High temperature collisional energy transfer in highly vibrationally excited molecules. III: Isotope effects in tert-butyl bromide systems

Changes in the magnitude of 〈ΔE_down〉, the average downward collisional energy transferred between a highly vibrationally excited reactant molecule and an inert bath gas, upon perdeuteration of the substrate are reported for tert-butyl bromide dilute in Ar, Kr, N₂, and CO₂. The technique of pressure-dependent very low-pressure pyrolysis (VLPP) was used to obtain the absolute values of 〈ΔE_down〉, which are for C₄H₉Br, 230 (Ar), 285 (Kr), 270 (N₂), and 365 (CO₂) while for C₄D₉Br, 200 (Ar), 250 (Kr), 220 (N₂), and 335 (CO₂), all in cm^?1 at ca. 720 K. The estimated uncertainties in these values are ca. ± 10%. These observed 〈ΔE_down〉, values and trends found with results from this series of isotope studies, are compared with current theoretical models. Extrapolated high-pressure temperature-dependent rate coefficients (s^?1) for the thermal decomposition of reactant are 10^13.8±0.3 exp(?175 ± 8 kJ mol^?1/RT) for C₄H₉Br and 10^14.3±0.3 exp(?183 ± 8 kJ mol^?1/RT) for C₄D₉Br. These results are in accord with other studies and the expected isotope effect.     

Collisional energy transfer in highly vibrationally excited molecules: A very low-pressure pyrolysis study of acetyl chloride

The average downward energy transfer (〈Δ E_down〉) is obtained for highly vibrationally excited acetyl chloride with Ne and C₂H₄ bath gases at ca. 870 K. Data are obtained by the technique of very low-pressure pyrolysis (VLPP). Fitting these data by solution of the appropriate reaction-diffusion integrodifferential master equation yields the gas/gas collisional energy transfer parameters: 〈Δ E_down〉 values are 220 ± 10 cm^?1 (Ne bath gas) and 330 ± 20 cm^?1 (C₂H₄). These energy transfer quantities are much less than those predicted by statistical theories, or those observed for similar sized molecules such as CH₃CH₂Cl. These results are explained by the qualitative predictions of the biased random walk model wherein the fundamental mechanism of energy transfer is the multiple interactions between the bath gas and the individual atoms of the reactant molecule, during the course of the collision event. The charge distribution of acetyl chloride decreases the number of such interactions, thereby reducing the amount of energy transferred per collision.     

High temperature collisional energy transfer in highly vibrationally excited molecules: Isotope effects in tert-butyl chloride systems

The average downward collisional energy transfer (<ΔE_down>) is obtained for highly vibrationally excited tert-butyl chloride, both undeuterated and per-deuterated, with Kr, N₂, CO₂, and C₂H₄ bath gases, at ca. 760 K. Data are obtained using the technique of pressure-dependent very low-pressure pyrolysis. Reactant internal energies to which the data are sensitive are in the range 200–250 kJ mol^?1. For C₄H₉Cl, the <ΔE_down> values (cm^?1) are 255 (Kr), 265 (N₂), 440 (CO₂), and 585 (C₂H₄), and for C₄D₉Cl, 245 (N₂), 370 (CO₂), and 540 (C₂H₄). The uncertainties in these values are ca. 20% (40% for Kr); the uncertainties in the deuteration ratios are 10–15%. The value for Kr is in agreement with theoretical predictions of a biased random walk model for internal energy change in monatomic/substrate collisions. The effect of deuteration of <ΔE_down> is also in accord with that predicted by a modification of the theory. Extrapolated highpressure rate coefficients for the thermal decomposition of reactant are 10^13.6 exp(-187 kJ mol^?1/RT) s^?1 (C₄H₉Cl) and 10^14.2 exp(?196 kJ mol^?1/RT) s^?1 (C₄D₉Cl), in accord with other studies and the expected isotope effect.     

Thermal decomposition of CF3Br using Br-atom absorption

Br-atom atomic resonance absorption spectrometry (ARAS) has been developed and applied to measure thermal decomposition rate constants for CF₃Br (+ Kr)→CF₃+Br (+ Kr) over the temperature range, 1222–1624 K. The Br-atom curve-of-growth (145<λ<163 nm) was determined using this reaction. For [Br]≤1×10¹² molecules cm⁻³, absorbance, (ABS)=1.410×10⁻¹³ [Br], yielding σ=1.419×10⁻¹⁴ cm². The curve-of-growth was then used to convert (ABS) to Br-atom profiles which were then analyzed to give measured rate constants. These can be expressed in second-order by k₁=8.147×10⁻⁹ exp(−24488 K/T) cm³ molecule⁻¹ s⁻¹ (±33%, 1222≤T≤1624 K). A unimolecular theoretical approach was used to rationalize the data. Theory indicates that the dissociation rates are closer to second- than to first-order, i.e., the magnitudes are 30–53% of the low-pressure-limit rate constants over 1222–1624 K and 123–757 torr. With the known, E₀=ΔH₀⁰=70.1 kcal mole⁻¹, the optimized theoretical fit to the ARAS data requires 〈ΔE〉_down=550 cm⁻¹. These conclusions are consistent with recently published data and theory from Kiefer and Sathyanarayana. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 859–867, 1998     

Vibrational relaxation and dissociation in the perfluoromethyl halides,CF3Cl,CF3Br,andCF3I

The processes of vibrational relaxation and unimolecular dissociation of the perfluoromethyl halides CF₃Cl, CF₃Br, and CF₃I have been studied in the shock tube with the laser-schlieren technique. Vibrational relaxation was resolved in pure CF₃Cl and CF₃Br (400–484 K and 400–500 K, respectively), and in the mixtures; 2% CF₃Cl/Kr (500–1000 K), 10% CF₃Cl/Kr (440–670 K), 4% CF₃Br/Kr (450–850 K), and 2% CF₃I/Kr (620–860 K). Relaxation in the pure gases is extremely rapid, but shows a well-resolved, accurately exponential decay which provides very precise relaxation times in close agreement with ultrasonic results. Relaxation times as short as 0.1 μs-atm can be resolved, showing the method has a resolution within a factor 2–3 of the best ultrasonic methods. Relaxation dilute in rare gas shows a complex double exponential behavior consistent with a two-stage series process. Rates of CF₃(SINGLEBOND)X fission in these mixtures were measured over 1800–3000 K, P<0.55 atm, for CF₃Cl; 1600–2500 K, P<0.55 atm, in CF₃Br; and 1260–2100 K, P<0.34 atm, in CF₃I. Rates for dissociation were derived from a full profile modeling using a secondary mechanism of six CF₃ reactions. RRKM analysis showed all dissociations to lie near the low pressure limit. Using literature barriers, these rates are best fit with (ΔE)_all=−270 cm⁻¹ for CF₃Cl, 〈ΔE〉_down=0.3 T for CF₃Br, and 〈ΔE〉_down=800 cm⁻¹ for CF₃F. All these transfers are on the large side, similar to those found in other halogenated methanes. © 1997 John Wiley & Sons, Inc.     

Pyrolysis of vinylacetylene between 300 and 450°C

Vinylacetylene was pyrolyzed at 300–450°C in a packed and an unpacked static reactor with a pinhole bleed to a quadrupole mass spectrometer. The reactant and C₈H₈ products were monitored continuously during a reaction by mass spectrometry. In some runs, the products were also analyzed by gas chromatography after the run. In these runs CH₄, C₂H₆, C₃H₆, and C₂H₄ were also detected. The reaction for vinylacetylene removal and C₈H₈ formation is homogeneous, second order in reactant, and independent of the presence of a large excess of N₂ or He. However, C₈H₈ formation is about half-suppressed by the addition of the free-radical scavengers NO or O₂. The rate coefficient for total vinylacetylene removal is 1.7 × 10⁶ exp(?79 ± 13 kJ/mol RT) L/mol · s. The major reaction for C₄H₄ removal is polymerization. In addition four C₈H₈ isomers, carbon, and small hydrocarbons are formed. The three major C₈H₈ isomers are styrene, cyclooctatetraene (COT), and 1,5? dihydropentalene (DHP). The C₈H₈ compounds are formed by both molecular and free-radical processes in a second-order process with an overall k ? 3 × 10⁸ exp(?122 kJ/mol RT) L/mol · s (average of packed and unpacked cell results). The molecular process occurs with an overall k = 8.5 × 10⁷ exp (?118 kJ/mol RT) L/mol · s. The COT, DHP, and an unidentified isomer (d), are formed exclusively in molecular processes with respective rate coefficients of 4.4 × 10⁴ exp(?77 kJ/mol RT), 1.7 × 10⁵ exp(?89 kJ/mol RT), and 3.1 × 10⁹ exp(? 148 kJ/mol RT) L/mol · s. The styrene is formed both by a direct free-radical process and by isomerization of COT.     

Collisional de-excitation of the metastableD-states of Ba+ by He,Ne, N2 and H2

The rate constants 〈σ · υ〉 for collisional de-excitation of the metastable 5D states of Ba⁺ ions have been determined in an ion trap experiment. TheD-states are selectively populated by pulsed laser excitation of the 6P _1/2 or 6P _3/2 state and the decay at different background pressures is monitored by the change in fluorescence intensity of the excited ions. From the pressure dependence of the decay constants we calculate the de-excitation rate constants for different collision partners, averaged over the velocity distribution of the trapped ion cloud. For He, Ne, H₂ and N₂ we obtain in the c.m. energy range of 0.1–0.5 eV: 〈σ·υ〉 (He)=3.0±0.2·10^?13cm³/s, 〈σ·υ〉 (Ne)=5.1±0.4·10^?13cm³/s, 〈σ·υ〉 (H₂)=3.7±0.3·10^?11cm³/s, 〈σ·υ〉 (N₂)=4.4±0.3·10^?11cm³/s. The results can be understood qualitatively by a consideration of the ion-atom and ion-molecules interaction potential.     

Incubation in cyclohexene decomposition at high temperatures

A detailed master equation simulation has been carried out for the thermal unimolecular decomposition of C₆H₁₀ in a shock tube. At the highest temperatures studied experimentally [J. H. Kiefer and J. N. Shah, J. Phys. Chem., 91, 3024 (1987)], the average thermal vibrational energy is greater than the reaction threshold and therefore 〈ΔE〉 (up and down steps) is positive for molecules at that energy, rather than negative; the converse is true at lower temperatures. The calculated incubation time, in which the decomposition rate constant rises to 1/e of its steady state value, is found to be only weakly dependent on temperature (at constant pressure) between 1500 K and 2000 K and to depend almost exclusively on 〈ΔE〉_d (down steps, only), and not on collision probability model. Simulations of the experimental data show the magnitude of 〈ΔE〉_d depends weakly on assumed collision probability model, but is nearly independent of temperature. The second moment 〈ΔE〉^½ is found to be independent of both temperature and transition probability model. The experimental data are not very sensitive to the possible energy-dependence of 〈ΔE〉_d for a wide range of assumptions. It is concluded that the observed experimental “delay times” probably can be identified with the incubation time; further experiments are desirable to test this possibility and obtain more direct measures of the incubation time.     

Study of the reactions of oxygen atoms with carbonothioicdichloride,carbonothioicdifluoride, and tetrafluoro-1,3-dithietane

The reactions of ground-state oxygen atoms with carbonothioicdichloride, carbonothioicdifluoride, and tetrafluoro-1,3-dithietane have been studied in a crossed molecular jet reactor in order to determine the initial reaction products and in a fast-flow reactor in order to determine their overall rate constants at temperatures between 250 and 500 K. These rate constants are??(O + C₂CS) =(3.09 ± 0.54) × 10^?11 exp(+115 ± 106 cal/mol/RT),??(O + F₂CS) = (1.22 ± 0.19) × 10^?11 exp(-747 ± 95 cal/mol/RT), and??(O + F₄C₂S₂) = (2.36 ± 0.52) × 10^?11 exp(-1700 ± 128 cal/mol/RT) cm³/molec˙sec. The detected reaction products and their rate constants indicate that the primary reaction mechanism is the electrophilic addition of the oxygen atom to the sulfur atom contained in the reactant molecule to form an energy-rich adduct which then decomposes by C-S bond cleavage.     

Direct kinetic parameters for the reaction of Br atoms with neopentane

Laser flash photolysis coupled with resonance fluorescence detection of Br atoms was employed to investigate the temperature dependence of the reaction Br + neo‐C₅H₁₂ (1) between 688 and 775 K. The following Arrhenius preexponential factor and activation energy were determined (±1 σ): A₁ = (6.89 ± 2.27) 10¹⁴ cm³ mol⁻¹ s⁻¹ and E_A,1 = 57.61 ± 2.05 kJ mol⁻−¹ The only other kinetic parameters reported for the reaction of Br atoms with neo‐C₅H₁₂ were obtained from competitive kinetic experiments relative to Br + C₂H₆. Comparison with our direct results is hampered by uncertainties in the kinetic data for the reference reaction that may need reinvestigation. The standard enthalpy of formation for the neo‐C₅H₁₁ radical was estimated to be 34.7 and 41.6 kJ mol⁻¹, depending on the value of the activation energy assumed for the reverse reaction neo‐C₅H₁₁ + HBr (−1). © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 33: 49–55, 2001     

The unimolecular dissociation of 3,4-dihydro-2H-pyran over a wide temperature range

The thermal decomposition of 3,4-dihydro-2H-pyran (DHP, C₅H₈O) has been investigated by two methods: in shock waves with the laser-schlieren technique using mixtures of 5 and 10% DHP in krypton over 900–1500 K, 110–560 torr; in a flow tube having a reaction pressure 0.5 torr above atmospheric using the decomposition of allylethyl ether as an internal standard, and covering 663–773 K. The retro-Diels-Alder dissociation to the stable acrolein and ethylene is the dominant channel for all conditions. Precise rate constants (rms deviation of 10%) were obtained for this process over the indicated temperature ranges. Unimolecular falloff is evident in the shock-tube results, and RRKM calculations also predict a slight falloff at the lower temperatures. These RRKM calculations use a routine vibration model transition state and agree closely with the high-temperature data when 〈ΔE〉_down is a fixed 400 cm^?1. Arrhenius expressions for k_∞ derived from the two measurements are in close accord and also consistent with most previous studies of this reaction. © 1995 John Wiley & Sons, Inc.     

Kinetic isotope effect for hydrogen/deuterium abstraction by chlorine atoms from (CH3)2NNO2 and (CD3)2NNO2

The kinetic isotope effect for the abstraction of hydrogen/deuterium from dimethylnitramine and dimethylnitramine-d₆ by chlorine atoms has been studied in the temperature range 273–353 K. The rate constant ratio k_H0/k_D is given by the Arrhenius expression, k_H/k_D=(0.92 ± 0.07)exp(286 ± 250/RT), where R is expressed in cal mol^?1 K^?1. The absolute rate constant for the deuterium abstraction reaction is extrapolated as k_D=(1.50 ± 0.90) × 10^?10 exp(?1,486 ± 370/RT) cm³ molecule^?1 s^?1. The temperature dependence of the kinetic isotope effect was calculated using the conventional transition-state theory, and the obtained values for k_H/k_D and ΔE_{H, D} are in good agreement with the experimental value for a bent transition state geometry, with two new vibrational frequencies of 340 cm^?1 (272 cm^?1) corresponding to the in-plane and out-of-plane motions of hydrogen (deuterium) atoms in the Cl…H…C arrangement. © 1993 John Wiley & Sons, Inc.     

Ozone–olefin reactions in the gas phase 1. Rate constants and activation energies

Reactions of ozone with simple olefins have been studied between 6 and 800 mtorr total pressure in a 220-m³ reactor. Rate constants for the removal of ozone by an excess of olefin in the presence of 150 mtorr oxygen were determined over the temperature range 280 to 360° K by continuous optical absorption measurements at 2537 Å. The technique was tested by measuring the rate constants k₁ and k₂ of the reactions (1) NO + O₃ → NO₂ + O₂ and (2) NO₂ + O₃ rarr; NO₃ + O₂ which are known from the literature. The results for NO, NO₂, C₂H₄, C₃H₆, 2-butene (mixture of the isomers), 1,3→butadiene, isobutene, and 1,1 -difluoro-ethylene are 1.7 × 10^{?1 4} (290°K), 3.24 × 10^?17 (289°K), 1.2 × 10^{?1 4} exp (–4.95 ± 0.20/RT), 1.1 × 10^{?1 4} exp (–3.91 ± 0.20/RT), 0.94 × 10^{?1 4} exp ( –2.28 ± 0.15/RT), 5.45 ± 10^{?1 4} exp ( –5.33 ± 0.20/RT), 1.8 ×10^?17 (283°K), and 8 × 10^?20 cm³/molecule ·s(290°K). Productformation from the ozone–propylene reaction was studied by a mass spectrometric technique. The stoichiometry of the reaction is near unity in the presence of molecular oxygen.     

Kinetic H/D isotope effects for gas phase hydroxylation of benzene and chlorobenzene between 520–1080 K. Hydroxyl radical versus O(3P) atom attack

Gas phase slow combustion of (chloro)benzene in O₂/N₂ mixtures, and induced by addends such as tert butylhydroperoxide, cyclohexane, or methanol, leads to (chloro)-phenol as the only important aromatic product. Using C₆H₆/C₆D₆ mixtures, formation of phenol/perdeuterophenol was studied between 520–1080 K. The temperature dependence of this product ratio was found to obey the Arrhenius expression for the intermolecular isotope effect log k_H/k_D = ?0.14 ± 0.03 + (1240 ± 80)/2.303RT (R in cal/mol K). Essentially the same result was obtained for the intramolecular isotope effect, measuring the change in isomer distribution for the chlorophenols formed from p-deuterio-chlorobenzene versus those for chlorobenzene. These results are in accordance with H(D)-abstraction by ·OH, via a linear transition state, as the first and (relative) rate determining step. Whereas above 1000 K, at reduced pressure, the intramolecular isotope effect continues to prevail, C₆H₆/C₆D₆ do not show differences in rate of formation of C₆H₅OH/C₆D₅OH. Under these conditions, the only effective reaction of arene to phenol appears to be set in by addition of O(³P).     

Mode Selective Stereomutation and Parity Violation in Disulfane Isotopomers H2S2, D2S2, T2S2

We report quantitative calculations of stereomutation tunneling in the disulfane isotopomers H₂S₂, D₂S₂, and T₂S₂, which are chiral in their equilibrium geometry. The quasi‐adiabatic channel, quasi‐harmonic reaction path Hamiltonian approach used here treats stereomutation including all internal degrees of freedom. The torsional motion is handled as an anharmonic reaction coordinate in detail, whereas all the remaining degrees of freedom are taken into account approximately. We predict how stereomutation is catalyzed or inhibited by excitation of the various vibrational modes. The agreement of our theoretical results with spectroscopic data from the literature on H₂S₂ and D₂S₂ is excellent. We furthermore predict the influence of parity violation on stereomutation as characterized approximately by the ratio (ΔE_pv/ΔE_±) of the (local or vibrationally averaged) parity violating potential ΔE_pv and the tunneling splittings ΔE_± in the symmetrical case. This ratio is exceedingly small for the reference molecules H₂O₂ and D₂O₂, and still very small (2⋅10⁻⁶ cm⁻¹) for H₂S₂, which, thus, all exhibit essentially parity conservation in the dynamics. However, for D₂S₂ it is ca. 0.002, and for T₂S₂ it is ca. 1, which seems to be the first case where such intermediate mixing through parity violation is quantitatively predicted for spectroscopically accessible molecules. The consequences for the spectroscopic detection of molecular parity violation are discussed briefly also in relation to other molecules.     

The kinetics and equilibrium of the gas-phase reaction CH3CF2Br + I2 = CH3CF2I + IBr the C-Br bond dissociation energy in 1,1-difluorobromoethane

The kinetics and equilibrium of the gas-phase reaction of CH₃CF₂Br with I₂ were studied spectrophotometrically from 581 to 662°K and determined to be consistent with the following mechanism: A least squares analysis of the kinetic data taken in the initial stages of reaction resulted in log k₁ (M^?1 · sec^?1) = (11.0 ± 0.3) - (27.7 ± 0.8)/θ where θ = 2.303 RT kcal/mol. The error represents one standard deviation. The equilibrium data were subjected to a “third-law” analysis using entropies and heat capacities estimated from group additivity to derive ΔH_r° (623°K) = 10.3 ± 0.2 kcal/mol and ΔH_rr (298°K) = 10.2 ± 0.2 kcal/mol. The enthalpy change at 298°K was combined with relevant bond dissociation energies to yield DH°(CH₃CF₂ - Br) = 68.6 ± 1 kcal/mol which is in excellent agreement with the kinetic data assuming that E₂ = 0 ± 1 kcal/mol, namely; DH°(CH₃CF₂ - Br) = 68.6 ± 1.3 kcal/mol. These data also lead to ΔH_f°(CH₃CF₂Br, g, 298°K) = -119.7 ± 1.5 kcal/mol.     

稀土配合物[Pr(C3H7NO2)2(C3H4N2)(H2O)](ClO4)3的标准生成焓及热分解动力学研究

合成了高氯酸镨和咪唑(C₃H₄N₂), DL-α-丙氨酸(C₃H₇NO₂)混配配合物晶体. 经傅立叶变换红外光谱、化学分析和元素分析确定其组成为[Pr(C₃H₇NO₂)₂(C₃H₄N₂)(H₂O)](ClO₄)₃. 使用具有恒温环境的溶解-反应量热计, 以2.0 mol•L^－1 HCl为量热溶剂, 在T＝(298.150±0.001) K时测定出化学反应PrCl₃•6H₂O(s)＋2C₃H₇NO₂(s)＋C₃H₄N₂(s)＋3NaClO₄(s)＝[Pr(C₃H₇NO₂)₂(C₃H₄N₂)(H₂O)](ClO₄)₃(s)＋3NaCl(s)＋5H₂O(1)的标准摩尔反应焓为Δ_rH_m^ө＝(39.26±0.11) kJ•mol^－1. 根据盖斯定律, 计算出配合物的标准摩尔生成焓为Δ_fH_m^ө{[Pr(C₃H₇NO₂)₂(C₃H₄N₂)(H₂O)](ClO₄)₃(s), 298.150 K}＝(－2424.2±3.3) kJ•mol^－1. 采用TG-DTG技术研究了配合物在流动高纯氮气(99.99%)气氛中的非等温热分解动力学, 运用微分法(Achar-Brindley-sharp和Kissinger法)和积分法(Satava-Sestak和Coats-Redfern法)对非等温动力学数据进行分析, 求得分解反应的表观活化能E＝108.9 kJ•mol^－1, 动力学方程式为dα/dt＝2(5.90×10⁸/3)(1－α)[－ln(1－α)]^－1exp(－108.9×10³/RT).     

Kinetic isotope effects in the reactions of bromine atoms with CH4 and CD4

The competitive reactions of Br atoms with CH₄ and CD₄ were studied over the temperature range of 562° to 637°K. Over this temperature interval, the kinetic isotope effect, k_H/k_D, varied from 3.05 to 2.47 for the reactions The rate constant ratio k_H/k_D, expressed in Arrhenius form, was found to equal (1.10 ± 0.05) exp (1030 ± 60/RT). A comparison is presented between the experimental result and the result obtained theoretically from absolute rate theory using the London-Eyring-Polanyi-Sato (LEPS) method of constructing the potential energy surface of the reaction. The agreement between theory and experiment is very poor, and this is believed to arise from the highly unsymmetrical nature of the potential energy surface involved in these reactions. A comparison is also presented between the k_H/k_D values obtained in the Br + CH₄–CD₄ experiments and the available data on the corresponding Cl + CH₄–CD₄ reactions.     

A shock tube,laser‐schlieren study of the pyrolysis of isobutene: Relaxation,incubation, and dissociation rates

Dissociation, vibrational relaxation, and unimolecular incubation have all been observed in shock waves in isobutene with the laser‐schlieren technique. Experiments covered a wide range of high‐temperature conditions: 900–2300 K, and post‐incident shock pressures from 7 to 400 torr in 2, 5, and 10% mixtures with krypton. The surprising observation is that of vibrational relaxation, well resolved over the full temperature range. The resolved process is completely exponential, with relaxation times in the range 20–120 ns atm. Relaxation and dissociation are clearly separated for T > 1850 K, with estimated incubation times near 200 ns atm. Incubation is essential for modeling of the very low‐pressure decomposition. Modeling of gradients with a chain mechanism initiated by CH fission produces an excellent fit and accurate dissociation rates that show severe falloff. A restricted‐rotor, Gorin‐model RRKM analysis fits these rates quite well with the known bond‐energy as barrier and 〈ΔE〉_down = 680 cm^?1. The extrapolated k_∞ is log k_∞(s^?1) = 19.187–0.865 log T ?87.337 (kcal/mol)/RT, in good agreement with previous work. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 381–390, 2003