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.     

Ab initio MO and TST calculations for the rate constant of the HNO+NO2→HONO+NO reaction

Potential-energy surfaces for various channels of the HNO+NO₂ reaction have been studied at the G2M(RCC,MP2) level. The calculations show that direct hydrogen abstraction leading to the NO+cis-HONO products should be the most significant reaction mechanism. Based on TST calculations of the rate constant, this channel is predicted to have an activation energy of 6–7 kcal/mol and an A factor of ca. 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ at ambient temperature. Direct H-abstraction giving NO+trans-HONO has a high barrier on PES and the formation of trans-HONO would rather occur by the addition/1,3-H shift mechanism via the HN(O)NO₂ intermediate or by the secondary isomerization of cis-HONO. The formation of NO+HNO₂ can take place by direct hydrogen transfer with the barrier of ca. 3 kcal/mol higher than that for the NO+cis-HONO channel. The formation of HNO₂ by oxygen abstraction is predicted to be the least significant reaction channel. The rate constant calculated in the temperature range 300–5000 K for the lowest energy path producing NO+cis-HONO gave rise to © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 729–736, 1998     

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.     

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.     

Ab initio chemical kinetics for the NH2 + HNOx Reactions,Part I: Kinetics and Mechanism for NH2 + HNO

The kinetics and mechanism for the reaction of NH₂ with HNO have been investigated by ab initio calculations with rate constant prediction. The potential energy surface of this reaction has been computed by single‐point calculations at the CCSD(T)/6‐311+G(3df, 2p) level based on geometries optimized at the CCSD/6‐311++G(d, p) level. The major products of this reaction were found to be NH₃ + NO formed by H‐abstraction via a long‐lived H₂N???HNO complex and the H₂NN(H)O radical intermediate formed by association with 26.9 kcal/mol binding energy. The rate constants for formation of primary products in the temperature range of 300–3000 K were predicted by variational transition state or RRKM theories. The predicted total rate constants at the 760 Torr Ar pressure can be represented by k_total = 3.83 × 10^?20 × T^+2.47exp(1450/T) at T = 300–600 K; 2.58 × 10^?22 × T^+3.15 exp(1831/T) cm³ molecule^?1 s^?1 at T = 600?3000 K. The branching ratios of major channels at 760 Torr Ar pressure are predicted: k₁ + k₃ + k₄ producing NH₃ + NO accounts for 0.59–0.90 at T = 300–3000 K peaking around 1000 K, k₂ accounts for 0.41–0.03 at T = 300–600 K decreasing with temperature, and k₅ accounts for 0.07–0.27 at T > 600 K increasing gradually with temperature. The NH₃ + NO formation rate constant was found to be a factor of 3–10 smaller than that of the isoelectronic reaction CH₃ + HNO producing CH₄ + NO, which has been shown to take place by barrierless H‐abstraction without involving a hydrogen‐bonding complex as in the NH₂ case. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 677–677, 2009     

The rate constant of NO + O3 → NO2 + O2 in the temperature range of 283–443 K

The reaction NO + O₃ → NO₂ + O₂ has been studied in a 220-m³ spherical stainless steel reactor under stopped-flow conditions below 0.1 mtorr total pressure. Under the conditions used, the mixing time of the reactants was negligible compared with the chemical reaction time. The pseudo-first-order decay of the chemiluminescence owing to the reaction of ozone with a large excess of nitric oxide was measured with an infrared sensitive photomultiplier. One hundred twenty-nine decays at 18 different temperatures in the range of 283–443 K were evaluated. A weighted least-squares fit to the Arrhenius equation yielded k = (4.3 ± 0.6) × 10^?12 exp[-(1598 ± 50)/T] cm³/molecule sec (two standard deviations in brackets). The Arrhenius plot showed no curvature within experimental accuracy. Comparison with recent results of Birks and co-workers, however, suggests that a nonlinear fit, as proposed by these authors, is more appropriate over an extended temperature range.     

Theoretical study of reactions of N2O with NO and OH radicals

The reactions of N₂O with NO and OH radicals have been studied using ab initio molecular orbital theory. The energetics and molecular parameters, calculated by the modified Gaussian-2 method (G2M), have been used to compute the reaction rate constants on the basis of the TST and RRKM theories. The reaction N₂O + NO → N₂ + NO₂ (1) was found to proceed by direct oxygen abstraction and to have a barrier of 47 kcal/mol. The theoretical rate constant, k₁ = 8.74 × 10⁻¹⁹ × T^2.23 exp (−23,292/T) cm³ molecule⁻¹ s⁻¹, is in close agreement with earlier estimates. The reaction of N₂O with OH at low temperatures and atmospheric pressure is slow and dominated by association, resulting in the HONNO intermediate. The calculated rate constant for 300 K ≤ T ≤ 500 K is lower by a few orders than the upper limits previously reported in the literature. At temperatures higher than 1000 K, the N₂O + OH reaction is dominated by the N₂ + O₂H channel, while the HNO + NO channel is slower by 2–3 orders of magnitude. The calculated rate constants at the temperature range of 1000–5000 K for N₂O + OH → N₂ + O₂H (2A) and N₂O + OH → HNO + NO (2B) are fitted by the following expressions: in units of cm³ molecule ⁻¹s⁻¹. Both N₂O + NO and N₂O + OH reactions are confirmed to enhance, albeit inefficiently, the N₂O decomposition by reducing its activation energy. © 1996 John Wiley & Sons, Inc.     

Investigation of the rate coefficient for O(3p) + NO2 → O2 + NO

Measurements of the rate coefficient of the reaction (O³P) + NO₂ → O₂ + NO have been made at 296°K and 240°K, using the technique of NO₂* chemiluminescent decay. Values of 9.3 × 10^?12 cm³ molec^?1 sec^?1 at 296°K and 10.5 × 10^?12 cm³ molec^?1 sec^?1 at 240°K were obtained, in excellent agreement with the recent results of Davis, Herron, and Huie [1]. The earlier lower values may have resulted from loss of NO₂ on surfaces.     

Kinetics of the reactions of NCO with NO and NO2

The rate coefficients of the reactions of NCO radicals with NO and NO₂: (1) NCO + NO → products (293–836 K) and : (2) NCO + NO₂ → products (294–774 K) were measured by means of laser photolysis and laser induced fluorescence technique in the indicated temperature ranges. NCO radicals were produced from the reaction of CN, from photodissociation of ICN or BrCN, with O₂. The concentration of NCO was monitored with a dye laser set at 414.95 nm. We determined k₁ = 1.73 × 10^?5 T^?2.01 exp(?470/T) cm³ molecule^?1 s^?1 that agrees with published results at room temperature and confirms the temperature dependence of an early report. A non-Arrhenius negative temperature dependence of k₂ was observed in this work that agrees satisfactorily with results for a shock tube¹⁸ near 1250 K. We obtained k₂ = 6.4 × 10^?10 T^?0.646 exp(164/T) cm³ molecule^?1 s^?1 for 1250 K ≥ T ≥ 294 K by combining data of these two measurements. Our result at 294 K and the temperature dependence disagree with results of two previous investigations. © 1995 John Wiley & Sons, Inc.     

Absolute Rate constants for O + No + N2 → NO2 + N2 from 217–500 K

Flash photolysis of NO coupled with time resolved detection of O via resonance fluorescence has been used to obtain rate constants for the reaction O + NO + N₂ → NO₂ + N₂ at temperatures from 217 to 500 K. The measured rate constants obey the Arrhenius equation k = (15.5 ± 2.0) × 10^?33 exp(1160 ± 70)/1.987 T] cm⁶ molecule^?2 s^?1. An equally acceptable equation describing the temperature dependence of k is k = 3.80 × 10^?27/T^1.82 cm⁶ molecule^?2 s^?1. These results are discussed and compared with previous work.     

Cyanogen pyrolysis and the CN + NO reaction behind incident shock waves

The reaction chemistry of C₂N₂? Ar and C₂N₂? NO? Ar mixtures has been investigated behind incident shock waves. Progress of the reaction was monitored by observing the cyano radical (CN) in absorption at 388.3 nm. A quantitative spectroscopic model was used to determine concentration histories of CN. From initial slopes of CN concentration during cyanogen pyrolysis, the rate constant for C₂N₂ + M → 2CN + M (1) was determined to be k₁ = (4.11 ± 1.8) × 10¹⁶ exp(?47,070 ± 1400/T) cm³/mol · s. A reaction sequence for the C₂N₂? NO system was developed, and CN profiles were computed. By comparison with experimental CN profiles the rate constant for the reaction CN + NO → NCO + N (3) was determined to be k₃ = 10^{(14.0 ± 0.3)} exp(?21,190 ± 1500/T) cm³/mol · s. In addition, the rate of the four-centered reaction CN + NO → N₂ + CO (2) was estimated to be approximately three orders of magnitude below collision frequency.     

Kinetics and mechanisms for reactions of HNO with CH3 and C6H5 studied by quantum‐chemical and statistical‐theory calculations

Kinetics and mechanisms for the reactions of HNO with CH₃ and C₆H₅ have been investigated by ab initio molecular orbital (MO) and transition‐state theory (TST) and/or Rice‐Ramsperger‐Kassel‐Marcus/Master Equation (RRKM/ME) calculations. The G2M(RCC, MP2)//B3LYP/6‐31G(d) method was employed to evaluate the energetics for construction of their potential energy surfaces and prediction of reaction rate constants. The reactions R + HNO (R = CH₃ and C₆H₅) were found to proceed by two key product channels giving (1) RH + NO and (2) RNO + H, primarily by direct abstraction and indirect association/decomposition mechanisms, respectively. As both reactions initially occur barrierlessly, their rate constants were evaluated with a canonical variational approach in our TST and RRKM/ME calculations. For practical applications, the rate constants evaluated for the atmospheric‐pressure condition are represented by modified Arrhenius equations in units of cm³ mol^?1 s^?1 for the temperature range 298–2500 K: κ_1A = 1.47 × 10¹¹ T ^0.76 exp[?175/ T ], κ_2A = 8.06 × 10³ T ^2.40 exp[?3100/ T ], κ_1B = 3.78 × 10⁵ T ^2.28 exp[230/ T ], and κ_2B = 3.79 × 10⁹ T ^1.19 exp[?4800/ T ], where A and B represent CH₃ and C₆H₅ reactions, respectively. Based on the predicted rate constant at 1 atm pressure for R + HNO → RNO + H, we estimated their reverse rate constants for R + HNO production from H + RNO in units of cm³ mol^?1 s^?1: κ_?2A′ = 7.01 × 10¹⁰ T ^0.84 exp[120/ T ] and κ_?2B′ = 2.22 × 10¹⁹ T ^?1.01 exp[?9700/ T ]. The heats of formation at 0 K for CH₃NO, CH₃N(H)O, CH₃NOH, C₆H₅N(H)O, and C₆H₅NOH have been estimated to be 18.6, 18.1, 22.5, 47.2, and 50.7 kcal mol^?1 with an estimated ±1 kcal mol^?1 error. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 261–274, 2005     

EPR kinetic study of the reactions of CF3Br with H atoms and OH radicals

Reactions of CF₃Br with H atoms and OH radicals have been studied at room temperature at 1–2 torr pressures in a discharge flow reactor coupled to an EPR spectrometer. The rate constant of the reaction H + CF₃Br → CF₃ + HBr (1) was found to be k₁ = (3.27 ± 0.34) × 10^?14 cm³/molec·sec. For the reaction of OH with CF₃Br (8) an upper limit of 1 × 10^?15 cm³/molec·sec was determined for k₈. When H atoms were in excess compared to NO₂, used to produce OH radicals, a noticeable reactivity of OH was observed as a result of the reaction OH + HBr → H₂O + Br, HBr being produced from reaction (1).     

Direct rate measurements on the reactions N + OH → NO + H And O + OH → O2 + H

Rate constants for the radical-radical reactions N + OH → NO + H (1), and O + OH → O₂ + H (2) have been measured for the first time by a direct method. In each experiment, a known concentration of N or O atoms is established in a discharge-flow system. OH radicals are then created by flash photolysis of H₂O present in the flowing gas, and the disappearance of OH is monitored by time-resolved observations of its resonance fluorescence. The experiments yield K₁ = (5.0 = 1.2) × 10^?11 cm³ molecule^?1 s^?1 and k₂ = (3.8 = 0.9) × 10^?11 cm³ molecule^?1 s^?1, for the reactions at 298 = 5 K.     

Kinetic study of reactions of C2H5O2 with NO at 298 K and 0.55 – 2 torr

The kinetics of C₂H₅O₂ and C₂H₅O₂ radicals with NO have been studied at 298 K using the discharge flow technique coupled to laser induced fluorescence (LIF) and mass spectrometry analysis. The temporal profiles of C₂H₅O were monitored by LIF. The rate constant for C₂H₅O + NO → Products (2), measured in the presence of helium, has been found to be pressure dependent: k₂ = (1.25±0.04) × 10^?11, (1.66±0.06) × 10^?11, (1.81±0.06) × 10^?11 at P (He) = 0.55, 1 and 2 torr, respectively (units are cm³ molecule^?1 s^?1). The Lindemann-Hinshelwood analysis of these rate constant data and previous high pressure measurements indicates competition between association and disproportionation channels: C₂H₅O + NO + M → C₂H₅ONO + M (2a), C₂H₅O + NO → CH₃CHO + HNO (2b). The following calculated average values were obtained for the low and high pressure limits of k_2a and for k_2b : k = (2.6±1.0) × 10^?28 cm⁶ molecule^?2 s^?1, k = (3.1±0.8) × 10^?11 cm³ molecule^?1 s^?1 and k_2b ca. 8 × 10^?12 cm³ molecule^?1 s^?1. The present value of k, obtained with He as the third body, is significantly lower than the value (2.0±1.0) × 10^?27 cm⁶ molecule^?2 s^?1 recommended in air. The rate constant for the reaction C₂H₅O₂ + NO → C₂H₅O + NO₂ (3) has been measured at 1 torr of He from the simulation of experimental C₂H₅O profiles. The value obtained for k₃ = (8.2±1.6) × 10^?12 cm³ molecule^?1 s^?1 is in good agreement with previous studies using complementary methods. © 1995 John Wiley & Sons, Inc.     

Thermal reaction of HNCO with NO2 at moderate temperatures

The thermal reaction of HNCO with NO₂ has been studied in the temperature range of 623 to 773 K by FTIR spectrometry. Major products measured are CO₂ and NO with a small amount of N₂O. Kinetic modeling of the time resolved concentration profiles of the reactants and products, aided by the thermochemical data of various likely reactive intermediates computed by means of the BAC-MP4 method, allows us to conclude that the reaction is initiated exclusively by a new bimolecular process: with a rate constant, k₁ = 2.5 × 10¹²e^?13,100/T cm³/mols. The well-known bimolecular reaction is the only strong competitive process in this important reactive system throughout the temperature range studied. Kinetic modeling of NO formation and NO₂ decay rates gave rise to values of k₁₀ which were in close agreement with literature data. © 1993 John Wiley & Sons, Inc.     

Atmospheric chemistry of FCOx radicals: Kinetic and mechanistic study of the FC(O)O2 + NO2 reaction

A pulse radiolysis system was used to study the kinetics of the reaction of FC(O)O₂ radicals with NO₂. By monitoring the rate of the decay of NO₂ using its absorption at 400 nm the reaction rate constant was determined to be (5.5 ± 0.6) × 10^?12 cm³ molecule^?1 s^?1 at 296 K and 500–1000 mbar pressure of SF₆ diluent. A long path length Fourier transform infrared spectrometer was used to investigate the thermal stability of the product FC(O)O₂NO₂. The rate of thermal decomposition of FC(O)O₂NO₂ was independent of the total pressure of N₂ diluent over the range 100–700 torr and was fit by the expression k_?3 = 6.0 × 10¹⁶ exp(?14150/T) s^?1. The results are discussed in the context of the atmospheric chemistry of FCO_x radicals. © 1995 John Wiley & Sons, Inc.     

Vibrational emission of NO2 from the reaction of NO with O3

Vibrational chemiluminescence in the Δν₁ = Δν₃ = ?1 band of NO₂ is observed both in the O + NO and O₃ + NO reactions and shown to be emitted by molecules with up to 11 000 cm^?1 of vibrational energy. Quenching rate constants of NO₂³ are estimated ranging from about 6 × 10^?14 for Ar to about 3 × 10^?12 cm³ s^?1 for NO₂. The ratio of vibrational to electronic emission is 0.06 ± 0.03 for O + NO and 5.3 ± 1.0 for O₃ + NO. It is suggested that vibrationally excited NO₂ is a major product of that channel of the O₃ + NO reaction which forms ground-state NO₂(²A₁) directly.     

Kinetics of the reactions Br + NO2 + M and I + NO2+ M

The reactions Br + NO₂ + M → BrNO₂ + M (1) and I + NO₂ + M → INO₂ + M (2) have been studied at low pressure (0.6-2.2 torr) at room temperature and with helium as the third body by the discharge-flow technique with EPR and mass spectrometric analysis of the species. The following third order rate constants were found k₁₍₀₎ = (3.7 ± 0.7) × 10^?31 and k₂₍₀₎ = (0.95 ± 0.35) × 10^?31 (units are cm⁶ molecule^?2 s^?1). The secondary reactions X + XNO₂ → X₂ + NO₂ (X = Br, I) have been studied by mass spectrometry and their rate constants have been estimated from product analysis and computer modeling.     

Kinetics of the reaction O(3P) + SO2 + M → SO3 + M over the temperature range of 299°–440°K

Rate constants for the reaction O(³P) + SO₂ + M have been determined over the temperature range of 299°–440°K, using a flash photolysis–NO₂ chemiluminescence technique. For M?Ar, the Arrhenius expression was obtained. At room temperature k₂^Ar = (1.05 ± 0.21) × 10^?33 cm⁶/molec²·sec. In addition, the rate constants k₂ = (1.37 + 0.27) × 10^?33 cm⁶/molec²·sec, k₂ = (9.5 ± 3.0) ± 10^?33 cm⁶/molec²·sec, k₃ = (1.1 ± 0.2) ± 10^?31 cm⁶/molec²·sec, and k₃ = (2.6 ? 0.9) ± 10^?31 cm⁶/molec²·sec were obtained at room temperature where k₃^M is the rate constant for the reaction O + NO + M → NO₂ + M. The rate data are compared and discussed with literature values.