Overall rate constant measurements of the reaction of hydroxy- and chloroalkylperoxy radicals derived from methacrolein and methyl vinyl ketone with nitric oxide

The overall rate constants of the reactions of NO with hydroxy- and chloroalkylperoxy radicals, derived from the OH- and Cl-initiated oxidation of methacrolein and methyl vinyl ketone, respectively, were directly determined for the first time using the turbulent-flow technique and pseudo-first-order kinetics conditions with high-pressure chemical ionization mass spectrometry for the direct detection of peroxy radical reactants. The individual 100 Torr, 298 K hydroxyalkylperoxy + NO rate constants for the methacrolein [(0.93 +/- 0.12) (2sigma) x 10(-11) cm3 molecule(-1) s(-1)] and methyl vinyl ketone [(0.84 +/- 0.10) x 10(-11) cm3 molecule(-1) s(-1)] systems were found to be identical within the 95% confidence interval associated with each separate measurement, as were the chloroalkylperoxy + NO rate constants for both methacrolein [(1.17 +/- 0.11) x 10(-11) cm3 molecule(-1) s(-1)] and methyl vinyl ketone [(1.14 +/- 0.14) x 10(-11) cm3 molecule(-1) s(-1)]. However, the difference in the rate constants between the hydroxyperoxy + NO and chloroalkylperoxy + NO systems was found to be statistically significant, with the chloroalkylperoxy + NO rate constants about 30% higher than the corresponding hydroxyalkylperoxy + NO rate constants. This substituent effect was rationalized via a frontier molecular orbital model approach.     

Dichlorosilylene: Rate constant for the gas-phase reaction with nitric oxide

Using the recently detected intense UV absorption spectrum of it has been established that SiCl₂ reacts with nitric oxide in the gas phase with a bimolecular rate constant, k(SiCl₂+NO), of (1.6±0.1)×10⁹ M⁻¹s⁻¹ at 25°C.     

The reaction of methoxy radicals with nitric oxide and nitrogen dioxide

By allowing dimethyl peroxide (10^?4M) to decompose in the presence of nitric oxide (4.5 × 10^?5M), nitrogen dioxide (6.5 × 10^?5M) and carbon tetrafluoride (500 Torr), it has been shown that the ratio k₂/k_2′ = 2.03 ± 0.47: CH₃O + NO → CH₃ONO (reaction 2) and CH₃O + NO₂ → CH₃ONO₂ (reaction 2′). Deviations from this value in this and previous work is ascribed to the pressure dependence of both these reactions and heterogeneity in reaction (2). In contrast no heterogeneous effects were found for reaction (2′) making it an ideal reference reaction for studying other reactions of the methoxy radical. We conclude that the ratio k₂/k_2′ is independent of temperature and from k₁ = 10^10.2±0.4M^?1 sec^?1 we calculate that k_2′ = 10^9.9±0.4M^?1 sec^?1. Both k₂ and k_2′ are pressure dependent but have reached their limiting high-pressure values in the presence of 500 Torr of carbon tetrafluoride. Preliminary results show that k₄ = 10.^9.0±0.6 10^?4.5±1.1/ΘM^?1 sec^?1 (Θ = 2.303RT kcal mole^?1) and by k₄ = 10^8.6±0.6 10^?2.4±1.1/ΘM^?1 sec^?1: CH₃O + O₂ → CH₂O + HO₂ (reaction 4) and CH₃O + t-BuH → CH₃OH + (t-Bu) (reaction 4′).     

Electron paramagnetic resonance study of nitroxides generated from nitric oxide by reaction with transient radicals

Photochemical or thermal decomposition of azo‐compounds (such as 2,2^′‐azobisisobutyronitrile, 2,2^′‐azobis(2‐methylpropionamidine) dihydrochloride, dialkyl peroxides (such as tert‐butyl peroxide and diacyl peroxides (such as benzoyl peroxide) in anaerobic nitric oxide (NO)‐saturated dimethylsulfoxide (DMSO) or aqueous solutions yielded nitroxides. Well‐characterized electron paramagnetic resonance spectra of nitroxides revealed that NO was favorable for reacting with carbon‐centered and less stereo‐inhibited transient alkyl radicals, giving kinds of nitrosoalkane, typically nitrosomethane, which act sequentially as C‐nitroso compounds to trap transient radicals present in solution, yielding spin‐trapping adducts, i.e. nitroxides. Radicals, including sulfinyl radicals from UV‐irradiated DMSO, were trapped by the in situ formed CH₃NO. O‐centered radicals could not add to the freshly formed C‐nitroso compounds. Possible mechanisms are suggested. Copyright © 2002 John Wiley & Sons, Ltd.     

Absolute rate constant for the disproportionation reaction of formyl radicals from 295 to 475 K

The rate constant for the reaction HCO + HCO → CH₂O + CO was measured at temperatures between 298 and 475 K. The formyl radicals were produced by flash photolysis of formaldehyde and were detected by resonance absorption at 614.5 nm. At low pressure and room temperature, k₁ = (3.35 ± 0.85) × 10^?11 cm³ molecule^?1 s^?1. There is no discernable variation of k₁ with temperature up to 475 K.     

Formation of 3-Methylfuran from the gas-phase reaction of OH radicals with isoprene and the rate constant for its reaction with the OH radical

3-Methylfuran has been identified as a product of the gas-phase reaction of the OH radical with isoprene, and under simulated atmospheric conditions a formation yield of 0.044 ± 0.006 was determined. In an analogous manner, the OH radical reaction with 1,3-butadiene formed furan with a yield of 0.039 ± 0.011. Using a relative rate method, a rate constant for the reaction of the OH radical with 3-methylfuran of 9.35 × 10^?11 cm³ molecule^?1 s^?1 (with an estimated overall uncertainty of ±20%) at 296 ± 2 K was also determined. These data show that 3-methylfuran is a reactive compound which will be present in the troposphere at concentrations ?5% of those of its isoprene precursor.     

The reaction of nitric oxide with glutathionylcobalamin

UV-vis stopped-flow results show that glutathionylcobalamin can react with nitric oxide at pH 7 to form nitrosylcob(II)alamin in a reaction second-order overall. From kinetic studies we suggest that nitric oxide attacks glutathionylcobalamin to form a caged transition state followed by formation of the nitrosylcob(II)alamin.     

Protein thiyl free radicals produced by nitric oxide and nitric oxide donors

The spin-trapping technique was used to study the radical intermediates produced by reaction of nitric oxide (^*NO) and peroxynitrite with serum albumin and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Our results show that the major radical product induced by ^*NO and by peroxynitrite with serum albumin and GAPDH was a thiyl radical. The same radical can be detected in the ^*NO-transfer from S-nitroso albumin to low molecular weight thiols. Moreover, ^*NO or peroxynitrite treatment of GAPDH was able to induce NAD-dependent covalent modification of the enzyme in erythrocyte ghosts.     

Experimental evidence for the pressure dependence of the reaction rate constant between acetic acid and hydroxyl radicals

The reaction rate constant of acetic acid with the hydroxyl radical is measured at 93 Torr with our high-pressure flow system (HPFS) and found to display a negative temperature dependence that can be described by the Arrhenius expression, k(T) = (2.44 ± 0.22) × 10(-14) exp ((1027 ± 24)/T)) cm(3) molecule(-1) s(-1). Compared with our previously reported 7 Torr data, we find a noticeable pressure dependence. This dependence is observed to increase with decreasing temperature. This finding is consistent with a termolecular reaction mechanism. It is the first experimental evidence of the pressure dependence for this rate constant. A kinetics model is constructed, and the model results agree qualitatively with our experimental data. The extrapolated rate constant of the title reaction would be faster than previously believed at conditions of the upper troposphere/lower stratosphere, suggesting that the importance of acetic acid in its impact on HO(x) chemistry is currently underestimated.     

The rate of reaction of methyl radicals with ozone

The absolute rate constant for the reaction of methyl radicals with ozone has been measured as a function of temperature. Small concentrations of CH₃ were generated by flash photolyzing CH₃NO₂ at 193 nm with an ArF laser. A photoionization mass spectrometer was used to follow the rate of decay of CH₃ at various ozone concentrations. The resulting rate constants could be fit by the expressions over the temperature range of 243–384 K. These rate constants can be modeled by simple transition state theory using reasonable parameters for the activated complex. Use of this rate constant shows that less than 1% of the methyl radicals formed in the stratosphere react with ozone.     

The rate constant for the intramolecular isomerization of pentyl radicals

The thermal decomposition of n-pentane has been investigated in the temperature range 737 to 923 K. Making various assumptions, the detailed distribution of major products (methane, ethane, ethene, propene, and 1-butene) is used to evaluate the rate constant for the unimolecular isomerization which proceeds via a five-membered, cyclic transition state. Two alternative sets of assumptions are used. Common to both of them are assumptions concerning the thermochemistry and rate constants for decomposition of the C₅H₁₁ radicals. Method 1 assumes that all secondary C? H bonds are equally reactive towards hydrogen abstraction in which case, in addition to the value of ??₁₀, the ratio of the rate constants for abstraction from primary and secondary C? H bonds is evaluated. Values about a factor of two higher than published values for similar molecules are obtained. The alternative, method 2, assumes that the ratio of abstraction from the 1- and 2- positions of n?pentane is the same as that published for n?butane, in which case, in addition to the value of ??₁₀, the ratio of the rates of abstraction from the 3- and 2- positions of n-pentane is obtained. The value obtained is 0.401 which is to be compared with the statistically expected (and assumed in method 1) 0.5. Detailed discussions of the values of ??₁₀ obtained leads to the conclusion that method 1 leads to the best value where θ = 2.303RT in kcal/mol and error limits are two standard deviations. Combination of this value with values recalculated from published lower temperature data gives which, it is concluded, is the best value in the range 438 to 923 K.     

Electrophilic reaction of nitric oxide with Wittig reagents

Reaction of nitric oxide (NO) with p-substituted benzyl triphenylphosphonium chlorides or bromides (Wittig reagents) in CH₂Cl₂ under argon undergoes electrophilic attack of NO on the carbon center of phosphonium ylides, producing benzonitriles.     

Rate constant measurements for the reaction of Cl atoms with nitric acid over the temperature range 240–300 K

Rate constants for the reaction between Cl atoms and HONO₂ were measured at 243, 264, and 298 K by the flash photolysis resonance fluorescence technique. The data can be fit to the Arrhenius expression k₁ = 5.1 × 10^?12 exp(?1700/T) cm³ molecule^?1 s^?1 and indicate that the reaction is unimportant in stratospheric Cl atom removal. Sources of measurement error in this and earlier stidues are discussed.     

Carbon-bound diazeniumdiolates from the reaction of nitric oxide with amidines

[reaction: see text] The enediamine tautomer of a variety of substituted amidine free bases reacts with nitric oxide (NO) to produce compounds containing a carbon-bound diazeniumdiolate [R1R2R3C-N(O)=NO-] functional group (previously called "nitrosohydroxylamines"). The new reaction has been shown to be quite general, although the nature of the products does vary. Amidines containing more than one replaceable hydrogen produce polydiazeniumdiolates as intermolecular salts, while those in which only one diazeniumdiolation can occur provide zwitterionic salts. These diazeniumdiolated amidines are shown to be useful NO donor compounds which undergo very slow spontaneous dissociation on dissolution in pH 7.4 phosphate buffer to produce mixtures of NO and nitrous oxide containing mostly NO. The most advantageous manifestation of the new discovery is the preparation of the monodiazeniumdiolated amidine zwitterions. Reaction of the medically relevant alpha-adrenergic agonists tetrahydrozoline and idazoxan produced monodiazeniumdiolated amidine zwitterions from which NO release was observed for up to 28 days and showed little sign of ending. The reaction should be applicable to a variety of pharmaceutical agents, including NO synthase inhibitors, antitumor agents, and antibacterials.     

Rate constants of the combination of methyl radicals with nitric oxide and oxygen

Rate constants for the combination of methyl radicals with NO and O₂ have been measured by flash photolysis of azomethane coupled with product analysis by gas chromatography. Values of the rate constants have been obtained over the pressure region from 50 to 700 torr with He, N₂, and Ar as quenching molecules. The high-pressure limits were obtained through an RRKM model calculation and were found to be The rate constants were measured relative to the methyl combination reaction k₁ with k₁ = 9.5 × 10^?11 cm³/molec · sec. The RRKM model suggests D₀(CH₃? O₂) = 32 ± 3 kcal/mole.     

High-temperature rate constant determination for the reaction of OH with iso-butanol

This work presents the first direct experimental study of the rate constant for the reaction of OH with iso-butanol (2-methyl-1-propanol) at temperatures from 907 to 1147 K at near-atmospheric pressures. OH time-histories were measured behind reflected shock waves using a narrow-linewidth laser absorption method during reactions of dilute mixtures of tert-butylhydroperoxide (as a fast source of OH) with iso-butanol in excess. The title reaction's overall rate constant (OH + iso-butanol →(k(overall)) all products) minus the rate constant for the β-radical-producing channel (OH + iso-butanol →(k(β)) 1-hydroxy-2-methyl-prop-2-yl radical + H(2)O) was determined from the pseudo-first-order rate of OH decay. A two-parameter Arrhenius fit of the experimentally determined rate constant in the current temperature range yields the expression (k(overall) - k(β)) = 1.84 × 10(-10) exp(-2350/T[K]) cm(3) molecule(-1) s(-1). A recommendation for the overall rate constant, including k(β), is made, and comparisons of the results to rate constant recommendations from the literature are discussed.     

Absolute rate constant for the reaction of Phenyl radical with Acetylene

The absolute rate constant for the reaction of phenyl radical with acetylene has been measured at 20 torr total pressure in the temperature range of 297 to 523 K using the cavity-ring-down technique. These new kinetic data could be quantitatively correlated with the data obtained earlier with a relative rate method under low-pressure (10^?3–10^?2 torr) and high-temperature (1000–1330 K) conditions. These kinetic data were analyzed in terms of the RRKM theory employing the thermochemical and molecular structure data computed with the BAC-MP4 technique. The calculated results reveal that the total rate constant for the C₆H₅ + C₂H₂ reaction (k_t) is pressure-independent, whereas those for the formation of C₆H₅C₂H (k_b) and the C₆H₅C₂H₂ adduct (k_c) are strongly pressure-dependent. A least-squares analysis of the calculated values for 300–2000 K at the atmospheric pressure of N₂ or Ar can be given by and all in units of cm³/s. The latter equation effectively represents the two sets of experimental data. © 1994 John Wiley & Sons, Inc.