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

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 \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH}_3 {\rm COCH}_3 + {\rm I}^{\rm .} \stackrel{1}{\rightarrow}{\rm CH}_{\rm 3} {\rm COCH} + {\rm HI} $\end{document} 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 R T in kcal/mole. This activation energy yields an acetone C? H bond strength of 98 kcal/mole and δH (CH₃CO?H₂) radical = ?5.7 ± 2.6 kcal/mole. As the acetone bond strength is the same as the primary C? H 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.     

Kinetics of the gas-phase reaction of allyl alcohol with iodine. The heat of formation and stabilization energy of the hydroxyallyl radical

The reaction of iodine with allyl alcohol has been studied in a static system, following the absorption of visible light by iodine, in the temperature range 150-190°C and in the pressure range 10-200 torr. The rate-determining step has been found to be and k₃ is consistent with the equation From the activation energy and the assumption E_-3 = 1 ± 1 kcal mol^?1, it has been calculated that kcal mol^?1. The stabilization energy of the hydroxyallyl radical has been found to be 11.4 ± 2.2 kcal mol^?1.     

Kinetics of the thermal unimolecular decomposition of hex-1-ene-3-yne. Heat of formation and resonance stabilization energy of the 3-ethenylpropargyl radical

The thermal unimolecular decomposition of hex-1-ene-3-yne (HEY) has been investigated over the temperature range 949–1230 K using the technique of very low-pressure pyrolysis (VLPP). One reaction pathway is the expected C₅? C₆ bond fission to form the resonance-stabilized 3-ethenylpropargyl radical. There is a concurrent process producing molecular hydrogen which probably occurs via the intermediate formation of hexatrienes and cyclohexa-1,3-diene. RRKM calculations yield the extrapolated high-pressure rate parameters at 1100 K given by the expressions 10^16.0±0.3 exp(?300.4 ± 12.6 kJ mol^?1/RT) s^?1 for bond fission and 10^13.2+0.4 exp(?247.7 ± 8.4 kJ mol^?1/RT) for the overall formation of hydrogen. The A factors were assigned from the results of previous studies of related alkynes, alkenes, and alkadienes. The activation energy for the bond fission reaction leads to ΔH [H₂CCHCC?H₂] = 391.9, DH [H₂CCHCCCH₂? H] = 363.3, and a resonance stabilization energy of 56.9 ± 14.0 kJ mol^?1 for the 3-ethenylpropargyl radical, based on a value of 420.2 kJ mol^?1 for the primary C? H bond dissociation energy in alkanes. Comparison with the revised value of 46.6 kJ mol^?1 for the resonance energy of the unsubstituted propargyl radical indicates that the ethenyl substituent (CH₂?CH) on the terminal carbon atom has only a small effect on the propargyl resonance energy. © John Wiley & Sons, Inc.     

The gas-phase kinetics of the reaction of 1,1-difluoroiodoethane with hydrogen iodide: The C–I bond dissociation energy in 1,1-difluoroiodoethane

The kinetics of gas-phase reaction of CH₃CF₂I with HI were studied from 496 to 549K and have been shown to be consistent with the following mechanism: A least squares treatment of the data gave where θ = 2.303 RT kcal/mole. The observed activation energy E₁ was combined with E₂ = 0 ± 1 kcal/mole to yield The result, combined with data for several C? I bond dissociation energies, leads us to conclude that the C(sp³)? I bond is relatively insensitive to F for H substitution and that the C(sp²)–I bond has considerable double-bond character.     

Kinetics of the gas-phase reaction of OH with HCl

The reaction of hydroxyl radicals with hydrogen chloride (reaction 1) has been studied experimentally using a pulsed-laser photolysis/pulsed-laser-induced fluorescence technique over a wide range of temperatures, 298-1015 K, and at pressures between 5.33 and 26.48 kPa. The bimolecular rate coefficient data set obtained for reaction 1 demonstrates no dependence on pressure and exhibits positive temperature dependence that can be represented with modified three-parameter Arrhenius expression within the experimental temperature range: k1 = 3.20 x 10(-15)T0.99 exp(-62 K/T) cm3 molecule(-1) s(-1). The potential-energy surface has been studied using quantum chemical methods, and a transition-state theory model has been developed for the reaction 1 on the basis of calculations and experimental data. The model results in modified three-parameter Arrhenius expressions: k1 = 8.81 x 10(-16)T1.16 exp(58 K/T) cm3 molecule(-1) s(-1) for the temperature range 200-1015 K and k1 = 6.84 x 10(-19)T2.12 exp(646 K/T) cm3 molecule(-1) s(-1) for the temperature dependence of the reaction 1 rate coefficient extrapolation to high temperatures (500-3000 K). A temperature dependence of the rate coefficient of the Cl + H2O --> HCl + OH reaction has been derived on the basis of the experimental data, modeling, and thermochemical information.     

Very low-pressure pyrolysis (VLPP) of pentynes. III. Pent-2-yne. Heat of formation and resonance stabilization energy of the 3-methylpropargyl radical

The thermal unimolecular decomposition of pent-2-yne has been studied over the temperature range of 988–1234 K using the technique of very low-pressure pyrolysis (VLPP). The main reaction pathway is C₄? C₅ bond fission producing the resonance-stabilized 3-methylpropargyl radical. There is a concurrent process producing molecular hydrogen and penta-1,2,4-triene presumably via the intermediate formation of cis-penta-1,3-diene. The 1,4-hydrogen elimination from cis-penta-1,3-diene is the rate-determining step in the molecular pathway. This is supported by an independent VLPP study of cis- and trans-penta-1,3-diene. RRKM calculations show that the experimental rate constants for C? C bond fission are consistent with the following high-pressure rate expression at 1100 K: where θ = 2.303RT kcal/mol and the A factor was assigned from the results of shock-tube studies of related alkynes. The activation energy leads to ΔH[CH₃C?C?H₂] = 70.3 and DH[CH₃CCCH₂? H] = 87.4 kcal/mol. The resonance stabilization energy of the 3-methylpropargyl radical is 10.6 ± 2.5 kcal/mol, which is consistent with previous results for this and other propargylic radicals.     

Metal-mediated formation of gas-phase amino acid radical cations

The results from an investigation of the collision-induced dissociation (CID) of the ternary complexes [Cu(II)(terpy)(AA)](2+) are presented (terpy = 2,2':6',2' '-terpyridine; AA = one of the twenty common amino acids). These complexes show a rich gas-phase chemistry, which depends on the identity of the amino acid. For the histidine-, lysine- and tryptophan-containing complexes, oxidative dissociation of the amino acid is observed, yielding the amino acid radical cation. The results of further mass selection and CID of these amino acid radical cations are presented. The CID of the series [Fe(III)(salen)(AA)](+) (where salen = N,N'-ethylenebis(salicylideneaminato)) is also examined. These complexes undergo loss of the neutral amino acid in all cases, although the radical cation of arginine is also produced and its subsequent fragmentation examined. B3-LYP/6-31G(d) computations were carried out to test aspects of the proposed fragmentation mechanism of the histidine and arginine radical cations.     

A theoretical investigation of the gas-phase oxidation reaction of the saturated tert-butyl radical.

The radical-radical reaction mechanisms and dynamics of ground-state atomic oxygen [O(3P)] with the saturated tert-butyl radical (t-C4H9) are investigated using the density functional method and the complete basis set model. Two distinctive reaction pathways are predicted to be in competition: addition and abstraction. The barrierless addition of O(3P) to t-C4H9 leads to the formation of an energy-rich intermediate (OC4H9) on the lowest doublet potential energy surface, which undergoes subsequent direct elimination or isomerization-elimination leading to various products: C3H6O + CH3, iso-C4H8O + H, C3H7O + CH2, and iso-C4H8 + OH. The respective microscopic reaction processes examined with the aid of statistical calculations, predict that the major addition pathway is the formation of acetone (C3H6O) + CH3 through a low-barrier, single-step cleavage. For the direct, barrierless H-atom abstraction mechanism producing iso-C4H8 (isobutene) + OH, which was recently reported in gas-phase crossed-beam investigations, the reaction is described in terms of both an abstraction process (major) and a short-lived addition dynamic complex (minor).     

Mechanism for the gas-phase reaction between formaldehyde and hydroperoxyl radical. A theoretical study

We present a high-level theoretical study on the gas-phase reaction between formaldehyde and hydroperoxyl radical carried out using the DFT-B3LYP, QCISD, and CCSD(T) theoretical approaches in connection with the 6-311+G(d,p), 6-311+G(2df,2p), and aug-cc-pVTZ basis sets. The most favorable reaction path begins with the formation of a pre-reactive complex and produces the peroxy radical CH(2)(OO)OH in a process that is computed to be exothermic by 16.8 kcal/mol. This reaction involves a process in which the oxygen terminal of the HO(2) moiety adds to the carbon of formaldehyde, and, simultaneously, the hydrogen of the hydroperoxyl group is transferred to the oxygen of the carbonyl in a proton-coupled electron-transfer mechanism. Our calculations show that this transition state lies below the sum of the energy of the reactants, and we computed a rate constant at 300 K of 9.29 x 10(-14) cm(3) molecule(-1) s(-1), which is in good agreement with the experimental results. Also of interest in combustion chemistry, we studied the hydrogen abstraction process by HO(2), the result of which is the formation of HCO + H(2)O(2). We found two reaction paths with activation enthalpies close to 12 kcal/mol. For this process, we computed a rate constant of 1.48 x 10(-16) cm(3) molecule(-1) s(-1) at 700 K, which also agrees quite well with experimental results.     

Intramolecular hydrogen bonding and hydrogen atom abstraction in gas-phase aliphatic amine radical cations

The intramolecular hydrogen atom abstraction by the nitrogen atom in isolated aliphatic amine radical cations is examined experimentally and with composite high-level ab initio methods of the G3 family. The magnitude of the enthalpy barriers toward H-atom transfer varies with the shape and size of the cyclic transition state and with the degree of substitution at the nitrogen and carbon atoms involved. The lower barriers are found for 1,5- and 1,6-abstraction, for chairlike transition states, for abstraction reactions in ionized primary amines, and for abstraction of H from tertiary carbon atoms. In most cases, the internal energy required for 1,4-, 1,5-, and 1,6-hydrogen atom abstraction to occur is less than that required for gas-phase fragmentation by simple cleavage of C-C bonds, which explains why H-atom transfer can be reversible and result in extensive H/D exchange prior to the fragmentation of many low-energy deuterium labeled ionized amines. The H-atom transfer to nitrogen is exothermic for primary amine radical cations and endothermic for tertiary amines. It gives rise to a variety of distonic radical cations, and these may undergo further isomerization. The heat of formation of the gauche conformers of the gamma-, delta-, and epsilon-distonic isomers is up to 25 kJ mol(-1) lower than that of the corresponding trans forms, which is taken to reflect C-H-N hydrogen bonding between the protonated amino group and the alkyl radical site.     

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

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, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm I} + {\rm c} - {\rm C}_5 {\rm H}_8 \stackrel{4}{\rightarrow}{\rm HI} + {\rm c} - {\rm C}_5 {\rm H}_7 $\end{document}, log [k₄/(1 mole^?1 sec^?1)] = 10.25 ± 0.08 - (12.26 ± 0.18)/θ, where θ = 2.303 R T in kcal/mole. Surface effects are not important. This value of E₄ leads to a value of DH = 82.3 ± 1 kcal/mole and ΔH_f298 = 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.     

Kinetics of iodination of hydrogen sulfide by iodine and the heat of formation of the SH radical

The kinetics of the gas-phase thermal iodination of hydrogen sulfide by I₂ to yield HSI and HI has been investigated in the temperature range 555–595 K. The reaction was found to proceed through an I atom and radical chain mechanism. Analysis of the kinetic data yields log k (l/mol·sec) = (11.1 ± 0.18) – (20.5 ± 0.44)/θ, where θ = 2.303 RT, in kcal/mol. Combining this result with the assumption E_?1 = 1 ± 1 kcal/mol and known values for the heat of formation of H₂S, I₂, and HI, ΔH_f,298⁰(SH) = 33.6 ± 1.1 kcal/mol is obtained. Then one can calculate the dissociation energy of the HS? H bond as 90.5 ± 1.1 kcal/mol with the well-known values for ΔH_f,298⁰ of H and H₂S.     

The azoethane-initiated thermal reaction of isobutene. Heat of formation of the 2-methyl-2-pentyl radical

The azoethane-sensitized thermal reaction of isobutene has been studied at 526–565 K. The initial concentrations of azoethane and isobutene were in the ranges of 1.40–10.5 × 10^?4 and 6.78–26.6 × 10^?4 mol/dm³, respectively. From the initial rates of formation of ethane and 2-methylpentane the heat of formation of the 2-methyl-2-pentyl radical was determined. The result obtained is ±H(2-methyl-2-pentyl) = 0.8 ± 2.0 kcal/mol. The entropy of the radical, obtained from statistical mechanical calculations and experimentally, is S⁰(2-methyl-2 pentyl) = 92.8 ± 1.5 cal/mol°K. The results support the high heat of formation of the t-butyl radical suggested by different authors.     

Kinetics of the molybdate catalyzed oxidation of iodide by hydrogen peroxide

The kinetics of the oxidation of iodide by hydrogen peroxide catalyzed by acidic molybdate have been studied by a spectrophotometric stopped-flow method. The results are interpreted in terms of the mechanism and the implied rate law where [mol] is total analytical concentration of molybdate. The values obtained for the rate and equilibrium constants are k₄ = (3.3 ± 1) × 10² 1./mole · s, K₁ = (1.2 ± 0.6) × 10⁴ 1./mole, K₂ = (1.3 ± 0.7) × 10³ 1./mole, and K₃ = (4 ± 3) × 10² 1./mole at 298°K.     

Mechanism and kinetics of the reaction between hydrogen iodide and ozone

The reaction between hydrogen iodide and ozone at 295 K has been investigated by the resonance fluorescence method applied to the detection of iodine atoms. A chain mechanism is suggested for this reaction. The chain initiation rate constant is k ₁ = (5.45 ± 1.80) × 10^?17 cm³/s, and the chain propagation rate constant is k ₃ = (1.1 ± 0.4) × 10^?12 cm³/s.     

Kinetics of hydrogen abstraction reaction between trifluoromethyl formate and OH radical: A theoretical investigation

Kinetics and mechanism of the hydrogen abstraction reaction between trifluoromethyl formate, CF₃OCHO, and OH radical have been investigated by using ab initio molecular orbital theory up to G2(MP2) level. The hydrogen abstraction rate constant has been calculated for the first time over a temperature range of 250–450 K by using standard transition state theory including the tunneling correction. Arrhenius parameters of the reaction have been estimated from the temperature dependence of the calculated rate constant. The calculated value for the rate constant (2.0 × 10^?14 cm³ molecule^?1 s^?1) at 298 K is found to be in very good agreement with the recent experimental results. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 500–507, 2002     

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

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 \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm NO} + trans - {\rm 3,}trans{\rm - 5}\stackrel{1}{\rightarrow}trans - 3,cis - 5 + {\rm NO} $\end{document} can be represented (with standard errors) by log k₁ = (7.18 ± 0.06) – (16.75 ± 0.12)/θ, where θ = 2.303 R T in kcal/mole. The consecutive-step reaction mechanism involves addition of NO to the double bond (K_{a, b} = k_a/k_b), followed by rotation of the 5,6-C? C bond in the adduct radical (k_c.) Analysis of the observed activation parameters shows, that k_c is rate-controlling and consequently k₁ = k_cK_{a, b}. Estimates of k_c and K_{a, 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.     

Theoretical study on the gas-phase reaction of acetaldehyde with methoxy radical

Structural Chemistry - The reaction of acetaldehyde with methoxy radical has been investigated theoretically by means of quantum chemistry methods at the BMC-QCISD//B3LYP/6-311+G(d,p) level. The...     

Theoretical study of the gas-phase reaction of hydrogen isocyanate with water

Transition states for the two probable pathways for the gas-phase hydrolysis of hydrogen isocyanate have been determined using the MINDO /3 method. Activation barriers obtained showed that the one-step mechanism is preferred to the two-step mechanism involving an intermediate. It was shown that the reaction of polymeric water has lower activation barrier than the reaction of monomeric water. Energy and charge decomposition analysis showed that in the former less energy is required in the deformation of molecules for the transition state (TS ) formation due to the cyclic flow of electronic charges in the TS .