Showing papers in "International Journal of Chemical Kinetics in 1970"

Continuous-wave chemical laser

Abstract: This invention discloses a system for generating a continuous wave (CW) chemically pumped lasing action. A continuous radiation output is achieved by a technique which involves generation of a high speed jet containing a reactant and an inert diluent. A second reactant is diffused into the high speed jet. The chemical reaction between these reactants creates, directly, a vibrationally excited gaseous product with the requisite population inversion and lifetime needed for lasing. The vibrationally excited product gas is created within, or flows into, an optical cavity whose axis is transverse to the flow. Lasing is made to occur within the optical cavity. After lasing, the product gas is quickly convected out of the optical cavity by the high speed jet.

Reactions of Cl2P3/2: Absolute rate constants for reaction with H2, CH4, C2H6, CH2Cl2, C2Cl4, and cC6H12

Abstract: The absolute rate constants have been measured for several gas-phase chlorine atom-molecule reactions at 25°C by resonance fluorescence. These reactions and their corresponding rate constants in units of cm3 mole−1 sec−1 are: The effects of varying the substrate pressure, total pressure, light intensity and chlorine-atom source on the value of the bimolecular rate constants have been investigated for all these reactions. Conditions under which no competing side reaction occurs were established and the reported rate constants were measured under these conditions. For reactions (2), (5), (6), (7), and 8, there is a discrepancy of a factor of two between the rate constants measured in this work and values in the literature; it is suggested that this is due to an error in the previously measured value of k/k upon which the relative measurements in the literature ultimately depend.

A Quantitative study of alkyl radical reactions by kinetic spectroscopy. Part I. Mutual combination of methyl radicals and combination of methyl radicals with nitric oxide

Abstract: Combination reactions of the methyl radical have been studied by following the decay of the absorbance of the methyl radical during the course of the reaction by means of kinetic spectroscopy. The limiting values of the second-order rate constants at high pressure were determined for two reactions at room temperature: The extinction coefficient of the methyl radical was found to have a maximum value of (1.02 ± 0.06) X 104 1 mole−1 cm−1 at 216.4 nm. Integration of the extinction coefficient over the absorption band of the methyl radical gave an oscillator strength of 1.0 X 10−2.

Rate and mechanism of thermal decomposition of 4-methyl-l-pentyne in a single-pulse shock tube

Abstract: Dilute mixtures of 4-methyl-l-pentyne have been pyrolyzed in a single-pulse shock tube. The decomposition process involves bond breaking: as well as a molecular reaction: The rate parameters are: The heat of formation of propynyl radical is thus ΔHf300 = 338 kJ mol−1 (80.7 kcal mol−1)˙ This leads to a propynyl resonance energy of 40 kJ mol−1 (9.6 kcal mol−1).

The reactions of CF3 radicals with cyclanes and the determination of C?H bond dissociation energies in cyclanes

Abstract: The reactions of CF3 radicals with the C3 to C7 cyclanes and spiropentane were studied and the following Arrhenius parameters were obtained for the reaction CF3 + c-RH CF3H + c-R: Text Table Text. c-RH log A (cm3mole−1sec−1) E (kcal/mole) D(c-R—H) (kcal/mole) Cyclopropane 11.54 8.73 100.7 Cyclobutane 11.66 6.48 95.7 Cyclopentane 12.30 6.18 94.3 Cyclohexane 12.12 6.26 94.9 Cycloheptane 12.43 5.89 94.0 Spiropentane 11.91 8.12 98.8 The CF3 radicals were generated by photolysis of hexafluoroacetone or CF3I and a comparison is made of the utility of the two compounds as radical sources. The Arrhenius parameters are compared with those for corresponding reactions of CH3 radicals with cyclanes, and the general reactivity of cyclic compounds toward free radicals is discussed. An Evans-Polanyi treatment is used to derive CH bond dissociation energies in cyclanes; and these results, based on the reactions of CF3 with cyclanes, agree well with those previously obtained using CH3 plus cyclanes. The final mean values are shown above.

A thermochemical and kinetic analysis of the mechanisms of pyrolysis of polycyclic molecules

Abstract: The quantitative kinetics of pyrolysis of some cyclic and polycyclic compounds are examined from the point of view of biradical intermediates. Transition state methods for estimating the Arrhenius parameters of the lowest free energy biradical pathway to products in cyclic and polycyclic compound reactions are described and illustrated. A large number of the polycyclic reactions are found to have Arrhenius parameters consistent with the biradical mechanism estimates. Other reactions are found to have much faster experimental rates and must therefore be concerted. The value of activation energy and activation entropy estimates as discriminatory tests of mechanism, i.e., single step (concertedness) or consecutive step processes, is discussed.

Comparative rate single‐pulse shock tube studies on the thermal decomposition of cyclohexene, 2,2,3‐trimethylbutane, isopropyl bromide, and ethylcyclobutane

Abstract: A check of the data from comparative rate single-pulse shock tube experiments have been carried out through the use of a new standard reaction, the decyclization reaction of ethylcyclobutane. The rate expressions for cyclohexene and 2,2,3-trimethylbutane have been found to be in excellent agreement with previously published results. Most of the small discrepancy that does exist is apparently due to the differences between the present and earlier (decomposition of isopropyl bromide) "standard" reaction. For the latter process, the present study yields These results confirm the correctness of previously published comparative rate single-pulse shock tube experiments. They demonstrate once again that for the decomposition of paraffin hydrocarbons, calculated preexponential factors are at least an order of magnitude higher than the directly measured number and that the accepted value of the heat of formation of t-butyl radicals ΔHf300(tC4H9·) = 29 kJ (6.8 kcals) is at least 10 kJ too low. Finally, attention is called to recent studies on neopentane decomposition in flow and static systems which are in complete agreement with the present conclusions.

Reaction of O(1D) with N2O

Abstract: O(1D), produced from the photolysis of N2O at 2139 A, reacts with N2O in accord with: We have used the method of chemical difference to obtain an accurate measure of k2/k3 = 0.59 ± 0.01. Furthermore, the quantum yield of production of O(3P), either on direct photolysis or on deactivation of O(1D) by N2O, is less than 0.02 and probably zero.

A simple, self-consistent electrostatic model for quantitative prediction of the activation energies of four-center reactions. II.*†

Abstract: A simple electrostatic model of point dipoles is used which permits direct calculation of the activation energies for the addition of the molecules H2O, H2S, H3N, and H3P to olefins. These calculated values agree with the known experimental data to within ±2 kcal/mole on the average. It was found that the best fit could be obtained with a polar transition state that corresponded to a reduction in bond order from 1 to ½ for the bond-breaking coordinates and an increase in bond order from 0 to 0.18 for the bond-forming coordinates. The replacement of a hydrogen atom of the species H2O, H2S, H3N, or H3P by a polarizable methyl group is expected to stabilize the charge on the central atoms. The following stabilization energies for the pairs H2OCH3OH, H2SCH3SH, H3NCH3NH2, H3PCH3PH2 were calculated: −4.8 kcal/mole, −0.7 kcal/mole, −1.9 kcal/mole, −0.8 kcal/mole, respectively.

Kinetics and thermochemistry of the gas phase reaction of methyl ethyl ketone with iodine. I. The resonance energy of the methylacetonyl radical

Abstract: The gas phase reaction of iodine (2.8–43.3 torr) with methyl ethyl ketone (MEK) (7.4–303.4 torr) has been studied over the temperature range 280–355°C in a static system. The initial rate of disappearance of I2 is first order in MEK and half order in I2. The rate-determining step is the abstraction of a secondary hydrogen atom by an iodine atom: where k1 is given by and θ = 2.303RT in kcal/mole. This activation energy is equivalent to a secondary CH bond strength of 92.3 ± 1.4 kcal/mole and ΔH of the methylacetonyl radical = -16.8 ± 1.7 kcal/mole. By comparison with 95 kcal/mole for the secondary CH bond strength, when delocalization of the unpaired electron with a pi bond is not possible, the resonance stabilization of the methylacetonyl radical is calculated to be 2.7 ± 1.7 kcal/mole. This value is 10 kcal/mole less than the stabilization energy of the isoelectronic methylallyl radical. The difference in pi bond energies in the canonical forms of the methylacetonyl radical is shown to account for the variation in stabilization energies.

Kinetics of the gas‐phase reaction of acetone with iodine: Heat of formation and stabilization energy of the acetonyl radical

Abstract: The kinetics of the gas-phase reaction CH3COCH3 + I2 ⇄ CH3COCH2I + HI have been measured spectrophotometrically in a static system over the temperature range 340–430°. The pressure of CH3COCH3 was varied from 15 to 330 torr and of I2 from 4 to 48 torr, and the initial rate of the reaction was found to be consistent with as the rate-determining step. An Arrhenius plot of the variation of k1 with temperature showed considerable scatter of the points, depending on the conditioning of the reaction vessel. After allowance for surface catalysis, the best line drawn by inspection yielded the Arrhenius equation, log [k1/(M−1 sec−1)] = (11.2 ± 0.8) – (27.7 θ 2.3)/θ, where θ = 2.303 RT in kcal/mole. This activation energy yields an acetone CH bond strength of 98 kcal/mole and δH (CH3COĊH2) radical = −5.7 ± 2.6 kcal/mole. As the acetone bond strength is the same as the primary CH bond strength in isopropyl alcohol, there is no resonance stabilization of the acetonyl radical due to delocalization of the radical site. By contrast, the isoelectronic allyl resonance energy is 10 kcal/mole, and reasons for the difference are discussed in terms of the π-bond energies of acetone and propene.

Kinetic study of the gas‐phase reaction c‐C5H8 + I2 ⇄ c‐C5H6 + 2HI the heat of formation and the stabilization energy of the cyclopentenyl radical

Abstract: The gas-phase dehydrogenation of cyclopentene to cyclopentadiene catalyzed by iodine in the range 178–283°C has been found to obey a rate law consistent with the slow rate-determining step, , log [k4/(1 mole−1 sec−1)] = 10.25 ± 0.08 - (12.26 ± 0.18)/θ, where θ = 2.303RT in kcal/mole. Surface effects are not important. This value of E4 leads to a value of DH = 82.3 ± 1 kcal/mole and ΔHf298 = 38.4 ± 1 kcal/mole. From difference in bond strengths in the alkane and the alkene, the allylic resonance stabilization in the cyclopentenyl radical is 12.6 ± 1.0 kcal/mole, in excellent agreement with the value for the butenyl radical. Arrhenius parameters for the other steps in the mechanism are evaluated. The low value of A4 (compared with A4 for cyclopentane) suggests a “tighter” transition state for H-atom abstraction from alkenes than from alkanes.

The elimination of HF from vibrationally excited fluoroethanes. The decomposition of 1,1,1‐trifluoroethane‐d0 and d3

Abstract: The vibrationally excited molecules CF3CH and CF3CD have been synthesized by radical combination (produced by ketone photolysis), and HF and DF elimination from them studied as a function of temperature and pressure. Using RRK theory many calculations have recently been made of critical energies for the decomposition of "hot" fluoroethane molecules. Taking CF3CH as an example, it is concluded that the empiricism involved in such calculations renders results of doubtful significance. The non-equilibrium kinetic isotope effect is kH/kD = 3.1 at 470°K. Arrhenius parameters are also presented for radical abstraction reactions from the ketone source molecules.

Kinetic study of the gas‐phase reaction c‐C5H10 + I2 ⇄ c‐C5H8 + 2HI the heat of formation of cyclopentyl radical

Abstract: The kinetics of the gas-phase dehydrogenation of cyclopentane to cyclopentene is found to be consistent with a slow attack by an I atom (step 4, text) on cyclopentane in the range 282–382°C. The measured rate constants fit the Arrhenius equation, log k4 = 11.95 ± 0.08 – (24.9 ± 0.23)/θ 1 mole−1 sec−1, where θ = 2.303RT in kcal/mole. This leads to a value of ΔH = 24.3 ± 1 kcal/mole and a bond dissociation energy DH = 94.9 ± 1 kcal/mole. The latter value is identical with DH0(i-Pr-H) = 95 ± 1 kcal/mole and signifies that cyclopentane and the cyclopentyl radical have the same strain energy. Arrhenius parameters are deduced for all six steps in the reaction mechanism. Surface reactions are shown to be unimportant. Cyclopentyl iodide is an unstable intermediate in the reaction and the rate constant for its bimolecular formation from HI + cyclopentene is found to be log k6 = 8.40 ± 0.29 - (26.9 ± 0.8)/θ 1 mole−1 sec−1. Together with the equilibrium constant, this yields for the unimolecular elimination of HI from cyclopentyl iodide, the rate constant, log k5 = 13.3 ± 0.3 – (42.8 ± 1.2)/θ sec−1.

The free-radical reaction of tri-n-butyltin hydride with peroxides†

Abstract: Rate constants for the tri-n-butyltin radical (Sn ·) induced decomposition of a number of peroxides have been measured in benzene at 10°C. The values range from ∼100 M−1 sec−1 for di-t-butyl peroxide to 2.6 × 107M−1 sec−1 for di-t-butyl diperoxyisophthalate. The majority of the peroxides, including diethyl peroxide, diacetyl peroxide, and t-butyl peracetate, have rate constants of ∼105M−1 sec−1. It is shown that di-n-alkyl disulfides are ten times as reactive toward Sn · as di-n-alkyl peroxides, although the exothermicities of these reactions are ∼15 and ∼39 kcal/mole, respectively. The enhanced reactivity of the disulfides is attributed to the easier formation of an intermediate or transition state with 9 electrons around sulfur, compared with an analogous species with 9 electrons around oxygen. The following bond strengths (kcal/mole) have been estimated: D[SnOR] = 77; D[SnH] = 82; D[SnSR] = 83; and D[SnOC(O)R] = 86, where R = alkyl. Rate constants for reaction of Sn · with some benzyl esters have also been measured. It has been found that t-butoxy radicals can add to benzene and abstract hydrogen from benzene at ambient temperatures.

Reaction of CF3 radicals with group IV tetramethyls

Abstract: Arrhenius parameters have been measured for the abstraction of hydrogen from the C Si, Ge, and Sn tetramethyls: The rate constants correlate with the proton chemical shift, which is related to a polar effect In all cases except carbon, a hot-molecule β-fluorine rearrangement-elimination reaction occurs following radical combination: We suggest the occurrence of a radical exchange reaction for the Si, Sn, and Ge systems, with kexchange (CF3 + Sn(Me)4) ∼ 107 ml m−1 s−1

Mechanism of the bimolecular reactions of ethylene and propylene

Abstract: Rate constants for the bimolecular reactions of ethylene and propylene to form radicals and to form cyclobutane or its derivatives have been calculated using thermodynamic and kinetic data. Comparison of these rates with the kinetics of the thermal reactions of ethylene and propylene show that cyclobutane and its derivatives are probably not important intermediates in the processes forming radicals.

The pyrolysis of ethane in the presence of nitric oxide

Abstract: The kinetics of the ethane pyrolysis have been studied at temperatures from 550 to 596°C and with 0 to 62% of added nitric oxide. The rates of production of various products were studied by gas chromatography; ethylene, hydrogen, methane, nitrogen, water, nitrous oxide and acetonitrile were found as primary products, with hydrogen cyanide, carbon monoxide, acetaldehyde, n-butane, 1-butene, cis- and trans-2-butene and 1,3-butadiene as secondary products. For all the primary products the orders with respect to C2H6and NO were determined, as were the activation energies at two different percentages of NO (15.7 and 45.5%). Nitric oxide was found to be rapidly consumed with a finite initial rate, and the rate of production of H2O was close to that of C2H4 at higher nitric oxide pressures. A mechanism is proposed which gives good agreement with all of the observed results. Its main features are: (1) Initiation takes place mainly by the unimolecular dissociation of ethane; there is no evidence for or against the process NO + C2H6 HNO + C2H5; (2) NO scavenges ethyl radicals to form acetaldoxime which decomposes, and in this way the breakdown of C2H5 is hastened; (3) termination takes place mainly by the unimolecular decomposition of acetaldoxime to give inactive products. Some of the relevant rate parameters are evaluated. Reactions are proposed to account for the formation of the secondary products observed.

H2S inhibition of the pyrolysis of ethane

Abstract: At 540°C, H2S (0.1 to 5 mm Hg) diminishes the initial rate of pyrolysis of C2H6 (50 mm Hg) into C2H4 + H2 and, even more strongly, the rate of appearance of the traces of nC4H10; on the contrary, the initial rate of formation of the traces of CH4 is practically not modified. A mechanism is proposed in order to interpret these experimental facts.

The Kinetics of the gas‐phase isomerization of 1,trans‐3,trans‐5‐heptatriene into the cis‐5‐isomer catalyzed by Nitric Oxide and the stabilization energy in the pentadienyl radical

Abstract: The kinetics of the nitric oxide catalyzed, homogeneous, gas-phase isomerization of 1,trans-3,trans-5-heptatriene have been studied for temperatures ranging between 130°C and 241°C. The very clean reaction involves exclusive geometrical isomerization about the 5,6-π-bond. The observed rate constants for can be represented (with standard errors) by log k1 = (7.18 ± 0.06) – (16.75 ± 0.12)/θ, where θ = 2.303 RT in kcal/mole. The consecutive-step reaction mechanism involves addition of NO to the double bond (Ka, b = ka/kb), followed by rotation of the 5,6-CC bond in the adduct radical (kc.) Analysis of the observed activation parameters shows, that kc is rate-controlling and consequently k1 = kcKa, b. Estimates of kc and Ka, b lead to a value of k1 in good agreement with experiment. Comparing our data with those previously obtained for the similar 1,3-pentadiene system results in a value for the extra stabilization energy generated in the 1,3-heptadienyl radical of 18.5 ± 1.7 kcal/mole. This value is discussed in view of comparable data in the literature.

Thermal decomposition of benzotrifluoride. The CC bond strength and the heat of formation of the phenyl radical

Abstract: The kinetics of the decomposition of benzotrifluoride was studied from 720°c to 859°c in a flow system with and without carrier gas. Consideration of the product distribution made possible the study of the decomposition into CF3 and C6H5 radicals, which appeared to be truly homogeneous in character. The first-order rate constant of the CC bond fission, log k (sec−1) = (17.9 ± 0.5) (99.7 ± 2.5)/θ, did not change with change of initial concentration, pressure of the carrier gas, or contact time. The Arrhenius parameters have been related to the appropriate thermodynamic data. Assumption of 0 kcal/mole for the activation energy of the reverse combination reaction yielded DH(C6H5CF3) = 103.6 ± 2.5 kcal/mole and ΔH(C6H5) = 77.1 ± 3.0 kcal/mole. Applicability of the simple first-order formula to calculation of the rate constant has been also dealt with.

A Complication in the thermal reactions of 1,1,2,2-tetramethylcyclopropane

Abstract: Reinvestigation of the gas phase thermal reaction of 1,1,2,2-tetramethylcyclopropane (699-759°K) gave for the unimolecular disappearance of reactant, k(TMC) = 1015.27–63.93/θ sec−1, in good agreement with the original results of Frey and Marshall. However, evidence for a high activation energy (E = 79 ± 5 kcal/mole), competitive unimolecular decomposition to 2,3-dimethyl-1 and -2-butenes was also obtained. It is proposed that the serious discrepancy noted [1] between the experimentally observed Arrhenius parameters for the overall reaction kinetics, and those predicted by transition state calculations assuming a biradical mechanism for the isomerization reactions (previously believed to be the only primary reaction mode) can be explained in terms of the increasing importance of the decomposition reactions at higher reaction temperatures.

Pyrolysis of 1,2-dichloropropane

Abstract: The thermal decomposition of 1,2-dichloropropane at atmospheric pressure has been studied in the temperature range 227–590°C, in a flow system. Above 450°C, the reaction is homogenous and unimolecular with a rate constant: Below 450°C, a low activation energy, probably heterogenous process competes with the gas phase reaction The primary reaction products are HCl and the monochloropropene isomers; the relative amounts of each isomer depend on the temperature in the low but not in the high temperature region. The direction of the HCl elimination is discussed in terms of substituent effects at the α- and β-carbon positions and compared with literature data on similar reactions Secondary products are formed principally by further pyrolysis of allyl chloride. The first-order rate constant of this reaction is given by: .

Stern–volmer plots in the photolysis of perfluoroazoethane

Abstract: Nitrogen quantum yields are reported for the photolysis of C2F5NNC2F5 at 3660 A over the pressure range 2–10 cm from 25° to 150°c. The Stern–Volmer plots obtained are discussed and compared with those obtained with azoethane and azoisopropane.

Semi‐empirical potential energy surfaces for hydrogen‐atom transfer reactions

Abstract: A method for constructing potential energy surfaces previously proposed by the author has been extended to hydrogen transfer reactions between halide, oxygen, and carbon atoms. A qualitative relation was found between the repulsive energy and the number of anti-bonding electrons. In general, the calculated kinetic isotope effect is in satisfactory agreement with observed values and the contributions by H-atom tunneling to the rate of reaction is smaller than that obtained from other surfaces.

The pyrolysis of acetaldehyde in the presence of nitric oxide

Abstract: The kinetics of the acetaldehyde pyrolysis have been studied at temperatures from 450° to 525°C, at an acetaldehyde pressure of 176 torr and at 0 to 40 torr of added nitric oxide. The following products were identified and their rates of formation measured: CH4, H2, CO, CO2, C2H4, C2H6, H2O, C3H6, C2H5CHO, CH3COCH3, CH3COOCHCH2, N2, N2O, HCN, CH3NCO, and C2H5NCO. Acetaldehyde vapor was found to react with nitric oxide slowly in the dark at room temperature, the products being H2O, CH3COOCH3, CO, CO2, N2, NO2, HCN, CH3NO2, and CH3ONO2. The rates of formation of N2 and C2H5NCO depend on how long the CH3CHO-NO mixture is kept at room temperature before pyrolysis; the rates of formation of the other products depend only slightly on the mixing period. The pyrolysis of “clean” CH3CHO–NO mixtures (i.e., the results extrapolated to zero mixing time, which are independent of products formed in the cold reaction) are interpreted as follows: (1) There are two chain carriers, CH3 and CH2CHO, their concentrations being interdependent and influenced by NO in different ways: the CH3 radical is both generated and removed by reactions directly involving NO, whereas CH2CHO is generated only indirectly from CH3 but is also removed by direct reaction with NO. (2) An important mode of initiation by NO is its addition to the carbonyl group with the formation of which is converted into ; this splits off OH with the formation of CH3NCO or CH3 + OCN. (3) Important modes of termination are The steady-state equations derived from the mechanism are shown to give a good fit to the experimental rate versus [NO] curves and, in particular, explain why there is enhancement of rate by NO at higher CH3CHO pressures and, at lower CH3CHO pressures, inhibition at low [NO] followed by enhancement at higher [NO]. The cold reaction is explained in terms of chain-propagating and chain-branching steps resulting from the addition of several NO molecules to CH3CHO and the CH3CO radical. In the “unclean” reaction it is found that the rates of N2 and C2N5NCO formation are increased by CH3NO2, CH3ONO, and CH3ONO2 formed during the cold reaction. A mechanism is proposed, involving the participation of α-nitrosoethyl nitrite, CH3CH(NO)ONO. It is suggested that there are two modes of behavior in pyrolyses in the presence of NO: (1) In the paraffins, ethers, and ketones, the effects are attributed to the addition of NO to a radical with the formation of an oxime-like compound. (2) In the aldehydes and alkenes, where there is a hydrogen atom attached to a double-bonded carbon atom, the behavior is explained in terms of addition of NO to the double bond followed by the formation of an oxime-like species.