Optical detection of NO3 and NO2 in “pure” HNO3 vapor,the liquid-phase decomposition of HNO3

Flowing and static gas-phase samples of HNO₃ in O₂ and N₂ were analyzed by long-path ultraviolet/visible (UV/VIS) spectroscopy to reveal the presence of both NO₂ and NO₃, the concentrations of which were calculated using differential absorption cross sections. NO₂ is produced predominantly by the heterogeneous decomposition of HNO₃, whereas NO₃ is generated in the gas phase by the thermal decomposition of N₂O₅, a product of the self-disproportionation of liquid HNO₃. © 1993 John Wiley & Sons, Inc.     

Gas-phase reaction of NO3 radicals with isoprene: a kinetic and mechanistic study

The gas-phase reaction of NO₃ radicals with isoprene was investigated under flow conditions at 298 K in the pressure range 6.8<P(mbar)<100 using GC-MS/FID, direct MS, and long-path FT–IR spectroscopy as detection techniques. By means of a relative rate method, the rate constant for the primary attack of NO₃ radicals toward isoprene was determined to be (6.86±2.60)×10⁻¹³ cm³ molecule⁻¹ s⁻¹. The formation of the possible oxiranes, 2-methyl-2-vinyl-oxirane and 2-(1-methyl-vinyl)-oxirane, was observed in dependence on total pressure. In the presence of O₂ in the carrier gas, the product distribution was found to be strongly dependent on the reaction pathways of formed peroxy radicals. If the peroxy radicals mainly reacted in a self-reaction, the formation of organic nitrates was detected and 4-nitroxy-3-methyl-but-2-enal was identified as a main product. On the other hand, when NO was added to the gas mixture and the peroxy radicals were converted via RO₂+NO→RO+NO₂, the formation of methyl vinyl ketone as the main product as well as 3-methylfuran and meth-acrolein was observed. From the ratio of the product yields if NO was added to the gas mixture it was concluded that the attack of NO₃ radicals predominantly takes place in the 1-position. A reaction mechanism is proposed and the application of these results to the troposphere are discussed. © 1997 John Wiley & Sons, Inc.     

Kinetics study of the reaction HO + HNO3

The reaction has been studied using a discharge-flow tube with resonant fluorescence detection of HO. Measurements of k₁ have been made at temperatures between 237 and 404 K. Our results and earlier work suggest that the rate constant has a minimum at T ? 500 K. Below room temperature our results are given by the expression k = (2.0 ± 0.4) × 10^?14 exp[(430 ± 60)/T] cm³ molecule^?1 s^?1, where the error limits include an estimate of the measurement accuracy. No dependence of k₁ on pressure at 300 K is observed in studies between 1- and 10-torr helium. An upper limit of k_b < 1% k₁ at 300 K is given.     

Generation of HNO2 by bubbling NO in HNO3 system and its application in the PUREX process

This work is aimed at studying HNO₂ generation by bubbling NO in HNO₃ solution. The formation and decomposition of HNO₂ depend on the 2NO+HNO₃+H₂O3HNO₂ reaction. During the HNO₂ generation, the HNO₃ concentration was kept constant. The influence of HNO₃ concentration, temperature, NaNO₃ concentration and NO bubbling rate was studied. Its possible application in the PUREX process was evaluated.     

A kinetic study of S-nitrosothiol decomposition

Under anaerobic conditions S-nitrosothiols 1a-e undergo thermal decomposition by homolytic cleavage of the S-N bond; the reaction leads to nitric oxide and sulfanyl radicals formed in a reversible manner. The rate constants, k(t), have been determined at different temperatures from kinetic measurements performed in refluxing alkane solvents. The tertiary nitrosothiols 1c (k1(69 degrees C) = 13 x 10(-3) min(-1)) and 1d (k1(69 degrees C) = 91 x 10(-3) min(-1)) decomposed faster than the primary nitrosothiols 1a (k1(69 degrees C) = 3.0 x 10(-3) min(-1)) and 1b (k1(69 degrees C) = 6.5 x 10(-3) min(-1)). The activation energies (E# = 20.5-22.8 Kcal mol(-1)) have been calculated from the Arrhenius equation. Under aerobic conditions the decay of S-nitrosothiols 1a-e takes place by an autocatalytic chain-decomposition process catalyzed by N2O3. The latter is formed by reaction of dioxygen with endogenous and/or exogenous nitric oxide. The autocatalytic decomposition is strongly inhibited by removing the endogenous nitric oxide or by the presence of antioxidants, such as p-cresol, beta-styrene, and BHT. The rate of the chain reaction is independent of the RSNO concentration and decreases with increasing bulkiness of the alkyl group; this shows that steric effects are crucial in the propagation step.     

A kinetic study of the decomposition of N-bromoserine

This article reports the kinetics of the decomposition of N-bromoserine formed rapidly by bromation of serine by BrO^?. The main decomposition products are glycolaldehyde, ammonia, carbon dioxide, and bromide ions at pH < 11.5, and β-hydroxypyruvic acid, ammonia, and bromide ions at pH > 11.5. The reaction is of order one with respect to N-bromoserine, and is independent of ionic strength and excess serine. The rate constant increases with increasing pH at pH > 11 and with decreasing pH at pH < 8, and over the range pH 8–11 has the constant value 1.67 × 10^?3 s^?1 at 298 K.     

A computational study of the kinetics and mechanism for the reaction of HCO with HNO

The mechanism for the reaction of HCO with HNO has been studied at the G2M level of theory, based on the geometric parameters optimized by the BH&HLYP/6‐311G(d, p) method. There are three direct hydrogen abstraction channels producing (1) H₂CO + NO, (2) H₂NO + CO, and (3) HNOH + CO with barriers of 3.7, 3.9, and 10.4 kcal/mol, respectively. Another important reaction channel, (4), involves an association process forming HN(O)CHO (LM1) with a very small barrier and the subsequent isomerization and decomposition of LM1 producing HNOH + CO as major products. The rate constants of the dominant reaction channels (1), (2), and (4) in the temperature range 200–3000 K have been predicted by the microcanonical RRKM and transition state theory calculations with Eckart tunneling corrections. The theoretical result shows that in the high temperature range ( T > 1500 K), k₁ (H₂CO + NO) and k₂(H₂NO + CO) are preponderant, while in the low temperature range, both k₄(LM1) and k₄(HNOH + CO) appear to be dominant at high and low pressures, respectively. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 205–215, 2004     

A kinetic study of the reaction of O(3P) with toluene

The absolute rate constant of the reaction of O(³P) with toluene was measured by the microwave discharge-fast-flow method to obtain k_T = 10^{9.7–2.7/2.303RT} l./mole·sec at 100–375°C. This was in good agreement with the rate constant calculated from the combination of the relative rate constant obtained by Jones and Cvetanovic with the recently determined absolute rate constants of the reaction of O(³P) + olefins. The extrapolation of the above Arrhenius plot to 27°C was also in good agreement with the absolute value of k_T = 4.5 × 10⁷ l./mole·sec determined recently by Atkinson and Pitts at 27°C. The rate constant of the reaction of chlorobenzene with O(³P), obtained at 238°C as 10^8.3 l./mole·sec by a competitive method, was smaller than k_T by a factor of about two at the same temperature.     

Effects of nitric oxide on the thermal decomposition of methyl nitrite: Overall kinetics and rate constants for the HNO + HNO and HNO + 2NO reactions

The effects of NO on the decomposition of CH₃ONO have been investigated in the temperature range 450–520 K at a constant pressure of 710 torr using He as buffer gas. The measured time-dependent concentration profiles of CH₃ONO, NO, N₂O, and CH₂O can be quantitatively accounted for with a general mechanism consisting of various reactions of CH₃O, HNO, and (HNO)₂. The results of kinetic modeling with sensitivity analyses indicate that the disappearance rate of CH₃ONO is weakly affected by NO addition, whereas that of the HNO intermediate strongly altered by the added NO. In the presence of low NO concentrations, the modeling of N₂O yields leads to the rate constant for the bimolecular reaction, HNO + HNO → N₂O + H₂O (25): In the presence of high NO concentrations (P_NO > 50 torr), the modeling of CH₂O yields gives the rate constant for the termolecular radical formation channel, HNO + 2NO → HN₂O + NO₂ (35): Discussion on the mechanisms for reactions (25) and (35), and the alkyl homolog of (35), RNO + 2NO, is presented herein. © John Wiley & Sons, Inc.     

A mechanistic and kinetic study on the decomposition of morpholine

The combustion chemistry of morpholine (C(4)H(8)ONH) has been experimentally investigated recently as a representative model compound for O- and N-containing structural entities in biomass. Detailed profiles of species indicate the self-breakdown reactions prevailing over oxidative decomposition reactions. In this study, we derive thermodynamic and kinetic properties pertinent to all plausible reactions involved in the self-decomposition of morpholine and its derived morphyl radicals as a crucial task in the development of comprehensive combustion mechanism. Potential energy surfaces have been mapped out for the decomposition of morpholine and the three morphyl radicals. RRKM-based calculations predict the self-decomposition of morpholine to be dominated by 1,3-intramolecular hydrogen shift into the NH group at all temperatures and pressures. Self-decomposition of morpholine is shown to provide pathways for the formation of the experimentally detected products such as ethenol and ethenamine. Energetic requirements of all self-decomposition of morphyl radicals are predicted to be of modest values (i.e., 20-40 kcal/mol) which in turn support the occurrence of breaking-down reactions into two-heavy-atom species and the generation of doubly unsaturated four-heavy-atom segments. Calculated thermochemical parameters (in terms of standard enthalpies of formation, standard entropies, and heat capacities) and kinetic parameters (in terms of reaction rate constants at a high pressure limit) should be instrumental in building a robust kinetic model for the oxidation of morpholine.     

Study of the HNO + HNO and HNO + NO reactions by intracavity laser spectroscopy

Flash photolysis of CH₃CHO and H₂CO in the presence of NO has been investigated by the intracavity laser spectroscopy technique. The decay of HNO formed by the reaction HCO + NO → HNO + CO was studied at NO pressures of 6.8–380 torr. At low NO pressure HNO was found to decay by the reaction HNO + HNO → N₂O + H₂O. The rate constant of this reaction was determined to be k₁ = (1.5 ± 0.8) × 10^?15 cm³/s. At high NO pressure the reaction HNO + NO → products was more important, and its rate constant was measured to be k₂ = (5 ± 1.5) × 10^?19 cm³/s. NO₂ was detected as one of the products of this reaction. Alternative mechanisms for this reaction are discussed.     

Infrared signatures of HNO3 and NO3(-) at a model aqueous surface. A theoretical study

The infrared signatures of nitric acid HNO3 and its conjugate anion NO3(-) at the surface of an aqueous layer are derived from electronic structure calculations at the HF/SBK+* level of theory on the HNO3 x (H2O)3 --> NO3(-) x H3O(+) x (H2O)2 model reaction system embedded in clusters comprising 33, 40, 45, and 50 classical, polarizable waters, mimicking various degrees of solvation [Bianco, R.; Wang, S.; Hynes, J. T. J. Phys. Chem. A 2007, 111, 11033]. The molecular level character of the various bands is discussed, and the solvation patterns are described in terms of hydrogen bonding and resulting polarization of the species' intramolecular bonds. Connection is made with assorted experimental results, including surface-sensitive Sum Frequency Generation spectroscopy of aqueous nitric acid solutions, infrared spectroscopy of amorphous thin films of nitric acid monohydrate (NAM) and dihydrate (NAD), and infrared and Raman spectroscopic results for bulk aqueous solutions of nitric acid and nitrate salts.     

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.     

Theoretical interpretation of the kinetics and mechanisms of the HNO + HNO and HNO + 2NO reactions with a unified model

Previously measured decay rates of HNO in the presence of NO have been kinetically modeled on the basis of thermochemical data calculated with the BAC-MP4 technique. The results of this modeling, aided by TST-RRKM calculations for the association of HNO and the isomerization, decomposition, and stabilization of the many dimers of HNO, reveal that the decay of HNO under NO-lean conditions occurs primarily by association forming cis- and trans-(HNO)₂ at temperatures below 420 K. N₂O, which is a relatively minor product, is believed to be formed by H₂O elimination from cis-HON ? NOH, a product of succesive isomerization reactions: trans-(HNO)₂^? → HN(OH)NO^? → HN(O)NOH^? → cis-HON NOH^?. The calculated rate constants, which fit experimental data quantitatively, can be represented by k = 10^16.2 × T^?2.40e^?590/T cm³/mol sec for the HNO recombination reaction and k = 10^?2.44T^3.98e^?600/T cm³/mol sec for N₂O formation in the temperature range 80–420 K, at a total pressure of 710 torr H₂ or He. Under NO-rich conditions, HNO reacts predominantly by the exothermic termolecular reaction, HNO + 2NO → HN(NO)ONO → HN NO + NO₂, with a rate contant of (6 ± 1) × 10⁹ cm⁶/mol² sec at room temperature, based on both HNO decay and NO₂ production. All existing thermal kinetic data on HNO + HNO and HNO + 2NO processes can be satisfactorily rationalized with a unified model based on the thermochemical data obtained by BAC-MP4 calculations.     

A kinetic study of the reaction of substituted benzenethiols with pertechnetate

The kinetics of the reaction of pertechnetate with a series of para-substituted benzenethiols have been studied. The reaction follows simple second order kinetics. The rate of reaction decreases as the substituent becomes more electron-withdrawing. The kinetic data suggest that the reaction involves nucleophilic attack of a benzenethiol molecule on technetium.

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Temperature-dependent rate constant for the reaction of F atoms with HNO3

The kinetics of the reaction of F atom with HNO₃, source of NO₃ radicals widely used in laboratory studies, has been investigated at nearly 2.7 mbar total pressure of helium over a wide temperature range, T = 220-700 K, using a low-pressure discharge flow reactor combined with an electron impact ionization quadrupole mass spectrometer. The rate constant of the reaction F + HNO₃ → NO₃ + HF (1) was determined using both relative rate method and absolute measurements under pseudo–first-order conditions, monitoring the kinetics of F-atom consumption in excess of HNO₃, k₁ = (8.2 ± 0.4) × 10⁻¹² exp((315 ± 15)/T) cm³ molecule⁻¹ s⁻¹ (where the uncertainties represent precision at the 2σ level, the estimated total uncertainty on k₁ being 15% at all temperatures). The reaction rate constant was found to be in excellent agreement with the only previous temperature-dependent study. Experiments on detection of the reaction product, HF, have shown that NO₃ and HF forming channel of the title reaction is the dominant, if not unique, on the whole temperature range of the study.     

A kinetics study of the reaction of Cl with NO2

The third order rate coefficients for the addition reaction of Cl with NO₂, Cl + NO₂ + M → ClNO₂ (ClONO) + M; k₁, were measured to be k₁(He) = (7.5 ± 1.1) × 10^?31 cm⁶ molecule^?2 s^?1 and k₁(N₂) = (16.6 ± 3.0) × 10^?31 cm⁶ molecule^?2 s^?1 at 298 K using the flash photolysis-resonance fluorescence method. The pressure range of the study was 15 to 500 torr He and 19 to 200 torr N₂. The temperature dependence of the third order rate coefficients were also measured between 240 and 350 K. The 298 K results are compared with those from previous low pressure studies.     

The heats of formation of NO3− and NO3− association complexes with HNO3 and HBr

The equilibrium constant for the reaction has been determined between 331 and 480°K using a variable-temperature flowing afterglow. These data give ΔH°(1) = -1.03 ± 0.21 kcal/mol and ΔS°(1) = —4.6 ± 1.0 cal/mol°K. When combined with the known thermochemical values for HBr, Br^?, and HNO₃, this yields ΔH(NO₃^?) = -74.81 ± 0.54 kcal/mol and S(NO₃^?) = 59.4 cal/mol·°K. In addition ΔH_n-1,n and ΔS_n-1,nfor the gas-phase reactions were determined for n = 2 and 3. The implications of these measurements to gas-phase negative ion chemistry are discussed.     

A kinetic and product study of reaction of chlorine atom with CH3CH2OD

The reaction of atomic chlorine with CH₃CH₂OD has been examined using a discharge fast flow system coupled to a mass spectrometer combined with the relative rate method (RR/DF/MS). At 298 ± 2 K, the rate constant for the Cl + CH₃CH₂OD reaction was determined using cyclohexane as a reference and found to be k₃ = (1.13 ± 0.21) × 10^?10 cm³ molecule^?1 s^?1. Mass spectral studies of the reaction products resulted in yields greater than 97% for the combined hydrogen abstraction at the α and β sites (3a + 3b) and less than 3% at the hydroxyl site (3c). As a calibration of the apparatus and the RR/DF/MS technique, the rate constant of the Cl + CH₃CH₂OH reaction was also determined using cyclohexane as the reference, and a value of k₂ = (1.05 ± 0.07) × 10^?10 cm³ molecule^?1 s^?1 was obtained at 298 ± 2 K, which was in excellent agreement with the value given in current literature. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 584–590, 2004