Lifetimes and Quenching of the Ā 2A′(υ3′ = 0 – 2) → X̄ 2A″ Fluorescence of HSO

Fluorescence lifetimes of vibrational levels of the first electronically excited state (ā ² A′) of HSO, determined by the technique of discharge flow and laser-induced fluorescence, are 81 ± 10, 74 ± 8, and 76 ± 14 μs for υ₃′ = 0, 1 and 2, respectively. The rate coefficients of quenching of each level by He, N₂, O₂ and O₃ were measured; the coefficients for HSO (ā, υ₃′ = 0 – 2) quenched by O₃ arc 35–50 times as great as those of HSO by He.     

Temperature and third-body dependence of the rate constant for the reaction O + O2 + M → O3 + M

The flash photolysis resonance fluorescence technique was used to measure the rate constants of the reaction O + O₂ + M → O₃ + M (M = N₂, O₂, Ar, and He) as a function of temperature. The results for the rate constants are given by The activation energies with N₂, O₂, and Ar as third bodies are equal within the experimental error, (?1370 → 340 cal/mol), and the relative third-body efficiencies at 298 K for N₂, O₂, Ar, and He are 1.00, 0.99, 0.69, and 0.60, respectively.     

Kinetics of the reaction of CF3O2 with NO

The rate coefficient for the reaction of CF₃O₂ with NO has been measured at 295 K in helium using a flow tube sampled by a mass spectrometer. The value obtained for this rate coefficient was (17.8 ± 3.6) × 10^?12 cm^?3 s^?1 and found to be independent of [He] over the range (6.3 ? 16.8) × 10¹⁶ cm^?3. This value is approximately a factor of 2 higher than earlier measurements of the rate coefficients for CH₃O₂ and C₂H₅O₂ with NO and indicates that further measurements are required for this important class of reactions.     

Kinetics of the reactions of CCl3 with O and O2 and of CCl3O2 with NO at 295 K

The reactions of CCl₃ with O(³P) and O₂ and those of CCl₃O₂ with NO have been studied at 295 K using discharge flow methods with helium as the bath gas. The rate coefficient for the reaction of CCl₃ with O was found to be (4.2 ± 0.6) × 10^?11 cm³/s and that for CCl₃O₂ with NO was (18.6 ± 2.8) × 10^?12 cm³/s with both coefficients independent of [He]. For reaction between CCl₃ and O₂ the rate coefficient was found to increase from 1.51 7times; 10^?14 cm³/s to 7.88 × 10^?14 cm³/s as the [He] increased from 3.5 × 10¹⁶ cm^?3 to 2.7 × 10¹⁷ cm^?3. There was no evidence for a direct two-body reaction, and it is concluded that the only product of this reaction is CCl₃O₂. Examination of these results for CCl₃ + O₂ in terms of current simplified falloff treatment suggests that the high-pressure limit for this reaction is ～ 2.5 × 10^?12 cm³/s, which may be compared with a direct measurement of the high-pressure limit of 5 × 10^?12 cm³/s. A value of (5.8 ± 0.6) × 10^?31 cm⁶/s has been obtained for k₀, the coefficient in the low-pressure region. This value is compared with corresponding values found earlier for the (CH₃, O₂) and (CF₃, O₂) systems and with estimates based on unimolecular rate theory.     

Classical rotational and centrifugal sudden approximations for atom—molecule collisional energy transfer

The application of centrifugal and rotational sudden approximations to classical trajectory studies of rotational energy transfer in atom—molecule collisions to examined. Two different types of approximations are considered: a centrifugal sudden (CS) approximation, in which the orbital angular momentum is assumed to be constant during collisions, and a classical infinite order sudden (CIOS) approximation, in which the CS treatment is combined with an energy sudden approximation to totally decouple translational and rotational equations of motion. The treatment of both atom plus linear and nonlinear molecule collisions is described, including the use of rotational action-angle variables for the rotor equations of motion. Applications of both CS and CIOS approaches to rotational energy transfer in He + I₂ collisions are presented. We find the calculated CS and CIOS rotationally inelastic cross sections are in generally good agreement [errors of (typically) 10–50%] with accurate quasiclassical (QC) ones, with the CS results slightly more accurate than CIOS. Both methods are less accurate for small |Δj| transitions than for large |Δj| transitions. Computational savings for the CS and CIOS applications is about a factor of 3 (per trajectory) compared to QC. We also present applications using the CS method to rotational energy transfer in He, Ar, Xe + O₃ collisions, making comparisons with analogous QC results of Stace and Murrell (SM). The agreement between exact and approximate results in these applications is generally excellent, both for the average energy transfer at fixed impact parameters, and for rotationally inelastic cross sections. Results are better for He + O₃ and Ar + O₃ than for Xe + O₃, and better at low temperatures than at high. Since SM's quasiclassical treatment considered only total internal energy transfer without attempting a partitioning between vibration and rotation, while our CS calculation considers only rotational energy transfer, the observed good agreement between our and SM's cross sections indicates that most internal energy transfer in He, Ar, Xe + O₃ is rotational. The relation of this result to models of the activation process in thermal unimolecular rate constant determination is discussed.     

A further study of the near-thermoneutral reactions O2H+ + H2 ⇌ H3+ + O2

The forward and reverse rate coefficients for the reactions (1) O₂H⁺ + H2 ? H₃⁺ + O₂ and (2) O₂D⁺ + D₂ ? D₃⁺ + O₂ have been determined in a SIFT at 80 and 300 K, from which values of the enthalpy and entropy changes in the reactions have been obtained. The data indicate that the proton affinity of H₂ is greater than that of O₂ by 0.33 ± 0.04 kcal mole^?1; similary, the deuteron affinity of D₂ is 0.35 ± 0.04 kcal mole^?1 greater than that of O₂. The measurements of entropy changes confirm that O₂H⁺ has a triplet electronic ground state.     

Gas-phase reactions in plasmas of SF6 with O2 in He

Processes which occur in microwave discharges of dilute mixtures of SF₆ and O₂ in He have been examined using a flow reactor sampled by a mass spectrometer. Two classes of experiments were performed. In the first set of experiments, mixtures containing 6×10¹¹ cm^–3 SF₆, 6×10¹⁶ cm^–3 He, and O₂ in the range (0–3.6)×10¹³ cm^–3 were passed through a 20-W 2450-MHz microwave discharge. The gas mixtures arriving at a sample point downstream from the discharge were examined for SF₆, SF₄, SOF₂, SOF₄, SO₂F₂, SO₂, F, and O. In the second class of experiments, rate coefficients were measured for the reactions of SF₄ with O and O₂ and for the reaction of SF with O. The rate coefficient for the reaction of SF with O was found to be (4.2±1.5)×10^–11 cm^–3 s^–1. SF₄ was found to react so slowly with both oxygen atoms and oxygen molecules that only upper limits could be placed on the rate coefficients for these reactions. These values were 2×10^–14 cm³ s^–1 and 5×10^–15 cm³ s^–1 for reactions with O and O₂ respectively. The observed distribution of products from the discharged mixtures is discussed in terms of the measured rate coefficients.     

Association reactions of atoms and radicals

The pressure and temperature dependences of association reactions involving atoms and/or radicals is discussed and illustrated by reference to the reactions CH₃ + CH₃ → C₂H₆, CH₃ + O₂ → CH₃O₂, CH₃ + H → CH₄, and H + C₂H₄ → C₂H₅. Recent experimental measurements of the rate coefficients, k([M], T) are described, particular attention being paid to experiments designed to measure the rate coefficient over wide ranges of pressure and temperature. Methods of fitting the experimental data, to obtain estimates of the limiting rate coefficients, k⁰ and k^∞, and to permit extrapolation to regions beyond the experimental range, are discussed. These methods include the Troe factorization technique, a combination of master equation and variational RRKM theory, and recent calculations by Wagner and Wardlaw using the technique developed by Wardlaw and Marcus to describe loose transition states.     

Ab initio computation of vibrational relaxation rate coefficients for the collisions of CO2 with helium and neon atoms

Ab initio calculations of rate coefficients are reported for the vibrational relaxation of CO₂ molecules in collision with helium and neon atoms. Self consistent-field computations have been performed to parameterise simple three-dimensional potential energy functions which have been used in vibrational close-coupling, rotational infinite-order-sudden calculations of rate coefficients. Excellent agreement is obtained between the calculated and experimental rate coefficients for the deactivation of the (01₁0) vibrational level in the He + CO₂ system at temperatures of 300 K and above. The ab initio predictions of rate coefficients for relaxation of CO₂ vibrational levels such as (10⁰0) and (02⁰0) should be useful in computer simulations of CO₂ lasers.     

Drift tube investigations on the reactions of O 2 + with CH4 and of CO 2 + with NO in various buffer gases

The influence of internal excitation on the reactions of O ₂ ⁺ + CH₄ and of CO ₂ ⁺ + NO has been investigated using a slow flow drift tube. The rate coefficients for these reactions obtained as a function of relative kinetic energy in various buffer gases like He, Ne, Ar, and Kr showed higher values under conditions where the internal excitation of the reactant ions was enhanced. For both reactions the lowest reactivity at all kinetic energies was observed to occur in He, indicating that He is the least effective buffer for collisionally inducing internal excitation of molecular ions.     

Kinetics of the brominated alkyl radical (CHBr2, CH3CHBr) reactions with NO2 in the temperature range 250–480 K

The gas‐phase kinetics of CHBr₂ + NO₂ and CH₃CHBr + NO₂ reactions have been studied in direct time resolved measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals were generated by pulsed laser photolysis of bromoform and 1,1‐dibromoethane at 248 nm. The subsequent decays of the radical concentrations were monitored as a function of [NO₂] under pseudo–first‐order conditions. The rate coefficients of both reactions are independent of bath gas (He) pressure and display negative temperature dependence under the conditions of 2–6 Torr pressure (He) and 250–480 K. The obtained bimolecular rate coefficients are k(CHBr₂ + NO₂) = (9.8 ± 0.4) × 10^?12 (T/300 K)^{?1.65 ± 0.18} cm³ s^?1 (288–483 K) and k(CH₃CHBr + NO₂) = (2.27 ± 0.06) × 10^?11 (T/300 K)^{?1.28 ± 0.11} cm³ s^?1 (250–483 K), with the uncertainties given as one standard error. Estimated overall uncertainties in the measured bimolecular reaction rate coefficients are ±25%. The reaction products identified were CBr₂O for the CHBr₂ + NO₂ reaction and CHBrO and CH₃CHO with minor amounts of CH₃ for the CH₃CHBr + NO₂ reaction, respectively. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 767–777, 2012     

Rate constant for the reaction Br + O3 → BrO + O2 from 248 to 418 K: Kinetics and mechanism

The rate constant for the Br + O₃ → BrO + O₂ reaction was measured by the discharge flow technique, employing resonance fluorescence detection of Br. Over the temperature range 248 to 418 K, in 1 to 3 torr of He, decays of Br in excess O₃ yield the value k₁ = (3.28 ± 0.40) × 10^?11 e^[?944±30]/T cm³ molecule^?1 s^?1. Cited uncertainties are at the 95% confidence level and include an estimate of the systematic errors. The rate constants for the reactions of O₃ with Br, Cl, F, OH, O, and N correlate with the electron affinities of the radicals suggesting that the reactions proceed through early transition states dominated by transfer of electron density from the highest occupied molecular orbital of ozone to the singly occupied radical MO. The implications of this new measurement of k₁ for stratospheric chemistry are discussed.     

Kinetics of the CH3O2 + HO2 reaction: A temperature and pressure dependence study using chemical ionization mass spectrometry

A temperature and pressure kinetic study for the CH₃O₂ + HO₂ reaction has been performed using the turbulent flow technique with a chemical ionization mass spectrometry detection system. An Arrhenius expression was obtained for the overall rate coefficient of CH₃O₂ + HO₂ reaction: k(T) = (3.82^+2.79_?1.61) × 10^?13 exp[(?781 ± 127)/T] cm^?3 molecule^?1 s^?1. A direct quantification of the branching ratios for the O₃ and OH product channels, at pressures between 75 and 200 Torr and temperatures between 298 and 205 K, was also investigated. The atmospheric implications of considering the upper limit rate coefficients for the O₃ and OH branching channels are observed with a significant reduction of the concentration of CH₃OOH, which leads to a lower amount of methyl peroxy radical. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 571–579, 2007     

High-pressure range of the recombination O + O2 → O3

The recombination reaction O + O₂ → O₃ was studied by laser flash photolysis of pure O₂ in the pressure range 3–20 atm, and of N₂O? O₂ mixtures in the bath gases Ar, N₂, (CO₂, and SF₆) in the pressure range 3–200 atm. Fall-off curves of the reaction have been derived. Low-pressure rate coefficients were found to agree well with literature data. A high-pressure rate coefficient of k_∞ = (2.8 ± 1.0) × 10^?12 cm³ molecule^?1 s^?1 was obtained by extrapolation.     

Direct measurement of the C2H5C(O)O2 + NO reaction rate coefficient using chemical ionization mass spectrometry

The rate coefficient for the reaction of the peroxypropionyl radical (C₂H₅C(O)O₂) with NO was measured with a laminar flow reactor over the temperature range 226–406 K. The C₂H₅C(O)O₂ reactant was monitored with chemical ionization mass spectrometry. The measured rate coefficients are k(T) = (6.7 ± 1.7) × 10⁻¹² exp{(340 ± 80)/T} cm³ molecule⁻¹ s⁻¹ and k(298 K) = (2.1 ± 0.2) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. Our results are comparable to recommended rate coefficients for the analogous CH₃C(O)O₂ + NO reaction. Heterogeneous effects, pressure dependence, and concentration gradients inside the flow reactor are examined. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet: 31: 221–228, 1999     

Sensitivity and correlation analysis of the physical parameters in absorption,four-wave mixing,and Schlieren experiments

Experimental data that are used to determine rate coefficients depend not only upon reaction rates but also the physical properties of the measured species. Sensitivity coefficients are presented for the physical parameters of three general experimental techniques: a signal linearly dependent on the concentration of a species, a signal quadratically dependent on concentration, and a schlieren signal, which depends upon a bulk property of the system. With these, both the physical and chemical parameters of a model may be treated on a comparable basis. The similarities and differences between these techniques are illustrated in a simple example of radical formation via first-order precursor decomposition followed by second-order recombination. The results are then applied to two important examples: H₂ + O₂ and CH₃ + CH₃. In almost all cases, the experimental data contains more information about the physical parameters, such as the optical cross section, than the kinetic rate coefficients. Furthermore, if a physical parameter is not properly treated; strong correlations between it and rate coefficients will introduce significant systematic biases in the rate coefficient. © 1995 John Wiley & Sons, Inc.     

Gas permeation through hydrogels : II. Poly(vinylalcohol-co-itaconic acid) membranes

Permeability and diffusion coefficients of O₂, He, CO₂ and C₄H₆ were measured in water,swollen poly(vinylalcohol-co-itaconic acid) membranes having various water contents from 0.48 to 0.83. The permeability coefficients of CO₂ and C₄H₆ were found to depend on the upstream pressure, while the permeability coefficients of O₂ and He were independent of the pressure. With decreasing pressure the permeability coefficients of CO₂ and C₄H₆ increased, and the pressure dependence became larger with decreasing water content of the membranes. A parallel permeation model based on the two states of water in the water-swollen membranes could be applied successfully to CO₂ and C₄H₆.     

On the behaviour of the wave function for the 23S state of the two-electron atom in the region where both electron–nuclear distances are small

The expansion of the wave function for the 2³S state of the two-electron atom in the neighbourhood of the singularity at r₁ = r₂ = 0 is considered. The restrictions imposed on the variational functions by this expansion are discussed. For the 2³S state of He, Li⁺, N⁵⁺ the behaviour of the variational function based on the Fock expansion in the neighbourhood of this singularity is investigated. The agreement of the variational coefficients with the theoretical coefficients is satisfactory. The calculated values of E and 〈δ(r₂)〉 for He, Li⁺, N⁵⁺ are given.     

The reaction of nitric oxide with vibrationally excited ozone

The reaction between nitric oxide and vibrationally excited ozone was studied in a fast flow reactor by monitoring the visible emission from electronically excited NO₂¹. The antisymmetric mode (ν₃) of O₃ was excited with a Q-switched 9.6 μm CO₂ laser, and a laser-induced signal was detected, with a rise rate constant of (4.0 ± 0.5) × 10¹¹ cm³/mole sec and a decay rate constant of (1.1 ± 0.1) × 10¹¹ cm³/mole sec for an NO-rich mixture. The latter was unaffected by addition of large amounts of He or Ar, indicating that the signal was not a thermal effect. Most of the measurements were made at 350°K; however, the He and Ar dilution results suggest that the enhanced reaction rate is not very sensitive to temperature. In order to explain the observed rise times, it was necessary to postulate an intermediate step prior to the chemical reaction. A model which is consistent with our data has energy transferred from ν₃ to ν₂ (the bending mode) at a rate of (2.9 ± 0.5) × 10¹¹ cm³/mole sec for NO and a rate of (1.1 ± 0.2) × 10¹¹ cm³/mole sec for He. According to this model, the rate constant for the reaction of NO with O₃ (ν₂= 1) producing vibrationally excited ground state NO₂²,

$\text{NO + O}{}_3{}^2\text{(010)}\text{3}\text{NO}{}_2{}^2\text{+ O}{}_2$

is (1.5 ± 0.2) × 10¹¹ cm³/mole sec, and the relative rate for the reaction of O₃ (ν₂ = 1) and O₃(ν₂ = 0) with NO was estimated to be $\text{k}{}_3\text{(1)}\text{k}{}_3\text{(0)}\text{≈ 22}$.     

Ozone oxidation of cysteine in optically trapped aqueous micro-droplets

The ozone (O₃) oxidation kinetics of cysteine in aqueous micro-droplets at different acidities is investigated in this study via aerosol optical tweezers coupled with Raman spectroscopy. This study exploits the O₃ oxidation of cysteine near the interface of micro-droplets as a model system to elucidate the oxidation damage of amino acids in biosurfaces. For each optically trapped micro-droplet, Raman spectroscopy is used to determine its droplet radius, concentrations of solutes, and droplet pH, as well as their time evolutions during the kinetics measurements. The bimolecular rate coefficients of the cysteine + O₃ reaction measured in micro-droplets are around 4 × 10⁵ M⁻¹ s⁻¹ and 2 × 10⁴ M⁻¹ s⁻¹ for pH ≈ 5 and 0.5, respectively. These results agree with the previous bulk measurements, indicating that the observed aerosol kinetics can be solely rationalized via diffusion-limited kinetics. The results also indicate that a high-ionic strength could enhance the cysteine + O₃ reaction, particularly for the zwitterion form of cysteine. The results imply that when surfactant proteins in lung fluids are exposed to ambient O₃, the cysteine residues in proteins will be attacked by O₃ at first due to the high reactivity of the thiol moiety in cysteine.