cycles under given reaction conditions.     

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.     

Stationary points on the potential energy surface of O2−HF and O2−H2O

The determination of minima and saddle points on the potential energy surfaces of the hydrogen bonded species O₂^?HF and O₂^?H₂O is performed with unrestricted Hartree-Fock calculations. Geometries, electron density distributions, and relative energies for every stationary point are reported. Only one true minimum is found for O₂^?HF and for O₂^?H₂O, and this approximately corresponds to a structure where the partially positive hydrogen atom is located along one of the superoxide ion electron lone-pair directions. Calculated ΔH, ΔS, and ΔG values for the reaction between O₂^? and H₂O are in good agreement with experimental data.     

Dissociate electron attachment to monohalogenated benzenes

Formation of halogen negative ions by dissociative electron attachment on the halobenzenes C₆H₅Br, C₆H₅Cl and C₆H₅F is studied as a function of incident electron energy up to 7.5 eV by mass spectrometry. The threshold energies for Cl⁻ and Br⁻ provide a determination of the first electron affinities for C₆H₅Cl and C₆H₅Br. The absolute cross section for Ci⁻ formation from C₆H₅Cl was also measured.     

Electron Transfer in Dye‐Sensitised Semiconductors Modified with Molecular Cobalt Catalysts: Photoreduction of Aqueous Protons

A visible‐light driven H₂ evolution system comprising of a Ru^II dye ( RuP ) and Co^III proton reduction catalysts ( CoP ) immobilised on TiO₂ nanoparticles and mesoporous films is presented. The heterogeneous system evolves H₂ efficiently during visible‐light irradiation in a pH‐neutral aqueous solution at 25 °C in the presence of a hole scavenger. Photodegradation of the self‐assembled system occurs at the ligand framework of CoP , which can be readily repaired by addition of fresh ligand, resulting in turnover numbers above 300 mol H₂ (mol CoP )^?1 and above 200,000 mol H₂ (mol TiO₂ nanoparticles)^?1 in water. Our studies support that a molecular Co species, rather than metallic Co or a Co‐oxide precipitate, is responsible for H₂ formation on TiO₂. Electron transfer in this system was studied by transient absorption spectroscopy and time‐correlated single photon counting techniques. Essentially quantitative electron injection takes place from RuP into TiO₂ in approximately 180 ps. Thereby, upon dye regeneration by the sacrificial electron donor, a long‐lived TiO₂ conduction band electron is formed with a half‐lifetime of approximately 0.8 s. Electron transfer from the TiO₂ conduction band to the CoP catalysts occurs quantitatively on a 10 μs timescale and is about a hundred times faster than charge‐recombination with the oxidised RuP . This study provides a benchmark for future investigations in photocatalytic fuel generation with molecular catalysts integrated in semiconductors.     

Molecular states formed by ion beam neutralization of [C2H2]+ and [C2H3]+ on metal targets

A combination of charge-stripping and beam-scattering techniques has been used to study the molecular states formed when a fast beam of [C₂H₂]⁺ and [C₂H₃]⁺ in several isotopic forms are neutralized by electron transfer from metal target atoms (K, Na, Mg and Zn). For [C₂H₃]⁺ the isotopic compositions and relative abundances of product states were found to be insensitive to the method of ion preparation (electron impact and chemical ionization). Ground state neutrals are formed in partial abundance when Mg or Zn is used as a target atom. With low ionization potential targets (K and Na) excitel dissociative states of C₂H₂ and C₂H₃ are formed as major beam constituents. For these states decomposition products have been identified and fragmentation energies measured. The excited states of C₂H₂ and C₂H₃ lie alout 6.8 eV and 2.9 eV, respectively, above their stable ground states. The discussion focuses on the possible identity of the excited states and their structural relations to the precursor ions.     

Comparative analysis of the state of lithium and sodium atoms in water clusters

Structures and energetic characteristics of Li(H₂O) _n and Li⁺(H₂O) _n clusters with n = 1–6, 19, and 27 determined in the second order of the Møller-Plesset perturbation theory with 6–31++G(d,p) basis set are analyzed. The electron density redistribution, which takes place upon the electron addition to a Li⁺(H₂O) _n cluster, is found to be provided by hydrogen-bonded water molecules: initially almost neutral molecules, which are most distant from lithium, become negatively charged. The calculated energies of the electron capture by Li⁺(H₂O) _n clusters are approximated with the appropriate electrostatic model, and estimates of the lithium ionization energy in water clusters of various sizes are found. Similar estimates obtained earlier for sodium are made more accurate.     

Vicinage effect in secondary electron emission from a clean (001) surface of SnTe induced by keV hydrogen clusters

The total secondary electron emission yields from a clean (001) surface of SnTe crystal are studied at the bombardment of hydrogen clusters (H⁺, H ₂ ⁺ and H ₃ ⁺ ions) with energies ranging from 7 to 12 keV/amu. The observed yield is not proportional to the number of protons in the cluster ion. The yields are successfully explained by applying the stopping powers of a homogeneous electron gas for the constituents of the clusters in keV energy region, which provide the vicinage effect.     

Reactions of Doubly Ionized Benzene with Nitrogen and Water: A Nitrogen‐Mediated Entry into Superacid Chemistry

Even in the highly diluted gas phase, rather than electron transfer the benzene dication C₆H₆²⁺ undergoes association with dinitrogen to form a transient C₆H₆N₂²⁺ dication which is best described as a ring‐protonated phenyl diazonium ion. Isotopic labeling studies, photoionization experiments using synchrotron radiation, and quantum chemical computations fully support the formation of protonated diazonium, which is in turn a prototype species of superacidic chemistry in solution. Additionally, reactions of C₆H₆²⁺ with background water involve the transient formation of diprotonated phenol and, among other things, afford a long‐lived C₆H₆OH₂²⁺ dication, which is attributed to the hydration product of Hogeveen’s elusive pyramidal structure of C₆H₆²⁺, as the global minimum of doubly ionized benzene. Nitrogen is essential for the formation of the C₆H₆OH₂²⁺ dication in that it mediates the formation of the water adduct, while the bimolecular encounter of the C₆H₆²⁺ dication with water only leads to (dissociative) electron transfer.     

Polarization studies of Lyman-α radiation emitted in atom-H2-molecule collisions

A polarization study of Lyman-α radiation emitted in collisions of H⁺, H, He⁺, and He projectiles with H₂ molecules has been performed at projectile energies ranging from 1–25 keV. In H⁺?H₂ and H?H₂ collisions, the measured linear polarization is negative at low incident velocities, indicating a preferred alignment of the excited electron charge cloud perpendicular to the incident projectile direction. This is taken as significance for a rotational coupling which in these two collision systems predominantly populates the H(2p _±1) substates. In He⁺?H₂ and He?H₂ collisions, the measured linear polarization is about zero and independent of the projectile energy. In these more asymmetric systems,a′?a′ and a′?a″ couplings are now of comparable magnitude.