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

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Abstract
Thermochemical parameters of three C2H5O• radicals derived from ethanol were reevaluated using coupled-cluster 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 CH2O, 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 k∞(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 × 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.

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