Accelerated chemistry in the reaction between the hydroxyl radical and methanol at interstellar temperatures facilitated by tunnelling

Abstract

Understanding the abundances of molecules observed in dense interstellar clouds requires knowledge of the rates of gas phase reactions between two neutral species. However, reactions possessing an activation barrier were considered too slow to play any important role at the low temperatures in these clouds. Here we show that despite the presence of a barrier the rate coefficient for the reaction between the hydroxyl radical (OH) and methanol, one of the most abundant organic molecules in space, is almost two orders of magnitude larger at 63K than previously measured at ~200K. We also observe formation of the methoxy radical product that was recently detected in space. These results are interpreted through the formation of a hydrogen-bonded complex that is sufficiently long-lived to undergo quantum mechanical tunnelling to form products. We postulate that this tunnelling

Measurements of Rate Coefficients for Reactions of OH with Ethanol and Propan-2-ol at Very Low Temperatures

“…The rate coefficient for OH + propan-2-ol is also found to exhibit a notable inverse temperature dependence below 240 K, as shown in Figure 1(b), increasing by around an order of magnitude between 296 and 88 K. 5 The rate coefficients k OH + ethanol and k OH + propan-2-ol were measured over a range of total bath gas densities and, in contrast to the reaction of OH with methanol, 17 the measured rate coefficients demonstrate a pressure dependence at the low temperatures studied in this work, as shown in Figure 4. DISCUSSION Figure 5 shows the generic potential energy surface of the reaction of OH with ethanol or propan-2-ol, based on calculations reported in the literature, from which it can be seen that a weakly bound complex (between~18-27 kJmol -1 ) is formed prior to any barrier to abstraction.…”

“…However, at low temperatures when the quantum mechanical tunneling mechanism dominates reaction, the product branching ratios are predominantly influenced by the shape of the vibrationally adiabatic reaction path, in other words, the barrier widths. 17,30,31 Therefore, in the case of OH + ethanol it is likely that hydrogen atom abstraction process is not occurring through the lowest energy barrier, which corresponds to abstraction from the CH 2 moiety (R2b) to form CH 3 CHOH, but instead through the barrier which has the largest imaginary frequency (~2300-2900 cm -1 ), which is abstraction from the hydroxyl group (R2c) resulting in ethoxy radical formation, CH 3 CH 2 O. [18][19][20] Although this channel has been experimentally and theoretically determined to be relatively inactive at ambient temperatures, 6,19 our previous work on the OH + methanol reaction has shown that abstraction via the barrier with the largest imaginary frequency (also the OH group, to form CH 3 O)…”

“…However, recent low temperature studies of neutral-neutral reactions have shown that the temperature dependence of their rate coefficients is often complex and poorly understood, and so kinetic data close to ambient temperatures cannot simply be extrapolated. [14][15][16][17] From previous studies it is acknowledged that at temperatures above 200 K, the reactions of OH with alcohols appear to proceed via a direct, bimolecular hydrogen abstraction mechanism (R1). 2,6,17,18 + R1…”

“…17,22 At low temperatures, the reaction leads to the formation of water vapor and the methoxy radical (CH 3 O) as products. 17 The increase in the rate coefficient is attributed to quantum mechanical tunneling through the barrier to hydrogen abstraction, a mechanism which has also been observed in the reactions of OH with dimethyl ether and acetone, 15 and is expected to be applicable to reactions of OH with other oxygenated VOC reactions at very low temperatures. It is notable that the observed low temperature product of the OH reaction with methanol predicted by master equation modeling was not CH 2 OH, which would be obtained via tunneling through the lowest energy pathway.…”

“…It is notable that the observed low temperature product of the OH reaction with methanol predicted by master equation modeling was not CH 2 OH, which would be obtained via tunneling through the lowest energy pathway. 17 In this paper we report the results of a kinetics study of the reactions of OH with ethanol and with propan-2-ol in the range 54 to 148 K using a pulsed Laval nozzle apparatus, and discuss the possible mechanisms to explain the observed temperature and pressure dependencies of the rate coefficients.…”