Direct kinetic studies of the reactions of 2-butoxy radicals with NO and O2

This Letter reports the first kinetic study of 2-butoxy radicals to employ direct monitoring of the radical. The reactions of 2-butoxy with O₂ and NO are investigated using laser-induced fluorescence (LIF). The Arrhenius expressions for the reactions of 2-butoxy with NO (k₁) and O₂ (k₂) in the temperature range 223–311 K have been determined to be k₁=(7.50±1.69)×10⁻¹²×exp((2.98±0.47) kJmol⁻¹/RT) cm³ molecule⁻¹ s⁻¹ and k₂=(1.33±0.43)×10⁻¹⁵×exp((5.48±0.69) kJmol⁻¹/RT) cm³ molecule⁻¹ s⁻¹. No pressure dependence was found for the rate constants of the reaction of 2-butoxy with NO at 223 K between 50 and 175 Torr.     

Experimental and theoretical investigation of the reaction CF3 + NO + M → CF3NO + M from 0.5 to 12 Torr and from 253 to 373 K

The kinetics of the association reaction of CF₃ with NO was studied as a function of temperature near the low-pressure limit, using pulsed laser photolysis and time-resolved mass spectrometry. CF₃ radicals were generated by photolysis of CF₃I at 248 nm and the kinetics was determined by monitoring the time-resolved formation of CF₃NO. The bimolecular rate constants were measured from 0.5 to 12 Torr, using nitrogen as the buffer gas. The results are in very good agreement with recent data published by Vakhtin and Petrov, obtained at room temperature in a higher pressure range and, therefore, the two studies are quite complementary. A RRKM model was developed for fitting all the data, including those of Vakhtin and Petrov and for extrapolating the experimental results to the low- and high-pressure limits. The rate expressions obtained are the following: k₁(0) = (3.2 ± 0.8) × 10⁻²⁹ (T/298)^{−(3.4±0.6)} cm⁶ molecule⁻² s⁻¹ for nitrogen used as the bath gas and k₁(∞) = (2.0 ± 0.4) × 10⁻¹¹ (T/298)^(0±1) cm³ molecule⁻¹ s⁻¹. RRKM calculations also help to understand the differences in reactivity between CF₃ and other radicals, for the same association reaction with NO.     

Rate constants for the reaction of OH radicals with CH3CN, C2H5CN AND CH2=CH-CN in the temperature range 298–424 K

Rate constants for the reactions of OH with CH₃CN, CH₃CH₂CN and CH₂=CH-CN have been measured to be 5.86 × 10⁻¹³ exp(−1500 ± 250 cal mole⁻¹/RT), 2.69 × 10⁻¹³ exp(−1590 ± 350 cal mole⁻¹/RT and 4.04 × 10⁻¹² cm³ molecule⁻¹ s⁻¹, respectively in the temperature range 298–424 K. These results are discussed in terms of the atmospheric lifetimes of nitrfles.     

Time-resolved vibrational cemiluminescence: Rate constants for the reaction F + HC1 → HF + Cl and for the relaxtion of HF(v = 3) by HCl, CO2, N2O, CO, N2 and O2

The reaction: F + HCl→ HF (v 3) + Cl (1), has been initiated by photolysing F₂ using the fourth-harmonic output at 266 nm from a repetitively pulsed Nd: YAG laser By analysing the time-dependence of the HF(3,0) vibrational chemiluminescence, rate constants have been determined at (296 ± 5) K for reaction (1), k₁ = (7.0 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, and for the relaxation of HF(v = 3) by HCl, CO₂, N₂O, CO, N₂ and O₂: k_HCl = (1.18 ±0.14) × 10⁻¹¹ k_CO2 = (1.04 ± 0. 13) × 10⁻¹², k_N2O = (1.41 ± 0.13) × 10⁻¹¹ k_CO = (2.9 ± 0.3) × (10⁻¹², k_N2 = (7.1 ± 0.6) × 10⁻¹⁴ and k_O2 = (1.9 ± 0.6) × 10⁻¹⁴ cm³molecule⁻¹s⁻¹.     

Kinetic study of gas-phase Lu(D3/2) with O2, N2O and CO2

The second-order rate constants of gas-phase Lu(²D_3/2) with O₂, N₂O and CO₂ from 348 to 573 K are reported. In all cases, the reactions are relatively fast with small barriers. The disappearance rates are independent of total pressure indicating bimolecular abstraction processes. The bimolecular rate constants (in molecule⁻¹ cm³ s⁻¹) are described in Arrhenius form by k(O₂)=(2.3±0.4)×10⁻¹⁰exp(−3.1±0.7 kJmol⁻¹/RT), k(N₂O)=(2.2±0.4)×10⁻¹⁰exp(−7.1±0.8 kJmol⁻¹/RT), k(CO₂)=(2.0±0.6)×10⁻¹⁰exp(−7.6±1.3 kJmol⁻¹/RT), where the uncertainties are ±2σ.     

Rate coefficients for the reactions of cyclohexadienyl (c-C6H7) radicals with O2 and NO at room temperature

Rate coefficients for the reactions of cyclohexadienyl (c-C₆H₇) radicals with O₂ and NO were measured at 296 ± 2 K. The c-C₆H₇ radicals were detected selectively by laser-induced fluorescence. The rate coefficient for the reaction of c-C₆H₇ with O₂, (4.4 ± 0.5) × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹, was independent of the bath-gas (He) pressure (13–80 Torr). In the reaction of c-C₆H₇ with NO, thermal equilibrium among c-C₆H₇, NO, and C₆H₇NO was observed. The forward and reverse reactions were in the falloff region, and the equilibrium constant was (1.5 ± 0.6) × 10⁻¹⁵ cm³ molecule⁻¹.     

Kinetics study of the gas-phase reactions of C2F5OC(O)H and n-C3F7OC(O)H with OH radicals at 253–328 K

The rate constants, k₁ and k₂ for the reactions of C₂F₅OC(O)H and n-C₃F₇OC(O)H with OH radicals were measured using an FT-IR technique at 253–328 K. k₁ and k₂ were determined as (9.24 ± 1.33) × 10⁻¹³ exp[−(1230 ± 40)/T] and (1.41 ± 0.26) × 10⁻¹² exp[−(1260 ± 50)/T] cm³ molecule⁻¹ s⁻¹. The random errors reported are ±2 σ, and potential systematic errors of 10% could add to the k₁ and k₂. The atmospheric lifetimes of C₂F₅OC(O)H and n-C₃F₇OC(O)H with respect to reaction with OH radicals were estimated at 3.6 and 2.6 years, respectively.     

Lifetime and quenching rate constants for a flourescent excited state of tetramethylethylene (2,3-dimethyl-2-butene)

The collisionless lifetime of a flourescent excited state of tetramethylethylene (TME) vapor when excited at 235 nm is 20.8±0.9 ns and the self-quenching rate constant is (2.97±0.01) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The rate constants for quenching of TME vapor by O₂, benzene. CCl₄, Xe, N₂ and CH₄ are also repeated. In the vapor phase, the lifetime strongly depends on the excitation energy. The lifetime of liquid TME is 10±2 ns at 25±2°C.     

Kinetic processes of Hg6s6p(P1) in xenon

The principal route for decay of Hg 6s6p(³P₁) in xenon is shown to be bimolecular deactivation to the mercury ground state, with rate coefficient 9.1 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹; relaxation to the ³P₀ state plays a negligible role. The equilibrium constant of the reaction Hg(³P₁) + Xe HgXe(A ³O⁺), has been recorded as 1.73 × 10⁻²⁰ cm³ molecule⁻¹ at 293 K.     

Cooperative two-photon-induced chemical bond formation during a Kr+F2 collision to form KrF

The reactive Kr⁺F₂⁻ potential energy surface is probed by two-photon, laser-induced chemical bond formation during a Kr+F₂ collision. This is compared with the pulsed laser excitation (two-photon) of Kr(2p₉) followed by collision with F₂ leading to the formation of KrF(B, C). In addition to reporting the excitation spectrum for the two-phonon-induced collision process, these techniques were used to determine quenching rate constants of Kr₂F^*. Quenching by Xe gives XeF(B, C) with rate constant (1.5±0.2)×10⁻¹⁰ cm³ s⁻¹; the quenching rate constant for F₂ is (1.5±0.2)×10⁻¹⁰ cm³ s⁻¹, and the radiative lifetime of Kr₂F^* is 240±35 ns. The quenching rate constant for the coupled Kr(2p₈) and Kr(2p₉) levels by F₂ is (13±2)×10⁻¹⁰ cm³ s⁻¹.     

Phosphorescence of biacetyl sensitized by benzene in gas phase pulse radiolysis

The phophorescence of biacetyl induced by an energy transfer to biacetyl from triplet benzene produced in the pulse radiolysis of benzene-biacetyl mixtures has been studied. The time required to reach the maximum intensity of phosphorescence, t_max, after the electron pulse, varies as a function of biacetyl pressure at constant benzene pressure (40 torr), which gives the lifetime of triplet benzene τ = (6.7 ± 3.2) × 10⁻⁶ s and the rate constant of the energy transfer k_{C6H6^*(T1) + biacetyl} = (1.6 ± 0.7) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹.     

Flash photolysis/temperature jump study of the vibrational relaxation of N2O(010) by O(P) and NO

This survey begins with the photochemistry at 254 nm and 298 K in the system H₂O₂COO₂RH, the primary objective of which is to determine the rate constants for the reaction OH + RH → H₂O + R relative to the well-known rate constant for the reaction OH + CO → CO₂ + H. Inherent in the scheme is that the reaction HO₂+CO→OH+CO₂ is negligible compared with the OH reaction, and a literature consensus gives k_HO2 < 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹, or some 10⁶ less than k_OH at 298 K. Theoretical calculations establish that the first stage in the HO₂ reaction is the formation of a free radical intermediate HO₂ + CO → HOOCO (perhydroxooxomethyl) which decomposes to yield the products, and that the rate of formation of the intermediate is equal to the rate of formation of the products. The structure of the intermediate and a reaction profile are shown.

High temperature rate data reported subsequent to the data in the consensus and theoretical calculations lead here to a recommendation that, in the range 250–800 K, k_HO2 = 3.45 × 10⁻¹²T^1/2 exp(1.15 × 10⁴/T) cm³ molecule⁻¹ s⁻¹, the hard-sphere-collision Arrhenius modification. This yields k_HO2(298) = 1.0 × 10⁻²⁷ cm³ molecule⁻¹ s⁻¹ or some 10¹⁴ slower than k_OH(298).     

State-selected reaction and relaxation of CH with H2

The state-selected reaction of CH(X₂Πν″ = 0, 1) with H₂ has been studied, in which CH was generated by IRMPD of a precursor gas, CH₃OH. The subsequent evolution of CH (ν″ = 0, 1) was monitored by the sensitive LIF technique. For the ground state and vibrationally excited state CH, the reaction with H₂ is found to depend on the total pressure in the sample cell at room temperature, which suggests that the reaction proceeds through an intermediate adduct, CH₃. The backward dissociation process is found to depend on the buffer pressure, which can be rationalized via a collision-induced backward dissociation. The decay rates of CH (ν″ = 0, 1) due to collisions with H₂ and Ar at a buffer pressure of 10 Torr are k_H2 (ν″ = 1) = (2.3±0.1) × 10⁻¹ cm³ molecule⁻¹ s⁻¹ and k_Ar (ν″ = 1) = (4.4±0.1) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹. Possible effects of the vibrational excitation on the reaction rate of CH (ν″ = 1) are discussed.     

Rates of the reactions CN + H2CO and NCO + H2CO in the temperature range 294–769 K

The rate coefficients of the reactions: (1) CN + H₂CO → products and (2) NCO + H₂CO → products in the temperature range 294–769 K have been determined by means of the laser photolysis-laser induced fluorescence technique. Our measurements show that reaction (1) is rapid: k₁(294 K) = (1.64 ± 0.25) x 10⁻¹¹ cm³ molecule⁻¹ s⁻¹; the Arrhenius relation was determined as k₁ = (6.7 ± 1.0) x 10⁻¹¹ exp[(−412 ± 20)/T] cm³ molecule⁻¹ s⁻¹. Reaction (2) is approximately a tenth as rapid as reaction (1) and the temperature dependence of k₂ does not conform to the Arrhenius form: k₂ = 4.62 x 10⁻¹⁷T^1.71 exp(198/T) cm³ molecule⁻¹ s⁻¹. Our values are in reasonable agreement with the only reported measurement of k₁; the rate coefficients for reaction (2) have not been previously reported.     

Collisional quenching of electronically excited germanium atoms, Ge[4p(S0)], by small molecules investigated by time-resolved atomic resonance absorption spectroscopy

The collisional quenching of electronically excited germanium atoms, Ge[4p²(¹S₀)], 2.029 eV above the 4p²(³P₀) ground state, has been investigated by time-resolved atomic resonance absorption spectroscopy in the ultraviolet at λ = 274.04 nm [4d(¹P₁⁰) ← 4p²(¹S₀)]. In contrast to previous investigations using the ‘single-shot mode’ at high energy, Ge(¹S₀) has been generated by the repetitive pulsed irradiation of Ge(CH₃)₄ in the presence of excess helium gas and added gases in a slow flow system, kinetically equivalent to a static system. This technique was originally developed for the study of Ge[4p²(¹D₂)] which had eluded direct quantitative kinetic study until recently. Absolute second-order rate constants obtained using signal averaging techniques from data capture of total digitised atomic decay profiles are reported for the removal of Ge(¹S₀) with the following gases (k_R in cm³ molecule⁻¹ s⁻¹, 300 K): Xe, 7.1 ± 0.4 × 10⁻¹³; N₂, 4.7 ± 0.6 × 10⁻¹²; O₂, 3.6 ± 0.9 × 10⁻¹¹; NO, 1.5 ± 0.3 × 10⁻¹¹; CO, 3.4 ± 0.5 × 10⁻¹²; N₂O, 4.5 ± 0.5 × 10⁻¹²; CO₂, 1.1 ± 0.3 × 10⁻¹¹; CH₄, 1.7 ± 0.2 × 10⁻¹¹; CF₄, 4.8 ± 0.3 × 10⁻¹²; SF₆, 9.5 ± 1.0 × 10⁻¹³; C₂H₄, 3.3 ± 0.1 × 10⁻¹⁰; C₂H₂, 2.9 ± 0.2 × 10⁻¹⁰; Ge(CH₃)₄, 5.4 ± 0.2 × 10⁻¹¹. The results are compared with previous data for Ge(¹S₀) derived in the single-shot mode where there is general agreement though with some exceptions which are discussed. The present data are also compared with analogous quenching rate data for the collisional removal of the lower lying Ge[4p²(¹D₂)] state (0.883 eV), also characterized by signal averaging methods similar to that described here.     

The multichannel reaction NH2 + NH2 at ambient temperature and low pressures

NH₂ profiles were measured in a discharge flow reactor at ambient temperature by monitoring reactants and products with an electron impact mass spectrometer. At the low pressures used (0.7 and 1.0 mbar) the gas-phase self-reaction is dominated by a ‘bimolecular’ H₂-eliminating exit channel with a rate coefficient of k_3b(300 K) = (1.3 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and leading to N₂H₂ + H₂ or NNH₂ + H₂. Although the wall loss for NH₂ radicals is relatively small (k_w ≈ 6–14 s⁻¹), the contribution to the overall NH₂ decay is important due to the relatively slow gas-phase reaction. The heterogeneous reaction yields N₂H₄ molecules.     

Temperature dependence of the reaction C2H (C2D) + O2 between 295 and 700 K

The rate coefficients for the reactions of C₂H and C₂D with O₂ have been measured in the temperature range 295 K T 700 K. Both reactions show a slightly negative temperature dependence in this temperature range, with k_C2H+O2 = (3.15 ± 0.04) × 10⁻¹¹ (T/295 K)^{−(0.16 ± 0.02)} cm³ molecule⁻¹ s⁻¹. The kinetic isotope effect is k_C2H/k_C2D = 1.04 ± 0.03 and is constant with temperature to within experimental error. The temperature dependence and the C₂H + O₂ kinetic isotope effect are consistent with a capture-limited metathesis reaction, and suggest that formation of the initial HCCOO adduct is rate-limiting.     

Kinetics and mechanism of the gas phase reaction of Cl atoms with iodobenzene

Smog chamber/FTIR techniques were used to study the kinetics and mechanism of the reaction of Cl atoms with iodobenzene (C₆H₅I) in 20–700 Torr of N₂, air, or O₂ diluent at 296 K. The reaction proceeds with a rate constant k(Cl+C₆H₅I)=(3.3±0.7)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹ to give chlorobenzene (C₆H₅Cl) in a yield which is indistinguishable from 100%. The title reaction proceeds via a displacement mechanism (probably addition followed by elimination).     

Simultaneous observation of fluorescence quenching by singlet—singlet and by singlet—triplet exciton interactions in crystalline anthracene

The singlet exciton quenchings due to SS and ST interactions in crystalline anthracene have been separated by considering the kinetics of the two interactions. The corresponding rate constants are _γSS = (1 ± 0.5) × 10⁻⁸ and _γST = (f ± 3) × 10⁻⁹ cm³ sec⁻¹.     

Quenching of the translationally hot and thermalized NH(c Π) radicals by HN3

The cross section for the quenching of NH(c ¹Π, ν = 0) by HN₃ was measured by using a pulsed laser technique. A single rotational level of NH(c ¹Π, ν = 0) was formed by exciting NH(a ¹Δ, ν = 0) with a frequency doubled dye laser. NH(a¹Δ) was produced by photolyzing HN₃ with a XeCl excimer laser. The time profiles of the NH(c-a) fluorescence were measured at various pressures of HN₃. Experiments were performed both in the presence and in the absence of He buffer gas. In the absence of He, the NH radicals were found to be translationally hot; the average velocity was 3800±600 m s⁻¹. The quenching cross sections for the translationally hot and thermalized NH(c) radicals by HN₃ were determined to be (28±5) × 10⁻¹⁶ and (85±3) × 10⁻¹⁶ cm², respectively. No rotational level dependence could be observed in the quenching of the hot NH(c) radicals.