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

Thermochemical parameters of three C2H5O center dot 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 center dot 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) Delta H-f(C2H5OH) = -56.4 +/- 0.8 kcal/mol (exptl: -56.21 +/- 0.12 kcal/mol), Delta H-f((CH3CHOH)-H-center dot) = -13.1 +/- 0.8 kcal/mol, Delta H-f((CH2CH2OH)-H-center dot) = -6.2 +/- 0.8 kcal/mol, and Delta H-f(CH3CH2O center dot) = -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. beta-C-C bond scission in the ethoxy radical, CH3CH2O center dot, leading to the formation of (CH3)-H-center dot 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 E-0(double dagger) = 17.2 kcal/mol, an activation energy of E-a = 18.0 kcal/mol and a high-pressure thermal rate coefficient of k(infinity)(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 10(9)p(0.793) 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 N-2 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 beta-C-C bond scission in alkoxy radicals.