On the relation between the activation energy for electron attachment reactions and the size of their thermal rate coefficients

Abstract

Rate coefficients k(T) for dissociative electron attachment (DEA) to molecules in many cases exhibit a more or less strong rise with increasing temperature T (the electron temperature T(e) and the molecular temperature T(G) are assumed to be in thermal equilibrium, i.e., T = T(e) = T(G)). This rise is frequently modeled by the Arrhenius equation k(T) = k(A) exp[-E(a)∕(k(B)T)], and an activation energy E(a) is deduced from fits to the experimental data k(T). This behavior reflects the presence of an energy barrier for the anion on its path to the dissociated products. In a recent paper [J. Kopyra, J. Wnorowska, M. Foryś, and I. Szamrej, Int. J. Mass Spectrom. 268, 60 (2007)] it was suggested that the size of the rate coefficients for DEA reactions at room temperature exhibits an exponential dependence on the activation energy, i.e., k(E(a); T ≈ 300 K) = k(1) exp[-E(a)∕E(0)]. More recent experimental data for molecules with high barriers [T. M. Miller, J. F. Friedman, L. C. Schaffer, and A. A. Viggiano, J. Chem. Phys. 131, 084302 (2009)] are compatible with such a correlation. We investigate the validity and the possible origin of this dependence by analyzing the results of R-matrix calculations for temperature-dependent rate coefficients of exothermic DEA processes with intermediate barrier toward dissociation. These include results for model systems with systematically varied barrier height as well as results of molecule-specific calculations for CH(3)Cl, CH(3)Br, CF(3)Cl, and CH(2)Cl(2) (activation energies above 0.2 eV) involving appropriate molecular parameters. A comparison of the experimental and theoretical results for the considered class of molecules (halogenated alkanes) supports the idea that the exponential dependence of k(T = 300 K) on the activation energy reflects a general phenomenon associated with Franck-Condon factors for getting from the initial neutral vibrational levels to the dissociating final anion state in a direct DEA process. Cases are discussed for which the proposed relation does not apply.

Electron attachment to chlorinated alcohols

“…Hotop et al [25] carried out R-matrix calculations of temperature-dependence rate coefficients for exothermic dissociative electron attachment (DEA) processes for several halogenated alkanes. They theoretically justified experimental observations on an exponential dependence of thermal (T≈300 K) rate coefficients on the activation energy resulting from the Arrhenius equation.…”

“…it is evident that fluorinated alcohols are better electron scavengers than chlorinated ones (bigger values of the rate coefficients and smaller values of the activation energies).For example since the rate coefficients for CH 2 ClCH 2 OH and CH 2 ClCH 2 Cl (k=4.5×10 -12 cm 3 s -1[25]), or CH 2 ClCH 2 CH 2 OH and CH 2 ClCH 2 CH 2 Cl (k=2.0×10 -12 cm 3 s -1[16]) are practically the same so we can conclude that replacement of chlorine atom by a hydroxyl group does not significantly affect the efficiency of electron-capture process. This confirm E a for chlorinated alcohols are collected inTable 1.…”

Studies on the Processes of Electron Capture and Clustering of Benzyl Chloride by Ion Mobility Spectrometry

“…The obtained values of the electron attachment rates increase with temperature. Such a phenomenon is experimentally observed for many halogenated organic compounds [ 29 ]. The theoretical explanation of the phenomenon may be based on the dependence of the cross-section for dissociative capture on the energy of electrons.…”

Low‐energy electron interactions with chlorotrimethylsilane (Si(CH3)3Cl), dichlorodimethylsilane (Si(CH3)2Cl2) and chloromethyldimethylsilane (SiH(CH3)2(CH2Cl))

“…As was indicated by Hotop et al, 39 rate coefficients for DEA to molecules in many cases exhibit a more or less strong increase with increasing temperature T (the electron temperature T e and the molecular temperature T G are assumed to be in thermal equilibrium, i.e. T = T e = T G ).…”