Shock‐tube studies on the reactions of dimethyl ether with oxygen and hydrogen atoms

Abstract

The reactions of dimethyl ether (CH 3 OCH 3 , DME) with O( 3 P) and H atoms have been studied at high temperatures by using a shock tube apparatus coupled with atomic resonance absorption spectroscopy (ARAS). The rate coefficients for the reactions CH 3 OCH 3 + O( 3 P) → CH 3 OCH 2 + OH (1) and CH 3 OCH 3 + H → CH 3 OCH 2 + H 2 (2) were experimentally determined from the decay of O( 3 P) and H atoms as:These results show that DME can react with O( 3 P) atoms more easily than with H atoms. By combining these results with the previous lower temperature data, we obtained the following modified Arrhenius expressions applied over the wide temperature range between 300 and 1500 K: Both reactions of DME are faster than those of ethane, because the dissociation energy of the C H bond in DME is smaller. Furthermore, the rate coefficients for reactions (1) and (2) were calculated with the transition-state theory (TST). Structural parameters and vibrational frequencies of the reactants and the transition states required for the TST calculation were obtained from the MP2(full)/6-31G(d) ab initio molecular orbital (MO) calculation. The energy barrier, E ‡ 0 , was adjusted until the TST rate coefficient most closely matched the observed one. The fitting results of E ‡ 0 (1) = 23 kJ mol −1 and E ‡ 0 (2) = 34 kJ mol −1 were in agreement with the G2 energy barriers, within the expected uncertainty, demonstrating that the experimentally determined rate coefficients were theoretically valid. C 2006 Wiley Periodicals, Inc. Int J Chem Kinet

Kinetic investigations of the unimolecular decomposition of dimethylether behind shock waves

“…The best agreement in the entire pressure and temperature range between experiments and model calculations has been obtained with a temperature independent rate constant of k 3 = 1×10 13± 0.3 cm 3 mol -1 s -1 (6) in excellent agreement with a very recent, direct determination of k 3 by Takahashi et al [17]: measurements in the temperature range 1038 -1208 K led to an Arrhenius expression of k 3 =10 14.63±0.45 exp((-37±10) kJmol -1 /RT) cm 3 mol -1 s -1 , yielding rate constants of 10 13.1 <k 3 <10 13.4 in our temperature range. They also recommend another Arrhenius expression, valid over a wide temperature range, by taking into account older measurements at lower temperatures: k 3 =10 1.04 T -4.0 exp(-7.7 kJ mol -1 /RT) cm 3 mol -1 s -1 : this expression yields the following rate constants in our temperature range: 10 13.1 <k 3 <10 13.6 .…”

Roaming radicals in the thermal decomposition of dimethyl ether: Experiment and theory

“…The data of Fernandes et al [9] was also not included in this evaluation since they assumed a T-independent value for k 3 in order to extract k 1 values. The theoretical predictions of Takahashi et al [21] appear to be in better agreement with the lower-T measurements of Meagher et al [43] than the more direct resonance-fluorescence measurements of Lee et al [42]. The more rigorous QCISD(T)/6-311 + G(3df,3pd)// MP2/6-311 + G(d,p) based variational TST based theoretical predictions of Wu et al [45] are in reasonable agreement with the low-T data, but systematically overpredict the abstraction rate constants for (R3) by a factor of 2-3 at high-T (>500 K).…”

“…These higher concentration experiments expand our rate constant database for DME decomposition to lower-T, and allow for the determination of rate constants for H + CH 3 OCH 3 via the modeling of the temporal H-atom profiles. The measurements for the H + DME reaction extend the experimental rate database for that reaction to higher-T than the recent measurements of Takahashi et al [21].…”

“…(E10). Combining the present data (Table 3) for reaction (R3) with the data of Takahashi et al [21] and the lower-T results of Tranter and Walker [40], Faubel et al [41], Lee et al [42], and Meagher et al [43], rate constants have been evaluated over the temperature range, 273-1465 K, to give k 3 ¼ 6:54  10 À24 T 4:13 expðÀ896=TÞ cm 3 …”

“…7 are obtained by extrapolation of the results in Table 2 to lower temperatures by means of an Arrhenius expression for each particular reflected shock density range. Initial values of k 3 are taken from the work of Takahashi et al [21]. The black solid line is a fit to the profile in Fig.…”