bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics of cerium (IV) oxidation of mandelic acid. Ionic strength and specific cation effects
Authors: Arcoleo Gioacchina , Calvaruso Giuseppe , Cavasino F. Paolo , Dio Emanuele Di
Published: January 2005

The kinetics of the redox reaction between mandelic acid (MA) and ceric sulfate have been studied in aqueous sulfuric acid solutions and in H2SO4MClO4 (M+ = H+, Li+, Na+) and H2SO4MHSO4 (M+ = Li+, Na+, K+) mixtures under various experimental conditions of total electrolyte concentration (that is, ionic strength) and temperature. The oxidation reaction has been found to occur via two paths according to the following rate law: rate = k[MA] [Ce(IV)], where k = k1 + k2/(1 + a)2[HSO4−]2 = k1 + k2/(1 + 1/a)2[SO42−]2, a being a constant. The cations considered exhibit negative specific effects upon the overall oxidation rate following the order H+ ⩽ Li+ < Na+ < K+. The observed negative cation effects on the rate constant k1 are in the order Na+ < Li+ < H+, whereas the order is in reverse for k2, namely, H+ ⩽ Li+ < Na+. Lithium and hydrogen ions exhibit similar medium effects only when relatively small amounts of electrolytes are replaced. The type of the cation used does not affect significantly the activation parameters.
Mutual combination of ClO radicals
Authors: Basco N. , Hunt J. E.
Published: January 2005

The mutual combination reaction
equation image
is proposed as the rate‐limiting step in the removal of ClO radicals at moderate pressures. The third‐‐order rate constants measured at room temperature were k1(Ar) = 3.51 ± 0.14 × 109 l2/mol2·ec; k1(He) ≈ 2.8 × 109 l2/mol2·sec, and k1(O2) ≈ 7.9 × 109 l2/mol2·sec. There is also an independent second‐order reaction
equation image
for which k3 ≈ 8 × 106 l/mol·sec. A new absorption spectrum has been observed in the ultraviolet and attributed to Cl2O2. The extinction coefficient for Cl2O2 has been measured at six wavelengths, and, between 292 and 232 nm, it increases from 0.4 × 103 to 2.9 × 103 l/mol·cm. In the presence of the chlorine atom scavengers OClO or Cl2O, Cl2O2 exists in equilibrium with ClO. The equilibrium constant Ke1 = 3.1 ± 0.1 × 106 l/mol at 298 K, and, with ΔS10 estimated to be −133 ± 11 J/K·mol, ΔH10 = −69 ± 3 kJ/mol and ΔHf0(Cl2O2) = 136 ± 3 kJ/mol.
Quantum effects and isotope effects in chemical kinetics
Authors: Christov S. G. , Parlapanski M.
Published: January 2005

The relations of quantum and kinetic isotope effects are investigated using the exact quantum corrections to the collision and activated complex theories. The latter are computed for the collinear three‐atomic reaction H2+ H → H + H2and the related isotopic reactions using realistic potential energy surfaces. Taking into account the bent configurations of the collision complex H‐H‐H gives a very good agreement between the quantum collision theory and the experimental data for the absolute values and the isotopic ratios of rate constants. Classical trajectory calculations yield considerably lower results.
The thermal reactions of ethylene at 500°C in the presence and absence of small quantities of oxygen
Authors: Back M. H. , Martin R.
Published: January 2005

Initial rates of formation of primary products have been measured in the pyrolysis of ethylene at 500°C, and a mechanism for the reaction is proposed based on the isomerization of the n−hexyl radical. Additions of oxygen in amounts less than 0.1% markedly increased the rate of formation of all products, but oxygen was not completely consumed in the reaction. It is shown that secondary initiation processes are important at very low conversions, and butene‐1 is the most reactive product. Bimolecular reactions between olefins and with oxygen are important steps in the mechanism.
Wall effects in the pyrolysis of diethylether. A case of hetero‐homogeneous pyrolysis at 500°C
Authors: Foucaut J. F. , Martin R.
Published: January 2005

The pyrolysis of diethylether has been studied at 500°C and 50 Torr initial pressure in packed vessels (surface‐to‐volume ratio 11 cm−1). In untreated vessels the reaction appears to be essentially homogeneous as reported before. On the contrary, it is very sensitive to the walls of PbO‐coated or NaOH‐treated vessels. Thus most of the initial rates of product formation are increased in the PbO‐coated vessel and decreased in the NaOH‐treated vessel. The efficiency of the walls has been monitored for several weeks and was shown to decrease slowly with the vessel aging time. These phenomena are interpreted in terms of a free‐radical chain mechanism in which a part of the initiation or termination steps occurs at the walls, the propagating steps being homogeneous. The present work clearly proves the existence of hetero‐homogeneous chain reactions at 500°C. This new class of reaction could be of practical interest in the design of industrial reactors.
Reaction of NO with hypochlorous acid
Authors: Ghibaudi Elena , Barker John R. , Benson Sidney W.
Published: January 2005

The reaction between NO(g) at concentrations between 0.1 and 1.0 Torr in 1‐atm N2 and aqueous solutions of NaClO has been studied over the pH range of 6–12 and hypochlorite concentrations between 0.01 and 1.0M. A very rapid and efficient reaction occurs leading to the production of about 30%–40% of the NO as NO2 and with conversions of NO up to 98% at about 1‐sec contact time. It is shown that a fast chain reaction initiated by the endothermic step
equation image
can account for the data. The very exothermic reaction NO + ClO− → NO2 + Cl− is shown to be at least 30‐fold slower than i. The overall reaction seems very promising as a method of reducing NO and NO2 emissions from the exhausts of industrial plants.
Estimating the positions of transition states from experimental data
Authors: Basilevsky M. V. , Weinberg N. N. , Zhulin V. M.
Published: January 2005

A procedure is suggested to estimate the positions of a transition state (TS) along the reaction coordinate in reaction series involving several radical reactions. It is based on the dependence between experimental activation energies U± and heat effects Q. Taking into account the shift of a TS position, the following relation between these quantities is obtained: U± −; U = A (Q ‐ Q0)2 + B(Q ‐ Q0). The subscript 0 indicates the standard reference reaction. For three series, namely, the hydrogen abstraction from hydrocarbon substrates by radicals CH3˙, CF˙3, and Br·, the bond lengths characterizing the TS reaction center are evaluated. The TS positions appear to vary significantly in the reaction series, which accounts for significant changes in the experimentally observed activation volumes. The derivation of the Hammond rule and the range of its validity for the series with polar and steric substituent effects are discussed.
Kinetics of the reactions of cyclopropane derivatives. V. The gas‐phase unimolecular reactions of 1 α‐chloro‐2α,3α‐dimethylcyclopropane and 1α‐chloro‐2α,3β‐dimethylcyclopropane
Authors: Robinson P. J. , Waller M. J.
Published: January 2005

Thermolysis of the “all‐cis” compound 1α‐chloro‐2α,3α‐dimethylcyclopropane (A) at 550–607 K and 6–115 torr is a first‐order homogeneous non‐radical‐chain process giving penta‐1,3‐diene (PD) and HCl as products. The Arrhenius parameters are log10A(sec−1) = 13.92 ± 0.08 and E = 199.6 ± 0.9 kJ/mol. The isomer with trans‐methyl groups, 1α‐chloro‐2α,3β‐dimethylcyclopropane (B) reacts by two parallel first‐order processes giving as observed products trans‐4‐chloropent‐2‐ene (4CP) and PD + HCl, with log10A(sec−1) = 14.6 and 13.8, respectively, and E = 199.5 and 190.2 kJ/mol, respectively. The 4CP undergoes secondary decomposition to PD + HCl (as investigated previously). Comparison of the results for compounds (A) and (B) with those for other gas‐phase and solution reactions leads to the conclusion that the gas‐phase thermolyses proceed by rate‐determining ring opening to form olefins which may decompose further by thermal or chemically activated reactions, and that the ring opening is a semiionic electrocyclic reaction in which alkyl groups in the 2,3‐positions trans to the migrating chlorine semianion move apart, with appropriate consequences for the rate of reaction and the stereochemistry of the products.
Excited‐state kinetics of azoalkanes some further studies with azoethane, azo‐n‐propane, and azoisopropane
Authors: Pritchard G. O. , Marchant P. E. , Steel C.
Published: January 2005

The photochemistry of azo‐n‐propane is investigated at 366 nm up to 1 atm pressure, and over a range of temperature from 50 to 190°C. Some additional experiments with azoethane at room temperature and azoisopropane at 180 and 190°C are also reported. From a consideration of the pressure dependence of the quantum yields for photodissociation a generalized mechanism is proposed which accounts for the known experimental observations in acyclic azoalkane photochemistry. These observations include the extensive photoisomerization data which were previously obtained for azoisopropane. In the mechanistic scheme dissociation at low pressures is believed to occur mainly from S and T, the vibrationally excited and randomized first excited singlet and triplet states. At high pressures and low temperatures (≤100°C) the major dissociation channel is probably a nonrandom S1 state. In direct or singlet sensitized photolysis isomerization occurs predominatly at high pressure and is postulated to occur by internal conversion from S, the thermalized singlet, to the ground state. During the process partitioning to the cis or trans isomer is equally probable. In triplet sensitized photolysis isomerization occurs via intersystem crossing from T1to the ground state. At elevated temperatures (>150°C) dissociation from S, which has a significant activation energy, can compete with return to the ground state.
Kinetics of cyclodimerization of hexafluoropropene and of its cycloaddition to chlorotrifluoroethene
Authors: Atkinson Bernard , Tsiamis Christos
Published: January 2005

The thermal cyclodimerization of hexafluoropropene and its cycloaddition to chlorotrifluoroethene have been studied at 570–700 K and 8–62 kPa. Both reactions give 1,2‐substituted hexafluorocyclobutanes. For the intercombination product 1‐chloro‐2‐trifluoromethyl‐hexafluorocyclobutane infrared, nuclear magnetic resonance and mass spectra as well as vapor pressure are reported. The rates of reaction can be expressed by the following equations:

The reactions probably proceed by a biradical mechanism.
Combination and disproportionation reactions of allylic radicals with ethyl, propyl, and t‐Butyl Radicals
Authors: Baulch D. L. , Chown P. K. , Montague D. C.
Published: January 2005

1‐Methylallyl, 1,1‐dimethylallyl, 1,2‐dimethylallyl, 1,3‐dimethylallyl, 1,1,2‐trimethylallyl, and 1‐ethylallyl radicals have been generated in the gas phase at 20 ± 1°C by addition of H atoms, formed by Hg(63P1) photosensitization of H2, to appropriate dienes. Their combination reactions with ethyl radicals have been studied and the relative reactivities of the reaction centers in each allylic radical determined. Similar measurements have been made for some combination reactions of n‐propyl, i‐propyl, and t‐butyl with 1‐methylallyl and 1,1,2‐trimethylallyl radicals. The more substituted reaction centers are found to be the less reactive. In addition the self‐combination and disproportionation of 1‐methylallyl radicals has been investigated, as has cross disproportionation of each allylic radical with ethyl. The results establish a general pattern of reactivity for these radicals, which is interpreted primarily in terms of the effects of steric interaction during reaction.
Mechanistic details of the Belousov–Zhabotinsky oscillations. III. The induction period
Authors: Edelson D.
Published: January 2005

The oscillating Belousov–Zhabotinsky reaction exhibits an initial quiescent induction period during which the concentrations of various reactants and intermediates adjust to a quasisteady‐state, which eventually switches into oscillation. The duration of this induction period is strongly dependent on the initial bromide ion concentration. The previously reported detailed mechanism for this system is used to calculate this dependence, and the results are shown to agree with recent experimental work. The factors that account for this behavior are discussed.
Addition of trichloromethyl radicals to tetrachloroethylene
Authors: Horowitz A. , Baruch G.
Published: January 2005

The gamma‐radiation‐induced free‐radical chain reactions in liquid CCl4C2Cl4cC6H12 mixtures were studied in the temperature range of 363–448°K. The main products in this system are chloroform, hexachloropropene and chlorocyclohexane. These products are formed via reactions (1)–(5):
equation image
with G values (molec/100 eV) of the order of magnitude of 102 and 103 at the lowest and highest temperatures, respectively. Values of k2/k1 were determined from the product distribution. In turn, these values gave the following Arrhenius expression for k2/k1 (θ = 2.303RT, in kcal/mol):

From this result and the previously determined Arrhenius parameters of reaction (1), k2 is found to be given by

.
Evidence for α‐elimination from neopentyl chloride in the gas phase
Authors: Failes Robert L. , Mollah Yousuf M. A. , Shapiro Jacob S.
Published: January 2005

Experimental evidence is presented for a unimolecular gas‐phase Wagner‐Meerwein shift in neopentyl chloride pyrolysis. In the decomposition of α,α‐neopentyl chloride‐d2 at 445°C, maximally inhibited by cyclohexene, the initial products were isotopically pure 2‐methyl‐1‐butene‐d2 and 2‐methyl‐2‐butene‐d1. Rearrangement, accompanied by loss of either α‐ or γ‐hydrogen in the formation of hydrogen chloride, is consistent with an incipient ion‐pair type of transition state. The cyclohexene maximally inhibited pyrolysis of neopentyl chloride was also examined over the temperature range 424–478°C and Arrhenius parameters of E, 258.7 kJ/mole and logA/sec−1, 13.78, were determined.
Evaluation of the rate coefficients and arrhenius parameters of hydrogen atom transfer reactions. I. The method
Authors: Bérces Tibor , Dombi Jóazsef
Published: January 2005

A method is presented for the prediction of rate coefficients and Arrhenius parameters for bimolecular hydrogen atom transfer reactions A + BC → AB + C. The treatment sets out from structural considerations of the complex A ⃛ B ⃛ C and calculates the energy of the complex along the reaction path from empirical functions for a bonding energy term and an endgroup contribution. The treatment proceeds by assuming ultrasimple transition state models and assigning the force constants and vibrational frequencies. Finally the rate coefficient and Arrhenius parameters are obtained on the basis of separable activated complex theory. Application of the method requires known properties of reactant and product molecules and does not demand the use of adjustable parameters. The relation and differences between this method and the BEBO treatment as well as Zavitsas' method are dealt with.
Kinetics of oxidation of butane 2,3‐diol by osmium(VIII)
Authors: Anand B. H. , Menghani G. D.
Published: January 2005

The oxidation of butane 2,3‐, propane 1,2‐, ethane diol and 2‐methoxy ethanol in aqueous alkaline medium by Os(VIII) has been studied. The reaction is base catalyzed and shows first‐order kinetics in Os(VIII), whereas the order is less than 1 in butane 2,3‐diol [BD]. The rate of oxidation is BD > propane 1,2 > ethane diol ≈ 2‐methoxy ethanol. The change in ionic strength has no effect on the rate of reaction. Activation parameters ΔE, PZ, and ΔS* have been evaluated.
Kinetics of the diels–alder addition of ethene to cyclohexa‐1,3‐diene and its reverse reaction in the gas phase
Authors: Huybrechts G. , Rigaux D. , Vankeerberghen J. , Van Mele B.
Published: January 2005

The addition of ethene to cyclohexa‐1,3‐diene has been studied between 466 and 591 K at pressures ranging from 27 to 119 torr for ethene and 10 to 74 torr for cyclohexa‐1,3‐diene. The reaction is of the “Diels–Alder” type and leads to the formation of bicyclo[2.2.2]oct‐2‐ene. It is homogeneous and first order with respect to each reagent. The rate constant (in l./mol sec) is given by

The retron‐Diels–Alder pyrolysis of bicyclo[2.2.2]oct‐2‐ene has also been studied. In the ranges of 548–632 K and 4–21 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 bicyclo[2.2.2]oct‐2‐ene are estimated.
A modified model of diffusion‐controlled reactions
Authors: Burshtein A. I. , Yakobson B. I.
Published: January 2005

The treatment used for diffusion‐controlled reactions is extended to the case when the final jumplike step of the reagent approach differs essentially from the preceding diffusion. In terms of the model proposed it is possible to take into account, in a simple way, chemical inhomogeneity of the reagents when the in‐cage reaction rate depends on the pair state (for example, on the molecular orientation). In this connection the reaction can be controlled either by the reagent intrusion into the solvation shell of the partner, or by the in‐cage relaxation.
Chain ion molecular mechanism of ascorbic acid oxidation with hydrogen peroxide. Cu3+ ion as a chain carrier
Authors: Skurlatov Yu.
Published: January 2005

The kinetics and mechanism of ascorbic acid (DH2) oxidation have been studied under anaerobic conditions in the presence of Cu2+ ions. At 10−4 ≤ [Cu2+]0 < 10−3M, 10−3 ≤ [DH2]0 < 10−2M, 10−2 ≤ [H2O2] ≤ 0.1M, 3 ≤ pH < 4, the following expression for the initial rate of ascorbic acid oxidation was obtained:
equation image
where χ2 (25°C) = (6.5 ± 0.6) × 10−3 sec−1. The effective activation energy is E2 = 25 ± 1 kcal/mol. The chain mechanism of the reaction was established by addition of Cu+ acceptors (allyl alcohol and acetonitrile). The rate of the catalytic reaction is related to the rate of Cu+ initiation in the Cu2+ reaction with ascorbic acid by the expression
equation image
where C is a function of pH and of H2O2 concentration. The rate equation
equation image
where k1(25°C) = (5.3 ± 1) × 103M−1 sec−1 is true for the steady‐state catalytic reaction. The Cu+ ion and a species, which undergoes acid–base and unimolecular conversions at the chain propagation step, are involved in quadratic chain termination. Ethanol and terbutanol do not affect the rate of the chain reaction at concentrations up to ≈0.3M. When the Cu2+–DH2–H2O2 system is irradiated with UV light (λ = 313 nm), the rate of ascorbic acid oxidation increases by the value of the rate of the photochemical reaction in the absence of the catalyst. Hydroxyl radicals are not formed during the interaction of Cu+ with H2O2, and the chain mechanism of catalytic oxidation of ascorbic acid is quantitatively described by the following scheme.

Initiation:
equation image
Propagation:
equation image

equation image
Termination:
equation image
Deactivation of O2(A3Σ) by O2, O, and Ar
Authors: Kenner R. D. , Ogryzlo E. A.
Published: January 2005

O2 in the A3Σu+ state has been prepared in a discharge flow system by recombining oxygen atoms on a nickel surface. The decay of this excited state was followed by observing the emission between 280 and 400 nm. The wall deactivation was observed to approach unit efficiency. Rate constants were determined to be 0.9 × 10−11, 2.9 × 10−13, and 8.6 × 10−16 cm3/molecule sec for the quenching of O2(A3Σu+) by O, O2, and Ar, respectively.
Consistency of a set of chemical reactions
Authors: Oliver P.
Published: January 2005

It is a useful feature of many general chemical kinetics programs that the user's reaction scheme is tested as far as possible before a calculation is started. Attempts are made to answer the question: How far can a reaction scheme be tested for consistency, with respect to mass conservation, without knowing the molecular weights of the species involved? It is found that this problem can be transformed so that it is solvable by the standard methods used in linear programming.
Kinetics of the thermal decomposition of pyridine in a flow system
Authors: Houser Thomas J. , McCarville Michael E. , Biftu Tesfaye
Published: January 2005

The kinetics of the pyrolysis of pyridine was studied in the temperature range of 875–1050°C, at 1 atm total pressure, in a Vycor stirred‐flow reactor. Initial concentrations ranged from 0.25 to 2 mol % in helium and reaction times from 0.25 to 4 sec. It was found at the lower temperatures that the reaction was mixed first and second order. At the higher temperatures an autocatalytic term, zero order in the substrate and first order in the concentration of pyridine decomposed, was necessary in addition to the other two terms to describe the kinetic data. Most of the product was in the form of a primarily aromatic, nonvolatile tar; the major volatile products were HCN and C2H2. An increase in the surface‐to‐volume ratio of the reactor decreased the rate, indicating a radical chain or sequential reaction with radical consumption on the surface. Experiments with additives showed that C2H2 increased the rate. Using the kinetic and product data, some mechanistic considerations are discussed, and a mechanism is proposed to account for the first‐order term.
Determination of the mechanism of the oxidation of the trichloromethyl radical by the photolysis of chloral in the presence of oxygen
Authors: Ohta Tomohiro , Mizoguchi Isao
Published: January 2005

The photooxidation of chloral was studied by infrared spectroscopy under steady‐state conditions with irradiation of a blackblue fluorescent lamp (300 nm < λ < 400 nm, λmax = 360 nm) at 296 ± 2 K. The products were hydrogen chloride, carbon monoxide, carbon dioxide, and phosgen. The kinetic results reveal that the reaction proceeds via chain reaction of the Cl atom:
equation image

The results lead to the conclusion that mechanism (B) is confirmed to be more likely than mechanism (A), which was favored at one time by Heicklen for the mechanism of the oxidation of trichloromethyl radicals by oxygen molecules:
equation image

The ratio of the initial rates of CO and CO2 formation gave k7/k6 = 4.23M−1, and the lower limit of reaction (5) was found to be 3.7 × 108M−1 sec−1.
A smog chamber and modeling study of the gas phase NOx–air photooxidation of toluene and the cresols
Authors: Atkinson Roger , Carter William P. L. , Darnall Karen R. , Winer Arthur M. , Pitts James N.
Published: January 2005

An experimental and modeling study of irradiated toluene–NOx–air, toluene–benzaldehyde–NOx–air, and cresol–NOx–air mixtures at part‐per‐million concentrations has been carried out. These mixtures were irradiated at 303 ± 1 K in a 5800‐liter Teflon‐lined, evacuable environmental chamber, with temperature, humidity, light intensity, spectral distribution, and the concentrations of O3, NO, NO2, toluene, PAN, formaldehyde, benzaldehyde, o‐cresol, m‐nitrotoluene, and methyl nitrate beingmonitored as a function of time. For the toluene and toluene–benzaldehyde–NOx–air runs a variety of initial reactant concentrations were investigated. Cresol–NOx–air runs were observed to be much less reactive in terms of O3 formation and NO to NO2 conversion rates than toluene–NOx–air runs, with the relative reactivity of the cresol isomers being in the order meta » ortho > para. The addition of benzaldehyde to toluene–NOx–air mixtures decreased the reactivity, in agreement with previous studies. Alternative mechanistic pathways for the NOx photooxidations of aromaticsystems in general are discussed, and the effects of varying these mechanistic alternatives on the model predictions for the toluene and o‐cresol–NOx–air systems are examined. Fits of the calculations to most of the experimental concentration–time profiles could be obtained to within the experimental uncertainty for two of the mechanistic options considered. In both cases it is assumed that (1) O2 adds to the OH–toluene adduct ∼75% of the time forming, after a further addition of O2, a C7 bicyclic peroxy radical, and (2) this C7 bicyclic peroxy radical reacts with NO ∼75% of the time to ultimately form α‐dicarbonyls and conjugated γ‐dicarbonyls (e.g., methylglyoxal + 2‐butene‐1,4‐dial) and ∼25% of the time to form organic nitrates. The major uncertainties in the mechanisms concern (1) the structure of the bicyclicperoxy intermediate, and (2) the γ‐dicarbonyl photooxidation mechanism. Good fits to the o‐cresol concentration–time profiles in the toluene–NOx runs are obtained if it is assumed that o7‐cresol reacts rapidly with NO3 radicals. However, it is observed that the model underpredicts nitrotoluene yields by a factor of ∼10, but this is in any case a minor product. It is concluded that further experimental work will be required toadequately validate the assumptions incorporated in the aromatic photooxidation mechanisms presented here.
FTIR studies of the kinetics and mechanism for the reaction of Cl atom with formylchloride
Authors: Niki H. , Maker P. D. , Savage C. M. , Breitenbach L. P.
Published: January 2005

The rate constant for the reaction Cl + CHClO → HCl + CClO was determined from relative decay rates of CHClO and CH3Cl inthe photolysis of mixtures containing Cl2 (∼1 torr), CH3Cl (∼1 torr), and O2 (∼0.1 torr) in 700 torr N2. In such mixtures CHClO was generated in situ as a principal product prior to complete consumption of O2. The value of k(Cl + CHClO)/k(Cl + CH3Cl) = 1.6 ± 0.2(3σ) combined with the literature value of k(Cl + CH3Cl) = 4.9 × 10−13 cm3/molecule sec gives k(Cl + CHClO) = 7.8 × 10−13 cm3/molecule sec at 298 ± 2 K, in excellent agreement with a previous value of (7.9 ± 1.5) × 10−13 cm3/molecule sec determined by Sanhueza and Heicklen [J. Phys. Chem., 79, 7 (1975)]. Thus this reaction is approximately 100 times slower than the corresponding reactions of aldehydes and alkanes with comparable CH bond energies (≤95 kcal/mol).
H2S‐promoted thermal isomerization of butene‐2 CIS to butene‐1 or butene‐2 trans around 500°C
Authors: Richard C. , Boiveaut A. , Martin R.
Published: January 2005

H2S increases the thermal isomerization of butene‐2 cis (Bc) to butene‐1 (B1) and butene‐2 trans (Bt) around 500°C. This effect is interpreted on the basis of a free radical mechanism in which buten‐2‐yl and thiyl free radicals are the main chain carriers. B1 formation is essentially explainedby the metathetical steps:
equation image
whereas the free radical part of Bt formation results from the addition–elimination processes:
equation image
. It is shown that the initiation step of pure Bc thermal reaction is essentially unimolecular:
equation image
and that a new initiation step occurs in the presence of H2S:
equation image
. The rate constant ratio has been evaluated:

and the best values of k1 and k1', consistent with this work and with thermochemical data, are

. From thermochemical data of the literature and an “intrinsic value” of E−3 ⋍ 2 kcal/mol given by Benson, further values of rate constants may be proposed:

equation image
is shown to be E4 ⋍ 3.5 ± 2 kcal/mol, of the same order as the activation energy of the corresponding metathetical step.
Rate coefficients of decomposition of azoethane and azoisopropane
Authors: Ács G. , Péter A. , Huhn P.
Published: January 2005

The thermal decompositions of azoethane and azoisopropane were studied in a large excess of ethylene in the temperature interval of 523–623 K. It was demonstrated that under such conditions, the bulk of the alkyl radicals react with ethylene. Via measurements on the consumption of the azo compound and on the formation of gaseous nitrogen, it was possible to determine the rate coefficients of the initiation reactions:

. The resulting data were as follows: For azoethane,

. For azoisopropane,
Influence of butadiene‐1,3 on ethanal pyrolysis at 495°C
Authors: Richard Claude , Martin René
Published: January 2005

At 495°C and a low extent of reaction, ethanal pyrolysis is slightly inhibited by the addition of small quantities of butadiene‐1,3, whereas it is accelerated by more important quantities. The inhibiting effect is interpreted in terms of a free‐radical chain mechanism in which the main chain carriers of ethanal pyrolysis (CH3.free radicals) reversibly add to butadiene‐1,3 and yield penten‐2‐yl (R.) free radicals. These free radicals either react in a metathetical step:
equation image
or in terminating steps. But butadiene‐1,3 also gives rise to new initiation steps:
equation image
which account for the accelerating effect. Process (i3) seems to be more important than process (i2) in the experimental conditions, but its nature could not be identified. The results are consistent with literature data and the following value of k6:

(4.57T in cal/mol).
Photolysis of azomethane–propane mixtures. Arrhenius parameters for the hydrogen abstraction reactions of methyl with azomethane and with propane
Authors: Durban Philip C. , Marshall Roger M.
Published: January 2005

Mixtures of up to 14% azomethane in propane have been photolyzed using mainly 366 nm radiation in the ranges of 323–453 K and 25–200 torr. Detailed measurements were made of the yields of nitrogen, methane, and ethane. Other products observed were isobutane, n‐butane, ethene, and propene. A detailed mechanism is proposed and shown to account for the observed variation of product yields with experimental conditions. The quantum yield of the molecular process

is found to be given by the temperature‐independent equation

The values of rate constants obtained are

where the reactions are
equation image
and it is assumed that the rate constant for the reaction
equation image
is given by
Reactivity of amines toward tert‐butoxy radicals: Effect of solvent and amine structure
Authors: Encina M. V. , Díaz S. , Lissi E.
Published: January 2005

The reactivity of amines towards tert‐butoxy radicals depends upon the amine ionization potential and the solvent, indicating that polar structures contribute to the reactivity of these compounds. Nevertheless, the dependence with the amine potential is smaller than the one obtained for the quenching of carbonyl excited states by amines and other factors as the strength of the broken hydrogen bond and steric hindrance are also significant in determining the rate of the process.
Mechanism of the azide–bromine reaction in aqueous medium
Authors: Vivekanandam T. S. , Singh U. Chandra , Ramachandran M. S.
Published: January 2005

Results on the oxidation of N by Br2 in neutral and acid media are presented. The rate of the reaction is found to be proportional to [N] and [Br2]. The gaseous product of oxidation is found to be pure nitrogen. The stoichiometry of the reaction is

The reaction shows a positive salt effect. It is found that the addition of Br− stabilizes the complex BrN3, which decomposes into Br− and N2:

The spectroscopic measurements also support the kinetic observation. The equilibrium constant K, the rate constants and the thermodynamic parameters were calculated. It is observed that H+ ion inhibits the reaction. The mechanism is discussed in terms of the kinetic results.
Temperature‐dependent a factor in thermal unimolecular reactions II. Exponential dependence
Authors: Forst Wendell , Turrell Sylvia
Published: January 2005

It is assumed that A∞ in the limiting high‐pressure unimolecular rate constant expression k∞ = A∞ exp(−Ea∞/kT) is given by A∞ = A″∞eBkT. The test case is the decomposition butane → 2 ethyl, where a negative B reproduces the temperature dependence of A∞ about as well as the previously considered power law A∞ = A′∞(kT)n. It is shown that the term eBkT modifies significantly the high‐temperature falloff of the general‐pressure rate constant kuni and its various derivatives, and admits of a fairly simple interpretation in terms of Benson's restricted rotor theory. On physical grounds, the exponential law appears more reasonable than the power law.
From energy profiles to structure–reactivity correlations
Authors: Agmon N.
Published: January 2005

Models for the energy profile along the reaction coordinate are utilized to determine the barrier's height and location as a function of ΔG. These form the basis for structure–reactivity correlations and afford a unified formulation for various postulates in the field of physical organic chemistry. Current experimental evidence is examined for the resulting correlations, and some of their applications as an aid to the chemical kineticist are presented.
Kinetics of the oxidation of U (IV) by hydrogen peroxide in sulfuric acid medium
Authors: Bhattacharyya P. K. , Saini R. D. , Ruikar P. B.
Published: January 2005

The kinetics of the reaction
equation image
have been investigated in H2SO4 medium under different conditions. The observed bimolecular rate constant kobs, has been found to depend on [H+]−0.55 and to increase with the initial concentration ratio of the reactants R0 = [H2O2]0/[U (IV)]0 above 0.49. The activation energy of the overall reaction has been determined as 13.79 and 14.3 kcal/mol at R0 = 1 and 0.35, respectively. Consistent with experimental data, a detailed reaction mechanism has been proposed where the hydrolytic reaction (4) followed by the rate‐controlling reaction (10) and subsequent fast reactions of U (V) and OH radicals are involved:
equation image
A kinetic expression has been derived from which a graphical evaluation of (kK4)−1 and k−1 has been made at R0 = 1 as (12.30 ± 0.09) × 10−3 M min, (6.23 ± 2.19) × 10−4 M min; and at R0 = 0.35 as (12.63 ± 2.13) × 10−3 M min, (8.32 ± 6.62) × 10−4 M min, respectively. Indications of some participation of a chain reactionat R0 = 1 have been obtained without affecting thesecond‐order kinetics as observed.
Consistency of theory and experiment in the ethane–methyl radical system
Authors: Skinner Gordon B. , Rogers David , Patel Kalabhai B.
Published: January 2005

The thermal decomposition of ethane has been reinvestigated using the single pulse, reflected shock technique. Reflected shock temperatures were corrected for boundary layer‐induced nonidealities using the thermal decomposition of cyclohexene as a kinetic standard. The rate constant for the reaction
equation image
was calculated from the rate of formation of methane under conditions of very low extent of reaction, over a temperature range of 1000–1241 K. Ethane compositions of 1% and 3% in argon at total reaction pressures of 3 and 9 atm were used, and a small pressure dependence of k1 was observed.

An RRKM model is described which gives excellent agreement with this and other recent dissociation and recombination rate constant data in light of a recent revision to the thermochemistry of the methyl radical. In the range of 1000–1300 K an RRKM extrapolated k is given by the expression, log k = 17.2 − 91,000/2.3RT, while at 298 K the calculation gives log k (l/mol sec) = 10.44, where k is calculated from k and the equilibrium constant.
Nonisothermal decomposition of methyl nitrate: Anomalous reaction order and activation energies and their correction
Authors: Gray P. , Griffiths J. F. , Hasegawa K.
Published: January 2005

Methyl nitrate decomposition is accompanied by self‐heating. Using very fine thermocouples, direct measurements of excess temperatures have been made to establish the intensity and extent of self‐heating in nonisothermal reaction. The measurements are displayed as contourlines of equal degrees of temperature excess on the pressure–temperature ignition diagram.

A twofold kinetic investigation has been made of the overall reaction. One part concentrates on achieving isothermal conditions and full characterization of intermediate products. It uses mass‐spectrometric analyses both to establish stoichiometry throughout decomposition and to validate velocity constant measurements derived from continuous pressure–time records. The best value for E = 151 ± 3 kJ/mol is about 10 kJ/mol less than previously.

The other part deliberately invades the nonisothermal region to test the qualitative and quantitative predictions of theory (1) that uncorrected reaction orders and activation energies will exceed their isothermal values, (2) that their relative excess (δn/n, δE/E) will be about the same, and (3) that they will be dependent in a simple and predictable way on the reduced excess central temperature.
Isotope effects in abstraction and exchange reactions H + H′Br
Authors: Hepburn J. W. , Klimek D. , Liu K. , Macdonald R. G. , Mayne H. R. , Northrup F. J. , Polanyi J. C.
Published: January 2005

A preliminary report is given of relative reactive cross sections for four abstraction reactions H + H′Br ← HH′ + Br with attacking atom (A) H or D, and atom under attack (B) H or D. The pattern of reactive cross sections, as obtained in a crossed molecular beam experiment at a collision energy ET = 7 kcal/mol, indicates Sr(D,H) ≤ Sr(D,D) > Sr(H,H) > Sr(H,D). The atoms in parentheses are A and B. We describe a three‐dimensional classical trajectory (CT) study on a potential‐energy surface proposed in 1969 by Parr and Kuppermann (PK); the CT results are in fair accord with experiment. It is suggested that (D,H) has the largest cross section because it exhibits the most favorable relative timing of A approach to BC rotation. On the basis of CT it appears that the same sequence of cross sections and the same rationale may be applied to the exchange reactions H + BrH′ ← HBr + H′.
Minor products and initiation rate in the chain pyrolysis of propane
Authors: Juste C. , Scacchi G. , Niclause M.
Published: January 2005

With a continuous jet‐stirred tank reactor operating at small space time (0.05–1.2 s) the kinetics of the formation of six minor products (ethane, isobutane, butene‐1, 2,3‐dimethyl‐butane, 4‐methylpentene‐1, and 1,5‐hexadiene) are studied during the pyrolysis of propane, at small extents of reaction and over the temperature range of 600–780°C. The experimental results are in agreement with the free radical mechanism proposed by Jezequel, Baronnet, and Niclause for this reaction. They show that the two most important termination processes are
equation image
The measured rates of formation of the minor products are consistent with the quasi‐identical values estimated by Jezequel and co‐workers (between 475 and 505°C) and by Allara and Edelson (between 510 and 560°C) for kinetic parameters (A1 ≃ 1016.65 s−1 and E1 ≃ 84.7 kcal/mole) of the chain initiation process
equation image
On the frequency factor in electron transfer reactions and its role in the highly exothermic regime
Authors: Marcus R. A.
Published: January 2005

Consideration of current information on the dependence of the electron transfer rate on the radial separation distance and on the reactants′ radial distribution function suggests for adiabatic transfers a frequency factor closer to 1012M−1 s−1 than to 1011M−1 s−1. One effect is to raise the λ values estimated from self‐exchange rate constants, and to extend thereby the range of ΔG°'s in which the “inverted region′” is masked by a diffusion‐controlled reaction rate.
Study of the HNO + HNO and HNO + NO reactions by intracavity laser spectroscopy
Authors: Cheskis S. G. , Nadtochenko V. A. , Sarkisov O. M.
Published: January 2005

Flash photolysis of CH3CHO and H2CO in the presence of NO has been investigated by the intracavity laser spectroscopy technique. The decay of HNO formed by the reaction HCO + NO → HNO + CO was studied at NO pressures of 6.8–380 torr. At low NO pressure HNO was found to decay by the reaction HNO + HNO → N2O + H2O. The rate constant of this reaction was determined to be k1 = (1.5 ± 0.8) × 10−15 cm3/s. At high NO pressure the reaction HNO + NO → products was more important, and its rate constant was measured to be k2 = (5 ± 1.5) × 10−19 cm3/s. NO2 was detected as one of the products of this reaction. Alternative mechanisms for this reaction are discussed.
On the photodissociation of nitromethane at 266 nm
Authors: Kwok H. S. , He G. Z. , Sparks R. K. , Lee Y. T.
Published: January 2005

In a crossed laser–molecular beam study of nitromethane it was found that the excitation of nitromethane at 266 nm did not yield dissociation products under collision‐free conditions. When a small cluster of nitromethane was excited at the same frequency, the product was seen only at energies and masses consistent with the rupture of the van der Waals bond by vibrational predissociation of the excited state.
Nonisothermal kinetic analysis of the chemiluminescence from tetramethyl‐1,2‐dioxetane
Authors: Mendenhall G. D. , Tauschek J. C. , Ogle C. A.
Published: January 2005

A computerized method is given for the evaluation of Arrhenius parameters which describe the chemiluminescent decomposition of tetramethyl‐1,2‐dioxetane. The parameters were determined in several solvents by linear regression methods and the equation ln ln \documentclass{article}\pagestyle{empty}\begin{document}$ [(\sum\nolimits_0^\infty I - \sum\nolimits_0^t I)/(\sum\nolimits_0^\infty I - \sum\nolimits_0^t I - \sum\nolimits_0^{t + \Delta t} I)] = \ln\, (A_1 \Delta t) - E_1 /RT$\end{document}, where I refers to photons counted by increments of Δt, and E1 and A are the first‐order Arrhenius parameters. The average of E1 and log A1 (s−1) from this method from six runs in CCl4 with initial concentrations of 4.9 × 10−5‐8.45 × 10−4M were 27.21 ± 0.88 kcal/mol (113.7 ± 3.7 kJ/mol) and 13.88 ± 0.50, respectively. Simulated curves of chemiluminescence versus time were obtained with the use of a computer program and an auxiliary plotter.
Pyrolysis of 1‐iodopropane by the variable encounter method
Authors: Wolters F. C. , Chao K.‐J. , Rabinovitch B. S.
Published: January 2005

The pyrolysis of 1‐iodopropane has been studied by the variable encounter method (VEM) at temperatures from 625 to 840 K. Deactivating wall collisions are found to be stronger for this molecule than for the hydrocarbons previously studied in this temperature range. The results of this study are compared with earlier steady‐state work.
Absolute rate constants for the reactions between amino and alkyl radicals at 298 K
Authors: Demissy M. , Lesclaux R.
Published: January 2005

The absolute rate constants for the reactions of NH2 radicals with ethyl, isopropyl, and t‐butyl radicals have been measured at 298 K, using a flash photolysis–laser resonance absorption method. Radicals were generated by flashing ammonia in the presence of an olefin. A new measurement of the NH2 extinction coefficient and oscillator strength at 597.73 nm was performed. The decay curves were simulated by adjusting the rate constants of both the reaction of NH2 with the alkyl radical and the mutual interactions of alkyl radicals. The results are k(NH2 + alkyl) = 2.5 (±0.5), 2.0 (±0.4), and 2.5 (±0.5) × 1010 M−1·s−1 for ethyl, isopropyl, and t‐butyl radicals, respectively. The best simulations were obtained when taking k(alkyl + alkyl) = 1.2, 0.6, and 0.65 × 1010M−1·s−1 for ethyl, isopropyl, and t‐butyl radicals, respectively, in good agreement with literature values.
Rate constants for the gas‐phase reactions of O3 with selected organics at 296 K
Authors: Atkinson Roger , Aschmann Sara M. , Fitz Dennis R. , Winer Arthur M. , Pitts James N.
Published: January 2005

Rate constants for the gas‐phase reactions of O3 with ethene, propene, 1‐hexene, 1‐heptene, styrene, o‐, m‐, and p‐cresol, o‐ and m‐xylene, benzylchloride, acrylonitrile, and trichloroethene have been determined at 296 ± 2 K. The rate constants ranged from <5 × 10−21 cm3 molecule−1 s−1 for m‐xylene to 2.16 × 10−17 cm3 molecule−1 s−1 for styrene, with those for ethene, propene, and 1‐hexene being in excellent agreement with literature data.
Mechanism of HTO formation in gaseous tritium mixtures
Authors: Papagiannakopoulos P. J. , Easterly C. E.
Published: January 2005

The mechanisms for the conversion of molecular tritium gas to tritiated water are examined for tritium mixtures with (1) oxygen and nitrogen, (2) oxygen and argon, and (3) water and helium, for which previous experimental data exist. By analyzing results of these experiments in light of the radiation chemistry involved in a mixture of tritium and other gases, an understanding of the conversion mechanisms is reached. The formation of H and/or OH free radicals as intermediate species is of particular significance in the formation of HTO in that these radicals initiate a number of reactions which lead to the formation of water. These reactions are analyzed in terms of steady‐state kinetics to obtain predictive models which can be judged against the experimental observations. For the three experimentally observed mixtures, model agreement is found to be within a factor of 2–3.
Chemistry in mixtures of Kr with Ar and He illuminated with Kr resonance radiation
Authors: Young Robert A.
Published: January 2005

The formation and destruction of excited rare‐gas dimers have been studied using resonance optical excitation of high‐pressure gas mixtures containing Kr. From an analysis of the resulting emission, 19 reaction rate coefficients have been inferred. It is concluded that atom–atom interchange processes play a major role in rare‐gas eximer formation, and that stepwise relaxation down the eximer vibrational ladder is less important than multistep relaxation due to atom–atom exchange in the excited eximer.
Gas‐phase thermolysis of sulfur compounds. Part III: n‐butyl 2‐propenyl sulfide
Authors: Martin Gonzalo , Drayer Alberto , Ropero Marcos , Alonso Miguel E.
Published: January 2005

The pyrolysis of n‐butyl 2‐propenyl sulfide has been investigated in a static system in the initial pressure range of 50–350 torr. The reaction was found to be homogeneous and first order. The rate coefficients are given by the Arrhenius equation

between 262 and 293°C. The rate of the reaction remains unchanged in the presence of cyclohexene as radical inhibitor. The main reaction products were propene and a trimer of n‐butyl thioaldehyde. The results are interpreted in terms of a molecular mechanism involving a cyclic six‐centered transition state. This mechanism is supported by the pyrolysis of 1,1‐dideutero‐n‐butyl 2‐propenyl sulfide at 281°C. The kinetic deuterium isotope effect had a value of 2.7 ± 0.2. Nuclear magnetic resonance and mass spectroscopic analysis of the reaction products showed the deuterium to be distributed as expected from the proposed reaction mechanism.
Kinetics and energetics of the criegee intermediate in the gas phase. I. The criegee intermediate in ozone–alkene reactions
Authors: Herron John T. , Martinez Richard I. , Huie Robert E.
Published: January 2005

The chemistry and energetics of the Criegee intermediate, a primary product of the ozonolysis of alkenes, are discussed in terms of recent ab‐initio calculations and laboratory studies. The experimental observations in O3–alkene systems can be rationalized on the basis of a general mechanism: magnified image where the ‡ represents the range of internal excitation energies available to the planar dioxymethylenes (i.e., the Criegee intermediates) initially formed via exothermic O3‐alkene reactions. Estimates are given for the rate constants of these reactions, and a critique is provided of the possible role of the Criegee intermediate and its isomers in the formation of alkanoic acid anhydrides in O3–alkene systems and in the formation of H2SO4 aerosols in O3–alkene–SO2 systems.
The mechanism of O3‐aldehyde reactions
Authors: Martinez Richard I.
Published: January 2005

A new mechanism is proposed for gas‐phase O3‐aldehyde reactions. Certain aspects of the new mechanism may be applicable also to O3‐aldehyde reactions in solution. The proposed mechanism involves initiation by addition of O3 across the aldehydic CO bond. The initiation process and the subsequent chemistry together represent an exact analogue of the Criegee mechanism for O3‐alkene reactions.
Shock‐initiated decomposition of tetramethylgermane and the reaction of methyl radicals with toluene
Authors: Dzarnoski J. , Ring M. A. , O'Neal H. E.
Published: January 2005

The shock‐initiated decomposition of tetramethylgermane (1078–1242 K) has been found to involve successive elimination of methyl radicals with the rate constant k1 for the first step given by

In the presence of excess toluene the products were CH4 (major), C2H4, and C2H6. Results relevant to the reaction of methyl radicals with toluene compared to methyl radical recombination are discussed.