Showing papers in "International Journal of Chemical Kinetics in 1976"

Direct identification of reactive routes and measurement of rate constants in the reactions of oxygen atoms with methanethiol, ethanethiol, and methylsulfide

Abstract: The room temperature reactions between oxygen atoms and methanethiol, ethanethiol, and methylsulfide have been studied in crossed jets to directly detect and identify the free-radical and stable products they produce. Knowledge of the products was used to assign reactive routes. The overall rate constants for all three O-atom reactions were also measured at 300 ± 2°K using a fast-flow reactor. They are 1.9 (CH3SH), 2.8 (C2H5SH), and 63 (CH3SCH3) × 10−12cm3/mol · sec. The identity of the detected products and the trend in rate constants in these reactions support an electrophilic addition mechanism followed by decomposition of the excited adduct by S-R bond cleavage (R = H, CH3, or C2H5).

The gas‐phase pyrolysis of alkyl nitrites. IV. Ethyl nitrite

Abstract: The rate of decomposition of s-butyl nitrite (SBN) has been studied in the absence (130–160°C) and presence (160–200°C) of NO. Under the former conditions, for low concentrations of SBN (6 × 10−5 − 10−4M) and small extents of reaction (∼1.5%), the first-order homogeneous rates of acetaldehyde (AcH) formation are a direct measure of reaction (1) since k3c » k2(NO): . Unlike t-butyl nitrite (TBN), d(AcH)/dt is independent of added CF4 (∼0.9 atm). Thus k3c is always » k2 (NO) over this pressure range. Large amounts of NO (∼0.9 atm) (130–160°C) completely suppress AcH formation. k1 = 1016.2–40.9/θ sec−1. Since (E1 + RT) and ΔH°1 are identical, within experimental error, both may be equated with D(s-BuO-NO) = 41.5 ± 0.8 kcal/mol and E2 = 0 ± 0.8 kcal/mol. The thermochemistry leads to the result ΔH°f (s-) = − 16.6 ± 0.8 kcal/mol. From ΔS°1 and A1, k2 is calculated to be 1010.4M−1 · sec−1, identical to that for TBN. From an independent observation that k6/k2 = 0.26 ± 0.01 independent of temperature, , we find E6 = 0 ± 1 kcal/mol and k6 = 109.8M−1 · sec−1. Under the conditions first cited, methyl ethyl ketone (MEK) is also a product of the reaction, the rate of which becomes measurable at extents of conversion >2%. However, this rate is ∼0.1 that of AcH formation. Although MEK formation is affected by the ratio S/V for different reaction vessels, in a spherical reaction vessel, this MEK arises as the result of an essentially homogeneous first-order 4-centre elimination of HNO. ; k5 = 1012.8–35.8/θ sec−1. Sec-butyl alcohol (SBA), formed at a rate comparable to MEK, is thought to arise via the hydrolysis of SBN, the water being formed from HNO. The rate of disappearance of SBN, that is, d(MEK + SBA + AcH)/dt, is given by kglobal = 1015.7–39.6/θ sec−1. In NO (∼1 atm) the rate of formation of MEK was about twice that in the absence of NO, whereas the SBA was greatly reduced. This reaction was also affected by the ratio S/V of different reaction vessels. It was again concluded that in a spherical reaction vessel, the rate of MEK formation was essentially homogeneous and first order. This rate is given by kobs = 1012.9–35.4/θ sec−1, very similar to k5. However, although it is clear that the rate of formation of MEK is doubled in the presence of NO, the value for kobs makes it difficult to associate this extra MEK with the disproportionation of s- and NO: s-. NO at temperatures of 130–160°C completely suppresses AcH formation. AcH reappears at higher temperatures (165–200°C), enabling k3c to be determined. Ignoring reaction (6), d(AcH)/dt = k1k3 (SBN)/[k3c + k2(NO)]; k3c = 1014.8–15.3/θ sec−1. Inclusion of reaction (6) into the mechanism makes very little difference to the result. Reaction (3c) is expected to be a pressure-dependent process.

Reactions of O 3PJ atoms with halogens: The rate constants for the elementary reactions O + BrCl, O + Br2, and O + Cl2

Abstract: Direct determinations of the rate constants (cm3/molec · sec) k1, k2, and k3 from 298 to 299°K are reported, using atomic resonance fluorescence in discharge flow systems: 1 The rate constant k1, which has not been determined previously, was found to possess an insignificant temperature coefficient (EA = (0 ± 700) J/mole) in the range of 299 to 619°K. The present result for k2 agrees well with reinterpreted values from the one previous determination. Measurements of O atom consumption rates and Br atom production rates in the O + Br2 reaction are interpreted to give an estimate of the rate constant k4, which has not been reported previously, at 298°K: k3 has been measured at three temperatures between 299 and 602°K. The present and previous results for k3 were combined to give the following rate expression:

Rate constants for the bimolecular self‐reaction of tertbutyl radicals in nalkane solvents

Abstract: Rate constants for the self-reaction of tertbutyl radicals in six nalkanes are determined as functions of temperature by kinetic electron-spin resonance spectroscopy and from rates of product formation. They are well described by a Smoluchowski–Stokes–Einstein treatment including effects of microfriction and spin statistical factors, indicating complete diffusion control. The ratio of disproportionation to combination products is found to depend on temperature and solvent. This is tentatively ascribed to reorientational effects in the encounter pairs.

Thermal stability of alcohols

Abstract: 3,3-Dimethylbutanol-2 (3,3-DMB-ol-2) and 2,3-dimethylbutanol-2 (2,3-DMB-ol-2) have been decomposed in comparative-rate single-pulse shock-tube experiments. The mechanisms of the decompositions are The rate expressions are They lead to D(iC3H7H) – D((CH3)2(OH) CH) = 8.3 kJ and D(C2H5H) – D(CH3(OH) CHH) = 24.2 kJ. These data, in conjunction with reasonable assumptions, give and The rate expressions for the decomposition of 2,3-DMB-1 and 3,3-DMB-1 are and

A flash photolysis study of the rate of the reaction OH + CH4 → CH3 + H2O over an extended temperature range

Abstract: A flash photolysis system has been used to study the rate of reaction (1), OH + CH4 CH3 + H2O, using time-resolved resonance absorption to monitor OH. The temperature was varied between 300 and 900°K. It is found that the Arrhenius plot of k1 is strongly curved and k1 (T) can best be represented by the expression The apparent Arrhenius activation energy changes from 15±1 kJ/mole at 300°K to 32±2 kJ/mole at 1000°K. On either side of our temperature range, both absolute rates and their temperature dependence are in good agreement with the results from most previous investigations.

Pyrolysis of dimethyl peroxide

Abstract: By using isobutane (t-BuH) as a radical trapit has been possible to study the initial step in the decomposition of dimethyl peroxide (DMP) over the temperature range of 110–140°C in a static system. For low concentrations of DMP (2.5 × 10−5−10−4M) and high pressures of t−BuH (∼0.9 atm) the first-order homogeneous rate of formation of methanol (MeOH) is a direct measure of reaction (1): . For complete decomposition of DMP in t-BuH, virtually all of the DMP is converted to MeOH. Thus DMP is a clean thermal source of Me. In the decomposition of pure DMP complications arise due to the H-abstraction reactions of Me from DMP and the product CH2O. The rate constant for reaction (1) is given by k1 = 1015.5−37.0/θ sec−1, very similar to other dialkyl peroxides. The thermochemistry leads to the result D(MeOOMe) = 37.6 ± 0.2 kcal/mole and /H(Me) = 3.8 ± 0.2 kcal/mole. It is concluded that D(ROOR) and D(ROH) are unaffected by the nature of R. From ΔS and A1, k2 is calculated to be 1010.3±0.5M−1· sec−1: . For complete reaction, trace amounts of t-BuOMe lead to the result k2 ∼ 109M−1 ·sec−1: products. From the relationship k6 = 2(k2k5a)1/2 and with k5a = 108.4M−1 · sec−1, we arrive at the result k6 = 109.7M−1 · sec−1: .

An absolute measurement of the rate constant for ethyl radical combination

Abstract: An absolute value of kr of ethyl radicals at 860 ± 17°K of 4.5 × 109M−1·sec−1 was determined under VLPP conditions, where the value of kr/kr∞ should be about 1/2. Thus kr∞(M−1·sec−1) ∼ 1010 at 860°K. An error of as much as a factor of 2 in kr would be surprising, but possible. The value of 1010M−1·sec−1 seems to be a factor of from 2 to 5 too high to be compatible with extensive data on the reverse reaction and the accepted thermochemistry. Changes in the heat of formation and entropy of the ethyl radical can change the situation somewhat, but even these changes when applied to the work of Hiatt and Benson [3] indicate that ethyl combination should be ∼ 109.3M−1·sec−1. More work is necessary if a better value is desired.

A shock‐tube study of the kinetics of reaction of hydroxyl radicals with H2, CO, CH4, CF3H, C2H4, and C2H6

Abstract: Concentration-time profiles have been measured for hydroxyl radicals generated by the shock-tube decomposition of hydrogen peroxide in the presence of a variety of additives. At temperatures close to 1300°K the rate constants for the reaction are found to be in the ratio 0.18:0.19:0.59:1.00:2.33:2.88 for the additives CO:CF3H:H2:CH4:C2H4:C2H6, respectively.

An investigation of nonequilibrium kinetic isotope effects in chemically activated vinyl radicals

Abstract: New experimental data have been obtained for H + C2H2, D + C2H2, H + C2D2, and D + C2D2 at room temperature. Two previously described apparatus were used in order to measure the pressure dependence of the reactions. The absolute rate constants are compared to results from other laboratories. The present results and those of Payne and Stief are used to obtain the high-pressure limiting rate constant at room temperature. When the activation energy from the work of Payne and Stief is considered, it is shown that the A factor for H + C2H2 is too low by a factor of ∼20. If a transmission coefficient is introduced which is constant for all isotopic variations, the pressure dependence can be explained in terms of the randomly energized radicals. RRKM theory is then invoked to explain the observed statistical nonequilibrium kinetic isotope effects.

Lyman‐α absorption photometry at high pressure and atom density kinetic results for H recombination

Abstract: Atomic absorption and fluorescence spectrophotometry have been routinely used in kinetic investigations as probes of relative, rather than absolute, atom concentration. The calibration of a Lyman-α photometer for measurement of absolute hydrogen atom concentrations at levels [H] ι ≤ 1.8 × 1014 atoms/cm2 and total pressure of 1.5 torr He is described. The photometer is characterized in terms of a two-level emission source and an absorption region in which only Doppler broadening of the transition is considered. The modifications due to pressure broadening by high pressures (500 ≤ P ≤ 1500 torr) in the absorption region are discussed in detail. Application of the technique is reported for the recombination of hydrogen atoms in the presence of six nonreactive heat bath gases. Experiments were performed in a static reaction cell at pressures of 500–1500 torr of heat bath gas, and hydrogen atoms were produced by Hg (3P1) photosensitization of H2. The technique is critically evaluated and the mechanistic implications of the hydrogen atom recombination results are examined. The measured room temperature recombination rate constants in H2, He, Ne, Ar, Kr, and N2 are 8.5 ± 1.2, 6.9 ± 1.5, 5.9 ± 1.5, 8.0 ± 0.8, 10.2 ± 0.9, and 9.6 ± 1.4, respectively, where the units are 1033 cm6/molec2 · sec.

The reaction of OH with CO

Abstract: NO2 was photolyzed with 2288A radiation at 300 and 423 K in the presence of H2O, CO, and in some cases excess He. The photolysis produces O(1D) atoms which react with H2O to give HO radicals or are deactivated by CO to O(3P) atoms. The ratio k sub 5/k sub 3 is temperature dependent, being 0.33 at 300 K and 0.60 at 423 K. From these two points the Arrhenius expression is estimated to be k sub 5/k sub 3 = 2.6 exp(-1200/RT) where R is in calories/mole - K. The OH radical is either removed by NO2 or reacts with CO.

Flash photolysis study of the recombination of chlorine atoms in the presence of various inert gases and NO

Abstract: The recombination of chlorine atoms has been investigated by flash photolysis in the inert gases He, Ne, Ar, N2, CO2, CF4, SiF4, SF6, and C2F6. The pressure dependence of the reaction has been measured between 0.5 and about 100 atm for He, N2, and CO2. Experiments on the NO-catalyzed recombination of chlorine in the presence of He (0.5–100 atm) permitted a determination of the falloff curve of the reaction Cl+NO(+He)ClNO(+He).

Charge transfer reactions in alkane and cycloalkane systems. Estimated ionization potentials

Abstract: Rate constants of change transfer reactions kCT, involving C3C9 alkanes and cycloalkanes, have been determined in an ion cyclotron resonance mass spectrometer. The rate constants are significantly lower than the corresponding rate constants for collision when the reaction is less than about 0.5 eV exothermic for linear alkane ions, or less than about 0.2 eV exothermic for cycloalkane ions. In this region of low reaction efficiency, the efficiency of reaction with linear or branched alkanes seems to depend primarily on reaction exothermicity. (The efficiencies of reaction of a given ion with cyclic alkanes also depend on ΔHrn, but are higher than for reactions with other compounds). Although the lowered reaction efficiencies probably result, at least in part, from unfavorable Franck–Condon factors in the energy range near the ionization onset, quantitative correlations between reaction efficiency and estimated relative Franck–Condon factors were not observed. When the enthalpy of reaction is small (less than about −0.15 eV), it is seen that the reverse charge transfer can also occur, and equilibrium is established under the conditions of these experiments. From the observed equilibrium constants, values for the standard free energy change are derived, and assuming that ΔS is small for electron transfer equilibria, values of ΔHrn are estimated. In the case of the equilibria involving cyclohexane ion, these values of ΔHrn lead to estimates of the ionization potentials of methylcyclopentane, 3-methylpentane, n-octane, 2,2-dimethylbutane, and 2,3-dimethylbutane, which are lower than the ionization potentials of cyclohexane, that is, <9.88 eV, although all these compounds had previously been reported to have ionization potentials above 10.03 eV. That the ionization potentials are indeed lower than 10.03 eV is confirmed by determining the quantum yields of ionization with 10.03-eV photons. It is pointed out that the conclusions reached here apparently also apply to the charge transfer reactions of alkane ions in the liquid phase.

The isothermal flash photolysis of hydrazoic acid

Abstract: The flash photolysis of HN3 was studied by coordinated time-resolved spectroscopic measurements of HN3 NH(a1Δ), NH(X3Σ), NH(c1π), NH(A3π), NH2, and N3 following flash photolysis of mixtures of HN3 with argon or helium. The primary photolysis is complex, but when the wavelength distribution of the flash is limited to values greater than about 200 nm, the major reactive product is NH(1Δ), or states which quickly decay to NH(1Δ). Disappearance of NH(1Δ) occurs predominantly by the process The process has little, if any, energy of activation, and no detectable dependence on the pressure of inert gas below 1 atm. The rate of formation of NH2 in its ground vibrational state depends on the inert gas pressure in a way that can be accounted for by vibrational relaxation from initial excited vibrational states. The total amount of NH2 is roughly comparable with the amount of HN3 decomposed by primary photolysis. The observed N3 can be attributed to the NH(1Δ) + HN3 reaction, although a smaller amount could also be formed by primary photolysis. The value of k2 is revised upward from the value given in a preliminary report on the basis of a more careful consideration of the effects of Beer's law failure in absorption measurements involving narrow spectral lines.

Energy disposal in the three‐centered elimination of df from 1,1,2‐trifluoroethane‐1‐d1

Abstract: The chemical activation data for three- and four-centered hydrogen fluoride elimination from CH2FCDF2 have been analyzed to assign the energy released to the olefin fragment in the three-centered process and to estimate the threshold energies for elimination channels. Based upon the cis–trans isomerization rates of CHF = CHF, 78% of the total available energy was released to the olefin fragment for the αα channel. The analysis suggests the existence of an appreciable barrier (∼10 kcal/mole) for the reverse reaction, addition of the CH2FCF carbene to DF. The threshold energies for αα, αβ, and βα elimination from 1,1,2-trifluoroethane-1-d1 were assigned as 71, 68, and 68 kcal/mole, respectively. Analysis of the chemical activation data for 1,1,2,2,-tetrafluoroethane, without distinguishing between the three- and four-centered elimination channels, suggests a threshold energy of ∼75 kcal/mole.

Analysis of nonlinear reactions in modulated molecular beam surface experiments

Abstract: An approximate method of analyzing nonlinear reaction models in modulated molecular beam surface kinetic studies is developed. The exact method for treating nonlinear surface mechanisms is tedious and almost always requires computer analysis. The proposed approximate method is a simple extension of the Fourier expansion technique valid for linear surface reactions; it quickly provides analytical expressions for the phase lag and amplitude of the reaction product for any type of nonlinear surface mechanism, which greatly facilitates comparison of theory and experiment. The approximate and exact methods are compared for a number of prototypical adsorption–desorption reactions which include coverage-dependent adsorption and desorption kinetics of order greater than unity. Except for certain extreme forms of coverage-dependent adsorption, the approximate method provides a good representation of the exact solution. The errors increase as the nonlinearities become stronger. Fortunately, when the discrepancy between the two methods is substantial, the reaction product signal is so highly demodulated that reliable experimental data usually cannot be obtained in these regions anyway.

Effect of thermal activation on the reaction of toluene with hydrogen atoms

Abstract: The effect of temperature on the reaction of toluene with hydrogen atoms is interpreted by the RRKM(Marcus–Rice) unimolecular rate theory. It was found that the product distribution changed markedly with temperature in the range of 300–780°K. Methyl-cyclohexadienes and methylcyclohexenes were the main products in the vicinity of room temperature, which were taken over by benzene and ethylbenzene as temperature increased. The calculation based on RRKM theory shows that the experimental results are explained in terms of the effect of temperature on the unimolecular reactions of the chemically activated methylcyclohexadienyl radicals.

Shock tube studies of the N2O/Ar and N2O/H2/Ar systems

Abstract: N2O decay has been monitored via infrared emission for a series of mixtures containing N2O/Ar and N2O/H2/Ar. These mixtures were studied behind reflected shock waves in the temperature interval of 1950–3075°K with total concentrations ranging from 1.2 to 2.5 × 1018 molec/cm3. In all cases the N2O decayed exponentially, and a rate constant kobs was obtained. Runs without added H2 could be described by the following Arrhenius parameters: log A = −9.72 ± 0.08 (in units of cm3/molec · sec) and EA = 203.5 ± 3.6 kJ/mole. Addition of 0.01% and 0.1% H2 was observed to increase the decay rate; the largest increase occurred between 2250 and 2500°K with 0.1% H2, where kobs doubled. Mixtures with no added H2 were analyzed by numerical integration of the following reactions: Quantitative agreement between calculations and observations were obtained with both high and low choices for k2 and k3. The additional reactions were included in the analysis of the mixtures containing H2. Here agreement was obtained only when low values were assigned to k2 and k3. The combinations of k1 k3 which agreed with all the data were k1 = 3.25 × 10−10 exp (−215 kJ/RT) and k2 = k3 = 1.91 × 10−11 exp (-105 kJ/RT).

Molecular beam study of f2 + i2

Abstract: Crossed molecular beam techniques have been used to study the endoergic reaction between F2 and I2. Above a threshold energy of 4 kcal/mole the observed products are I2F and F. At higher energies IF is also produced. Angular and velocity distributions indicate that the IF does not result from a four-center exchange reaction.

Thermal isomerization of cis‐ or trans‐2‐butene. The unimolecular elimination of hydrogen from cis‐2‐butene around 500°C

Abstract: An analytical and kinetic study of the thermal reaction of cis- or trans-2-butene has been performed in a static system over the temperature range of 480–550°C and at a low extent of reaction and initial pressures of 10–100 torr. The rate constant of the unimolecular cis–trans isomerization of cis-2-butene, determined under the conditions (2.3 RT in cal/mole) is in good agreement with previous measurements made at lower pressures. A comparison between the formation rates of hydrogen from the thermal reactions of cis- and trans-2 butene around 500°C leads to the rate constant value (2.3 RT in cal/mole) for the unimolecular 1,4hydrogen elimination from cis2butene.

Comparison between experimental and calculated results on the decomposition of chemically activated alcohols

Abstract: The life times of chemically activated alcohols have been determined using the high-pressure unimolecular rate parameters for thermal decomposition of alcohols from shocktube studies and RRKM calculations. They are compared with literature numbers (from insertion of 0(1D) into hydrocarbons). It is suggested that in some cases singlet oxygen carries excess energy into the hydrocarbon. The consequences of such an assumption are explored and discrepancies with previously published conclusions discussed.

The gas‐phase reaction of O3 with H2CO

Abstract: Explosions occur when O3 and H2CO are mixed in a fresh vessel, even in the presence of several hundred torr of N2 or O2. However, in an aged vessel the reaction is well behaved. The reaction between O3 and H2CO was studied at room temperature in an aged vessel in the presence of about 400 torr of either N2 or O2. The initial rate of O3 decay in the presence of N2 is about 103 times faster than in the presence of O2, and very small amounts of O2 quickly reduce the initial rate of O3 decay in the N2 case. A chain mechanism is postulated to account for the results in which chain initiation can occur both by thermal decomposition of O3, followed by reaction of O(3P) with H2CO to produce HO and HCO, as well as by which may occur both homogeneously and heterogeneously. The rate coefficient k1 ≃ 2.1 × 10−24 cm3/molec · sec represents an upper limit (to within a factor of 2 uncertainty) to the direct gas-phase reaction between O3 and H2CO.

Quenching of N2(A3Σu+) by O2, O, N, and H

Abstract: Metastable N2(A3Σu+), υ = 0, υ = 1, molecules are produced by a pulsed Tesla-type discharge of a dilute N2/Ar gas mixture. Rate coefficients for quenching these metastable levels by O2, O, N, and H were obtained by time-resolved emission measurements of the (0, 6) and (1, 5) Vegard–Kaplan bands. In units of cm3/mole · sec at 300°K and with an experimental uncertainty of ±20%, these rate coefficients for N2(A3Σu+) are Within the limits of error these coefficients apply to quenching N2(A3Σu+) υ′ = 1 as well.

Radiation‐induced dechlorination of carbon tetrachloride in cyclohexane Solutions. The kinetics of liquid‐phase reactions of trichloromethyl radicals

Abstract: The kinetics of the gamma-radiation induced free-radical chain reaction in solutions of carbon tetrachloride in cyclohexane (RH) has been investigated in the temperature range of 303–383°K. Trichloromethyl radicals were produced by the reaction of radiolytically generated cyclohexyl radicals with carbon tetrachloride. The kinetics of the following reactions were investigated: The following rate expression was obtained: The error limits are the standard deviation from the least mean square Arrhenius plots. Effects of phase on the kinetics of reactions (3) and (4) are considered.

The photochemistry of hexafluoroacetylacetone in the vapour phase. Occurrence of a novel HF elimination reaction

Abstract: Photolysis of the vapour of hexafluoroacetylacetone HFAA in its enolic form involves decomposition by two independent primary processes, one of which is a novel elimination of HF giving 2,2-difluoro-2,3-dihydro-5-trifluoromethylfuran-3-one: The HF is not vibrationally excited. Photolysis of the cyclic product of reaction (5) yields CF2 radicals which, if HFAA is present, undergo an insertion into the enolic OH bond, Structure (9). The infrared, ultraviolet, nuclear magnetic resonance, and mass spectra of HFAA and of the products of reactions (5) and (6) have been measured. Approximate quantum yields for reactions (1) and (5) have been obtained. Both ϕ1 and ϕ5 depend on pressure and the ratio ϕ1/ϕ5 increases with temperature and decreased wavelength of photolysing light. It is proposed that the ratio ϕ1/ϕ5 increases as the vibrational energy of electronically excited HFAA increases.