bluuebunny/crossref_metadata_2025 huggingface dataset:
Flash photolysis of s‐trinitrobenzene solutions
Authors: Capellos C. , Suryanarayanan K.
Published: January 2005

Flash photolysis of s‐trinitrobenzene (TNB) aerated solutions in alcohols generates a transient species with absorption maxima at 430 and 510 nm. The yield of the transient is a function of oxygen concentration, and its rate of formation is viscosity dependent. In deaerated solutions instead of the transient, a brown permanent product, identified as a charge transfer (CT) complex with absorption maxima at 470 and 550 nm, is produced. These species are formed only in polar solvents with relatively high proton affinity. The data suggest an intermolecular proton transfer, from electronically excited TNB to the solvent forming the anion magnified image The anion thereby produced interacts with oxygen in aerated alcohols to form the transient attributed to CT complex magnified image while in deaerated alcohols the anion reacts with the solvent to produce RO−, which leads to the CT complex RO…−TNB. This mechanism is supported by detailed kinetic and spectroscopic studies.
A new mechanism for gas phase ozone–olefin reactions
Authors: O'Neal H. Edward , Blumstein Carl
Published: January 2005

A new mechanism for gas phase ozone‐olefin reactions is proposed. The mechanism involves biradical intermediates which can react in a variety of ways. One of the possible reaction modes corresponds to the Criegie mechanism originally proposed to explain solution ozonolysis reactions and generally also accepted in the past for gas phase reactions. However, an examination of the gas phase data on ozone–olefin reactions and of the thermochemical and kinetic requirements for these reactions indicates that the Criegie reaction mode may be the least important of various other reaction possibilities. Those other reaction possibilities involve intramolecular H abstractions and rearrangements in biradical intermediates. The proposed mechanism provides very reasonable explanations for a number of unusual observations on gas phase ozonolysis. These are the formation of peroxidic bound products, aldehyde and 1,2‐dicarbonyl product fluorescences, and unexpected carbonyl product formations.
Homogeneous gas‐phase pyrolyses with a wall‐less reactor. III. The oxygen–ethane reaction. A double reversal in oxygen and surface effects
Authors: Taylor Jay E. , Kulich Donald M.
Published: January 2005

The impact of surface and oxygen on the oxidative pyrolysis of ethane at temperatures above 590°C was studied using a wall‐less reactor. At very low conversions under homogeneous conditions, ethene formation begins at the same temperature regardless of whether oxygen is present or absent. Between 0.00 and 0.13% conversion (592–632°C), the rate with oxygen is actually less than the rate in the absence of oxygen. A reversal occurs at about 633°C above which oxygen has a promoting effect. It is concluded that under homogeneous conditions the initiation step in the oxygen‐promoted pyrolysis is the same as in the oxygen‐free pyrolysis; therefore, initiation by direct attack of oxygen on ethane does not make an important contribution. The decrease in rate observed upon addition of oxygen implies the formation of the relatively unreactive HO2 · radical. As conversion of the HO2 · radical to the more reactive HO · radical becomes significant, the reaction is highly accelerated. If a stainless steel surface is added, the reaction is inhibited at higher conversions in the presence of oxygen. Again at low conversions, a second reversal occurs, and the stainless steel surface acts as a promoter below 649°C. The rate of surfacecatalyzed ethene formation at 590°C equals the rate of homogeneous ethene formation at 630°C.
Very low‐pressure pyrolysis. IX. The decomposition of azoethane, azoisopropane, and 2,2′‐azoisobutane
Authors: Perona Michael J. , Beadle Peter C. , Golden David M.
Published: January 2005

The very low‐pressure (VLPP) technique was used to study the pyrolysis of azoethane (AE), azoisopropane (AIP), and 2,2′‐azoisobutane (AIB). The low pressure rate constants were related to the high‐pressure Arrhenius parameters by means of the RRKM theory. This procedure in itself does not yield an unambiguous set of parameters. However, thermochemical and kinetic arguments are given which support the following values of log k∞ for the pyrolysis of AE, AIP, and AIB, respectively: 16.4–49.7/θ 16.6–47.9/θ, and 16.4–42.8/θ, where θ = 2.303RT in kcal/mole. The flow dependence of kuni was used to estimate the collisional efficiencies of the azo compounds relative to the wall.
Comparisons between experimental and calculated rate constants for dissociation and combination reactions involving small polyatomic molecules
Authors: Tsang Wing
Published: January 2005

The experimental results on decomposition and combination reactions involving O3, HNO3, NH3, C2N2, and NO2Cl over extended temperature and pressure ranges are compared with the deductions from RRKM calculations. Quantitative fits of the data over the entire range are possible only if the external (overall) rotations are assumed to be involved in the reactions. Recommended rate constants for the reactions O + O2+ N2→ O3+ N2and OH + NO2+ N2→ HNO3+ N2are presented.
Low‐pressure thermal decomposition of ONBr and ONCl in shock waves
Authors: Maloney K. K. , Palmer H. B.
Published: January 2005

Rate constants for the low‐pressure unimolecular decomposition of ONBr and ONCl in an argon bath have been determined at temperatures in the vicinity of 1000°K. Both molecules exhibit the usual depression of the observed activation energy below the bond dissociation energy. The Arrhenius expressions obtained are (units of cc mole−1 sec−1):

Treatment of the data by the classical RRK theory yields s ≅ 2.7 ± 1 for ONCl and 3.0 ± 0.6 for ONBr. Coupling the shock tube results for ONCl with lower‐temperature data from Ashmore and Burnett [3], one obtains s ≅ 2.5 ± 0.5 and λ ≈︁ 1. If it is assumed that s is also 2.5 for ONBr, then one finds the surprising (but tentative) result that λONClAr/λONBrAr ≈︁ 3 to 4.
Basic elimination of HCl from chlorinated ethanes
Authors: Walraevens R. , Trouillet P. , Devos A.
Published: January 2005

Chloroethanes react with aqueous caustic to yield either elimination or substitution products. The reaction rates were measured for the dichloroethanes, trichloroethanes, tetrachloroethanes, and pentachloroethane between 283 and 353°K. The constants of HCl eleminations referring to the rate equation

are given by

all rate constants being in 1./mole·s and R in cal/mole· deg.

With ethyl chloride, 1,1‐dichloroethane, and 1,1,l‐trichloroethane, the elimination is not observed and a slow substitution takes place. The influence of chlorine substituents on both sides of the molecule on mechanism and rate parameters is discussed.
The steady‐state approximation: Fact or fiction?
Authors: Farrow L. A. , Edelson D.
Published: January 2005

The kinetic equations for an 81‐reaction model of a photochemical smog chamber have been solved using a complete numerical integration as well as a quasi‐steady‐state approximation (QSSA) procedure. The two sets of results differ markedly in their prediction of experimentally significant factors such as the hydrocarbon depletion rate and the ozone and (NO)x peaking times. The sources of the discrepancy are traced to the fact that the assumed steady‐state conditions were not satisfied, leading to errors in the concentrations of intermediate radicals which in turn affect critical rates in the reaction model.

The occurrence of such discrepancies in various types of reaction models, and with different QSSA strategies, is discussed, and it is concluded that the extent of such errors in QSSA calculations cannot be reliably predicted. Their impact on conclusions regarding reaction mechanisms and rate constants can surpass uncertainties in experimental data; conversely the credibility of predictions derived through QSSA calculations becomes highly suspect. Since recently devloped methods for complete numerical integration of systems of kinetic equations are now available, it is recommended that these be adopted in future work, and that the use of QSSA be abandoned.
The free energy–reaction coordinate profile for α‐chymotryptic hydrolysis of a series of N‐acetyl‐α‐L‐amino acid methyl esters
Authors: Martinek Karel , Klyosov A. A. , Kazanskaya N. F. , Berezin I. V.
Published: January 2005

The effect of added nucleophiles (methanol and 1,4‐butanediol) on the steady‐state kinetics of α‐chymotryptic hydrolysis of a series of N‐acetyl‐L‐amino acid methyl esters, R‐CH(NHCOCH3)C(O)OCH3, has been studied. As a result, the rate and equilibrium constants of the ‘elementary’ steps of the enzyme process have been determined.

It has also been demonstrated how the free energy–reaction coordinate profile changes if the structure (the size of the hydrocarbon chain) of the ‘chemically inert’ substrate fragment R is varied. The effects observed can be described by the following equation:

equation image

where ΔGs and ΔGa are the free energies of formation of metastable intermediates, i.e., the enzyme–substrate complex and the acylenzyme, respectively, ΔG2≠ and ΔG3≠ are the free energies of activation for the chemical steps, i.e., enzyme acylation and acylenzyme hydrolysis, respectively; and ΔGtrans(R) is the free energy of transfer of substrate group R from water into a nonaqueous solvent.

To explain the results obtained, a mechanism for enzyme–substrate interaction is suggested according to which the potential free energy of sorption of substrate group R on the enzyme is 2 ΔGtrans(R). Such a high gain in the free energy of hydrophobic interaction may only be realized if (a) in the free enzyme the sorption region has a thermodynamically unfavorable contact with the aqueous medium, and (b) water is forced out of the active center as a result of the hydrophobic interaction of substrate group R with the enzyme. Such a model is in agreement with the published x‐ray data on the structure of the crystalline enzyme.

The kinetic experiment has proved that not all the potential free energy of sorption is realized as binding force. Thus the true free energy of the binding of substrate group R with the protein does not exceed half the maximum value, both in the enzyme–substrate complex and acylenzyme.
Yet another direct measurement of the rate constant for the recombination of methyl radicals
Authors: James F. C. , Simons J. P.
Published: January 2005

The decaying absorption of CH3 radicals at 216.4 nm has been followed over more than three half‐lives using a photoelectric split‐beam kinetic spectrometer. The rate constant for recombination kr was found to be (5.60 ± 0.76) × 10−11 cm3/molecule·s.
Chlorination of substituted phenols with chloramine T. A kinetic study
Authors: Balasubramanian V. , Thiagarajan V.
Published: January 2005

Kinetics of chlorination of substituted phenols with a particular emphasis on p‐nitrophenol (PNP) have been extensively studied using chloramine T (CAT). The effect of added mineral acids, neutral salts, and chloride have been investigated in detail. In aqueous acetic acid at high acidities the reactive phenols follow a zero‐order process, while PNP or the disubstituted derivatives give a fractional‐order dependence on substrate concentration. The concentration dependence of rate with respect to PNP, the chlorinating agent, and acid reveals the rate law

1/kobs versus 1/[PNP] gave a straight line with a finite intercept. In aqueous dimethylformamide (DMF) and dimethylsulfoxide (DMSO) the reaction shows a second‐order dependence on CAT and a first‐order dependence on PNP in the case of DMF and a slight increment in order in DMSO. Addition of water increases the rate both in aqueous acetic acid and in dipolar aprotic solvents such as DMF and DMSO. The order of the reaction with respect to CAT is found to be dependent on pH as well as the reactivity of the phenols. In buffered acetic acid medium a second‐order dependence on CAT was followed up to pH 7. The rate variations with temperature in the range of 30°¨Dot;50°C have been studied for all the substituted phenols, and the respective activation parameters have been calculated. The empirical rate law is accounted for by a mechanism involving species generated from CAT complexing PNP. Protonated CAT, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm H}_{\rm 2} \mathop {\rm O}\limits^{\rm + } {\rm Cl} $\end{document}, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm Cl}\mathop {\rm O}\limits^{\rm + } {\rm AcH} $\end{document}, and dichloramine T (DCT) are considered important depending on reaction media. The effect of salts, pH, structural variations, and solvent dependence have been accounted for by the proposed mechanism. An attack by positive chlorine on oxygen of the phenol is the preferred mode of attack.
Radiation‐induced dehalogenation of 1,1,1,2 tetrachloroethane and 1,1,2 trichloro‐1 bromoethane reactions of 1,1,2 trichloroethyl radicals
Authors: Aloni R. , Katz M. G. , Rajbenbach L. A.
Published: January 2005

The kinetics of the radiation‐induced free radical chain reactions in solutions of CCl3CClH2 and CCl2BrCH2Cl in cyclohexane (RH) were studied in the temperature range of 90–225°C. 1,1,2 trichloroethyl and 1,1,1,2 tetrachloroethyl radicals were produced by the reaction of radiolytically generated cyclohexyl (R) radicals with solutes. The reactions studied were
equation image
The following rate expressins were obtained:

where θ = 2.303RT in kcal/mole. From the activation parameters of the k4/k5 rate constant ratio and the assumption that E4 = E, E5 was calculated to be 20.2 ± 0.2 kcal/mole. The Arrhenius parameters for the Cl atom elimination reaction from chloroethyl radicals derived from liquid and gas‐phase studies are compared.
The two‐parameter taft equation in kinetics of free radical reactions
Authors: Afanas'Ev I. B.
Published: January 2005

A new method was developed for the calculation of the resonance substituent constants of the two‐parameter Taft equation logksub/k0=ρ*σ*+r*σr. It is based on a relationship between the spin density in free radicals and the rate constants of radical substitution reactions of CH3. Possibilities and limitations of the application of this correlation equation to the investigation of substitution and addition radical reactions are discussed.
Energy disposal in the three‐centered elimination of df from 1,1,2‐trifluoroethane‐1‐d1
Authors: Holmes B. E. , Setser D. W. , Pritchard G. O.
Published: January 2005

The chemical activation data for three‐ and four‐centered hydrogen fluoride elimination from CH2FCDF2have 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 thecis–transisomerization 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‐d1were 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.
Pyrolysis of dimethyl peroxide
Authors: Batt L. , McCulloch R. D.
Published: January 2005

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): \documentclass{article}\pagestyle{empty}\begin{document}${\rm DMP}\mathop \to \limits^1 2{\rm Me}\mathop {\rm O}\limits^{\rm .},{\rm Me}\mathop {\rm O}\limits^{\rm .} + t{\rm - BuH}\mathop \to \limits^4 {\rm MeOH} + t{\rm -}\mathop {\rm B}\limits^{\rm .} {\rm u}$\end{document}. 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\documentclass{article}\pagestyle{empty}\begin{document}$\mathop {\rm O}\limits^{\rm .}$\end{document}. In the decomposition of pure DMP complications arise due to the H‐abstraction reactions of Me\documentclass{article}\pagestyle{empty}\begin{document}$\mathop {\rm O}\limits^{\rm .}$\end{document} 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(MeOOMe) = 37.6 ± 0.2 kcal/mole and /H(Me\documentclass{article}\pagestyle{empty}\begin{document}$\mathop {\rm O}\limits^{\rm .}$\end{document}) = 3.8 ± 0.2 kcal/mole. It is concluded that D(ROOR) and D(ROH) are unaffected by the nature of R. From ΔS and A1, k2 is calculated to be 1010.3±0.5 M−1· sec−1: \documentclass{article}\pagestyle{empty}\begin{document}$2{\rm Me}\mathop {\rm O}\limits^{\rm .} \mathop \to \limits^2 {\rm DMP}$\end{document}. For complete reaction, trace amounts of t‐BuOMe lead to the result k2 ∼ 109 M−1 ·sec−1: \documentclass{article}\pagestyle{empty}\begin{document}$2t{\rm - Bu}\mathop \to \limits^5$\end{document} products. From the relationship k6 = 2(k2k5a)1/2 and with k5a = 108.4 M−1 · sec−1, we arrive at the result k6 = 109.7 M−1 · sec−1: \documentclass{article}\pagestyle{empty}\begin{document}$2t{\rm - u}\mathop {\rm B}\limits^{\rm .} \to (t{\rm - Bu)}_{\rm 2}{\rm,}t{\rm -}\mathop {\rm B}\limits^{\rm .} {\rm u} + {\rm Me}\mathop {\rm O}\limits^{\rm .} \mathop \to \limits^6 t{\rm - BuOMe}$\end{document}.
The SO2(3B1) photosensitized isomerization of cis‐ and trans‐1,2‐dichloroethylene
Authors: Wampler Fred B. , Böttenheim Jan W.
Published: January 2005

The photolysis of SO2 at 3712 Å in the presence of the 1,2‐dichloroethylenes has been investigated at 22deg;C. The data are consistent with the SO2(3B1) photosensitized isomerization of the 1,2‐dichloroethylene isomer. A kinetic treatment of the initial quantum yield data was consistent with the formation of a polarized charge‐transfer intermediate whenever SO2(3B1) molecules and one of the 1,2‐dichloroethylene isomers collide which ultimately decays unimolecularly to the cis‐isomer with a probability of 0.70 ± 0.26 and to the trans‐isomer with a 0.37 ± 0.16 probability. Quenching rate constants for removal of SO2(3B1) molecules by cis‐ and trans‐1,2‐dichloroethylene have been estimated from quantum yield data and from laser excited phosphorescence lifetimes using an excitation wavelength of 3130 Å. Estimates of the quenching rate constant (units of 1./mole ± sec) are for the cis‐isomer, (1.63 ± 0.71) × 1010, quantum yield data, and (2.44 ± 0.11) × 1010, lifetime data; and for the trans‐isomer,(2.59 ± 0.09)×1010, lifetime data, and (2.35 ±0.89) × 1010, quantum yield data. An experimentally determined photostationary composition,[cis‐C2Cl2H2]/[trans‐C2Cl2H2] = 1.8 ‐ 0.1, was in good agreement with a value of 2.00 ‐ 1.15 which was predicted from rate constants derived in this study.
Kinetics of the Diels–Alder addition of acrolein to cyclohexa‐1,3‐diene and its reverse reaction in the gas phase
Authors: Huybrechts G. , Paternoster G. , Baetens P.
Published: January 2005

The Diels–Alder addition of acrolein to cyclohexa‐1,3‐diene has been studied between 486 and 571°K at pressures ranging from 55 to 240 torr. The products are endo‐ and exo‐5‐formylbicyclo[2.2.2]oct‐2‐ene (endo‐ and exo‐FBO), and their formations are second order. The rate constants (in l./mole · sec) are given by

The retro‐Diels–Alder pyrolysis of endo‐FBO has also been studied. In the ranges of 565–638°K and 6–38 torr, the reaction is first order, and its rate constant (in sec−1) is given by

The reaction mechanism is discussed. The heat of formation and the entropy of endo‐FBO are estimated.
Abstraction of chlorine atoms from chloroalkanes. III. The kinetics of liquid phase reactions of trichlorosilyl radicals with chloromethanes
Authors: Aloni R. , Horowitz A. , Rajbenbach L. A.
Published: January 2005

The kinetics of chlorine atom abstraction from the chloromethanes (ClM), CCl4, CHCl3, CH2Cl2, and CH3Cl by radiolytically generated trichlorosilyl radicals was studied in the liquid phase by a competitive method. Arrhenius parameters of chlorine atom abstraction from chloromethanes relative to that of bromine atom abstraction from cyclohexyl bromide (RBr) were as follows:
equation image

The error limits are two standard deviations (2σ) from least mean square Arrhenius plots. From the linear correlation between Ecl values derived from the reactions of SiCl3 and cyclohexyl radicals with the ClM series it is estimated that Ecl (R + CH3Cl) ≃ 16 kcal/mole. In addition the relative Arrhenius parameters for the hydrogen atom abstraction from SiHCl3 and chlorine atom abstraction from CCl4 by cyclohexyl radicals were obtained log AH/Acl = 0.12 ± 0.15 and EH − Ecl = 0.24 ± 0.26. The EH − Ecl value was combined with existing data on E(R + CCl4) to yield the EH(R + SiHCl3) value.
Quenching rate constants for the removal of SO2(3B1) by various fluorinated olefins
Authors: Wampler Fred B.
Published: January 2005

A competitive technique employing the SO2(3B1) photosensitized isomerization of cis‐C2F2H2 to trans‐C2F2H2 in the presence of selected fluorinated olefins has been used at 3712 Å and 22°C to determine the quenching rate constants of the reaction \documentclass{article}\pagestyle{empty}\begin{document}${\rm SO}_{\rm 2} ({}^3B_1){\rm M}\mathop \to \limits^{k_{_4}}$\end{document} removal. With PSo2 = 25.4 torr and Pcis‐C2F2H2 = 0.239 torr Stern–Volmer plots for M = C2H4, C2H2F, 1,1‐C2F2H2, C2F4, and C3F6 yielded k4 (units of 1010 l./mole · sec) values of 5.29 ± 0.16, 4.21 ± 0.53, 1.92 ± 0.23, 0.575 ± 0.060, and 0.0335 ± 0.0027, respectively. The results were consistent with the ability of an olefin to quench SO2(3B1) being inversely proportional to the polarizability of the olefin's π bond and the effect can be clearly noted as each H atom in C2H4 is individually replaced by an F atom.
The reactions of methyl radicals with atomic and molecular oxygen
Authors: Washida N. , Bayes Kyle D.
Published: January 2005

The reaction of methyl radicals with atomic and molecular oxygen was studied with a photoionization mass spectrometer. The methyl radicals were generated by reacting oxygen atoms with ethylene in a fast‐flow tube reactor. The rate constant for the reaction of methyl radicals with oxygen atoms was (1.0 ± 0.2) × 10−10 cm3/molec · sec with no significant variation with temperature over the range of 259–341°K. The reaction of methyl radicals with molecular oxygen involves both a two‐body reaction, having a rate constant \documentclass{article}\pagestyle{empty}\begin{document}$\begin{array}{*{20}c} {k_{{\rm 3a}} = (10^{- 12.54 \pm 0.35})\exp [(- 940 \pm 250)T^{- 1}]} & {{\rm cm}^{\rm 3} /{\rm molec} \cdot {\rm sec}} \end{array}$\end{document}
and a three‐body recombination having a negative temperature dependence. The methyl peroxy radical could be observed at its steady‐state concentration. The rate constants determined at low pressures are compatible with the values determined at higher pressures by flash photolysis. Formaldehyde appears to be a major product of the two‐body reaction of CH3 with O2, and also of the reaction of CH3O2 with oxygen atoms.
The gas‐phase reaction of O3 with H2CO
Authors: Braslavsky S. , Heicklen Julian
Published: January 2005

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

equation image

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.
The photochemistry of hexafluoroacetylacetone in the vapour phase. Occurrence of a novel HF elimination reaction
Authors: Bassett John E. , Whittle Eric
Published: January 2005

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: magnified image

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, magnified image

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.
Photoelimination of HF by hexafluoroacetylacetone: A tandem laser study
Authors: Pimentel George C. , G.‐Tablas F. M. , Hartmann Josef , Whittle Eric
Published: January 2005

Tandem chemical laser techniques have been used to detect HF formed through photoelimination from 1,1,1,5,5,5‐hexafluoroacetylacetone, HFAA. Information about the HF vibrational population ratios is deduced: N2/N1 < 0.37 and N1/N0 < 0.45. This work supports the deduction by Bassett and Whittle that photodecomposition of the enol form of HFAA results mainly in HF elimination and ring closure to a dihydrofuranone, a new type of reaction.
The photolysis of SO2 at 3130 Å in the presence of cis‐ and trans‐2‐pentene
Authors: Wampler Fred B.
Published: January 2005

The photolysis of SO2 at 3130 Å, FWHM = 165 Å, and 22°C has been investigated in the presence of cis‐ and trans‐2‐pentene. Quantum yields for the SO2 photosensitized isomerization of one isomer to the other have been made for a variation in the [SO2]/[C5H10] ratio of 3.41–366 for cis‐2‐C5H10 and of 1.28–367 for trans‐2‐C5H10. A kinetic analysis of each of these systems permitted new estimates to be made for the SO2 collisionally induced intersystem crossing ratio at 3130 Å from SO2(1B1) to SO2(3B1). The estimates of k1a/(k1a + k1b) obtained are 0.12 ± 0.01 and 0.12 ± 0.02 (two different kinetic analyses in the cis‐2‐C5H10 study) and 0.20 ± 0.05 and 0.20 ± 0.04 (two different kinetic analyses in the trans‐2‐C5H10 study). Collisionally induced intersystem crossing ratios of k2a/(k2a + k2b) = 0.51 ± 0.10 and k3a/(k3a + k3b) = 0.62 ± 0.12 were obtained for cis‐ and trans‐2‐pentene, respectively. Quenching rate constants at 22°C for removal of SO2(3B1) molecules by cis‐ and trans‐2‐C5H10 were estimated as (1.00 ± 0.29) × 1011 l./mole·sec and (0.857 ± 0.160) × 1011 l./mole/sec, respectively. Prolonged irradiations, extrapolated to infinite irradiation times, for mixtures initially containing SO2 and pure isomer, either the cis or trans, yielded a photostationary composition of [trans‐2‐pentene]/[cis‐2‐pentene] = 2.1 ± 0.1.
Kinetics of the reaction of atomic hydrogen with methyl hydroperoxide
Authors: Šslemr F. , Warneck P.
Published: January 2005

The reaction H + CH3OOH was investigated under conditions of excess atomic hydrogen concentration using a flow reactor attached to a photoionization mass spectrometer. The rate coefficient of the reaction was determined as

The three important reaction channels were found to be
equation image
with the individual contributions determined as indicated. The product methoxy and methylperoxy radicals react mainly with atomic hydrogen under the employed experimental conditions according to
equation image
where the estimates for the percentage contributions of the various channels were derived from the measured product yields.
The Very low‐pressure pyrolysis of 2‐phenylethylamine. The enthalpy of formation of the aminomethyl radical
Authors: Colussi A. J. , Benson S. W.
Published: January 2005

The thermal unimolecular decomposition of 2‐phenylethylamine (PhCH2CH2NH2) into benzyl and aminomethyl radicals has been studied under very‐low‐pressure conditions, and the enthalpy of formation of the aminomethyl radicals, ΔH°f, 298K (H2NCH2·) = 37.0 ± 2.0 kcal/mol, has been derived from the kinetic data. This result leads to a value for the C—H bond dissociation energy in methylamine, BDE(H2NCH2—H) = 94.6 ± 2.0 kcal/mol, which is about 3.4 kcal/mol lower than in C2H6 (98 kcal/mol), indicating a sizable stabilization in α‐aminoalkyl radicals.
Application of Forst's method to the calculation of thermal unimolecular reaction rates and isotope effects in the falloff region
Authors: Dill Bernd , Heydtmann Horst
Published: January 2005

A method proposed in 1972 by W. Forst is used to calculate the experimentally accessible pressure dependence of thermal unimolecular rate constants. The specification of an activated complex always employed in RRKM calculations is avoided. This allows for a more consistent comparison between the results obtained by the application to various unimolecular processes. In order to bring experimental and calculated curves into agreement, fourcenter eliminations of hydrogen halides from alkyl halides require the formal introduction of a collision efficiency factor λ ≃ 0.2, and for the concerted ring opening of 1,1‐dichlorocyclopropane λ ≃ 0.4 must be assumed. The isotope effects for the decomposition of CD3CD2Cl and CH3CD2Cl have been studied, and the pressure dependence of kH/kD is reported. Studying the biradical ring opening of oxetan, cyclobutane, and cyclopropane, the falloff curves and isotope effects are predicted within the experimental uncertainty by the use of λ ≃ 1.0. This different behavior of concerted and biradical reactions against falloff calculations can hardly be attributed to experimental uncertainties in the Arrhenius parameters and/or the collision frequency alone.
Kinetics of the gas‐phase thermal decomposition of 1,1,2‐trifluoro‐2‐trifluoromethylcyclopropane
Authors: Quero E. D. , Ferrero J. C. , Staricco E. H.
Published: January 2005

The kinetics of the gas‐phase thermal decomposition of 1,1,2‐trifluoro‐2‐trifluoromethyl‐cyclopropane to 2,3,3,3‐tetrafluoropropene and CF2was studied in the temperature range of 539–604°K and with a total pressure of 17 to 300 torr of a 1:20 mixture of reactant and perfluoroethylene. Some runs were also made with the reactant only, with initial pressures of 0.2 to 20 torr. The reaction is first order, homogeneous, and the rate constants fit the following Arrhenius relationship:.
The thermal and initiated thermal decomposition of biacetyl‐d6
Authors: Schliebs R. , Knoll H. , Scherzer K.
Published: January 2005

The thermal and methyl radical initiated decomposition of biacetyl‐d6 were investigated at conversions 5% over the temperature range of 387‐412°C and 251–292°C, respectively. The deuterated sample contained 10.5% biacetyl‐d5. The reaction products were analyzed by mass spectrometry.

The following data could be determined from product ratios:
equation image
where E is in kcal/mol

With reasonable assumptions and literature data Arrhenius parameters for elementary steps were derived and discussed in particular with respect to isotopic effects in H/D‐abstraction reactions.
Comment on the evaluation of the arrhenius parameters by the least squares method
Authors: Cvetanovic R. J. , Singleton D. L.
Published: January 2005

Arrhenius parameters are frequently evaluated incorrectly by applying the least squares method to the logarithmic form of the Arrhenius equation without simultaneously transforming the statistical weights as required for the change of variable from k to ln k. This has been mentioned briefly in an earlier paper. In the present communication the correct procedures are discussed and illustrated by several examples of evaluations. In particular, Arrhenius parameters calculated by the Taylor series expansion of the exponential form of the Arrhenius equation are compared with those calculated from the logarithmic form, using an exact and two approximate transformations of the statistical weights. The comparisons indicate thatthe preferred procedure for obtaining Arrhenius parameters is either the Taylor series method or the logarithmic method with proper transformation of the experimentally determined statistical weights of the rate constants ki. The common approximation of assuming equal statistical weights of ln ki when the logarithmic form of the Arrhenius expression is used is shown not to be always appropriate, and reasons forthis are given.
Consecutive, irreversible first‐order reactions. Ambiguities and practical aspects of kinetic analyses
Authors: Jackson W. G. , Harrowfield J. Macb. , Vowles P. D.
Published: January 2005

The general problem of eliciting reliable rate constants from experimental data is considered in detail for consecutive reactions. Practical aspects are emphasized.
Pyrolytic production of Se (3P) from carbon diselenide. I. Equilibrium
Authors: Marquart J. R. , Belford R. L. , Fraenkel H. A.
Published: January 2005

Thermodynamic characteristics of the dissociation of carbon diselenide are examined. Results of a shock‐tube study of CSe2 pyrolysis are combined with previously unreported photoionization spectroscopy data on CSe2 and with a critical examination of the available literature on Se(g), Se2(g), CSe(g), CSe2(g), and CSe2(t). Based on the best currently available information, recommendations are made for the values of enthalpies, entropies, heat capacities, and derived functions. In particular, the most likely value for ΔH0 (CSe2(g) → CSe(g) + Se(g, 3P)) is +83 kcal/mol, and for Kp at 2000°K it is 0.018 atm.
Photolysis of 1,1,1‐difluorochloroethane (Freon 142) at 147 nm
Authors: Ichimura T. , Kirk A. W. , Tschuikow‐Roux E.
Published: January 2005

1,1,1‐Difluorochloroethane (Freon 142) was photolyzed at 147 nm in the pressure range of 3.6–20.6 torr. The effects of added NO, H2S, and CF4 were investigated. The extinction coefficient at 147 nm and 296°K was determined to be 64 ± 8 atm−1 · cm−1.

The molecule photodecomposes largely by α,β elimination of HCl to give 1,1‐difluoroethylene (Φ = 0.74 ± 0.06). There is no observable elimination of HF, but there is strong evidence for the elimination of the elements of FCl though the relative importance of this process is minor, as are contributions from carboncarbon and carbonhalogen bond fission.

The 1,1‐difluoroethylene formed is undoubtedly vibratonally excited and is the source of a pressure‐dependent small yield of fluoroacetylene. Over the pressure range studied there is no evidence that the major primary process itself is affected by changes in total pressure as is the case in the 147‐nm photolysis of ethyl chloride.
The reaction of ozone with thiophene in the gas phase
Authors: Kaduk Bruce A. , Toby Sidney
Published: January 2005

The reaction between ozone and thiophene was studied from 30 to 125°C over a pressure range of 0.005‐0.3 torr ozone and 0.1‐1 torr thiophene. The most abundant product was O2 with smaller amounts of CO2 and SO2. The mass balance was 100% for oxygen and approached 100% for sulfur at the higher values of [O3]0. The carbon balance, however, was only 25% and no H‐containing products were found, suggesting that the missing product is a hydrocarbon which may be a polymer. The rate law found was ‐d[O3]/dt = k1[Th] [O3] + k2 [Th] [O3]2 where log k1(M−1 · sec−1) = 7.8 ± 0.5 ‐ (8400 ± 700)/2.3RT, and log k2(M−2 · sec−1) = 12.4 ± 0.4 ‐ (4700 ± 400)/2.3RT. Added O2 had no effect on k1 but reduced k2 to a limiting value. It is thus not possible to measure the primary rate constant in this system by measuring the overall rate in the presence of oxygen, and this restriction may also apply to other ozone systems. A mechanism is postulated involving two chain sequences, one of which is inhibited by added O2. A comparison with other ozone systems is made, and the chain lengths are far greater for ozone + thiophene than other systems, under the conditions employed. Possible intermediates in the mechanism are discussed.
Modeling study of two‐step photodissociation method of isotope enrichment
Authors: Hwang W. C. , Kamada R. F. , Badcock C. C.
Published: January 2005

A general comprehensive model of the two‐step photodissociation method of isotope enrichment for diatomic reactants was constructed. The model includes kinetic and absorption processes, isotopic species, and temperature. Criteria to measure different aspects of enrichment were established.

The results show that, in general, isotopic mixing by kinetic processes can be controlled. The relative importance of intensities of excitation sources for the two steps depends on the degree of saturation of the first‐step absorption transition.

For the specific system modeled, i.e., the dissociation of HBr followed by scavenging with NO, the model predicts that detection of bromine enrichment is possible. The important processes and their influence on enrichment were identified, and optimization of experimental parameters was modeled.
Thermal stability of primary amines
Authors: Tsang Wing
Published: January 2005

Tertiary‐amyl amine has been decomposed in single‐pulse shock‐tube experiments. Rate expressions for several of the important primary steps are

This leads to D(CH3H) – D(NH2H) = −10.5 kJ and D[(CH3)3CH] – D[(CH3)2NH2CH] = + 6 kJ.

The present and earlier comparative rate single‐pulse shock‐tube data when combined with high‐pressure hydrazine decomposition results‐(after correcting for fall off effects through RRKM calculations) gives

where kr(…) is the recombination rate involving the appropriate radicals. This suggests that in this context amino radical behavior is analogous to that of alkyl radicals. If this agreement is exact, then

Rate expressions for the primary step in the decomposition of a variety of primary amines have been computed. In the case of benzyl amine where data exist the agreement is satisfactory. The following differences in bond energies have been estimated:
Electrocyclic versus homolytic decompositions of peroxides: Solvent and phase effects
Authors: Hiatt R. , Rahimi P. M.
Published: January 2005

The thermal decomposition of isopropyl peroxide in solvents is shownto be a competition between unimolecular homolysis (kr) and an electrocyclic reaction yielding H2plus acetone (kH). The value ofkHincreases markedly with increasing solvent polarity: PhCH3<i‐PrOH < MeOH < H2O. LogkHcorrelates linearly withEt(orZ). The effect onkris similar but less pronounced. Logkr's for bothi‐Pr2O2andt‐Bu2O2are shown to correlate with the molar solubility parameter δ. The negligible contribution of the electrocyclic reaction to thermal decompositions of peroxides in the vapor phase is thus not due to peculiar behavior of that reaction, which clearly has a polartransition state. It is argued that the transition state for unimolecular homolysis has no polar character despite the apparent sensitivity of homolysis rates to solvent polarity.
Radiationless transitions between excited singlet states of biacetyl
Authors: Richard C. , Bouchy M. , Andre J. C. , Niclause M.
Published: January 2005

Emission processes from lower excited states S1 (fluorescence) and T1 (phosphorescence) have been studied in the gas and liquid phases when biacetyl is excited into the second singlet state S2. (In agreement with Kasha's rule no fluorescence is observed from the S2 state.)

In the liquid phase, when biacetyl is excited into the singlet states S1 and S2, no difference is observed between these emission processes. This phenomenon certainly results from an efficient nonradiative transition between the second excited singlet state S2 and the first excited state S1 with practically no excess vibrational energy. The quantum yield of this transition is almost unity and does not depend on the nature of the solvent.

In the gas phase no emission processes are observed when biacetyl is excited into the S2 state at low pressure (less than 10 mm Hg). High pressure of inert gas is necessary in order to observe these processes. As for excitation into the S1 state with vibrational energy, loss of vibrational energy through collisions occurs from the S2 state. The quantum yield of the S2 → S1 transition by excitation at 290 nm is estimated around 0.5–0.6 at 6 atm of inert gas (ethane, ethylene, or carbon dioxide).
The mechanism of the vapor‐phase chlorination of benzene derivatives
Authors: Dorrepaal Wim , Louw Robert
Published: January 2005

The title reaction, displaying peculiar characteristics as to relative rates and isomer distributions, has been studied in detail. Prior to this study, different mechanisms had been advanced by several groups. Kinetic features (isomer patterns, relative and absolute rates, reaction orders, influences of additives, H/D isotope effects) strongly point to a free‐radical (chain) process, in which (1) is a crucial step. This abstraction reaction, endothermal by about 6 kcal/mol, apparently proceeds via a transition state closely resembling the free aryl radical. Relative rates and isomer distributions therefore reflect differences in stabilization energies, or in DH°(Ar – H). With high arene–Cl2 intake ratios or, more pronounced, with CCl4 as the reagent, aryl radicals also lead to biaryl, where arene successfully competes with the halogenating agent. This interpretation is quantitatively supported by our observation that “added,˝” recognizable aryl radicals yield the same chlorination–arylation product ratio, and by the results of competitive chlorination of benzene and chloroform over a temperature range of 200°C, where the latter study substantiates the value DH0(C6H5 – H) ≈ 109 kcal/mol.
Application of RRKM theory to the reactions OH + NO2+ N2→ HONO2+ N2(1) and ClO + NO2+ N2→ ClONO2+ N2(2); a modified gorin model transition state
Authors: Smith G. P. , Golden D. M.
Published: January 2005

Rate constants as a function of both temperature and pressure were calculated for the title reactions using RRKM theory in conjunction with a modified Gorin transition state. The modification introduces a hindrance parameter which accounts for repulsive interactions between the rotating fragments. At the highest stratospheric pressures (∼50 torr) and at stratospheric temperature (∼220°K), the extent of “falloff” from first‐order [N2] dependence is ∼70% for reaction (1) and ∼35% for reaction (2).
The Kinetics of alkyl radical reactions with aliphatic alcohol glasses as studied by electron spin resonance
Authors: Stepanov A. A. , Tkatchenko V. A. , Bol'Shakov B. V. , Tolkatchev V. A.
Published: January 2005

Methyl radical reactions with matrix molecules in glasses C2H5OH, (CH2OH)2, n‐ and i‐C3H7OH, n‐ and i‐C4H9OH, n‐ and i‐C5H11OH, C2D5OH, and i‐C3D7OD, and the reactions of Ċ2H5, Ċ3H7, Ċ4H9, Ċ5H11 with methanol glasses have been studied. Alkyl radicals were produced by photolysis of diphenylamine–alkylhalide–alcohol mixtures using ultraviolet light. In all cases the alkyl radical decay follows the law c = c0 exp(‐k √t). The √t law should not be associated with alkyl radical diffusion in a matrix. A method of processing the kinetics of those reactions in which one paramagnetic species changes into another with the total concentration being constant and the electron spin resonance spectra of both species overlapping, is described.
The kinetics of the reactions of methyl radicals with Br2, Cl2, and BrCl
Authors: Evans Brian S. , Whittle Eric
Published: January 2005

A general competitive method is described for the study of the kinetics of the reactions of radicals with halogens and interhalogen compounds in the gas phase. The method is applied to the reactions
equation image
over the temperature range of 48°–199°C. It is found that

where Θ=2.303RT J/mol. Also

There is no correlation between differences in activation energies and differences in enthalpy changes for these reactions, but polar effects may be important in reactions (7) and (8).
Reaction of H with C2N2 at pressures near 1 torr
Authors: Phillips L. F.
Published: January 2005

The rate of disappearance of C2N2 in the presence of a large excess of H atoms has been measured in a discharge‐flow system at pressures near 1 torr and temperatures in the range of 282–338 K. Under these conditions the reaction has a small negative temperature coefficient. A transition from second‐order to third‐order kinetics with decreasing pressure occurs at pressures near 1 torr. The results are discussed in terms of the mechanism
equation image
where k7 = (1.5 ± 0.2) × 10–15 cm3/molec1·sec is found for the forward rate of reaction (7). The results also give k7k8/k−7 = 3.7 × 10−31 cm6/molec2·sec and k7k9/k−7 = 3.0 × 10−32 cm6/molec2·sec, the first being probably an upper limit and the second probably a lower limit; hence k8/k9 = 12 is found as an upper limit.
Quasi‐steady‐state approximations in air pollution modeling: Comparison of two numerical schemes for oxidant prediction
Authors: Hesstvedt Eigil , Hov Öystein , Isaksen Ivar S. A.
Published: January 2005

A numerical method to solve the set of differential equations which describes the chemical development in polluted air is presented. The photochemical lifetimes, continuously monitored for all compounds, determine how the integrations are performed at all times. Components with lifetimes less than 10% of the time step, which is taken as 30 sec, are assumed to be in photochemical equilibrium, while compounds with photochemical lifetimes greater than 100 times the time step are computed according to Euler's method. All other components are calculated from the exponential solution of the continuity equation. The computational accuracy may be improved by iteration on components assumed to be determined by the instant values of other components. The convergence of the iteration is speeded up by ordering the short‐lived compounds in a hierarchical sequence. Since computational errors connected with QSSA methods are difficult to assess, comparison with an automatic scheme is necessary. Our method has been compared with Gear's method for a range of model mixtures of hydrocarbons, nitrogen oxides, and air, thought to cover most conceivable situations of atmospheric pollution. The agreement with Gear's method is within a few percent for all components all the time, in most cases even within 1%. No accumulation of deviations occur during long‐term integrations (e.g., 48 hr with day and night shiftings), and differences which appear during periods with strong concentration gradients (e.g., after sunrise) vanish when the activity has culminated. The method presented here is considerably more efficient than Gear's method, with respect to both computer time and storage.
Absolute rate constants for the reactions of O(3P) atoms and OH radicals with SiH4 over the temperature range of 297–438°K
Authors: Atkinson R. , Pitts J. N.
Published: January 2005

Absolute rate constants for the reaction of SiH4 with O(3P) atoms and OH radicals have been determined over the temperature range 297°–438°K using flash photolysis–NO2 chemiluminescence and flash photolysis–resonance fluorescence techniques, respectively. The Arrhenius expressions obtained are

where the error limits in the Arrhenius activation energies are the estimated overall error limits. Rate data for the reactions of SiH4, CH4, and H2S with O(3P), H, and F atoms and with OH, CH3, and CF3 radicals are compared, showing that H2S and SiH4, which have similar bond energies, have reasonably similar reactivities toward these atoms and radicals.