## Abstract

The present study reports temperature dependent rate constants *k*_{1}
for the title reaction across the temperature range 213 to
293 K obtained in a Knudsen flow reactor equipped with an
external free radical source based on the reaction
C_{2}H_{5}I + H^{•} → C_{2}H_{5}^{•} + HI
and single VUV-photon ionization mass spectrometry using
Lyman-*α* radiation of 10.2 eV. Combined with
previously obtained high-temperature data of *k*_{1} in the range
298–623 K using the identical experimental equipment and
based on the kinetics of C_{2}H_{5}^{•} disappearance
with increasing HI concentration we arrive at the following
temperature dependence best described by a three-parameter fit to the
combined data set: *k*_{1} = (1.89 ± 1.19)10^{−13}(*T*/298)^{2.92±0.51} exp ((3570 ± 1500)/*RT*),
*R* = 8.314 J mol^{–1} K^{–1} in the range
213–623 K. The present results confirm the general
properties of kinetic data obtained either in static or Knudsen flow
reactors and do nothing to reconcile the significant body of data
obtained in laminar flow reactors using photolytic free radical
generation and suitable free radical precursors. The resulting rate
constant for wall-catalyzed disappearance of ethyl radical across the
full temperature range is discussed.

Highly correlated *ab initio* quantum chemistry methods and
canonical transition state theory were applied for the reaction energy
profiles and rate constants. Geometry optimizations of reactants,
products, molecular complexes, and transition states are determined at
the CCSD/cc-pVDZ level of theory. Subsequent single-point energy
calculations employed the DK-CCSD(T)/ANO-RCC level. Further
improvement of electronic energies has been achieved by applying
spin-orbit coupling corrections towards full configuration interaction
and hindered rotation analysis of vibrational contributions. The
resulting theoretical rate constants in the temperature range
213–623 K lie in the range
E-11–E-12 cm^{3} molecule^{–1} s^{–1}, however
experiments and theoretical modelling seem at great odds with each
other.

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