Estimating Rate Constants for the Reactions of Ethers + OH: A New Application of the Electrotopological State
Résumé
Earlier this year, a new method was introduced to estimate OH rate constants using a fundamental property of sites within molecules: the electrotopological state [1]. This technique demonstrated robust predictions over a wide temperature range, but, for the purposes of method development, was restricted to the reactions of alkanes and haloalkanes. Nevertheless, there remains a pressing need to estimate the behaviour of many other functionalities within atmospheric and combustion chemistry.
In the current work, we show how this methodology can be extended towards etheric compounds (i.e. ethers and halogenated ethers with one or more ether linkages), a set of oxygenated compounds whose kinetics is relatively well defined [2]. Under atmospheric conditions, many of these ethers are found to be highly reactive towards OH, which can be attributed to the important role of hydrogen-bonded van der Waals (vdW) complexes in mediating these reactions at lower temperatures. By parameterizing this effect for 5- and 6-membered cyclic complexes, we find that rate coefficients can be estimated accurately compared with the well-established method of Atkinson [3].
An interesting outcome of this approach is – in leaving the original parameterization for the alkanes and haloalkanes unmodified – it presents a way to decouple several factors that control the kinetics. Here, the vdW parameterization can be viewed as being representative of an effective tunnelling parameter, κ, in the following equation:
k_"total" (T)=∑_i▒〖κ_i A_i "exp" (-B_i/T) (T/300)^(n_i ) 〗
with the remaining parameters (A, B and n corresponding to the A-factor, E/R and a curvature term respectively for the ith reaction site) being described by the original alkane and haloalkane parameterization. This is partly a consequence of the a priori information that the electrotopological state provides, which may not be afforded by a purely empirical technique, e.g. [3].
The implications of this new method could be many, and may help resolve a sticky issue in atmospheric chemistry regarding the reactivity of a multitude of multifunctional organic species [4].
References
[1] M.R. McGillen, L. Michelat, J.J. Orlando, W.P.L. Carter, Environ. Sci.: Atmos., 4 18–34 (2024).
[2] M.R. McGillen, W.P.L. Carter, A. Mellouki, J.J. Orlando, B. Picquet-Varrault, T.J. Wallington, Earth Syst. Sci. Data, 12 1203–1216 (2020).
[3] E.S.C. Kwok, R. Atkinson, Atmos. Environ., 29 1685–1695 (1995).
[4] L. Vereecken, B. Aumont, I. Barnes, J.W. Bozzelli, M.J. Goldman, W.H. Green, S. Madronich, M.R. McGillen, A. Mellouki, J.J. Orlando, B. Picquet-Varrault, A.K. Rickard, W.R. Stockwell, T.J. Wallington, W.P.L. Carter Int. J. Chem. Kinet., 50 435–469 (2018).
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