bluuebunny/crossref_metadata_2025 huggingface dataset:
Cerium (IV) oxidation rate of p‐chloromandelic acid in perchlorate and sulfate media
Authors: Calvaruso Giuseppe , Cavasino F. Paolo , Sbriziolo Carmelo
Published: January 2005

The rate of the cerium (IV) oxidation of p‐chloromandelic acid has been studied in perchlorate media at an ionic strength of 1.50 mol/dm3 by the stopped‐flow technique and in H2SO4MHSO4 (M+ = Li+, Na+, K+) and H2SO4MClO4 (M+ = H+, Li+, Na+) mixtures at constant total electrolyte concentrations of 1.00 and 2.00 mol/dm3 using the conventional spectrophotometric method. In perchlorate media the kinetic data indicate the formation of two intermediate complexes between cerium (IV) and the organic substrate, but only one is significantly involved in the intramolecular electron‐transfer process. The oxidation rate is markedly lower in sulfate media, where two reaction paths have been found to contribute to the overall redox reaction. The univalent cations examined exhibit negative specific effects upon the overall oxidation rate increasing in the order H+ < Li+ < Na+ < K+. Activation parameters have been also estimated.
A quantitative study of alkyl radical reactions by kinetic spectroscopy. IV. The flash photolysis of azopropanes
Authors: Adachi Hiroyuki , Basco Norman
Published: January 2005

The flash photolysis of azo‒n‒propane and of azoisopropane has been studied by kinetic spectroscopy. Transient absorption spectra in theregion of 220–260 nm have been assigned to the n‐propyl and isopropyl radicals. For the n‐propyl radical, ϵmax = 744 ± 39 l/mol cm at 245 nm and the rate constants for the mutual reactions were measured to be kc = (1.0 ± 0.1) × 1010 l/mol sec (combination) and kd = (1.9 ± 0.2) × 109 l/mol sec (disproportionation). For the isopropyl radical, ϵmax = 1280 ± 110 l/mol cm at 238 nm, with kc = (7.7 ± 1.6) × 109 l/mol sec and kd = (5.0 ± 1.2) × 109 l/mol sec

The rate constant for the dissociation of the vibrationally excited triplet state of the azopropanes into radicals was measured from the variation in the quantum yield of radicals with pressure. For azo‐n‐propane k = (6.6 ± 1.3) × 107 sec−1, and for azoisopropane k = (1.6 ± 0.4) × 108 sec−1. Collisional deactivation of the vibrationally excited singlet and triplet states was found to occur on every collision for n‐pentane; but nitrogen and argon were inefficient with a rate constant of 1.1 × 1010 l/mol sec.

Spectra observed in the region of 220–260 and 370–400 nm areattributed to the cis isomers of the parent trans‐azopropanes. These are formed, as permanent products, in increasing amounts as the pressure is increased.
Analytical solution of vibrational–rotational energy transfer in the dissociation and relaxation of N2, Br2, and CO at high temperature
Authors: Forst Wendell
Published: January 2005

An approximate analytical solution of relaxation in a low‐pressure system with exponential transition probabilities is given for vibrational–rotational energy transfer in the dissociation of diatomics. The main assumption is that the rotational degrees of freedom are in thermal equilibrium at all times, and that the barrier to dissociation in the vibrational–rotational plane is linear and asymmetric. The theory is applied to high‐temperature dissociations of N2, Br2, and CO in excess argon, with satisfactory agreement with available experimental data.
Reaction of hydrogen atoms with diethyldisulfide and ethylmethyldisulfide
Authors: Ekwenchi M. M. , Safarik I. , Strausz O. P.
Published: January 2005

H atoms react with C2H5SSC2H5 to give C2H5SH as the sole retrievable product with ϕ = 2.32 at 25°C and 2.84 at 145°C. The primary reaction is postulated to be H + C2H5SSC2H5 ← C2H5SH + C2H5S with k1 = (4.73 ± 0.64) × 1013 exp [−(1710 ± 69)/RT] cm3/mol·s relative to the rate constant of the H + C2H4 ← C2H5 reaction. The high value of the entropy of activation suggests the presence of partial hydrogen bonding in diethyldisulfide which is broken in the transition state.

Ethylmethyldisulfide reacts similarly: H + C2H5SSCH3 ← C2H5SH + CH3S or CH3SH + C2H5S. The thiyl radicals propagate a chain of radical exchange reactions forming the symmetrical disulfides with exposure‐time‐dependent quantum yields. The overall kinetics conform to a 16‐step mechanism from which the rate constants of the elementary reactions could be established by computer modeling. Thiyl radicals react considerably more slowly with disulfides than H atoms.
Arrhenius parameters for the alkoxy radical decomposition reactions
Authors: Choo Kwang Yul , Benson Sidney W.
Published: January 2005

Arrhenius parameters for the decomposition reactions of various alkoxy radicals are evaluated. The recommended values for tert‐butoxy radical decomposition reaction are log A (s−1) = 14.1, Ea = 15.3 kcal/mol. magnified image The parameters give reasonably self‐consistent results with related rate constants, product distributions of hydrocarbon oxidation reactions, and the values reported in different laboratories in different environments.

A reworking of data by Batt et al. on RO + NO and RO + NO2 leads to a lowering of his A factors for these reactions by a factor of 5. It also leads to a similar lowering of the A factors for RO decompositions. The intrinsic activation energies E for the addition of alkyl radicals R to the carbonyl group are not only found to be a function of the exothermicity of reaction, but they are also dependent on the nucleophilicity of the alkyl radicals. For each radical R (Me, Et, i‐Pr) the data can be fit to an Evans–Polanyi plot E = a + b (δH), where a uniform slope b = 0.58 can be found for all R. The intercepts a are then found to decrease nearly linearly with the decreasing ionization potential of R, namely, a = 2.1 [IP] – 6.2 (kcal/mol), with the ionization potential given in electron volts.
Modeling of biological reactions
Authors: Ma Shao‐mu , Eyring Henry , Ueda Issaku , Kaneshina Shoji
Published: January 2005

Cell membranes consist of lipid bilayers in which proteins are embedded. Many cell functions are carried out at the cell boundary which interface with water. Here we describe the response to an anesthetic of a lipid bilayer and of an enzyme separately. While both systems are markedly affected by anesthetics at appropriately high concentrations, the result at the clinical concentrations seems best explained as principally an effect on the protein somewhat accentuated by its presence in the lipid bilayer. Thus the lipoprotein complex seems to have the properties of the protein alone, except with somewhat greater induced sensitivity due to the lipid matrix in which it is immersed.
Kinetic investigation of the bromate–ascorbic acid–malonic acid system
Authors: Rábai Gy. , Bazsa Gy. , Beck M. T.
Published: January 2005

According to our experiments the bromide ion concentration exhibits in the bromate–ascorbic acid–malonic acid–perchloric acid system three extrema as a function of time. To describe this peculiar phenomenon, the kinetics of four component reactions have been studied separately. The following rate equations were obtained:

Bromate–ascorbic acid reaction:

Bromate–bromide ion reaction:

Bromide–ascorbic acid reaction:

Bromine–malonic acid reaction:

k4 = 6 × 10−3 s−1, k‐4 ≥ 1.7 × 103 s−1, k5 ≥ 1 × 107M−1 · s−1

Taking into account the stoichiometry of the component reactions and using these rate equations, the concentration versus time curves of the composite system were calculated. Although the agreement is not as good as in the case of the component reactions, it is remarkable that this kinetic structure exhibits the three extrema found.
Kinetics and mechanism of Os(VIII)‐catalyzed hexacyanoferrate(III) oxidation of α amino acids in alkaline medium
Authors: Acharya R. C. , Saran N. K. , Rao S. R. , Das M. N.
Published: January 2005

The kinetics of the Os(VIII)‐catalyzed oxidation of glycine, alanine, valine, phenylalanine, isoleucine, lycine, and glutamic acid by alkaline hexacyanoferrate(III) reveal that these reactions are zero order in hexacyanoferrate(III) and first order in Os(VIII). The order in amino acid as well as in alkali is 1 at [amino acid] ⩽2.5 × 10−2M and [OH−] ⩽1.3 × 10−M, but less than unity at higher concentrations of amino acids or alkali. The active oxidizing species under the experimental conditions is OsO4(H2O) (OH)−. The ferricyanide is merely used up to regenerate the Os(VIII) species from Os(VI) formed during the reaction. The structural influence of amino acids on the reactivity has been discussed. The amino acids during oxidation are shown to be degraded through intermediate keto acids. The kinetic data are accommodated by considering the interaction between the conjugate base of the amino acids and the active oxidizing species of Os(VIII) to form a transient complex in the primary rate‐determining step. The catalytic effect of hexacyanoferrate(II) has been rationalized.
Kinetics and energetics of the criegee intermediate in the gas phase. II. The criegee intermediate in the photooxidation of formaldehyde, in alkyldioxy disproportionation and O + oxoalkane addition reactions
Authors: Herron John T. , Martinez Richard I. , Huie Robert E.
Published: January 2005

The gas‐phase kinetics and energetics of the Criegee intermediate, deduced from studies of O3‐alkene systems, suggest that a hydroxy‐substituted Criegee intermediate probably participates in the photooxidation of formaldehyde. In contradistinction, the existing evidence suggests that the Criegee intermediate and its isomers are probably not involved in alkyldioxy disproportionation reactions. In the case of O + oxoalkane addition reactions, the Criegee intermediate and its isomers are discussed in terms of a complex equilibrium: .
Analysis of the unimolecular reaction N2O5 + M ⇌ NO2 + NO3 + M
Authors: Malko M. W. , Troe J.
Published: January 2005

Recent experimental results on the thermal decomposition of N2O5 in N2 are evaluated in terms of unimolecular rate theory. A theoretically consistent set of fall‐off curves is constructed which allows to identify experimental errors or misinterpretations. Limiting rate constants k0 = [N2] 2.2 × 10−3 (T/300)−4.4 exp(−11,080/T) cm3/molec·s over the range of 220–300 K, k∞ = 9.7 × 1014 (T/300)+0.1 exp(−11,080/T) s−1 over the range of 220–300 K, and broadening factors of the fall‐off curve Fcent = exp(‐T/250) + exp(−1050/T) over the range of 220–520 K have been derived. NO2 + NO3 recombination rate constants over the range of 200–300 K are krec,0 = [N2] 3.7 × 10−30 (T/300)−4.1 cm6/molec2·s and krec,∞ = 1.6 × 10−12 (T/300)+0.2 cm3/molec·s.
Direct detection of CF2 and computer modeling of its appearance in the fluorination of CH2F2
Authors: Seeger C. , Rotzoll G. , Lübbert A. , Schügerl K.
Published: January 2005

The branched chain reaction between CH2F2 and F2 is studied in a miniature tubular reactor having a diameter of 1.1 mm. The gases at the end of the tube expand into a low‐pressure chamber and are analyzed by molecular beam detection. The branching intermediate CF2 is detected directly at low ionization energies. An approximate quantitative reaction model for the fluorination of CH2F2 is proposed and discussed. The influence of the unknown rate constants in this scheme on the course of the reaction is examined. For the most reasonable set of rate constants the unknown rate constant for the branching reaction CF2 + F2 → CF3 + F at 550 K is found to be log k = 10.7 [cm3/mol·s].
A kinetic study of the reaction between formic acid and HoBr
Authors: Brusa M. A. , Colussi A. J.
Published: January 2005

The reaction between formic acid and HOBr in strongly acid aqueous media was studied by absorption spectrophotometry at 298 K. Bromine, the monitored species, displays a transient behavior, gradually rising up to a maximum and then decaying with first‐order kinetics. Reaction rates, expressed as R = ‐dCBr2/dt, depend on the concentrations of HCOOH (0.1–1.4M), HOBr (0.3–1.5 × 10−3 M), and H+ (0.1–1.0M). The mechanism
equation image
with k1 =1.04 ± 0.20M−1· s−1, k3 = 20.2M−1, quantitatively accounts for all observations within experimental error.
The reaction of atomic oxygen O(3P) with isobutane
Authors: Jewell Steven P. , Holbrook Kenneth A. , Oldershaw Geoffrey A.
Published: January 2005

The reaction of O(3P) atoms with isobutane has been studied by using the discharge‐flow system described previously [1]. The rate constant was measured from determinations of the isobutane concentration in the presence of an excess of O atoms and is given by k1 = (7.9 ± 1.4) × 107 dm3/mol·s at 307 K. In order to explain the observed reaction products, the mechanism requires that the principal process be the successive abstraction of H atoms from isobutane and from the t‐butyl radical to give isobutene. A minor part of the reaction between O(3P) and the t‐butyl radical gives the t‐butoxy radical, which decomposes to acetone. The branching ratios are
equation image
.
Very low‐pressure pyrolysis (VLPP) of pentynes. III. Pent‐2‐yne. Heat of formation and resonance stabilization energy of the 3‐methylpropargyl radical
Authors: Nguyen Tam T. , King Keith D.
Published: January 2005

The thermal unimolecular decomposition of pent‐2‐yne has been studied over the temperature range of 988–1234 K using the technique of very low‐pressure pyrolysis (VLPP). The main reaction pathway is C4C5 bond fission producing the resonance‐stabilized 3‐methylpropargyl radical. There is a concurrent process producing molecular hydrogen and penta‐1,2,4‐triene presumably via the intermediate formation of cis‐penta‐1,3‐diene. The 1,4‐hydrogen elimination from cis‐penta‐1,3‐diene is the rate‐determining step in the molecular pathway. This is supported by an independent VLPP study of cis‐ and trans‐penta‐1,3‐diene. RRKM calculations show that the experimental rate constants for CC bond fission are consistent with the following high‐pressure rate expression at 1100 K:

where θ = 2.303RT kcal/mol and the A factor was assigned from the results of shock‐tube studies of related alkynes. The activation energy leads to ΔH[CH3CCĊH2] = 70.3 and DH[CH3CCCH2H] = 87.4 kcal/mol. The resonance stabilization energy of the 3‐methylpropargyl radical is 10.6 ± 2.5 kcal/mol, which is consistent with previous results for this and other propargylic radicals.
Very low‐pressure pyrolysis (VLPP) of penta‐1,3‐dienes. Kinetics of the unimolecular 1,4‐hydrogen elimination from cis‐penta‐1,3‐diene
Authors: Nguyen Tam T. , King Keith D.
Published: January 2005

The thermal unimolecular reactions of cis‐ and trans‐penta‐1,3‐diene (c‐PTD and t‐PTD) have been studied over the temperature range of 1002–1235 K using the technique of very low‐pressure pyrolysis (VLPP). c‐PTD decomposes via 1,4‐hydrogen elimination analogous to that previously reported for cis‐but‐2‐ene. RRKM calculations incorporating a six‐center transition state show that the experimental rate constants are consistent with the following high‐pressure rate expression at 1100 K:

where θ = 2.303RT kcal/mol, and the A factor was assumed to be the same as that for cis‐but‐2‐ene. The activation energy is in excellent agreement with that obtained for cis‐but‐2‐ene. t‐PTD also undergoes decomposition by H2 elimination presumably via the prior rapid isomerization to c‐PTD the results are in exact agreement with those for c‐PTD.
Kinetics and mechanism of the gas‐phase thermal reaction between bis(fluoroxy) difluoromethane CF2(OF)2 and carbon monoxide
Authors: Croce A. E. , Castellano E.
Published: January 2005

The kinetics of the gas‐phase thermal reaction between CF2(OF)2 and CO has been studied in a static system at temperatures ranging between 110 and 140°C. The only reaction products were CF2O and CO2, giving the following stoichiometry:

The reaction is homogeneous. The rate is strictly second order in CF2(OF)2 and CO, and is not affected by the total pressure or by the presence of reaction products. Oxygen promotes a sensitized oxidation of CO and inhibits the formation of CF2O.

The experimental results in the absence of oxygen can be explained by a chain mechanism similar to that proposed for the reaction between F2O and CO with an overall rate constant of

From the experimental data obtained on the oxygen‐inhibited reaction, the rate constant for the primary process can be calculated:

The chain length v = 2.5 is independent of the temperature. Taking for collision diameters σ = 6 Å and σCO = 3.74 Å, a value α = 5.3 × 10−3 for the steric factor is obtained.
Mechanism of oxidation of phenylthioacetic acids by potassium peroxodiphosphate
Authors: Srinivasan C. , Pitchumani K.
Published: January 2005

The rates of oxidation of phenylthioacetic acid (PTAA) and several substituted phenylthioacetic acids by potassium peroxodiphosphate (PP) in 50% (v/v) aqueous acetic acid have been studied in detail. The rate of oxidation is expressed as

An analysis of the dependence of the rate on [H+] reveals that H3P2O is the active oxidizing species in the oxidation. The effect of ring substituents on the rate gives a ρ+ value of ‐0.45 ± 0.03 (r = 0.998, s = 0.02 at 40°C), pointing to the development of an electron‐deficient center in the transition state. The results are discussed in terms of a mechanism involving the rate‐determining formation of an intermediate between PP and phenylthioacetic acids, followed by the decomposition of the intermediate. These kinetic results are compared with those obtained in the oxidation of phenylthioacetic acids by peroxodisulfate.
Thiocyanate‐assisted demetallation of α,β,γ,δ‐tetra(p‐sulfophenyl)porphinatoindium(III) in aqueous media
Authors: Reynolds Warren L. , Gall Daniel , Arndt Peter
Published: January 2005

The rate law for the demetallation of the title indium(III)‐porphin complex in aqueous acidic thiocyanate media at 3.00M ionic strength was found to be of the form

where [H4P2−] is the concentration of the diacid product formed, [InP]t is the total concentration of all forms of indium(III)‐porphin complex present, and a and b are constants. The constant a is a pseudo‐third‐order rate constant with the value (0.057 ± 0.005)M−2 s−1 and b has the value 0.704M−2 at 50.5°C. If the mechanism for demetallation involves ringpuckering with the attachment of two H+ ions, then 1/b can be identified with the product K1K2 for the stepwise dissociation of two protons from two ring pyrrolic nitrogen atoms of H2InP−. In the sulfonated tetraphenylporphin used for these studies the ring pyrrolic nitrogen atoms seem to be the most probable sites for protonation. If this identification is correct, the value of 1.42 ± 0.13 found for the product K1K2 shows the enormous effect that the presence of the In3+ center has on the ionization constants of these two protons. That the kinetic studies show saturation effects with respect to proton addition to InP3− may result from the fact that In3+ sits about 0.6 Å above the porphin ring.
Kinetics and mechanism of reaction of p‐methoxystyrene and tetracyanoethylene. I. Evidence for competitive 1,2‐ and 1,4‐cycloadditions
Authors: Uosaki Y. , Nakahara M. , Osugi J.
Published: January 2005

Time‐resolved absorption spectra for a reaction mixture of p‐methoxystyrene and tetracyanoethylene (TCNE) are found to have a band maximum at 325 nm which is assigned to the 1,4‐cycloadduct. The reaction in chloroform at 15, 20, and 25°C is followed by the charge‐transfer band at 600 nm. The 1,4‐cycloadduct, besides the so far known 1,2‐cycloadduct and EDA complex, is taken into account to derive the rate equation for the EDA complex that is a linear second‐order differential equation. The rate constants for the elementary steps involved in the reaction are obtained. The 1,4‐cycloaddition has an activation entropy of ‐63 J/K·mol for the cycloreversion and a reaction constant ρ of ‐4.7, both of which indicate the polar transition state. On the other hand, activation entropy of the 1,2‐cycloaddition is 73 J/K·mol more negative than that of the 1,4‐cycloaddition, supporting the zwitterionic mechanism for the 1,2‐cycloaddition.
Kinetics and mechanisms of oxidation of organic compounds by silver(II) species. Part VI: Iminodiacetic acid and N‐methyliminodiacetic acid
Authors: Baiocchi Claudio , Bovio Giuseppe , Mentasti Edoardo
Published: January 2005

The kinetics and mechanism of oxidation of iminodiacetic acid and N‐methyliminodiacetic acid by aquasilver(II) and Ag(II)‐2,2′‐bipyridine complexes has been investigated. The results are discussed with reference to the active reaction pathways, the equilibrium quotient of the title reactions, the protolytic equilibria which involve the oxidizing complex, and the intrinsic self‐exchange rates of the oxidants.
Experimental investigation of chamber‐dependent radical sources
Authors: Carter W. P. L. , Atkinson R. , Winer A. M. , Pitts J. N.
Published: January 2005

While environmental chamber data have been widely used to generate and validate computer models of the chemistry occurring in polluted atmospheres, the effects of the chambers on the gas‐phase chemistry being studied have been poorly characterized. In order to investigate such chamber effects, a series of NOx—air irradiations, with trace levels of organics present to monitor OH radical concentrations, have been carried out in four different environmental chambers (ranging in volume from ∼100 to 40,000 L) at varying temperatures, humidities, pressures, and reaction conditions. In addition, a number of control experiments have been carried out to validate the technique for measuring OH radical levels in these irradiations. The data show that unknown sources of OH radicals are present in all of the chambers studied. The data are consistent with the presence of two distinct radical sources: (1) the photolysis of initially present HONO, whose importance increases with increasing NO2/NO concentration ratios, but which is a minor contributor to the overall radical flux after 30–60 min of irradiation, and (2) a constant (for these NOx—air irradiations) radical source which dominates beyond approximately the first 60 min of irradiation. The radical input rates, after the first ∽30–60 min of irradiation, are independent of the NO concentration, increase with increasing temperature, humidity, and NO2 concentration, are proportional to light intensity, and are dependent on the chamber employed. Although the exact nature of this radical source is still undetermined, results of experiments reported here allow a number of possible mechanisms to be ruled, out, and these are discussed.
Kinetics and mechanism of the oxidation of halotoluenes by acidic hexacyanoferrate(III)
Authors: Bhattacharjee A. K. , Mahanti M. K.
Published: January 2005

The oxidation of halotoluenes by hexacyanoferrate(III) in aqueous acetic acid containing perchloric acid (0.5M) at 50°C gave the corresponding aldehyde as the major product, and a small amount of polymeric material. The order with respect to each of the reactants—substrate, oxidant, and acid—was found to be unity. Increasing proportions of acetic acid increased the rate of the reaction. The reaction was influenced by changes in temperature, and the activation parameters have been evaluated. The Hammett plot yielded a ρ+ value of −1.8. A kinetic isotope effect kH/kD = 6.0 has been observed. The pathway for the conversion of the halotoluenes to the products has been mechanistically visualized as proceeding through the benzylic radical intermediate, formed in the rate‐determining step of the reaction. The radical undergoes rapid conversion to the products.
F‐to‐HF reactions. IX. Nonthermal complications for the CH4/C3F6/C2F6 moderated nuclear recoil system
Authors: Mathis Chester A. , Root John W.
Published: January 2005

Results are reported from moderated nuclear recoil 18F experiments with the CH4/C3F6/C2F6 mixture system. At a 99.5% confidence level measurement precision of ±3.4%, non‐thermal F‐to‐HF reactions are phenomenologically suppressed at C2F6 moderator concentrations in the range of 95.0–99.95 mol‐%. Effectively equilibrium reaction conditions can be established in well‐designed experiments of this type.
Experimental determination of the equilibrium constant of the reaction CH3 + O2 ⇄ CH3O2 during the gas‐phase oxidation of methane
Authors: Khachatryan L. A. , Niazyan O. M. , Mantashyan A. A. , Vedeneev V. I. , Teitel'boim M. A.
Published: January 2005

For the experimental determination of the equilibrium constant of the reaction CH3 + O2 ⇄ CH3O2 (1), the process of methane oxidation has been studied over the temperature range of 706–786 K. The concentration of CH3O2 has been measured by the radical freezing method, and that of CH3 from the rate of accumulation of ethane, assuming that C2H6 is produced by the reaction CH3 + CH3 → C2H6 (2). The equilibrium constant of reaction (1) has been obtained at four temperatures. For the heat of the reaction the value Δ˚H298 = ‐32.2 ± 1.5 kcal/mol is recommended.
Kinetics and thermochemistry of electron attachment to SF6
Authors: Heneghan Shawn P. , Benson Sidney W.
Published: January 2005

Thermochemical analysis of the electron capture process of SF6 leads to a rate constant for the reverse process \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm SF}_6^ - \mathop \to \limits^2 {\rm SF}_6 + e^ -,k_2 = 1.5 \times 10^{13 - 31.4/\theta } {\rm s}^{{\rm - 1}} $\end{document}, where θ = 2.303RT, in kcal/mol. The electron affinity of 32±3 kcal/mol is deduced from the observed bimolecularity of the capture process down to 0.1 torr Ar bath gas and estimated entropies of SF6 and SF. The capture process is discussed from the view point of the formation of a metastable SF electron (SF6·e) Langevin complex which appears to have a lifetime of about 2 × 10−13 s. Curve crossing from the SF6·e complex to vibrationally excited (SF)* appears to have a normal rate and A factor. This is interpreted to indicate near‐resonant coupling between the orbiting electron and the vibronic motions of SF6, together with similarity in structure of SF6 and SF. It is shown that the apparent slowness of thermal electron ejection from SF is a result of an unfavorable equilibrium constant rather than a slow rate.
Thermochemistry and kinetics of the reaction of methyl mercaptan with iodine
Authors: Shum Lilian G. S. , Benson Sidney W.
Published: January 2005

The gas‐phase reaction CH3SH + I2 has been studied spectrophotometrically over the temperature range of 476–604 K. It was found that the reaction undergoes H abstraction by I at ≤575 K, leading to the formation of MeSI and followed by a secondary reaction which leads to the formation of MeSSMe:
equation image
Taking into consideration the effect of reaction (2), the equilibrium constant K1 (554 K) has been evaluated to be 0.025 ± 0.004. This value was combined with the estimated values S (CH3SI, g) = 73.7 ± 1.0 eu and 〈ΔC〉 = 0.87 ± 0.3 eu to obtain ΔH = 4.03 ± 0.73 kcal/mol. This yields ΔH (CH3SI, g) = 7.16 ± 0.73 kcal/mol when combined with known thermochemical values for CH3SH, HI, and I2. A kinetic study was vitiated by the concurrent heterogeneous reaction of MeSH and I2 at lower temperatures and the rather complicated chemistry occurring at elevated temperatures. However, attempts at measuring rate constants at 554 K lead to a lower limit of ΔH (CH3S·, g) ≥ 29.5 ± 2 kcal/mol when an estimated value of A = 1010.8 ± 0.2 L/mol·s for the reactionc
equation image
is used. DH (CH3S–I) is estimated to be 49.3 ± 1.7 kcal/mol. The bond strengths of some divalent sulfurs and the reaction mechanisms are discussed. A crude estimate of DH0(H–CH2SH) = 96 ± 1 kcal has been obtained from the kinetic data.
Rate constants for the gas‐phase reactions of OH radicals with β‐dimethylstyrene and acetone. Mechanism of ββdimethylstyrene NOx – air photooxidation
Authors: Chiorboli C. , Bignozzi C. A. , Maldotti A. , Giardini P. F. , Rossi A. , Carassiti V.
Published: January 2005

The rate constants of gas‐phase reactions of the hydroxyl radical with β‐dimethylstyrene and acetone have been determined by a relative method at 298 K. The values obtained are β‐dimethylstyrene (3.3 ± 0.5) × 10−11 cm3/molecule·s and acetone (6.6 ± 0.9) × 10−13 cm3/molecule·s. A simplified kinetic treatment of the experimental data shows that β‐dimethylstyrene is stoichiometrically converted to benzaldehyde and acetone. In the photooxidation study of benzaldehyde, carbon dioxide was the only detected product. The ratio between carbon dioxide produced and benzaldehyde reacted was ≥1.
The kinetics and mechanism of thermal decomposition of alkyl isocyanates
Authors: Blake P. G. , Ijadi‐Maghsoodi S.
Published: January 2005

The kinetics of the gas‐phase decomposition of ethyl, isopropyl, and t‐butyl isocyanates have been studied in the temperature range of 380–530°C. t‐Butyl isocyanate decomposes almost exclusively by a unimolecular route to isobutene and HNCO, but in EtNCO and i‐PrNCO this route competes with a free‐radical chain which produces CO, CH4, and HCN or CH3CN. In i‐PrNCO, however, the chain process is very rapidly inhibited by the propene formed in the parallel unimolecular route. A minor heterogeneous bimolecular decomposition in each case gives rise to carbon dioxide and a carbodiimide. Mechanisms and trends in the alkyl isocyanates from methyl through t‐butyl are discussed.
Rate constant and products of the reaction CS2 + OH in the presence of O2
Authors: Barnes I. , Becker K. H. , Fink E. H. , Reimer A. , Zabel F. , Niki H.
Published: January 2005

The reaction of OH radicals with CS2 has been investigated by the application of Fourier transform infrared spectroscopy using both photolytic and nonphotolytic competitive techniques in a 420‐L reaction chamber at different pressures over the temperature range of 264–293 K. The measured effective rate constant was found to be dependent on total pressure, temperature, and the mole fraction of O2 present in the system. The products of the reaction were found to be COS and SO2 with one molecule of each being formed for every reacted CS2. A value of (2.7 ± 0.6) × 10−12 cm3/molecule·s was obtained as effective rate constant for the reaction at 293 K in 760 torr of synthetic air.
Direct conversion of synthesis gas to light hydrocarbons on heterogeneous catalysts
Authors: Gadalla Ahmed M. , Chan Ting , Anthony Rayford G.
Published: January 2005

Six catalysts consisting of mixtures of catalysts for methanol synthesis and its conversion to hydrocarbons were prepared by mechanical mixing, impregnation, and coprecipitation, followed by extrusion. Phase changes occurring during various stages of preparation and after use were followed by x‐ray diffraction. The catalysts were found to be nontrivial bifunctional catalysts which selectively convert synthesis gas directly to low molecular weight hydrocarbons in the range of C1 to C4. Methanol and dimethylether were detected as intermediates. Olefins were formed, and their further hydrogenation yielded products with high paraffinic content. Carbon dioxide retards the conversion of synthesis gas to hydrocarbons, whereas steam retards the hydrogenation of olefins. On heating the commercial zinc chromite catalyst, dehydration, reduction, and solid‐state reactions occur with the formation of ZnO‐ZnCr2O4 solid solution. Exchanging cations within the zeolite component did not affect the space lattice, but these ions tend to occupy special sites, and during service more order was detected. Copper ions tend to diffuse in ZnO, forming a solid solution, and during service metallic copper was produced.
Kinetics and mechanism of reaction of p‐methoxystyrene and tetracyanoethylene. II. Effect of pressure
Authors: Uosaki Y. , Nakahara M. , Osugi J.
Published: January 2005

The reaction of p‐methoxystyrene and TCNE is studied in 50% CHCl3–50% CCl4 (v/v) at 25°C at high pressures up to 1000 bar by following the disappearance of the absorbance at 600 nm due to the EDA complex. By using the mixed solvent, equations required for the high‐pressure kinetics are simplified. The volume of activation is −42 to −46 cm3/mol for the 1,4‐cycloaddition and −55 cm3/mol for the 1,2‐cycloaddition. The activation volume for the 1,4‐cycloreversion is assumed to be −8 to −12 cm3/mol based on a previous study on a similar system. The rate of the cycloreversion process is affected by solvent polarity. The rate of the 1,2‐cycloaddition is influenced by solvent more significantly than that of the 1,4‐cycloaddition. It is concluded that the transition state is polar in both cycloadditions and that its zwitterionic character is much stronger in the 1,2‐cycloaddition.
Kinetics and equilibria in the system Br + t‐BuO2H ⇆ HBr + t‐BuO2. OH bond dissociation energy in t‐BuO2‐H
Authors: Heneghan Shawn P. , Benson Sidney W.
Published: January 2005

The kinetics and equilibria in the system Br + t‐BuO2H ⇆ HBr + t‐BuO2· have been measured in the range of 300–350 K using the very low pressure reactor (VLPR) technique. Using an estimated entropy change in reaction (1) ΔS1 = 3.0 ± 0.4 cal/mol·K together with the measured ΔG1, we find ΔH1 = 1.9 ± 0.2 kcal/mol and DHº (t‐BuO2‐H) = 89.4 ± 0.2 kcal/mol ΔHf·(tBuO2·) = 20.7 kcal/mol and DHº (t‐Bu‐O2) = 29.1 kcal/mol. The latter values make use of recent values of ΔHf·(t‐Bu) = 8.4 ± 0.5 kcal/mol and the known thermochemistry of the other species. The activation energy E1 is found to be 3.3 ± 0.6 kcal/mol, about 1 kcal lower than the value found for Br attack on H2O2. It suggests a bond 1 kcal stronger in H2O2 than in tBuO2H.
Induced heterogeneity, a novel technique for the study of gas‐phase reactions. 2. Direct study of CC bond scission in ethane
Authors: Kanan Khalid , Purnell Howard , Sepherad Abolgassem
Published: January 2005

Deliberate activation of the reaction vessel surface leads to the domination of chain termination in ethane pyrolysis by the reaction

As a result, chains are dramatically reduced in length, methane yields are entirely primary and larger in proportion to other products, and values of k1

can be directly determined from methane yield data without ambiguity. Experiments carried out in the temperature range of 841–913K at initial ethane pressures of 1–20 torr, without and with added nitrogen, yield the infinite pressure Arrhenius equation

It is shown that most previously published data can be combined with those of this study to yield

Fall‐off curves for k1 as a function of pressure are in good agreement with those from other laboratories. From these the relevant data for k−1 can be extracted for use in other kinetic studies.
The reaction of cyclopentyl radicals with carbon tetrachloride
Authors: Matheson Ian , Tedder John , Sidebottom Howard
Published: January 2005

The photolysis of azocyclopentane in the presence of cyclopentane–carbon tetrachloride mixtures has been investigated in the gas phase. Product analysis data have been used to determine the Arrhenius parameters for the reactions

The rate data for chlorine atom abstraction from CCl4 by the cyclopentyl radical were compared with available data for other alkyl radicals in both the gas and the solution phases. The results indicate that the rate constant for chlorine atom abstraction in the gas phase is fairly insensitive to the nature of the attacking alkyl radical and that the activation energy for a secondary radical is about 4 kcal/mol higher than the corresponding reaction in the solution phase.
Studies in nickel(IV) chemistry. Kinetics of electron transfer from dimethyl sulfoxide to oxohydroxonickel(IV) in aqueous acid medium
Authors: Neogi G. , Acharya S. , Panda R. K. , Ramaswamy D.
Published: January 2005

The kinetics of decomposition of “oxohydroxonickel(IV)” [Ni(IV)] with concomitant intramolecular electron transfer to produce hexaaquanickel(II) and dioxygen in aqueous acid solutions show pseudo‐first‐order dissappearance of the Ni(IV). The pseudo‐first‐order rate constants for the acid decomposition (kad) satisfy

where KMH and kd refer to the equilibrium protonation constant and the decomposition constant of the protonated species of the Ni(IV) respectively. The values of KMH and kd in aqueous medium at 45°C and μ = 2.0M are 25.5 ± 1M−1 and (1.7 ± 0.1) × 10−5 s−1, respectively.

The kinetics of the intermolecular electron transfer from dimethyl sulfoxide (DMSO) to the Ni(IV), producing Ni(H2O)62+ and dimethyl sulfone as products, have been investigated by monitoring the formation of Ni(H2O)62+. The pseudo‐first‐order rate constants for the electron transfer kobs are linearly dependent on [DMSO]0 or [H+], attaining limiting values at higher relative [DMSO]0 or [H+], in accordance with

where K1c and K2c represent the formation constants of the precursors involving DMSO and the unprotonated and one‐protonated Ni(IV) species, respectively, and k1x and k2x are the corresponding decomposition rate constants of the precursors. The values of K2c and k2x are (2.3 ± 0.1) × 104M−1 and 19 ± 1 s−1, respectively, at 45°C and μ = 1.0M. Results are interpreted in terms of probable mechanisms involving (1) a rate‐determining decomposition of the protonated Ni(IV) followed by rapid product formation steps, and (2) precursor complex formation between DMSO and the unprotonated or the protonated species of the Ni(IV) followed by rate‐determining decomposition with electron transfer.
A Mass‐spectrometric study of propionaldehyde oxidation in the negative temperature coefficient region
Authors: Kaiser E. W.
Published: January 2005

The oxidation of propionaldehyde has been investigated in a 1‐L Pyrex reactor at total pressures of 50–120 torr and temperatures 553–713 K. Detection of reactants and products was principally by molecular beam mass spectrometry, although certain species could only be measured by gas‐chromatographic analysis. At 553 K the yield of water was ∼83% of the propionaldehyde consumed, leading to the conclusion that OH is the principal chain carrier near the beginning of the negative temperature coefficient region. Many oxygenated organics (CH2O, CH3CHO, C2H5OH, C2H5O2H, CH3O2H) and C2H4 are formed during the oxidation process. These oxidation products are consistent with the important role of O2 addition to C2H5 radicals at 553 K followed by subsequent reactions of the C2H5O2 radical. As the temperature is increased, the product concentrations smoothly change to a much simpler distribution in which C2H4, H2O2, and CO are the dominant products.
Kinetics of the reaction between methane and iodine from 830 to 1150 K in the presence and absence of oxygen
Authors: Pardini Steven P. , Martin Don S.
Published: January 2005

The formation of methyl iodide was determined by radiochemical methods and by massspectroscopic analyses in mixtures of Ar–CH4–I2 and ArCH4I2O2, heated by a reflected shock wave to temperatures of 830–1150 K. The rate of formation of CH3I was consistent with the chain mechanism

where the indicated rate constant for reaction between I and CH4 is given by k2(cm3/mol · s) = 1014.17 exp(−32.9 ± 0.8 kcal/mol/RT). No effect on the reaction rate by the presence of O2 was detected. However, in one experiment at 1097 K with 3.86 mol % O2 the formation of CH2O was indicated by the mass‐spectroscopic analysis, presumably from the reaction of O2 with CH3.
Catalytic isomerization of 1‐hexene on hy zeolite
Authors: Ko A. N. , Wojciechowski B. W.
Published: January 2005

The isomerization of 1‐hexene on 70/80 mesh HY zeolite was studied at 200°C. The observed reaction products are formed via a variety of processes including double bond shift, cis–trans isomerization, skeletal rearrangement, cracking, hydrogen transfer, polymerization, cyclization, and coke formation. By applying the time‐on‐stream theory, the products have been classified as primary, secondary, or both, according to their OPE curves on product selectivity plots. 2‐Ethyl‐1‐butene, which is present as an impurity in the feed, is found to react about 30 times faster than 1‐hexene. Both 2‐hexenes and 3 hexenes are formed primarily from 1‐hexene, while 3 methyl 2 pentenes and 3‐methyl‐1‐pentene formed from 2‐ethyl‐1‐butene. The ratio of the initial rate of deprotonation to that of hydrogen shift in these reactions is ∼15 and ∼100, respectively. All products of skeletal rearrangement are observed to be secondary. Cracking products are produced mainly from precoke, which is also the source of hydrogen in the formation of paraffins. A detailed reaction network along with its associated mechanisms are presented and discussed.
Pyrolysis of 5‐chloropentan‐2‐one and 4‐chloro‐1‐phenylbutan‐1‐one. Participation of the carbonyl group
Authors: Chuchani Gabriel , Domínguez Rosa M.
Published: January 2005

The rates of elimination of 5‐chloropentan‐2‐one and 4‐chloro‐1‐phenylbutan‐1‐one in the gas phase have been determined in a static system, seasoned with allyl bromide, and in the presence of the chain inhibitor propene. The reactions are unimolecular and follow a first‐order rate law. The working temperature and pressure ranges were 339.4–401.1°C and 46–117 torr, respectively. The rate coefficients for the homogeneous reactions are given by the following Arrhenius equations: for 5‐chloropentan‐2‐one, log k1(s−1) = (13.12 ± 0.88) ‐ (207.8 ± 11.0)kJ/mol/2.303RT; and for 4‐chloro‐1‐phenylbutan‐1‐one, log k1(s−1) = (12.28 ± 1.09) ‐ (185.2 ± 12.0)kJ/mol/2.303RT. The carbonyl group at the γ position of the CCl bond of haloketones apparently participates in the rate of pyrolysis. The five‐membered conformation appears to be a favorable structure for anchimeric assistance of the CO group in the gas‐phase elimination of chloroketones.
Kinetics and mechanism of photochemical reactions of peroxomonosulfate in the presence and absence of 2‐propanol
Authors: Kanakaraj Perumal , Maruthamuthu Pichai
Published: January 2005

The photochemical decomposition of peroxomonosulfate (PMS) in the presence and absence of 2‐propanol at 25°C was found to obey an overall first‐order rate – d[PMS]/dt = kϕ[PMS]. In the absence of 2‐propanol, the quantum yield ≤ for the decomposition of PMS was found to depend upon the concentration of PMS at [PMS] > 2 × 10−M, and is independent of concentration at [PMS] > 2 × 10−2M. The quantum yield in the presence of 2‐propanol was found to be 3.03 at [PMS] = 1 × 10−2M and 4.45 at higher concentrations of PMS. In the pH range of 2–9.0 the quantum yield was found to be independent of pH, and the overall rate constant kϕ was found to be 6.49 × 10−3 s−1 and 1.68 × 10−3 s−1, respectively, in the presence and absence of isopropanol. A suitable chain mechanism is proposed and explained.
Kinetics and mechanism of the decomposition of N‐brominated alanine in aqueous solution
Authors: Stanbro William D. , Lenkevich Michael J.
Published: January 2005

Aqueous bromine reacts with alkyl‐sidechain amino acids through a series of steps resulting in the formation of the corresponding alkyl aldelydes and nitriles. The kinetics and the mechanism of the interaction of bromine with alanine are examined. The products and the rates of this reaction are dependent in a complex way on the initial reactant concentration and pH. Acetaldeyde production is favored at low bromine‐to‐alanine ratios, low bromine concentrations, and pH values above 6. The first‐order rate constant for the formation of acetaldelyde from alanine under these conditions is k4 = 1.98 × 1015 e−22,500/RT min−1. At higher concentration the nitrile is formed through a bromoimine intermediate. Under most conditions the nitrile appears to form from a catalyzed decomposition of the bromoimine which is too fast to be followed by the methods used in this study. However, residual amounts of the bromoimine decay by a slower first‐order mechanism. The rate constant for this slower reaction in the case of alanine at pH 6.8–6.9 and alanine concentrations of 1 × 10−4M is k6 = 1.75 × 105 e−10,400/RT min−1.
The gas‐phase decomposition of methylsilane. Part I. Mechanism of decomposition under shock‐tube conditions
Authors: Sawrey B. A. , O'Neal H. E. , Ring M. A. , Coffey D.
Published: January 2005

The gas‐phase decompositions of methylsilane and methylsilane‐d3 have been investigated in a single‐pulse shock tube at 4700 torr total pressure in the temperature range of 1125–1250 K. For CH3SiD3 at 1200 K three primary steps occur in the homogeneous decomposition with efficiencies in parentheses:

,

, and

. For CH3SiH3 at 1200 K the primary CH4 elimination efficiency is 0.09 while the total primary H2 elimination efficiency is 0.91. Minor product formations of C2H4, acetylene, dimethylsilane, and SiH4 are discussed.
Kinetics, mechanism, and endo selectivity of Diels–Alder reactions of alkylmonosubstituted ethenes with cyclohexa‐1,3‐diene in the gas phase
Authors: Huybrechts G. , Poppelsdorf H. , Maesschalck L. , Van Mele B.
Published: January 2005

The reactions magnified image where Y = CH3 (M), C2H5 (E), iC3H7 (I), and tC4H9 (T) have been studied between 488 and 606 K. The pressures of CHD ranged from 16 to 124 torr and those of YE from 57 to 625 torr. These reactions are homogeneous and first order with respect to each reagent. The rate constants (in L/mol·s) are given by

The Arrhenius parameters are used as a test for a biradical mechanism and to discuss the endo selectivity of the reactions.
Pyrolysis of methyl chloride, a pathway in the chlorine‐catalyzed polymerization of methane
Authors: Weissman Maia , Benson Sydney W.
Published: January 2005

The reaction of CH4 + Cl2 produces predominantly CH3Cl + HCl, which above 1200 K goes to olefins, aromatics, and HCl. Results obtained in laboratory experiments and detailed modeling of the chlorine‐catalyzed polymerization of methane at 1260 and 1310 K are presented. The reaction can be separated into two stages, the chlorination of methane and pyrolysis of methylchloride. The pyrolysis of CH3Cl formed C2H4 and C2H2 in increasing yields as the degree of conversion decreased and the excess of methane increased. Changes of temperature, pressure, or additions of HCl had little effect. In the absence of CH4 C2H4 and C2H2 are formed by the recombination of ĊH3 and ĊH2Cl radicals. With added CH4 recombination of ĊH3 forms C2H6, which dehydrogenates to C2H4 + H2. C2H4 in turn dehydrogenates to C2H2 + H2. While HCl, C, CH4, and H2 are the ultimate stable products, C2H4, C2H2, and C6H6 are produced as intermediates and appear to approach stationary concentrations in the system. Their secondary reactions can be described by radical reactions, which can lead to soot formation. ĊH3 ‐ initiated polymerization of ethylene is negligible relative to the Ċ2H3 formation through H abstraction by Cl. The fastest reaction of Ċ2H3 is its decomposition to C2H2. About 20% of the consumption of C2H2 can be accounted for by the addition of Ċ2H3 to it with formation of the butadienyl radical. The addition of the latter to C2H2 is slow relative to its decomposition to vinylacetylene. Successive H abstraction by Cl from C4H4 leading to diacetylene has rates compatible with the experimental values. About 10% of Ċ4H5 abstracts H from HCl and forms butadiene. Successive additions of Ċ2H3 to butadiene and the products of addition can account for the formation of benzene, styrene, naphthalene, and higher polyaromatics. The following rate parameters have been derived on the basis of the experimentally measured reaction rates, the estimated frequency factors, and the currently available heat of formation of the Ċ2H3 radical (69 kcal/mol):
The role of electronically excited states in recombination reactions
Authors: Smith Ian W. M.
Published: January 2005

Methods are described for including the participation of bound electronically excited states in calculations on radical recombination reactions. These methods are illustrated by applying them to the reactions

For O2, accurate ab initio potentials are used in calculations which show that the electronic degeneracy and long‐range part of the potential are likely to be crucial in determining the contribution of a given electronic state to the overall reaction, as long as the state is not so weakly bound that it dissociates thermally before being electronically quenched. Weak collision effects are allowed for using a Monte Carlo technique and an assumed exponential form for the distribution of energies transferred in collisions with a third body. For larger systems it is evident that the role of bound excited states in the low‐pressure regime falls rapidly as the size of the system increases. As the high‐pressure limit is approached, however, the contribution of excited states is likely to come close to that expected simply on the basis of electronic degeneracy.