Absolute determinations of the kinetics and temperature dependences of the reactions of OH with a series of alkynes

Abstract

The rates of reaction of OH with propyne, but-l-yne, but-2-yne, pent-l-yne and hex-l-yne have been measured directly by using a discharge-flow resonance-fluorescence technique over a range of temperatures from 253 to 343 K. The room-temperature rate coefficients-are compared with previous determinations; the new values for pent-l-yne and hex-l-yne fit into a general pattern of reactivity consistent with a mechanism involving initial electrophilic attack of OH at the triple bond. Similar trends are also presented for other series of reactions between radicals and unsaturated organic molecules. The temperature dependences of the rate constants for the OH-talkyne reactions are small or negative, suggesting that adduct formation is the initial step in the reaction mechanism.

Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

“…These results are the first obtained below 253 K, and overall k 4 shows a weak temperature dependence, significantly less than for ethene, but again, consistent with the rule of Smith et al 2 Previously, this reaction has been studied by Boodaghians et al 49 who observed a weak temperature 1060 dependence between between 253-343 K using the dischargeflow resonance fluorescence method, but did not observe any significant pressure dependence over the limited range studied (1.7 -6. twice that of Boodaghians et al 49 , suggesting that both the study of Boodaghians et al 49 and of our own are in the fall-off region. The very high value for k 4 obtained by Bradley et al 52 in 1973 using ESR at 2.5 Torr appears to be inconsistent with our and all previous data.…”

Adventures on the C3H5O potential energy surface: OH + propyne, OH + allene and related reactions

“…The experimental result of Bradley et al [17] is subject to a factor of 4 error due to a mistake in the data analysis as pointed out by Boodaghians et al [14]; here we show the corrected (higher) value. Although our unadjusted ME calculation underpredicts the rate coefficients, it clearly shows the effect of pressure and also the slight negative temperature dependence observed by Boodaghians et al [14]. A very modest, À0.35 kcal mol À1 , adjustment to both addition transition states results in almost perfect agreement with the experiments, especially when the small, but non-negligible, contribution from abstraction is also included.…”

“…2 shows the calculated rate coefficients below $350 K for the propyne + OH reaction at various pressures. The apparent scatter in the experimental data is mostly due to pressure effects: the experiments of Atkinson and Aschmann [12] and Hatakeyama et al [16] were done at around one atmosphere, while the other ones [14,15,17] were done at low (1-6.4 Torr) pressures. The experimental result of Bradley et al [17] is subject to a factor of 4 error due to a mistake in the data analysis as pointed out by Boodaghians et al [14]; here we show the corrected (higher) value.…”

“…The apparent scatter in the experimental data is mostly due to pressure effects: the experiments of Atkinson and Aschmann [12] and Hatakeyama et al [16] were done at around one atmosphere, while the other ones [14,15,17] were done at low (1-6.4 Torr) pressures. The experimental result of Bradley et al [17] is subject to a factor of 4 error due to a mistake in the data analysis as pointed out by Boodaghians et al [14]; here we show the corrected (higher) value. Although our unadjusted ME calculation underpredicts the rate coefficients, it clearly shows the effect of pressure and also the slight negative temperature dependence observed by Boodaghians et al [14].…”

“…Recently Daranlot et al [10] studied the allene + OH reaction at and below room temperature and used an adjusted M06-2X/augcc-pVTZ surface and variational calculations to calculate the capture rate coefficient and product branching fractions at low temperatures. There are a few experimental works at hightemperatures on allene + OH [11,12], while propyne + OH has only been studied experimentally up to 343 K [13][14][15][16][17].…”