M.-C. Su

Rate Constants for the Thermal Decomposition of Ethanol and Its Bimolecular Reactions with OH and D: Reflected Shock Tube and Theoretical Studies

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.

Shock Tube and Theoretical Studies on the Thermal Decomposition of Propane: Evidence for a Roaming Radical Channel

The thermal decomposition of propane has been studied using both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for propane have been measured at high temperatures behind reflected shock waves using high-sensitivity H-ARAS detection and CH(3) optical absorption. The two major dissociation channels at high temperature are C(3)H(8) → CH(3) + C(2)H(5) (eq 1a) and C(3)H(8) → CH(4) + C(2)H(4) (eq 1b). Ultra high-sensitivity ARAS detection of H-atoms produced from the decomposition of the product, C(2)H(5), in (1a), allowed measurements of both the total decomposition rate constants, k(total), and the branching to radical products, k(1a)/k(total). Theoretical analyses indicate that the molecular products are formed exclusively through the roaming radical mechanism and that radical products are formed exclusively through channel 1a. The experiments were performed over the temperature range 1417-1819 K and gave a minor contribution of (10 ± 8%) due to roaming. A multipass CH(3) absorption diagnostic using a Zn resonance lamp was also developed and characterized in this work using the thermal decomposition of CH(3)I as a reference reaction. The measured rate constants for CH(3)I decomposition agreed with earlier determinations from this laboratory that were based on I-atom ARAS measurements. This CH(3) diagnostic was then used to detect radicals from channel 1a allowing lower temperature (1202-1543 K) measurements of k(1a) to be determined. Variable reaction coordinate-transition state theory was used to predict the high pressure limits for channel (1a) and other bond fission reactions in C(3)H(8). Conventional transition state theory calculations were also used to estimate rate constants for other tight transition state processes. These calculations predict a negligible contribution (<1%) from all other bond fission and tight transition state processes, indicating that the bond fission channel (1a) and the roaming channel (1b) are indeed the only active channels at the temperature and pressure ranges of the present experiments. The predicted reaction exo- and endothermicities are in excellent agreement with the current version of the Active Thermochemical Tables. Master equation calculations incorporating these transition state theory results yield predictions for the temperature and pressure dependence of the dissociation rate constants for channel 1a. The final theoretical results reliably reproduce the measured dissociation rate constants that are reported here and in the literature. The experimental data are well reproduced over the 500-2500 K and 1 × 10(-4) to 100 bar range (errors of ∼15% or less) by the following Troe parameters for Ar as the bath gas: k(∞) = 1.55 × 10(24)T(-2.034) exp(-45 490/T) s(-1), k(0) = 7.92 × 10(53)T(-16.67) exp(-50 380/T) cm(3) s(-1), and F(c) = 0.190 exp(-T/3091) + 0.810 exp(-T/128) + exp(-8829/T).

The thermal decomposition of C2H5I

The high-temperature thermal dissociation of C2H5I has been characterized in this study. Kinetics and overall yield experiments were performed over the temperature range, 946–2046 K, using the atomic resonance absorption spectrometric technique (ARAS) for the temporal detection of both product H and I atoms behind reflected shock waves. The C2H5I decomposition proceeds by both C-I fission and HI elimination. Rate constants for the C-I fission process, measured over the temperature and density ranges, 946–1303 K and 0.82–4.4×1018 cm−3, respectively, can be represented to within ±37% by the firstorder expression: k=6.34×109 exp(−15,894 K/T) s−1. Overall yield data for atomic product gave a branching ratio for C-I fission of (0.87±0.11) suggesting that 13% of the reaction proceeds through molecular HI elimination. This conclusion is consistent with earlier studies that showed C-I fission to be the dominant dissociation channel. The temperature and pressure dependences of the dissociation rate constants and the yield data have been described theoretically using three formulations of unimolecular rate theory. The best description was obtained with a full Masters equation analysis. However, all three calculations confirm that the HI-elimination pathway is lower lying than the C-I fission process by ≈3 kcal mol−1.

Thermal Decomposition of NH2OH and Subsequent Reactions: Ab Initio Transition State Theory and Reflected Shock Tube Experiments

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.

Thermal decomposition of iodobenzene using I-atom absorption☆

Abstract The I-atom atomic resonance absorption spectrometric (ARAS) technique has been used in Kr bath gas to study the thermal decomposition kinetics of iodobenzene, C 6 H 5 I, over T - and P -ranges of 1082–1416 K and 103–731 Torr, giving the first order expression, k (±60%) = 1.982 × 10 11 exp(−23120 K / T )s −1 . Two unimolecular theoretical approaches were used to rationalize the data implying E 0 = δ H 0 0 = (66.7 ± 0.7) kcal mol −1 and 〈 δE 〉 down = (447 ± 92) cm −1 .

Multipass optical detection in reflected shock waves: Application to OH radicals

An UV multipass optical absorption method to increase the sensitivity for radical species detection has been developed for high temperature chemical kinetics experiments in a shock tube. The specific illustration is for OH radicals in the reflected shock wave regime. With a resonance lamp source, 12 optical passes were found to give a sufficient signal‐to‐noise ratio for a large range of [OH]. Two different calibration procedures using the reaction systems H2/O2 and C2H5I/NO2 were used, and a curve of growth was determined. The measured absorbance (ABS), was found to be dependent on both temperature and [OH]. The results can be expressed in a modified Beer’s law form as,(ABS)=9.49×10−12T−0.5281[OH]0.8736.Using this curve of growth, the absorbance data from the above kinetics experiments were converted to concentration profiles. These were fully modeled with previously established mechanisms, giving excellent fits. The multipass method is compared to earlier systems that used both resonance lamp and laser ...

Thermal decomposition of CF3I using I-atom absorption

I-atom atomic resonance absorption spectrometry (ARAS) has been developed and applied to measure the thermal decomposition rate constant for CF 3 I (+M) → CF 3 + I (+M). The I-atom curve-of-growth (λ = 183 nm) was determined using this reaction, and, for [I] ⩽3 × 10 12 molecules cm −3 , (ABS) = 1.9215 × 10 −13 [I], yielding σ = 1.933 × 10 −14 cm 2 . Measured rate constants can be expressed by k 1 = 3.24 × 10 −9 exp(−17286 K/T) cm 3 molecule −1 s −1 (±56%, 1033 ⩽ T ⩽ 1285 K). RRKM theory has been applied to rationalize this result.

C2D5I dissociation and D+CH3 → CH2D+H at high temperature: Implications to the high-pressure rate constant for CH4 dissociation

The shock tube technique with H- and D-atom atomic resonance absorption spectrometry detection has been used to study the thermal decomposition of C 2 D 5 I and the reaction, CH 3 +D → CH 2 D+H over the temperature ranges 924–1370 and 1294–1753 K, respectively. First-order rate constants for the thermal decomposition of C 2 D 5 I can be expressed by the Arrhenius equation, log k C2D51 = (10.397± 0.297)−(7700±334 K)/ T , giving k C2D51 =2.49 × 10 10 exp(−17,729 K/ T ) s −1 . The branching ratio between product channels, C 2 D 5 +I and C 2 D 4 +DI, was also determined. These results coupled with the fast decomposition of C 2 D 5 radicals were then used to specify [D] t in subsequent kinetics experiments with CH 3 where [CH 3 ] 0 was prepared from the concurrent thermal decomposition of CH 3 I. Within experimental error, the rate constants for reaction 1 were found to be temperature independent with k 1 = (2.20±0.22) × 10 −10 cm 3 molecule −1 s − . The present data have been combined with earlier lower temperature determinations and the joint database has been examined with unimolecular rate theory. The implications of the present study can be generalized to supply a reliable value for the high-pressure limiting rate constant for methane dissociation.

Reflected shock tube studies of high-temperature rate constants for OH + C2H2 and OH + C2H4

The reflected shock tube technique with multi-pass absorption spectrometric detection of OH-radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the reactions, OH + C(2)H(2)--> products (1) and OH + C(2)H(4)--> C(2)H(3) + H(2)O (2). The present optical configuration gives a S/N ratio of approximately 1 at approximately 0.5-1.0 x 10(12) radicals cm(-3). Hence, kinetics experiments could be performed at [OH](0) = approximately 4-20 ppm thereby minimizing secondary reactions. OH was produced rapidly from the dissociations of either CH(3)OH or NH(2)OH (hydroxylamine). A mechanism was then used to obtain profile fits that agreed with the experiment to within <+/-5%. The derived Arrhenius expressions, in units of cm(3) molecule(-1) s(-1) are: k(1) = (1.03 +/- 0.24) x 10(-10) exp(-7212 +/- 417 K/T) for 1509-2362 K and k(2) = (10.2 +/- 5.8) x 10(-10) exp(-7411 +/- 871 K/T) for 1463-1931 K. The present study is the first ever direct measurement for reaction (1) at temperatures >1275 K while the present results extend the temperature range for (2) by approximately 700 K. These values are compared with earlier determinations and with recent theoretical calculations. The calculations agree with the present data for both reactions to within +/-10% over the entire T-range.

Optogalvanic wavelength calibration in the 555–575 nm region using argon

Abstract Optogalvanic spectra of neutral atomic argon observed in a commercial hollow cathode lamp provide a convenient means of dye laser wavelength calibration in the Rhodamine 6G dye region (555–575 nm).