The thermal decomposition of ethanol and its reactions with OH and D have been studied with both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for ethanol have been measured at high T in reflected shock waves using OH optical absorption and high-sensitivity H-atom ARAS detection. The three dissociation processes that are dominant at high T are C2H5OH--> C2H4+H2O (A) -->CH3+CH2OH (B) -->C2H5+OH (C).The rate coefficient for reaction C was measured directly with high sensitivity at 308 nm using a multipass optical White cell. Meanwhile, H-atom ARAS measurements yield the overall rate coefficient and that for the sum of reactions B and C , since H-atoms are instantaneously formed from the decompositions of CH(2)OH and C(2)H(5) into CH(2)O + H and C(2)H(4) + H, respectively. By difference, rate constants for reaction 1 could be obtained. One potential complication is the scavenging of OH by unreacted ethanol in the OH experiments, and therefore, rate constants for OH+C2H5OH-->products (D)were measured using tert-butyl hydroperoxide (tBH) as the thermal source for OH. The present experiments can be represented by the Arrhenius expression k=(2.5+/-0.43) x 10(-11) exp(-911+/-191 K/T) cm3 molecule(-1) s(-1) over the T range 857-1297 K. For completeness, we have also measured the rate coefficient for the reaction of D atoms with ethanol D+C2H5OH-->products (E) whose H analogue is another key reaction in the combustion of ethanol. Over the T range 1054-1359 K, the rate constants from the present experiments can be represented by the Arrhenius expression, k=(3.98+/-0.76) x10(-10) exp(-4494+/-235 K/T) cm3 molecule(-1) s(-1). The high-pressure rate coefficients for reactions B and C were studied with variable reaction coordinate transition state theory employing directly determined CASPT2/cc-pvdz interaction energies. Reactions A , D , and E were studied with conventional transition state theory employing QCISD(T)/CBS energies. For the saddle point in reaction A , additional high-level corrections are evaluated. The predicted reaction exo- and endothermicities are in good agreement with the current Active Thermochemical Tables values. The transition state theory predictions for the microcanonical rate coefficients in ethanol decomposition are incorporated in master equation calculations to yield predictions for the temperature and pressure dependences of reactions A - C . With modest adjustments (<1 kcal/mol) to a few key barrier heights, the present experimental and adjusted theoretical results yield a consistent description of both the decomposition (1-3) and abstraction kinetics (4 and 5). The present results are compared with earlier experimental and theoretical work.
Primary and secondary reactions involved in the thermal decomposition of NH2OH are studied with a combination of shock tube experiments and transition state theory based theoretical kinetics. This coupled theory and experiment study demonstrates the utility of NH2OH as a high temperature source of OH radicals. The reflected shock technique is employed in the determination of OH radical time profiles via multipass electronic absorption spectrometry. O-atoms are searched for with atomic resonance absorption spectrometry. The experiments provide a direct measurement of the rate coefficient, k1, for the thermal decomposition of NH2OH. Secondary rate measurements are obtained for the NH2 + OH (5a) and NH2OH + OH (6a) abstraction reactions. The experimental data are obtained for temperatures in the range from 1355 to 1889 K and are well represented by the respective rate expressions: log[k/(cm3 molecule(-1) s(-1))] = (-10.12 +/- 0.20) + (-6793 +/- 317 K/T) (k1); log[k/(cm3 molecule(-1) s(-1))] = (-10.00 +/- 0.06) + (-879 +/- 101 K/T) (k5a); log[k/(cm3 molecule(-1) s(-1))] = (-9.75 +/- 0.08) + (-1248 +/- 123 K/T) (k6a). Theoretical predictions are made for these rate coefficients as well for the reactions of NH2OH + NH2, NH2OH + NH, NH + OH, NH2 + NH2, NH2 + NH, and NH + NH, each of which could be of secondary importance in NH2OH thermal decomposition. The theoretical analyses employ a combination of ab initio transition state theory and master equation simulations. Comparisons between theory and experiment are made where possible. Modest adjustments of predicted barrier heights (i.e., by 2 kcal/mol or less) generally yield good agreement between theory and experiment. The rate coefficients obtained here should be of utility in modeling NOx in various combustion environments.
The third-order reaction, H + O2 + M → HO2 + M, has been measured near the low-pressure limit at room temperature for M = He, Ne, Ar, Kr, O2, N2, and H2O and over an extended range of temperatures in a shock tube for M = Ar, O2, and N2. In all cases, H atoms were produced by the laser photolysis of NH3 and detected by atomic resonance absorption spectroscopy. The measurements are consistent with the available experimental record and, in particular, confirm the exceptionally high recombination rate constant when M = H2O. The standard theoretical analysis is applied to this entire experimental record to derive the value of the average energy change per collision, −ΔE all. The resulting −ΔE all values are sensible for all M but H2O. The problem with H2O motivates a change in the standard theoretical analysis that both rationalizes the behavior of H2O and also quantitatively changes the derived −ΔE all values for the other species of M. These changes involve three modifications of the standard treatment: (1) explicit temperature dependence in the number of active rotational degrees of freedom contributing to the HO2* state density, (2) the replacement of Lennard-Jones potential for the HO2* + M interaction with an electrostatic + dispersion potential, and (3) the calculation of the collision rate between HO2* + M by a free rotor model for “complex formation” between the M and HO2*. The optimized values of −ΔE all that are produced from this new analysis have the following characteristics: (1) the value of −ΔE all is the same for all rare gases, and (2) −ΔE all for di- and polyatomic molecules are enhanced relative to the rare gas atoms. This work supports the conclusions of previous trajectory studies that collision rates between activated complexes and bath gases are often underestimated while −ΔE all derived from recombination kinetics measurements are often overestimated.
High-temperature rate constant experiments on OH with the five large (C(5)-C(8)) saturated hydrocarbons n-heptane, 2,2,3,3-tetramethylbutane (2,2,3,3-TMB), n-pentane, n-hexane, and 2,3-dimethylbutane (2,3-DMB) were performed with the reflected-shock-tube technique using multipass absorption spectrometric detection of OH radicals at 308 nm. Single-point determinations at approximately 1200 K on n-heptane, 2,2,3,3-TMB, n-hexane, and 2,3-DMB were previously reported by Cohen and co-workers; however, the present work substantially extends the database to both lower and higher temperature. The present experiments span a wide temperature range, 789-1308 K, and represent the first direct measurements of rate constants at T > 800 K for n-pentane. The present work utilized 48 optical passes corresponding to a total path length of approximately 4.2 m. As a result of this increased path length, the high OH concentration detection sensitivity permitted pseudo-first-order analyses for unambiguously measuring rate constants. The experimental results can be expressed in Arrhenius form in units of cm(3) molecule(-1) s(-1) as follows: kOH+n-heptane)(2.48(0.17) x 10(-10) exp[(-1927(69 K) / T] (838-1287 K)kOH+2,2,3,3-TMB)(8.26(0.89)x10(-11) exp[(-1337(94 K)/ T] (789-1061 K)kOH+n-pentane)(1.60(0.25) x 10(-10) exp[(-1903(146 K) / T] (823-1308 K)kOH+n-hexane)(2.79(0.39) x 10(-10) exp[(-2301(134 K) /T] (798-1299 K)kOH+2,3-DMB)(1.27(0.16) x 10(-10) exp[(-1617(118 K)/ T] (843-1292 K) The available experimental data, along with lower-T determinations, were used to obtain evaluations of the experimental rate constants over the temperature range from approximately 230 to 1300 K for most of the title reactions. These extended-temperature-range evaluations, given as three-parameter fits, are as follows: kOH+n-heptane)2.059 x 10(-15)T1.401 exp(33 K/ T) cm3 molecule-1 s-1 (241-1287 K)kOH+2,2,3,3-TMB)6.835 x 10(-17)T1.886 exp(-365 K/ T) cm3 molecule-1 s-1 (290-1180 K)kOH+n-pentane)2.495 x 10(-16)T1.649 exp(80 K/T) cm3 molecule-1 s-1 (224-1308 K)kOH+n-hexane)3.959 x 10(-18)T2.218 exp(443 K/T) cm3 molecule-1 s-1 (292-1299 K)kOH+2,3-DMB)2.287 x 10(-17)T1.958 exp(365 K/ T) cm3 molecule-1 s-1 (220-1292 K). The experimental data and the evaluations obtained for these five larger alkanes in the present work were used along with prior data/evaluations obtained in this laboratory for H abstractions by OH from a series of smaller alkanes (C(3)-C(5)) to devise rate rules for abstractions from various types of primary, secondary, and tertiary H atoms. Specifically, the current scheme was applied with good success to H abstractions by OH from a series of n-alkanes (n-octane through n-hexadecane). The total rate constants using this group scheme for reactions of OH with selected large alkanes are given as three-parameter fits in this article. The rate constants for the various abstraction channels in any large n-alkane can also be obtained using the groups listed in this article. The present group scheme serves to reduce the uncertainties in rate constants fo...
The thermal dissociation of acetaldehyde has been studied with the reflected shock tube technique using H(D)-atom atomic resonance absorption spectrometry detection. The use of an unreversed light source yields extraordinarily sensitive H atom detection. As a result, we are able to measure both the total decomposition rate and the branching to radical versus molecular channels. This branching provides a direct measure of the contribution from the roaming radical mechanism since the contributions from the usual tight transition states are predicted by theory to be negligible. The experimental observations also provide a measure of the rate coefficient for H + CH(3)CHO. Another set of experiments employing C(2)H(5)I as an H-atom source provides additional data for this rate coefficient that extends to lower temperature. An evaluation of the available experimental results for H + CH(3)CHO can be expressed by a three-parameter Arrhenius expression as k = 7.66 x 10(-20)T(2.75) exp((-486 K)/T) cm(3) molecule(-1) s(-1) (298-1415 K). Analogous experiments employing C(2)D(5)I as a D-atom source allow for the study of the isotopically substituted reaction. The present experiments are the only direct measure for this reaction rate constant, and the results can be expressed by an Arrhenius expression as k = 5.20 x 10(-10) exp((-4430 K)/T) cm(3) molecule(-1) s(-1) (1151-1354 K). The H/D + CH(3)CHO reactions are also studied with ab initio transition-state theory, and the results are in remarkably good agreement with the current experimental data.
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