Pressure dependence of secondary ion emission from selected 3d, 4d, and 5d transition metals under N2O, NO, and NO2

The reactant gas pressure dependence of secondary ion emission from surfaces of polycrystalline Cr, Fe, Co, Mo, Rh, W, Re, and Ir under the action of N₂O, NO and NO₂ was observed by means of moderate dynamic SIMS. The mass spectra for constant reactant gas pressure indicate the existence of two different groups of transition metals showing either dissociative or partial molecular adsorption behavior. This is confirmed at least above some suitable reactant gas pressure. Besides some special details (Fe/NO; Co/NO) several of the relative secondary ion intensities vs. reactant gas pressure exhibit similar curvature as for O₂, thus indicating the NO_x gases to be modified sources of oxygen. At higher pressures molecular secondary ions with and without metal atoms come to be appreciable. Received: 28 May 1997 / Revised: 2 February 1998 / Accepted: 4 February 1998     

Pressure dependence of secondary ion emission from selected transition metals under nitrogen dioxide

For Ar⁺ bombarded polycrystalline surfaces of Ta, Co, Ni, Pd and Pt the emission of positive secondary ions was observed using nitrogen dioxide as reactant gas with varied partial pressure (0.001 mPa <p _{spno}2 < 10 mPa) and dynamic SIMS conditions (2 keV; 32 A/cm²). The results indicate that NO₂ molecules appear to be completely destroyed in adsorption to Ta. Different behaviour was found for the other target metals. This can be explained by assuming surface species of partially molecular type. In some cases the results indicate two different modes of surface interaction with the reactant gas.     

Emission of secondary ions from 3d, 4d, and 5d transition metals under chloro-trifluoro-methane CClF3 as reactant gas

Surface interactions of CClF₃ with polycrystalline samples of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, Rh, Pd, Ag, Ta, W, Re, Ir, and Pt were investigated by means of moderate dynamic SIMS. As observed with other reactant gases these transition metals in most cases appear to be discernible into “dissociative” and (partial) “molecular” adsorbents. Small signals of oxidic secondary ions which are detectable for residual gas conditions vanished under the action of CClF₃. However, due to strong polarization by either of the halogens, the emission of Me²⁺ ions is enhanced for Ti, V, and Nb. Received: 6 August 1997 / Revised: 22 December 1997 / Accepted: 29 December 1997     

Emission of secondary ions from 3d, 4d, and 5d transition metals under chloro-trifluoro-methane CClF3 as reactant gas

Surface interactions of CClF₃ with polycrystalline samples of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, Rh, Pd, Ag, Ta, W, Re, Ir, and Pt were investigated by means of moderate dynamic SIMS. As observed with other reactant gases these transition metals in most cases appear to be discernible into “dissociative” and (partial) “molecular” adsorbents. Small signals of oxidic secondary ions which are detectable for residual gas conditions vanished under the action of CClF₃. However, due to strong polarization by either of the halogens, the emission of Me²⁺ ions is enhanced for Ti, V, and Nb.     

Secondary ion emission of selected transition metals under nitrogen oxides

The emission of various positive secondary ions has been investigated for polycrystalline targets of Ti, V, Cr, Nb, Ta, Co, Ni, Cu, Pd and Pt, which were bombarded by Ar⁺ ions under dynamic SIMS (DSIMS) conditions in the presence of the gaseous nitrogen oxides N₂O, NO and NO₂ at fixed pressure and under residual gas. Besides ions of the Me⁺ type several fragmentary ions (e.g. N⁺, O⁺, NO⁺, MeN⁺ and MeO⁺) and also cluster ions Me _x O _y ⁺ (x 2, 0 y 2) were detected. Signals of a more molecular type with respect to the reactant gas, e.g. MeNO⁺, were only found for Co, Ni, Cu, Pd and Pt. From this, one may infer that for the other targets the nitrogen oxides will exist preferentially in a dissociatively adsorbed state at the metal surface. Several aspects of secondary ion emission can be explained assuming a different degree of oxidation for the metals under the influence of reactant gas.Part of the dissertation     

Ab initio calculations of nitrogen oxide reactions: formation of N2O2, N2O3, N2O4, N2O5, and N4O2 from NO, NO2, NO3, and N2O

Ab initio computational methods were used to obtain Delta(r)H(o), Delta(r)G(o), and Delta(r)S(o) for the reactions 2 NO <=> N(2)O(2) (I), NO+NO(2) <=> N(2)O(3) (II), 2 NO(2) <=> N(2)O(4) (III), NO(2)+NO(3) <=> N(2)O(5) (IV), and 2 N(2)O <=> N(4)O(2) (V) at 298.15 K. Optimized geometries and frequencies were obtained at the CCSD(T) level for all molecules except for NO, NO(2), and NO(3), for which UCCSD(T) was used. In all cases the aug-cc-pVDZ (avdz) basis set was employed. The electronic energies of all species were obtained from complete basis set extrapolations (to aug-cc-pV5Z) using five different extrapolation methods. The [U]CCSD(T)/avdz geometries and frequencies of the N(x)O(y) compounds are compared with literature values, and problems associated with the values and assignments of low-frequency modes are discussed. The standard entropies are compared with values cited in the NIST/JANAF tables [NIST-JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data Monograph No. 9, 4th ed. edited by M. W. Chase, Jr. (American Chemical Society and American Institute of Physics, Woodbury, NY, 1988)]. With the exception of I, in which the dimer is weakly bound, and V, for which thermodynamic data appears to be lacking, the calculated standard thermodynamic functions of reaction are in good agreement with literature values obtained both from statistical mechanical and various equilibrium methods. A multireference-configuration interaction calculation (MRCI+Q) for I provides a D(e) value that is consistent with previous calculations. The combined uncertainties of the NIST/JANAF values for Delta(r)H(o), Delta(r)G(o), and Delta(r)S(o) of II, III, and IV are discussed. The potential surface for the dissociation of N(2)O(4) was explored using multireference methods. No evidence of a barrier to dissociation was found.     

Oxygen-transfer reactions between 3d transition metals and N(2)O and NO(2)

Density functional calculations have been performed to describe reactions of ground-state 3d transition metal atoms (Sc-Ni) with N(2)O and NO(2) molecules. From the analysis of the calculated reaction surfaces, a general reaction mechanism evolved. The reactions are initiated by electron transfer from metal to the oxidant molecule, which weakens the N-O bond and facilitates an O(-)((2)P) abstraction. 4s-3d hybridization taking place in the metal electronic structure plays an essential role in the net 4s(beta) electron transfer from the metal atom to the nitrogen-oxide molecule. These key steps contribute to connect the reactant and product channels on a single potential energy surface. The calculations revealed that reaction with NO(2) yields stable oxo-nitrosyl insertion products, and their equilibrium structural properties can be understood by inspecting the 4pi* metal-oxide orbital occupancies. Correlation is obtained between the metal 3d ionization energies and the reaction rates as well as activation energies. This correlation provides additional support for the reaction mechanism called electron-transfer-assisted oxygen abstraction. This novel mechanism exhibits the basic features of the simple electron transfer and direct abstraction kinetic models and sheds new light on the so-called resonance interaction model as well.     

An investigation of the reactions of H3O+ and O2+ with NO, NO2, N2O and HNO2 in support of selected ion flow tube mass spectrometry

A selected ion flow tube (SIFT) experimental investigation has been carried out of the reactions of H3O+, NO+ and O2+ with NO, NO2, N2O and HNO2, in order to obtain the essential kinetic data for the analyses of these compounds in air using selected ion flow tube mass spectrometry (SIFT-MS). These investigations show that NO+ ions do not react at a significant rate with any of these NOx compounds and that H3O+ ions react only with HNO2 (product ions H2NO2+ (75%) and NO+ (25%)). O2+ ions react with NO (product ion NO+), NO2 (product ion NO2+) and HNO2 (product ions NO+ (75%), NO2+ (25%)), but not with N2O. We conclude that both NO and NO2 can be accurately quantified in air using only O2+ precursor ions and SIFT-MS when HNO2 is not present. However, when HNO2 is present it invariably co-exists with both NO and NO2 and then both H3O+ and O2+ precursor ions are needed to determine the partial pressures of NO, NO2 and HNO2 in the air mixture. We also conclude that currently N2O cannot be analysed in air using SIFT-MS.     

Heterogeneous oxidation kinetics of organic biomass burning aerosol surrogates by O3, NO2, N2O5, and NO3

The reactive uptake coefficients (γ) of O(3), NO(2), N(2)O(5), and NO(3) by levoglucosan, abietic acid, nitroguaiacol, and an atmospherically relevant mixture of those species serving as surrogates for biomass burning aerosol have been determined employing a chemical ionization mass spectrometer coupled to a rotating-wall flow-tube reactor. γ of O(3), NO(2), N(2)O(5), and NO(3) in the presence of O(2) are in the range of 1-8 × 10(-5), <10(-6)-5 × 10(-5), 4-6 × 10(-5), and 1-26 × 10(-3), respectively, for the investigated organic substrates. Within experimental uncertainties the uptake of NO(3) was not sensitive to relative humidity levels of 30 and 60%. NO(3) uptake experiments involving substrates of levoglucosan, abietic acid, and the mixture exhibit an initial strong uptake of NO(3) followed by NO(3) gas-phase recovery as a function of NO(3) exposure. In contrast, the uptake of NO(3) by nitroguaiacol continuously proceeds at the same efficiency for investigated NO(3) exposures. The derived oxidative power, i.e. the product of γ and atmospheric oxidant concentration, for applied oxidants is similar or significantly larger in magnitude than for OH, emphasizing the potential importance of these oxidants for particle oxidation. Estimated atmospheric lifetimes for the topmost organic layer with respect to O(3), NO(2), N(2)O(5), and NO(3) oxidation for typical polluted conditions range between 1-112 min, indicating the potential for significant chemical transformation during atmospheric transport. The contact angles determined prior to, and after heterogeneous oxidation by NO(3), representative of 50 ppt for 1 day, do not decrease and thus do not indicate a significant increase in hygroscopicity with potential impacts on water uptake and cloud formation processes.     

Removal of NO in NO/N2, NO/N2/O2, NO/CH4/N2, and NO/CH4/O2/N2 systems by flowing microwave discharges

In this paper, continuing previous work, we report on experiments carried out to investigate the removal of NO from simulated flue gas in nonthermal plasmas. The plasma-induced decomposition of small concentrations of NO in N2 used as the carrier gas and O2 and CH4 as minority components has been studied in a surface wave discharge induced with a surfatron launcher. The reaction products and efficiency have been monitored by mass spectrometry as a function of the composition of the mixture. NO is effectively decomposed into N2 and O2 even in the presence of O2, provided always that enough CH4 is also present in the mixture. Other majority products of the plasma reactions under these conditions are NH3, CO, and H2. In the absence of O2, decomposition of NO also occurs, although in that case HCN accompanies the other reaction products as a majority component. The plasma for the different reaction mixtures has been characterized by optical emission spectroscopy. Intermediate excited species of NO*, C*, CN*, NH*, and CH* have been monitored depending on the gas mixture. The type of species detected and their evolution with the gas composition are in agreement with the reaction products detected in each case. The observations by mass spectrometry and optical emission spectroscopy are in agreement with the kinetic reaction models available in literature for simple plasma reactions in simple reaction mixtures.     

A spectroscopic study of the equilibrium NO2 + NO3 + M 2 N2O5 + M and the kinetics of the O3/N2O5/NO3/NO2/ air system

The equilibrium constant, K_eq of the reaction NO₂ + NO₃ + M 2 N₂O₅ + M has been determined for a small range of temperatures around room temperature in air at 740 torr by direct spectroscopical measurements of NO₂, NO₃, and N₂O₅. At 298 K, K_eq was determined as (3.73 ± 0.61) × 10⁻¹¹ cm³ molecule⁻¹. Averaging this and 11 other independent evaluations of K_eq yields K_eq = (3.31 ± 0.82) × 10⁻¹¹ cm³ molecule⁻¹, where the uncertainty is given as one standard deviation. The kinetics of the O₃/NO₂/N₂O₅/NO₃/ air system was studied in a static chamber at room temperature and 740 torr total pressure. Evidence of a unimolecular decay reaction of NO₃, NO₃ → NO + O₂, was found and its rate coefficient was estimated as (1.6 ± 0.7) × 10⁻³ s⁻¹ at 295 ± 2 K.     

Ionization gauge sensitivities of N2, O2, N2O,NO, NO2, NH3, CClF3 and CH3OH

A Bayard-Alpert (BA) gauge was used to determine apparent relative sensitivites S_rel,X for O₂, N₂O, NO, NO₂, NH₃, CClF₃ and CH₃OH from gauge calibration measurements in the range 1.3×10^?1 Pa≤p≤1.3·10^?3Pa. Nitrogen was used as a calibration standard.     

Molecular magnetic materials based on 4d and 5d transition metals

The study of paramagnetic compounds based on 4d and 5d transition metals is an emerging research topic in the field of molecular magnetism. An essential driving force for the interest in this area is the fact that heavier metal ions introduce important attributes to the physical properties of paramagnetic compounds. Among the attractive characteristics of heavier elements vis-à-vis magnetism are the diffuse nature of their d orbitals, their strong magnetic anisotropy owing to enhanced spin-orbit coupling, and their diverse structural and redox properties. This critical review is intended to introduce readers to the topic and to report recent progress in this area. It is not fully comprehensive in scope although we strived to include all relevant topics and a large subset of references in the area. Herein we provide a survey of the history and current status of research that has been conducted on the topic of second and third row transition metal molecular magnetism. The article is organized according to the nature of the precursor building blocks with special topics being highlighted as illustrations of the special role of heavier transition metal ions in the field. This paper is addressed to readers who are interested in molecular magnetism and the application of coordination chemistry principles to materials synthesis (231 references).     

Reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of polycyclic aromatic hydrocarbon surfaces

We investigated the reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of solid polycyclic aromatic hydrocarbons (PAHs) using a coated wall flow tube reactor coupled to a chemical ionization mass spectrometer. The PAH surfaces studied were the 4-ring systems pyrene, benz[a]anthracene, and fluoranthene. Reaction of NO3 radicals with all three PAHs was observed to be very fast with the reactive uptake coefficient, gamma, ranging from 0.059 (+0.11/-0.049) for benz[a]anthracene at 273 K to 0.79 (+0.21/-0.67) for pyrene at room temperature. In contrast to the NO3 reactions, reactions of the different PAHs with the other gas-phase species (N2O5, NO2, HNO3, and O3) were at or below the detection limit (gamma 相似文献   

对流层大气NO3与N2O5的实地测量

大气硝基(NO₃)自由基和五氧化二氮(N₂O₅)是对流层大气化学反应的核心物种,对于理解大气氧化性、二次有机气溶胶生成、活性卤素化学和全球硫素循环等对流层大气化学研究的关键问题具有重要意义。大气NO₃自由基和N₂O反应活性高、大气寿命短、浓度低,其定量分析非常具有挑战性。本文总结了大气NO₃自由基与N₂O₅的实地测量方法,并对差分光学吸收光谱(DOAS)、腔衰荡光谱(CRDS)、腔增强光谱(CEAS)、激光诱导荧光光谱(LIF)、电子顺磁共振谱(MIESR)和化学离子化质谱(CIMS)等六类技术方法的准确度、精确度、时间分辨率、测量干扰、标定方法和系统稳定性等方面进行了系统比较。综合分析认为吸收光谱法是对流层NO₃自由基与N₂O₅测量技术的发展方向,其中近期发展起来的腔技术(CRDS和CEAS)显示出较好的应用潜力。但是我国实际大气环境中普遍存在高浓度的颗粒物污染,如何在高颗粒物条件下实现NO₃自由基与N₂O₅的精密准确测量具有挑战性。本文进一步归纳了NO₃自由基与N₂O₅在城市、森林、自由大气和海洋海岸等典型大气环境条件下的浓度水平和主要科学发现,探讨了一些亟待解决的问题和可能的重点研究方向。     

The reactions of isotopically labeled N15NO+ with CO,NO, O2, N2, NO2, and N2O

The reactions of labeled N¹⁵NO⁺ with CO, NO, O₂, ¹⁸O₂, N₂, NO₂, and N₂O have been investigated using a tandem ICR instrument. In each case the total rate coefficient, product distribution, and kinetic energy dependence were measured. The results indicate that very specific reaction mechanisms govern these reactions. This conclusion is suggested by the lack of isotopic scrambling in many cases and by the complete absence of energetically allowed products in almost all of the systems. The kinetic energy studies indicate that most of the reaction channels proceed through an intermediate complex at low energies and via a direct mechanism at higher kinetic energies. Such direct mechanisms include long range charge transfer and atom or ion transfer.     

Synthesis of O2'-methyluridine, O2'-methylcytidine, N4,O2'-dimethylcytidine and N4,N4,O2'-trimethylcytidine from a common intermediate

A facile synthesis of the title compounds from a readily accessible precursor, 3',5'-O-bis-protected O4-(2-nitrophenyl)uridine, is described.