Detection of Two Highly Stable Silicon Nitrides: HSiNSi and H₃SiNSi

The formation mechanisms of silicon nitride and silicon nitrogen hydrogen films, both produced by chemical vapor deposition (CVD) techniques and widely used in electronic device fabrication, are poorly understood. Identification of gas-phase intermediates formed from starting materials, typically silane, ammonia, and/or nitrogen, is a critical step in assessing the interplay between gas and surface processes in film formation. Two potential intermediates in this process, HSiNSi and H₃SiNSi, have now been detected in a molecular beam by means of rotational spectroscopy. Both molecules were produced in electrical discharges of CVD-like gas mixtures and are the most readily observed silicon–nitrogen-containing molecules in the 6–20 GHz frequency range, though neither has been the subject of prior experimental or theoretical studies. HSiNSi and H₃SiNSi are likely formed from reactions involving the silanitrile radical (SiN, isoelectronic to CN), implying that similar gas-phase reactions may be involved in film growth

Spectroscopic Detection and Structure of Hydroxidooxidosulfur (HOSO) Radical, An Important Intermediate in the Chemistry of Sulfur-Bearing Compounds

The rotational spectrum of hydroxidooxidosulfur, HOSO, an intermediate of particular interest in the combustion of sulfur-rich fuels, has been determined to high accuracy from gas-phase measurements. Detection of specific isotopic species using isotopically enriched gases suggests that HOSO is formed in our discharge nozzle via the reaction H + SO₂ (+M) → HOSO (+M). A precise experimental <i>r</i>₀ geometry has also been derived from the isotopic analysis; HOSO has a <i>cis</i> configuration, but the subtle structural question of its planarity remains unresolved. From the derived spectroscopic constants, <i>in situ</i> and remote sensing for this fundamental radical can now be undertaken in a variety of environments, including in combustion reactors, the troposphere of Earth, and Io, the innermost Galilean moon of Jupiter

Temperature and Pressure Dependences of the Reactions of Fe⁺ with Methyl Halides CH₃X (X = Cl, Br, I): Experiments and Kinetic Modeling Results

The pressure and temperature dependences of the reactions of Fe⁺ with methyl halides CH₃X (X = Cl, Br, I) in He were measured in a selected ion flow tube over the ranges 0.4 to 1.2 Torr and 300–600 K. FeX⁺ was observed for all three halides and FeCH₃⁺ was observed for the CH₃I reaction. FeCH₃X⁺ adducts (for all X) were detected in all reactions. The results were interpreted assuming two-state reactivity with spin-inversions between sextet and quartet potentials. Kinetic modeling allowed for a quantitative representation of the experiments and for extrapolation to conditions outside the experimentally accessible range. The modeling required quantum-chemical calculations of molecular parameters and detailed accounting of angular momentum effects. The results show that the FeX⁺ products come via an insertion mechanism, while the FeCH₃⁺ can be produced from either insertion or S_N2 mechanisms, but the latter we conclude is unlikely at thermal energies. A statistical modeling cannot reproduce the competition between the bimolecular pathways in the CH₃I reaction, indicating that some more direct process must be important

Kinetics of Cations with C₂ Hydrofluorocarbon Radicals

Reactions of the cations Ar⁺, O₂⁺, CO₂⁺, and CF₃⁺ with the C₂ radicals C₂H₅, H₂C₂F₃, C₂F₃, and C₂F₅ were investigated using the variable electron and neutral density attachment mass spectrometry technique in a flowing afterglow–Langmuir probe apparatus at room temperature. Rate coefficients for observed product channels for these 16 reactions are reported as well as rate coefficients and product branching fractions for the 16 reactions of the same cations with each of the stable neutrals used as radical precursors (the species RI, where R is the radical studied). Reactions with the stable neutrals proceed by charge transfer at or near the collisional rate coefficient where energetically allowed; where charge transfer is endothermic, bond-breaking/bond-making chemistry occurs. While also efficient, reactions with the radicals are more likely to occur at a smaller fraction of the collisional rate coefficient, and bond-breaking/bond-making chemistry occurs even in some cases where charge transfer is exothermic. It is noted that unlike radical reactions with neutral species, which occur with rate coefficients that are generally elevated compared to those of stable species, ion–radical reactivity is generally decreased relative to that of reactions with stable species. In particular, long-range charge transfer appears more likely to be frustrated in the ion–radical systems

Analysis of the Pressure and Temperature Dependence of the Complex-Forming Bimolecular Reaction CH₃OCH₃ + Fe⁺

The kinetics of the reaction CH₃OCH₃ + Fe⁺ has been studied between 250 and 600 K in the buffer gas He at pressures between 0.4 and 1.6 Torr. Total rate constants and branching ratios for the formation of Fe⁺O­(CH₃)₂ adducts and of Fe⁺OCH₂ + CH₄ products were determined. Quantum–chemical calculations provided the parameters required for an analysis in terms of statistical unimolecular rate theory. The analysis employed a recently developed simplified representation of the rates of complex-forming bimolecular reactions, separating association and chemical activation contributions. Satisfactory agreement between experimental results and kinetic modeling was obtained that allows for an extrapolation of the data over wide ranges of conditions. Possible reaction pathways with or without spin-inversion are discussed in relation to the kinetic modeling results

Determining Rate Constants and Mechanisms for Sequential Reactions of Fe⁺ with Ozone at 500 K

We present rate constants and product branching ratios for the reactions of FeO_<i>x</i>⁺ (<i>x</i> = 0–4) with ozone at 500 K. Fe⁺ is observed to react with ozone at the collision rate to produce FeO⁺ + O₂. The FeO⁺ in turn reacts with ozone at the collision rate to yield both Fe⁺ and FeO₂⁺ product channels. Ions up to FeO₄⁺ display similar reactivity patterns. Three-body clustering reactions with O₂ prevent us from measuring accurate rate constants at 300 K although the data do suggest that the efficiency is also high. Therefore, it is probable that little to no temperature dependence exists over this range. Implications of our measurements to the regulation of atmospheric iron and ozone are discussed. Density functional calculations on the reaction of Fe⁺ with ozone show no substantial kinetic barriers to make the FeO⁺ + O₂ product channel, which is consistent with the reaction’s efficiency. While a pathway to make FeO₂⁺ + O is also found to be barrierless, our experiments indicate no primary FeO₂⁺ formation for this reaction

The Simplest Criegee Intermediate (H₂CO–O): Isotopic Spectroscopy, Equilibrium Structure, and Possible Formation from Atmospheric Lightning

A number of research groups have recently succeeded in producing the simple carbonyl oxides H₂COO and CH₃CHOO in sufficient quantity to observe them spectroscopically and to probe the kinetics of their reactions with NO₂ and SO₂. These latter studies provide evidence that the carbonyl oxides play an important role in the atmosphere, likely contributing to pollutant removal, aerosol formation, and planetary cooling. In this work, Fourier transform microwave and double-resonance spectroscopy are combined with theory to study five isotopic species of H₂CO–O, and a precise equilibrium structure is reported for this ephemeral yet crucial reactive intermediate. In contrast to the other investigations, which have exclusively produced H₂CO–O by halogen chemistry, passing a mixture of methane and excess molecular oxygen through an electrical discharge generates this isomer of H₂CO₂ with high selectivity, thereby suggesting that the molecule is produced in the direct vicinity of atmospheric lightning

Further Insight into the Reaction FeO⁺ + H₂ → Fe⁺ + H₂O: Temperature Dependent Kinetics, Isotope Effects, and Statistical Modeling

The reactions of FeO⁺ with H₂, D₂, and HD were studied in detail from 170 to 670 K by employing a variable temperature selected ion flow tube apparatus. High level electronic structure calculations were performed and compared to previous theoretical treatments. Statistical modeling of the temperature and isotope dependent rate constants was found to reproduce all data, suggesting the reaction could be well explained by efficient crossing from the sextet to quartet surface, with a rigid near thermoneutral barrier accounting for both the inefficiency and strong negative temperature dependence of the reactions over the measured range of thermal energies. The modeling equally well reproduced earlier guided ion beam results up to translational temperatures of about 4000 K