Rates and equilibria in the reaction system Br + i-C4H10 ⇌ HBr + t-C4H9. The heat of formation of the t-butyl radical

The very low pressure reactor (VLPR) technique has been used to measure the bimolecular rate constant of the title reaction at 300 K. The rate constant is given by log k₁ (1/mol s) = (11.6 ± 0.4) ? (5.9 ± 0.6)/θ the equilibrium constant has also been measured at the same temperature and is given by K₁ = (5.6 ± 1) × 10^?3 and hence log k_?1 (1/mol s) = 9.5 ± 0.1. The results show that the reaction Br + t? C₄H₉ → HBr + i? C₄H₈ is unimportant under the present experimental conditions. Assigning the entropy of t-butyl radical to be 74 ± 2 eu which is in the possible range, the value of K₁ gives ΔH (t-butyl) = 9.1 ± 0.6 kcal/mol^?1. This yields for the bond dissociation, DH° (t-butyl-H) = 93.4 ± 0.6 kcal/mol. Both of these values are found to be in good agreement with recent VLPP studies.     

The kinetics of the reaction: Br + C3H6 ⇌ HBr + C3H5

Studies of the reaction of Br + propylene to produce HBr and allyl radical were made using VLPR (Very Low Pressure Reactor) over the range 263–363 K. Apparent bimolecular rate constants k were found to vary in an inverse manner with the initial concentration of bromine atoms introduced into the reactor. Plots of k against [Br] give straight lines whose intercepts were taken to be the true bimolecular, metathesis rate constant k₁. The reaction scheme is where k₂ ? k₁ and k_?1 [HBr] is negligibly small under our conditions. Arrhenius parameters for k₁ were assigned for linear and bent transition states and shown to give excellent fits to the observed intercepts. where θ = 2.303 RT (kcal mol^?1). The dependence of k on [Br] is accounted for in terms of the reactivity of Br* (²P_1/2) produced in the microwave discharge. The activation energy for the metathesis reaction of Br* with propylene is shown to be very small.     

Kinetics of the gas-phase reaction CH3F + I2 ⇆ CH2FI + HI: The C?H bond dissociation energy in methyl and methylene fluorides

The kinetics of the gas-phase reaction of CH₃F with I₂ have been studied spectrophotometrically from 629 to 710 K, and were determined to be consistent with the following mechanism: (1) A least-squares analysis of the kinetic data taken in the initial stages of reaction resulted in where θ = 4.575T/1000 kcal/mol. The errors represent one standard deviation. The experimental activation energy E₄ = 30.8 ± 0.2 kcal/mol was combined with the assumption E₃ = 1 ± 1 kcal/mol and estimated heat capacities to obtain The enthalpy change at 298 K was combined with selected thermochemical data to derive The kinetic studies of ?HF₂ and CH₂F₂ have been reevaluated to yield These results are combined with literature data to yield the C? H, C? F, and C? Cl bond dissociation energies in their respective fluoromethanes, and the effect of α-fluorine substitution is discussed.     

Negative collision energy dependence of Br formation in the OH + HBr reaction

The reaction between HBr and OH leading to H(2)O and Br in its ground state is studied by means of a crossed molecular beam experiment for a collision energy varying from 0.05 to 0.26 eV, the initial OH being selected in the state |JOmega> = |3/2 3/2> by an electrostatic hexapole field. The reaction cross-section is found to decrease with increasing collision energy. This negative dependence suggests that there is no barrier on the potential energy surface for the formation pathway considered. The experimental results are compared with the previously reported quantum scattering calculations of Clary et al. (D. C. Clary, G. Nyman and R. Hernandez, J. Phys. Chem., 1994, 101, 3704), and briefly discussed in the light of skewed potential energy surfaces associated with heavy-light-heavy type reactions.     

Insights Gained by Modeling the Kinetics of Archetypal Hydrogen Atom Transfer Reactions: NH3 + ·H ⇆ H2N· + H2 and CH4 + ·H ⇆ H3C· + H2

Experimental measurements of the kinetics of the title reactions extend to temperature ranges of 1360 K for the ammonia‐hydrogen reaction and of 1602 K for the methane‐hydrogen reaction. Curved plots of ln(k) versus 1/T are obtained. Many theoretical calculations modeling these reactions routinely use tunneling corrections to match experiment. The steepness and curvatures of the plots are modeled successfully in this work and are shown to be caused solely by changes in the bond dissociation energies of the bonds involved in the reactions without invoking tunneling or any other adjustable parameters. The conclusion that tunneling does not contribute significantly to the rates in the temperature range of the measurements is in stark contrast with those theoretical calculations invoking large tunneling factors in the experimental temperature range. Support for the conclusion is provided by theoretical calculations of harmonic quantum transition state theory implementing instanton theory. There is direct experimental evidence that significant tunneling occurs in some H atom transfers, as with isotopomers of H₂ + ·H and other H transfers at very low temperatures. However, there is no direct experimental evidence of significant tunneling contributions to the rates of the title reactions in the temperature range of the measurements. Insights are gained into what specific forces must be overcome by the enthalpy of activation for reaction to occur.     

A consensus view of the temperature dependence of the gas phase reaction: OH + HBr → H2O + Br

The temperature dependence of the rate coefficient for the reaction, OH + HBr has been reinvestigated at low temperatures (T = 48–224 K) by using uniform supersonic flow reactors with laser induced fluorescence detection. This paper presents two forms of global fits: k(T) = 1.11 × 10^?11 (T/298)^?0.91 cm³ s^?1 and k(T) = 1.06 × 10^?11 (T/298)^?1.09 cm³ s^?1, both of which accurately describe the temperature dependence of the rate coefficient for the title reaction within the temperature range 20–350 K. These fits indicate that at temperatures below 200 K, the rate coefficient for this reaction shows inverse temperature dependence, while above 200 K the reaction shows insignificant temperature dependence. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 339–344, 2002     

Quantifying the non-RRKM effect in the H + O2 ⇄ OH + O reaction

In the present investigation the non-RRKM behavior in the title reaction is quantified in two different ways: (1) Quasiclassical trajectory calculations of the thermal rate coefficient are compared with results from a microcanonical variational transition-state theory/RRKM model. Results on both the Varandas DMBE IV and Melius-Blint potentials indicate that the non-RRKM behavior acts to reduce the thermal rate coefficient by about a factor of two, independent of temperature from 250 K to 5500 K. The QCT thermal rate coefficients on the two potentials are in remarkably good agreement with experiment and with each other over the entire temperature range. (2) The non-RRKM behavior as a classical phenomenon is demonstrated and quantified on both potentials by a direct test of the fundamental assumption. Complex-forming classical trajectories, started as either O + OH or H + O₂, are shown preferentially to return to the region of configuration space from which they were started. This test is discussed in detail in the text. The transition of the non-RRKM behavior from classical to quantum mechanics is also discussed. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 275–287, 1997.     

Theoretical study on the rate constants for the C2H5 + HBr --> C2H6 + Br reaction

The reaction C(2)H(5) + HBr --> C(2)H(6) + Br has been theoretically studied over the temperature range from 200 to 1400 K. The electronic structure information is calculated at the BHLYP/6-311+G(d,p) and QCISD/6-31+G(d) levels. With the aid of intrinsic reaction coordinate theory, the minimum energy paths (MEPs) are obtained at the both levels, and the energies along the MEP are further refined by performing the single-point calculations at the PMP4(SDTQ)/6-311+G(3df,2p)//BHLYP and QCISD(T)/6-311++G(2df,2pd)//QCISD levels. The calculated ICVT/SCT rate constants are in good agreement with available experimental values, and the calculate results further indicate that the C(2)H(5) + HBr reaction has negative temperature dependence at T < 850 K, but clearly shows positive temperature dependence at T > 850 K. The current work predicts that the kinetic isotope effect for the title reaction is inverse in the temperature range from 200 to 482 K, i.e., k(HBr)/k(DBr) < 1.     

The kinetics of the thermal gas-phase reaction Br2 + C6F5I → BrI + C6F5Br. An attempt to determine the bond dissociation energy D(C6F5-I)

The kinetics of the thermal bromination reaction have been studied in the range of 261°–391°C. The observed rate law is compatible with initiation by the step for which we obtain where Θ = 2.303RT cal/mol. Using the above value of E₆, we have This result disagrees with values of D(C₆F₅-I) obtained in other ways and we conclude that reaction (3) probably does not involve initiation by reaction (6). Instead, initiation may involve an addition of Br to the ring in C₆F₅I followed by decomposition of the adduct to give C₆F₅Br. If correct, this implies that the Arrhenius parameters above refer to the addition reaction rather than to reaction (6).     

Kinetics and equilibria of the reaction between bromine and ethyl ether. The C?H bond dissociation energy in ethyl ether

The kinetics and equilibria of the reaction: have been studied in the temperature range 298–333 K by using the very low pressure reactor (VLPR) technique. Combining the estimated entropy change of reaction (1), ΔS = 8.1 ± 1.0 eu, with the measured ΔG, we find ΔH = 4.2 ± 0.4 kcal/mol; ΔH(CH₃CHOC₂H₅) = ?20.2 kcal/mol, and DH° [Et OCH(Me)-H] = 91.7 ± 0.4 kcal/mol. We find: where θ = 2.3 RT in kcal/mol. It has been shown that the reaction proceeds via a loose transition state and the “contact TS” model calculation gives a very good agreement with the observed value.     

Reaction products with internal energy beyond the kinematic limit result from trajectories far from the minimum energy path: an example from H + HBr --> H2 + Br

The importance of reactive trajectories straying far from the minimum energy path is demonstrated for the bimolecular reaction H + HBr --> H2(v', j') + Br at 53 kcal/mol collision energy. Product quantum state distributions are measured and calculated using the quasi-classical trajectory technique, and the calculations indicate that highly internally excited H2 products result from indirect reactive trajectories with bent transition states. A general argument is made suggesting that reaction products with internal energy exceeding a kinematic constraint can, in general, be attributed to reactive collisions straying far from the minimum energy path.     

A Shock Tube Study of H2 + OH → H2O + H Using OH Laser Absorption

The rate constant for the reaction of hydroxyl radicals (OH) with molecular hydrogen (H₂) was measured behind reflected shock waves using UV laser absorption of OH radicals near 306.69 nm. Test gas mixtures of H₂ and tert‐butyl hydroperoxide (TBHP) diluted in argon were shock‐heated to temperatures ranging from 902 to 1518 K at pressures of 1.15–1.52 atm. OH radicals were produced by rapid thermal decomposition of TBHP at high temperatures. The rate constant for the title reaction was inferred by best fitting the measured OH time histories with the simulated profiles from the comprehensive reaction mechanism of Wang et al. (USC‐Mech v2.0) (2007). The measured values can be expressed in the Arrhenius equation as k₁(T) = 4.38 × 10¹³ exp(–3518/T) cm³ mol^?1 s^?1 over the temperature range studied. A detailed error analysis was performed to estimate the overall uncertainty of the title reaction, and the estimated (2 – σ) uncertainties were found to be ±17% at 972 and 1228 K. The present measurements are in excellent agreement with the previous experimental studies from Frank and Just (Ber Bunsen‐Ges Phys Chem 1985, 89, 181–187), Michael and Sutherland (J Phys Chem 1988, 92, 3853–3857), Davidson et al. (Symp (Int) Combust 1988, 22, 1877–1885), Oldenborg et al. (J Phys Chem 1992, 96, 8426–8430), and Krasnoperov and Michael (J Phys Chem A 2004, 108, 5643–5648).In addition, the measured rate constant is in close accord with the non‐Arrhenius expression from GRI‐Mech 3.0 ( http://www.me.berkeley.edu/gri_mech/ ) and the theoretical calculation using semiclassical transition state theory from Nguyen et al. (Chem Phys Lett 2010, 499, 9–15).     

Phenol O–H bond dissociation energy in water clusters

We are reporting ab initio and density functional theory (DFT) calculations for the phenol O–H bond dissociation energy in the gas phase and in phenol–water clusters. We have tested a series of recently proposed functionals and verified that DFT systematically underestimates the O–H bond dissociation energy of phenol. However, O–H bond dissociation energies in water clusters are in reasonable agreement with experimental data for phenol in solution. We have evaluated electronic difference densities in phenol–water, phenoxy–water, and water, and we are suggesting that the representation of this quantity gives an interesting picture of the electronic density rearrangement induced by hydrogen bond interactions in phenol–water clusters. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Kinetics of the hydrogen abstraction R−OH + H → R•−OH + H2 reaction class: An application of the reaction class transition state theory

This paper presents an application of the reaction class transition state theory (RC‐TST) to predict thermal rate constants for hydrogen abstraction reactions of the type R‐OH + H → R^?‐OH + H₂. We have derived all parameters for the RC‐TST method with linear energy relationships (LERs) and the barrier height grouping (BHG) approach for this reaction class from rate constants of 37 representative reactions divided in two types of hydrogen abstraction, namely from α carbon sites and non‐α carbon sites two training sets. Error analyses indicate that the RC‐TST/LER, where only reaction energy is needed, and RC‐TST/ BHG, where no other information is needed, can predict rate constants for any reaction in this reaction class with satisfactory accuracy for combustion modeling. Specifically for this reaction class, the RC‐TST/LER and RC‐TST/BHG methods have, respectively, less than 40% and 90% systematic errors in the predicted rate constants, when compared to the explicit full TST/Eckart method. The branching ratio analysis shows that in the low‐temperature regime α abstractions are dominant, whereas, for T > 1500 K, abstractions at other sites become more important. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 78–98, 2011     

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.     

Kinetic study of the reactions Br + IBr→I + Br2 and I + Br2→Br + IBr

The kinetics of the title reactions have been studied using the discharge-flow mass spectrometic method at 296 K and 1 torr of helium. The rate constant obtained for the forward reaction Br+IBr→I+Br₂ (1), using three different experimental approaches (kinetics of Br consumption in excess of IBr, IBr consumption in excess of Br, and I formation), is: k₁=(2.7±0.4)×10⁻¹¹ cm³ molecule⁻¹s⁻¹. The rate constant of the reverse reaction: I+Br₂→Br+IBr (−1) has been obtained from the Br₂ consumption rate (with an excess of I atoms) and the IBr formation rate: k₋₁=(1.65±0.2)×10⁻¹³ cm³molecule⁻¹s⁻¹. The equilibrium constant for the reactions (1,−1), resulting from these direct determinations of k₁ and k₋₁ and, also, from the measurements of the equilibrium concentrations of Br, IBr, I, and Br₂, is: K₁=k₁/k₋₁=161.2±19.7. These data have been used to determine the enthalpy of reaction (1), ΔH₂₉₈°=−(3.6±0.1) kcal mol⁻¹ and the heat of formation of the IBr molecule, ΔH_f,298°(IBr)=(9.8±0.1) kcal mol⁻¹. © 1998 John Wiley & sons, Inc. Int J Chem Kinet 30: 933–940, 1998     

Interaction energy and the shift in OH stretch frequency on hydrogen bonding for the H2O → H2O,CH3OH → H2O,and H2O → CH3OH dimers

The ability to use calculated OH frequencies to assign experimentally observed peaks in hydrogen bonded systems hinges on the accuracy of the calculation. Here we test the ability of several commonly employed model chemistries—HF, MP2, and several density functionals paired with the 6‐31+G(d) and 6‐311++G(d,p) basis sets—to calculate the interaction energy (D_e) and shift in OH stretch fundamental frequency on dimerization (δ(ν)) for the H₂O → H₂O, CH₃OH → H₂O, and H₂O → CH₃OH dimers (where for X → Y, X is the hydrogen bond donor and Y the acceptor). We quantify the error in D_e and δ(ν) by comparison to experiment and high level calculation and, using a simple model, evaluate how error in D_e propagates to δ(ν). We find that B3LYP and MPWB1K perform best of the density functional methods studied, that their accuracy in calculating δ(ν) is ≈ 30–50 cm^?1 and that correcting for error in D_e does little to heighten agreement between the calculated and experimental δ(ν). Accuracy of calculated δ(ν) is also shown to vary as a function of hydrogen bond donor: while the PBE and TPSS functionals perform best in the calculation of δ(ν) for the CH₃OH → H₂O dimer their performance is relatively poor in describing H₂O → H₂O and H₂O → CH₃OH. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Kinetics study of the OH + alkene --> H2O + alkenyl reaction class

In this paper we report on the kinetics of hydrogen abstraction for the OH + alkene reaction class, using the reaction class transition state theory (RC-TST) combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approaches. Parameters for the RC-TST were derived from theoretical calculations using a set of 15 reactions representing the hydrogen abstractions from the terminal and nonterminal carbon sites of the double bond of alkene compounds. Both the RC-TST/LER, where only reaction energy is needed at either density functional theory BH&HLYP or semiempirical AM1 levels, and RC-TST/BHG, where no additional information is required, are found to be promising methods for predicting rate constants for a large number of reactions in this reaction class. Detailed error analyses show that, when compared to explicit theoretical calculations, the averaged systematic errors in the calculated rate constants using both the RC-TST/LER and RC-TST/BHG methods are less than 25% in the temperature range 300-3000 K. The estimated rate constants using these approaches are in good agreement with available data in the literature.     

A shock tube study of the OH + OH → H2O + O reaction

The rate coefficient for the reaction has been determined in mixtures of nitric acid (HNO₃) and argon in incident shock wave experiments. Quantitative OH time-histories were obtained by cw narrow-linewidth uv laser absorption of the R₁(5) line of the A² σ⁺ ← X² Π_i (0,0) transition at 32606.56 cm^?1 (vacuum). The experiments were conducted over the temperature range 1050–2380 K and the pressure range 0.18–0.60 atm. The second-order rate coefficient was determined to be with overall uncertainties of +11%, ?16% at high temperatures and +25%, ?22% at low temperatures. By incorporating data from previous investigations in the temperature range 298–578 K, the following expression is determined for the temperature range 298–2380 K © 1994 John Wiley & Sons, Inc.