Thermal reduction of NO by H2: Kinetic measurement and computer modeling of the HNO + NO reaction

The kinetics and mechanism of the thermal reduction of NO by H₂ have been investigated by FTIR spectrometry in the temperature range of 900 to 1225 K at a constant pressure of 700 torr using mixtures of varying NO/H₂ ratios. In about half of our experimental runs, CO was introduced to capture the OH radical formed in the system with the well-known, fast reaction, OH + CO → H + CO₂. The rates of NO decay and CO₂ formation were kinetically modeled to extract the rate constant for the rate-controlling step, (2) HNO + NO → N₂O + OH. Combining the modeled values with those from the computer simulation of earlier kinetic data reported by Hinshelwood and co-workers (refs. [3] and [4]), Graven (ref.[5]), and Kaufman and Decker (ref. [6]) gives rise to the following expression: . This encompasses 45 data points and covers the temperature range of 900 to 1425 K. RRKM calculations based on the latest ab initio MO results indicate that the reaction is controlled by the addition/stabilization processes forming the HN(O)NO intermediate at low temperatures and by the addition/isomerization/decomposition processes producing N₂O + OH above 900 K. The calculated value of k₂ agrees satisfactorily with the experimental result. © 1995 John Wiley & Sons, Inc.     

Kinetics and mechanism of the reaction H + CH3ONO

The reaction of hydrogen atoms with methyl nitrite was studied in a fast-flow system using photoionization mass spectrometry and excess atomic hydrogen. The associated bimolecular rate coefficient can be expressed by in the temperature range of 223-398°K. NO, CH₃OH, CH₄, C₂H₆, CH₂O, and H₂O are the main products; OH and CH₃ radicals were detectable intermediates. The mechanism was deduced from the observed product yields using normal and deuterated reactants. The primary reaction steps were identified as followed by a rapid unimolecular decomposition of CH₂ONO into CH₂O and NO. Since the extent of reaction channel (1b) could not be determined independently, only extreme limits could be obtained for the individual contributions of the two channels of reaction (3) which follows the generation of CH₃O radicals: The most probable values, k_3a/k₃ = 0.31 ± 0.30 and k_3b/k₃ = 0.69 ± 0.30, support the previous results on this reaction, although the range of uncertainties is much greater here.     

The gas-phase decomposition of methylsilane. Part I. Mechanism of decomposition under shock-tube conditions

The gas-phase decompositions of methylsilane and methylsilane-d₃ have been investigated in a single-pulse shock tube at 4700 torr total pressure in the temperature range of 1125–1250 K. For CH₃SiD₃ at 1200 K three primary steps occur in the homogeneous decomposition with efficiencies in parentheses: , , and . For CH₃SiH₃ at 1200 K the primary CH₄ elimination efficiency is 0.09 while the total primary H₂ elimination efficiency is 0.91. Minor product formations of C₂H₄, acetylene, dimethylsilane, and SiH₄ are discussed.     

Stable γ-distonic oxonium ions: R\documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm O}\limits^ + $\end{document}HCHR′ĊH2CHR″ and their characteristic reaction

The γ-distonic radical ions R$ \mathop {\rm O}\limits^ + $CHR′CH₂?HR″ and their molecular ion counterparts R$ \mathop {\rm O}\limits^{{\rm + } \cdot } $CHR′CH₂CH₂R″ have been studied by isotopic labelling and collision-induced dissociation, applying a potential to the collision cell in order to separate activated from spontaneous decompositions. The stability of CH₃$ \mathop {\rm O}\limits^ + $HCH(CH₃)CH₂?HCH₃, C₂H₅$ \mathop {\rm O}\limits^ + $HCH(CH₃)CH₂?HCH₃, CH₃$ \mathop {\rm O}\limits^ + $HCH(CH₃)CH₂?H₂, CH₃$ \mathop {\rm O}\limits^ + $HCH₂CH₂?HCH₃ and C₂H₅$ \mathop {\rm O}\limits^ + $HCH₂CH₂?HCH₃, has been demonstrated and their characteristic decomposition, alcohol loss, identified. For all these γ-distonic ions, the 1,4-H abstraction leading to their molecular ion counterpart exhibits a primary isotope effect.     

Thermal stability of primary amines

Tertiary-amyl amine has been decomposed in single-pulse shock-tube experiments. Rate expressions for several of the important primary steps are This leads to D(CH₃? H) – D(NH₂? H) = ?10.5 kJ and D[(CH₃)₃C? H] – D[(CH₃)₂NH₂C? H] = + 6 kJ. The present and earlier comparative rate single-pulse shock-tube data when combined with high-pressure hydrazine decomposition results-(after correcting for fall off effects through RRKM calculations) gives where k_r(…) is the recombination rate involving the appropriate radicals. This suggests that in this context amino radical behavior is analogous to that of alkyl radicals. If this agreement is exact, then Rate expressions for the primary step in the decomposition of a variety of primary amines have been computed. In the case of benzyl amine where data exist the agreement is satisfactory. The following differences in bond energies have been estimated:      

Kinetics and mechanism of the thermal gas-phase reaction between NO2 and trichloroethene at 303–362.2 K

The kinetics of the gas-phase reaction between NO₂ and trichloroethene has been investigated in the temperature range 303–362.2 K. The pressure of NO₂ was varied betwen 5.1 and 48.7 torr and that of trichloroethene between 7.3 and 69.5 torr. The reaction was homogeneous. Two products were formed: nitrosyl chloride, ClNO, and glyoxyloxyl chloride, HC[O]C[O]Cl, which was identified by its infrared spectrum and its molecular weight determined by chromatography. The rate of consumption of the reactants was independent of the total pressure and can be represented by a second-order reaction: The following mechanism was proposed to explain the experimental results: The following expression was obtained for k: . © John Wiley & Sons, Inc.     

The reaction of O(1D) with CH4

The room-temperature photolysis of N₂O (10–100 torr) at 2139 Å to produce O(¹D) has been studied in the presence of CH₄ (10–891 torr). The reactions of O(¹D) with CH₄ were found to be The method of chemical difference was used to measure the rate constant ratio k₄/(k₂ + k₃), where reactions (2) and (3) are The CH₃ radicals produced in reaction (4) react with the O₂ and NO produced in reactions (2) and (3). Thus, near the endpoint of the internal titration, ?{C₂H₆} gives an accurate measure of k₄/(k₂ + k₃). For the translationally energetic O(¹D) atoms produced in the photolysis, k₄/(k₂ + k₃) = 2.28 ± 0.20. However, if He is added to remove the excess translational energy, then k₄/(k₂ + k₃) drops to 1.35 ± 0.3.     

The reactions of ozone with methyl and ethyl nitrites

The reactions of O₃ with CH₃ONO and C₂H₅ONO were studied using infrared absorption spectroscopy in a static reactor at temperatures between 298 and 352K. Both reactions followed simple second-order kinetics forming the corresponding nitrate: The rate coefficients are given by .     

High-temperature determination of the rate coefficient for the reaction H2 + CN → H + HCN

The rate coefficient of the reaction has been determined in the temperature range of 2700–3500 K using a shock tube technique. C₂N₂? H₂? Ar mixtures were heated behind incident shock waves and the early-time CN history was monitored using broad-band absorption spectroscopy. The rate coefficient providing the best fit to the data was in good agreement with extrapolations of previously published low-temperature results.     

A shock tube study of the CH2O + NO2 reaction at high temperatures

The reaction of CH₂O with NO₂ has been studied with a shock tube equipped with two stabilized ew CO lasers. The production of CO, NO, and H₂O has been monitored with the CO lasers in the temperature range of 1140–1650 K using three different Ar-diluted CH₂O-NO₂ mixtures. Kinetic modeling and sensitivity analysis of the observed CO, NO, and H₂O production profiles over the entire range of reaction conditions employed indicate that the bimolecular metathetical reaction, NO₂ + CH₂O → HONO + CHO (1) affects most strongly the yields of these products. Combination of the kinetically modeled values of ??₁ with those obtained recently from a low temperature pyrolytic study, ref. [8], leads to for the broad temperature range of 300–2000 K.     

The Competitive dehydrohalogenation of 1,1,1-trifluoro-2-chloroethane in reflected shock waves

The thermal decomposition of 1,1,1-trifluoro-2-chloroethane has been investigated in the single-pulse shock tube between 1120° and 1300deg;K at total reflected shock pressures from ～2610 to 3350 torr. Under these conditions, the major reaction is the α,α-elimination of hydrogen chloride, with The decomposition also involves the slower α,β-elimination of hydrogen fluoride, with the first-order rate constant given by At temperatures above 1270°K, two additional minor products were observed. These were identified as CF₂CFCl and CF₃CHCl₂ and suggest C? Cl rupture as a third reaction channel leading to complicated kinetics.     

Quenching of N2(A3Σu+) by O2, O,N, and H

Metastable N₂(A³Σ_u⁺), υ = 0, υ = 1, molecules are produced by a pulsed Tesla-type discharge of a dilute N₂/Ar gas mixture. Rate coefficients for quenching these metastable levels by O₂, O, N, and H were obtained by time-resolved emission measurements of the (0, 6) and (1, 5) Vegard–Kaplan bands. In units of cm³/mole · sec at 300°K and with an experimental uncertainty of ±20%, these rate coefficients for N₂(A³Σu⁺) are Within the limits of error these coefficients apply to quenching N₂(A³Σ_u⁺) υ′ = 1 as well.     

High temperature determination of the rate coefficient for the reaction H2O + CN → HCN + OH

The rate coefficient, k, of the reaction has been determined in the temperature range 2460–2840 K using a shock tube technique. C₂N₂? H₂O? Ar mixtures were heated behind incident shock waves and the CN and OH concentration time histories were monitored simultaneously using broad-band absorption near 388 nm (CN) and narrow-line laser absorption at 306.67 nm (OH). The rate coefficient expression providing the best fit to the data was with uncertainty limits of about ±45% in the temperature range 2460–2840 K. The rate coefficient of the reverse reaction was calculated using detailed balancing, and its extrapolation to lower temperatures was compared with previously published results.     

Reaction of O(1D) with N2O

O(¹D), produced from the photolysis of N₂O at 2139 Å, reacts with N₂O in accord with: We have used the method of chemical difference to obtain an accurate measure of k₂/k₃ = 0.59 ± 0.01. Furthermore, the quantum yield of production of O(³P), either on direct photolysis or on deactivation of O(¹D) by N₂O, is less than 0.02 and probably zero.     

Kinetics and mechanism of the reaction between N-and N,N′-methyl ethylenediamines and palladium(II) chloro complexes in aqueous acid medium

The substitution of N-alkyl substituted ethylenediamines for chloride ions in the rapidly equilibrating system has been investigated in aqueous acid medium. The kinetic data can be accommodated by the general rate law where n = 0, 1, or 2 and m = 0, 1, or 2, depending on whether none, one, or two methyl groups are attached to the two nitrogen atoms of ethylenediamine. Reaction with the most heavily substituted ethylenediamine, namely, N₂N₂en discloses a change of the mentioned rate law to on going from a lower to a higher chloride ion concentration range. This change in the mathematical form of the rate law can be explained in terms of an ion-pair association of N₂N₂enH⁺ and free chloride ions.     

HNO,an Intermediate in (Light-induced) Rearrangement Reactions of Nitrosooxy Compounds and Nitrosamines

IR-spectroscopic investigations of light-induced rearrangement reactions of nitrosooxymethane (CH³ONO³), nitrosooxyethane (CH₃CH₂ONO) and N,N-dimethylnitrosamine ((CH₃)₂NNO) in low-temperature rare-gas matrices have established that these molecules are transformed in two photolysis steps to the previously unknown C-nitroso compounds nitrosomethanol (CH₂(OH)(NO)), 1-nitrosoethanol (CH₃CH(OH)(NO)), and methyl(nitrosomethyl)amine CH₂(NO)(NH)(CH₃). Evidence for a similar rearrangement reaction has been advanced for N-Nitrosopyrrolidine which is converted to C-nitrosopyrrolidine . The matrix-isolation technique in combination with wavelength-selective irradiation allowed to trap and characterize an intermediate of rearrangement which revealed to be nitroxyl (HNO) complex (CH₂…HNO, CH₃CHO…HNO, CH₃N = CH₂…HNO, and ). Since these findings have a close resemblance with rearrangement reactions of more complex nitrosooxy compounds, nitrosamines, or nitrosohydrazines used in organic synthesis, it is suggested that also in these reactions nitroxyl is present as an intermediate species.     

A reinvestigation of the temperature dependence of the rate constant for the reaction O + O2 + M → O3 + M (for M = O2, N2, and Ar) by the flash photolysis resonance fluorescence technique

The flash photolysis resonance fluorescence technique has been used to reinvestigate the kinetics of the oxygen atom–oxygen molecule combination reaction. Third-order rate constants for O₂, N₂, and Ar as deactivant molecules were determined over the temperature range of 219–368 K. The results presented herein are the most extensive data sets available for atmospheric modeling and are used to formulate a recommendation for such purposes. The recommended rate expressions are or Comparisons of these results with existing literature data are presented.     

Thermal stability of intermediate sized acetylenic compounds and the heats of formation of propargyl radicals

4-Methylhexyne-1, 5-methylhexyne-1, hexyne-1, and 6-methylheptyne-2 have been decomposed in comparative-rate single-pulse shock-tube experiments. Rate expressions for the initial decomposition reactions at 1100°K and from 2 to 6 atm pressure are In combination with previous results, rate expressions for propargyl C? C bond cleavage are related to that for the alkanes by the expression These results yield a propargyl resonance energy of D(nC₃H₇-H) – D(C₃H₃-H) = 36 ± 2 kJ, in excellent agreement with a previous shock-tube study. They also lead to D(CH₃C≡CCH₂-H) – D(C₃H₃-H) = 0.6 ± 3 kJ, D(sC₄H₉-H) – D(iC₃H₇-H) = 0 ± 3 kJ, D(iC₄H₉-H) – D(nC₃H₇-H) = 2 ± 3 kJ, and D(nC₃H₇-H) – D(iC₃H₇-H) = 13.9 ± 3 kJ (all values are for 300°K). The systematics of the molecular decomposition process are explored.     

Pyrolysis of 4,4-bisperfluoromethylbutene-1,4 01 in the gas phase

Pyrolysis of (CF₃)₂C(OH)CH₂CH=CH₂, the reverse of the reaction between perfluoroacetone and propene, has been studied in the gas phase between 475° and 598°K. Even at 573°K, the unimolecular reaction rate constant appears to be in its pressure-independent region at 20.0 torr pressure. In a quartz vessel, the decomposition is homogeneous. The specific unimolecular rate constant is where the limits are for one standard deviation. Combining these results with the previously reported results on the reverse reaction, the equilibrium constant for the reaction is It is noteworthy that in the temperature range of the study of the forward reaction (448° to 573°K), the percentage of back reaction in the times of the experiments varies from less than 0.1 to 1.5. Using group additivities and the above ΔH⁰, ΔH of (CF₃)₂CO is calculated to be ?325.2 kcal/mole at 600°K and the average C? C bond is 42.0 kcal/mole.     

Cluster ions in the secondary ion mass spectrometry of choline and acetylcholine halides

Liquid secondary ion mass spectra of choline and acetylcholine halides exhibit several series of cluster ions whose origins were investigated using B/E and B²/E linked-scan techniques. In the case of choline halides three series of cluster ions were identified as (Me₃$ \mathop {\rm N}\limits^ + $CH₂CH₂OH + nM), (Me₃$ \mathop {\rm N}\limits^ + $CH₂CH₂OMe + nM) and (Me₃N$ \mathop {\rm N}\limits^ + $CH₂CH₂OH · Me₃$ \mathop {\rm N}\limits^ + $CH₂CH₂O^? + nM), while (CH₃COOCH₂CH₂$ \mathop {\rm N}\limits^ + $Me₃ + nM), (Me₃$ \mathop {\rm N}\limits^ + $CH₂CH₂OH + nM) and (CH₂ = CH$ \mathop {\rm N}\limits^ + $Me₃ + nM) were observed in the spectra of acetylcholine halides. For these cluster ions, bimolecular reactions induced on ion bombardment under secondary ion mass spectrometric conditions are discussed.