Theoretical Prediction of the Heats of Formation of C₂H₅O• Radicals Derived from Ethanol and of the Kinetics of β-C-C Scission in the Ethoxy Radical
- Univ. of Alabama, Tuscaloosa, AL (United States)
- Univ. of Alabama, Tuscaloosa, AL (United States); Univ. of Leuven (Belgium)
The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Thermochemical parameters of three C₂H₅O• radicals derived from ethanol were reevaluated using coupledcluster theory CCSD(T) calculations, with the aug-cc-pVnZ (n = D, T, Q) basis sets, that allow the CC energies to be extrapolated at the CBS limit. Theoretical results obtained for methanol and two CH₃O• radicals were found to agree within ±0.5 kcal/mol with the experiment values. A set of consistent values was determined for ethanol and its radicals: (a) heats of formation (298 K) ΔHf(C₂H₅OH) = -56.4 ±0.8 kcal/mol (exptl: -56.21 ± 0.12 kcal/mol), ΔHf(CH₃C•HOH) = -13.1 ±0.8 kcal/mol, ΔHf(C•H₂CH₂OH) = -6.2 ±0.8 kcal/mol, and ΔHf(CH₃CH₂O•) = -2.7 ± 0.8 kcal/mol; (b) bond dissociation energies (BDEs) of ethanol (0 K) BDE(CH₃CHOH-H) = 93.9 ± 0.8 kcal/mol, BDE(CH₂CH₂OH-H) = 100.6 ± 0.8 kcal/mol, and BDE- (CH₃CH₂O-H) = 104.5 ± 0.8 kcal/mol. The present results support the experimental ionization energies and electron affinities of the radicals, and appearance energy of (CH₃CHOH+) cation. β-C-C bond scission in the ethoxy radical, CH₃CH₂O•, leading to the formation of C•H₃ and CH₂=O, is characterized by a C-C bond energy of 9.6 kcal/mol at 0 K, a zero-point-corrected energy barrier of E0 ‡ = 17.2 kcal/mol, an activation energy of Ea = 18.0 kcal/mol and a high-pressure thermal rate coefficient of kω(298 K) = 3.9 s-1, including a tunneling correction. The latter value is in excellent agreement with the value of 5.2 s-1 from the most recent experimental kinetic data. Using RRKM theory, we obtain a general rate expression of k(T,p) = 1.26 x 109p0.793 exp(-15.5/RT) s-1 in the temperature range (T) from 198 to 1998 K and pressure range (p) from 0.1 to 8360.1 Torr with N₂ as the collision partners, where k(298 K, 760 Torr) = 2.7 s-1, without tunneling and k = 3.2 s-1 with the tunneling correction. Evidence is provided that heavy atom tunneling can play a role in the rate constant for β-C-C bond scission in alkoxy radicals.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (United States), Environmental Molecular Sciences Laboratory (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 921828
- Journal Information:
- Journal of Physical Chemistry. C, Journal Name: Journal of Physical Chemistry. C Journal Issue: 1 Vol. 111; ISSN 1932-7447
- Publisher:
- American Chemical Society
- Country of Publication:
- United States
- Language:
- English
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