Accurate gaseous ion mobility measurements

It is shown that theoretical values of the mobility of atomic ions in gases can be used to calibrate a drift-tube mass spectrometer, leading to subsequent measurements that are accurate to 0.6% for He⁺ in He near room temperature as the ratio of the electrostatic field strength to the gas number density ranges up to 2 × 10^?19 Vm². Values of the ratio of the parallel diffusion coefficient to the mobility are also obtained.     

Impact of the gaseous environment on the kinetics of solid-state decompositions

The main purpose of this study is to review the current state of the problem of the impact of gaseous environment on the kinetics of solid-state decompositions. Three different theoretical approaches to the interpretation of the decomposition kinetics have been considered. As it follows from the literature published over the past 80 years, the Arrhenius and Knudsen–Langmuir approaches based on the assumption of two different reaction mechanisms (congruent and incongruent) could not solve the problem. At the same time, successes in the application of the thermochemical approach that is based on the assumption of a unitary congruent dissociative vaporization mechanism with condensation of oversaturated vapor remain unnoticed by the TA community. Taking into account this situation, the author has outlined the key points of the thermochemical kinetics in a compact but rigorous and complete form once more. The revised kinetic equations for the different modes of decomposition, several important interrelations between the kinetic parameters, and, finally, the results in the interpretation or reappraisal of the main effects related to the impact of gaseous environment on the kinetics have been considered. In the framework of the thermochemical approach, the problem being discussed may be considered nowadays practically resolved.     

Energy and substituent effects on gaseous ionic decompositions: Electron and charge exchange ionization of benzophenones and benzils

The correlation with substituent constants reported previously for [YØCO]⁺/[ØCO]⁺ ratios in the electron ionization mass spectra of substituted benzophenones and acetophenones has also been observed in the electron ionization spectra of substituted benzils. The [YØCO]⁺/[ØCO]⁺ ratio for the substituted benzils varied with energy of the ionizing electrons according to predictions from a simple kinetic and thermochemical analysis. [YØCO]⁺/[ØCO]⁺ ratios in the charge exchange spectra of benzophenones obtained with Xe, Kr, CO, N₂ and Ar gave good correlations with sub-stituent constants in agreement with the same analysis. Good correlations were also noted for [YØCO]⁺/[ØCO]⁺ ratios with substituent constants for [M]⁺ ions of the benzophenones of the same excess energy (5.5 eV). [YØCO]⁺/[ØCO]⁺ ratios for benzils obtained by charge exchange with [CO]⁺ also showed good correlations with substituent constant. It is suggested that [Ø]⁺ and [YØ]⁺ ions from the benzophenones may be formed primarily by one step decompositions of the molecular ions, but that the [Ø]⁺ and [YØ]⁺ ions from the benzils are formed primarily by decomposition of [ØCO]⁺ and [YØCO]⁺ ions.     

Some stereochemical aspects of the pellizzari rearrangement

Variable temperature nmr spectroscopy and X-ray crystallography have been employed to study the stereochemistry of 2-cyano-3,3-dimethyl-1-phenylpyrazolidin-5-one which thermally rearranges to 2,3-dihydro-2,2-dimethylpyrimido[1,2-a]benzimidazol-4(1H)-one.     

Three-temperature theory of gaseous ion transport

A three-temperature kinetic theory of gaseous ion transport through neutral atomic gases, valid for electric fields of arbitrary strength without restriction on the ion—atom mass ratio or interaction potential, is presented. The theory is based on the use of a set of basis functions in which the ions are allowed to have different temperatures parallel and perpendicular to the field, neither of which is necessarily equal to the gas temperature, and for which the ion velocity distribution function is displaced from the origin. Although the theory is more difficult to use than is the two-temperature theory of Viehland and Mason and of Burnett, it has the important advantage of yielding accurate results in low orders of approximation to quantities that are intrinsically anisotropic, notably ion diffusion coefficients.     

Direct observation of ion emission from charged aqueous nanodrops: effects on gaseous macromolecular charging

Mechanistic information about how gaseous ions are formed from charged droplets has been difficult to establish because direct observation of nanodrops in a size range relevant to gaseous macromolecular ion formation by optical or traditional mass spectrometry methods is challenging owing to their small size and heterogeneity. Here, the mass and charge of individual aqueous nanodrops between 1–10 MDa (15–32 nm diameter) with ∼50–300 charges are dynamically monitored for 1 s using charge detection mass spectrometry. Discrete losses of minimally solvated singly charged ions occur, marking the first direct observation of ion emission from aqueous nanodrops in late stages of droplet evaporation relevant to macromolecular ion formation in native mass spectrometry. Nanodrop charge depends on the identity of constituent ions, with pure water nanodrops charged slightly above the Rayleigh limit and aqueous solutions containing alkali metal ions charged progressively below the Rayleigh limit with increasing cation size. MS2 capsid ions (∼3.5 MDa; ∼27 nm diameter) are more highly charged from aqueous ammonium acetate than from its biochemically preferred, 100 mM NaCl/10 mM Na phosphate solution, consistent with ion emission reducing the nanodrop and resulting capsid charge. The extent of charging indicates that the capsid partially collapses inside the nanodrops prior to the charging and formation of the dehydrated gaseous ions. These results demonstrate that ion emission can affect macromolecular charging and that conformational changes to macromolecular structure can occur in nanodrops prior to the formation of naked gaseous ions.

Ion evaporation from aqueous nanodrops is measured for the first time using charge detection mass spectrometry, and the origin of solute ion dependent charging of large (MDa) macromolecules is revealed.     

Inverse stereochemical effects in the mass spectra of carboalkoxycyclopropylsilanes

The electron impact induced decomposition of the cis and trans isomeric pairs of five 1-trialkylsilyl-2-alkoxy-carbonylcyclopropanes were investigated. Compared with carbon analogues, the silicon atom inhibited ring opening and dramatically altered the fragmentation. The main primary decomposition routes were alkyl loss from silicon, and trialkylsilyl ion formation as well as trialkylalkoxysilane elimination. An unexpected inverse stereochemical effect was found for this latter reaction which was far less significant for the cis isomers, where the interacting SiQ₃ and OR groups are close to each other, than for the trans isomers. This is explained by assuming a hidden silicon-oxygen bond formation, which is possible only in the cis isomers for geometric reasons. This interaction favours alkyl elimination over other fragmentations, and has a marked effect on the further decomposition of the [M—alkyl]⁺ ions, where observed differences in abundance as great as three orders of magnitude are accounted for.     

Photocatalytic decompositions of gaseous HCHO and methylene blue with highly ordered TiO_2 nanotube arrays

The highly ordered Ti O_2nanotubes(NTs) were fabricated by the anodic oxidation method.Their morphology,structure and crystalline phase were characterized by scanning electron microscopy(SEM) and X-ray diffractometer(XRD).The effects of morphology,specific surface area,pore structures and photocatalytic activity of the Ti O_2 NTs were investigated.UV–vis spectra analysis showed that its light absorption had been extended to the visible light range.The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic oxidation of gaseous HCHO and MB aqueous solution.The samples had better adhesion strength in the dark and showed a higher photocatalytic activity than nanoparticles.Especially,with ultraviolet light pretreatment,the nanotubes exhibited more stable active for photocatalytic decomposition and the photodecomposition rate remained at high level after 3 cycles of the photocatalysis experiment.Thus,how the number of surface active group ·OH increased and the mechanism for the great improvement for the photocatalytic activity are discussed.     

Application of self-ionization for enhancing stereochemical and positional effects from arylglycosides under electron ionization conditions in an ion trap mass spectrometer

Ion trap mass spectrometry has been used to structurally characterize and differentiate positional and stereo isomers of arylglycosides having potential antioxidant properties. The use of the self-ionization (SI) technique has allowed to evidence a strong reactivity of fragment ions produced from dissociations of the molecular ion towards the molecules introduced into the trap. Specific structural effects due to positional isomers and anomers have been also envisaged through the occurrence of bimolecular processes inside the ion trap analyzer. Under self-ionization conditions, even-electron ions are produced. The charge is retained on the sugar moiety, in agreement with its proton affinity higher than that of the substituted phenol moiety. Most of the fragmentation pathways involve elimination of acetic acid that protects the hydroxylic groups of the glycoside. SI also produces adduct ions, likely as covalent species, having higher m/z values than the molecular ion. The reaction site is mainly the double bond present in the pyranosidic ring. Even if some fragment ions have lost the initial stereochemistry, their formation can be related to the structure of the parent neutrals introduced into the cell. Collision-induced dissociation (CID) experiments, carried out on ions formed by ion-molecule reactions, have allowed to obtain further information on gas phase ion structures. The study of mass-selected ion-molecule reactions and their kinetics have evidenced a spectacularly different reactivity of the ion at m/z 111 towards the two anomers 2alpha and 2beta, with the latter showing a much more pronounced reactivity. The approach developed in this work revealed to be an useful tool in structural characterization, as well as in stereo and regiochemical differentiation of arylglycosides.     

First evidence for automerization of gaseous phenylium ion.

Tritium atom distribution in the anisole formed from the attack of a nucleogenic tritiated phenylium ion on gaseous methanol provides the first experimental evidence for a gas-phase automerization of phenylium ion.     

Application of the three-temperatue theory of gaseous ion transport

The three-temperature theory of gaseous ion transport is generalized to allow arbitrary choices for the parallel and perpendicular ion temperatures and the velocity displacement. A truncation-parametrization sheme is proposed which simplifies the numerical computation. Results obtained in this way compare favorably with exact and Monte Carlo results available for special cases. Finally, the theory is used to test a proposed interaction potential for Cs⁺-Ar by comparison of calculated and measured mobility and diffusion coefficients.     

Sorption of gaseous ammonia on sulfonated macroreticular ion exchangers

The amount of gaseous ammonia sorbed on dry sulfonated macroreticular ion exchangers with different contents of divinylbenzene was found to be greater than the number of sulfonic acid groups. The excess ammonia could be removed by washing with water. With ion exchangers containing more than 20% of divinylbenzene a certain amount of chemically bonded ammonia could not be removed, even by elution with strong acid. Sorption of ammonia occurred also on dry ion exchangers fully neutralized with sodium ion, and in that case also, ammonia could only partially be removed by water from higher crosslinked samples. On desorption into a gaseous phase the amount of desorbed ammonia was proportional to the surface area of the ion exchanger, since desorption occurred only from surface layers. The results obtained indicate that ammonia migrates among acid groups located within the mass of the ion exchanger.     

Ligand effects on the stereochemical outcome of Suzuki-Miyaura couplings

The ligands associated with various Pd catalysts play a crucial role in determining the stereochemistry of cross-couplings between boronic acids and Z-alkenyl halides. A ligand on palladium has been found that leads to the desired products under mild conditions and in high yields that, in most cases, retain their Z-olefin geometry.     

Subtle stereochemical and electronic effects in iridium-catalyzed isomerization of C-allyl glycosides

[reaction: see text] Stereoselective isomerization of C-allyl glycosides into (E)-C-vinyl glycosides or (Z)-exo-glycals was carried out in the presence of the cationic iridium(I) catalyst [(Ph(2)MeP)(2)Ir(cod)PF(6)]. The products of the isomerization were affected by the relative 1,2-stereochemical relationships and by the nature of the protecting groups. These effects are discussed along with a plausible reaction mechanism.     

Thermochemical ladders: scaling the ramparts of gaseous ion energetics

The use of multiple overlapping gaseous ion–molecule equilibrium measurements to construct thermochemical ladders is reviewed. The types of relative thermochemical data, which may be obtained, are summarized and the types of measurements required to convert the relative equilibrium affinity scales to absolute thermochemical quantities are discussed. The important thermochemical scales produced include proton affinities, gas phase acidities, ionization energies, electron affinities, carbocation stabilities, hydrogen bond energies, Lewis acidities, and metal cation affinities.     

Photocatalytic decompositions of gaseous HCHO and methylene blue with highly ordered TiO2 nanotube arrays

The highly ordered Ti O₂nanotubes（NTs） were fabricated by the anodic oxidation method.Their morphology,structure and crystalline phase were characterized by scanning electron microscopy（SEM） and X-ray diffractometer（XRD）.The effects of morphology,specific surface area,pore structures and photocatalytic activity of the Ti O₂ NTs were investigated.UV–vis spectra analysis showed that its light absorption had been extended to the visible light range.The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic oxidation of gaseous HCHO and MB aqueous solution.The samples had better adhesion strength in the dark and showed a higher photocatalytic activity than nanoparticles.Especially,with ultraviolet light pretreatment,the nanotubes exhibited more stable active for photocatalytic decomposition and the photodecomposition rate remained at high level after 3 cycles of the photocatalysis experiment.Thus,how the number of surface active group ·OH increased and the mechanism for the great improvement for the photocatalytic activity are discussed.     

Generalized Einstein relations from a three-temperature theory of gaseous ion transport

Generalized Einstein relations between mobility and diffusion coefficients are derived within the framework of a recently developed three-temperature kinetic theory of gaseous ion transport. A previously assumed connection between diffusion and differential mobility is firmly established within low-order approximations of the kinetic theory. Equations are obtained for the approximate calculation of the unobservable parallel and perpendicular ion temperatures, and for two higher moments of the ion velocity distribution function that appear as correction terms in the generalized Einstein relations. The present theory is tested on inverse-power potentials and on several alkali ion-noble gas systems, and is compared to two previous semi-empirical generalized Einstein relations. Simple procedures are recommended for the estimation of the parameters and correction terms that occur, to permit approximate calculation of gaseous ion diffusion coefficients from mobility data without the need of a computer or specific knowledge of the ion-neutral interaction potential.     

Glycals in stereochemical synthesis

Enantiomerically pure (14R)-14-methylhexadeca-8Z-enal (cis-trogodermal) — the aggregation pheromone of the khapra beetle (Trogoderma) — and its (14S,8Z)-enantiomer has been effected from di-O-benzoyl-D- and -L-arabinals.