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Title: 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

Abstract

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), ΔH f(CH₃C HOH) = -13.1 ±0.8 kcal/mol, ΔH f(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-Cmore » 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 10 9p0.793 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.« less

Authors:
 [1];  [2];  [1]
  1. Univ. of Alabama, Tuscaloosa, AL (United States)
  2. Univ. of Alabama, Tuscaloosa, AL (United States); Univ. of Leuven (Belgium)
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
921828
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry. C; Journal Volume: 111; Journal Issue: 1
Country of Publication:
United States
Language:
English
Subject:
10 SYNTHETIC FUELS; ACTIVATION ENERGY; ALKOXY RADICALS; ATOMS; DISSOCIATION; ELECTRONS; ETHANOL; ETHOXY RADICALS; FORECASTING; IONIZATION; KINETICS; METHANOL; PRESSURE RANGE; RADICALS; TUNNELING; Environmental Molecular Sciences Laboratory

Citation Formats

Matus, Myrna H., Nguyen, Minh T., and Dixon, David A.. 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. United States: N. p., 2006. Web. doi:10.1021/jp064086f.
Matus, Myrna H., Nguyen, Minh T., & Dixon, David A.. 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. United States. doi:10.1021/jp064086f.
Matus, Myrna H., Nguyen, Minh T., and Dixon, David A.. Fri . "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". United States. doi:10.1021/jp064086f.
@article{osti_921828,
title = {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},
author = {Matus, Myrna H. and Nguyen, Minh T. and Dixon, David A.},
abstractNote = {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.},
doi = {10.1021/jp064086f},
journal = {Journal of Physical Chemistry. C},
number = 1,
volume = 111,
place = {United States},
year = {Fri Dec 15 00:00:00 EST 2006},
month = {Fri Dec 15 00:00:00 EST 2006}
}
  • Alkanes constitute a significant fraction of automotive fuels, vehicle emissions, and air pollution in urban areas. Atmospheric oxidation of alkanes is initiated primarily via attack by OH radicals giving alkyl radicals which then add O{sub 2} to give alkyl peroxy radicals. Subsequent reactions of the alkyl peroxy radicals determine the atmospheric degradation mechanism, and hence ozone forming potential, of organic compounds. A pulse radiolysis technique was used to measure the UV absorption spectra of c-C{sub 6}H{sub 11}{sup {sm_bullet}} and (c-C{sub 6}H{sub 11})O{sub 2}{sup {sm_bullet}} radicals over the ranges 230--290 and 220--300 nm, {sigma}(c-C{sub 6}H{sub 11}{sup {sm_bullet}}){sub 250 nm} = (7.0more » {+-} 0.8) {times} 10{sup {minus}18} and {sigma}((c-C{sub 6}H{sub 11})O{sub 2}{sup {sm_bullet}}){sub 250 nm} = (5.7 {+-} 0.6) {times} 10{sup {minus}18} cm{sup 2} molecule{sup {minus}1}. The rate constant for the self-reaction of c-C{sub 6}H{sub 11}{sup {sm_bullet}} radicals was {kappa}{sub 3} = (3.0 {+-} 0.4) {times} 10{sup {minus}11} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. Rate constants for reactions of (c-C{sub 6}H{sub 11})O{sub 2}{sup {sm_bullet}} radicals with NO and NO{sub 2} were {kappa}{sub 4} = (6.7 {+-} 0.9) {times} 10{sup {minus}12} and {kappa}{sub 5} = (9.5 {+-} 1.5) {times} 10{sup {minus}12} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}, respectively. FTIR-smog chamber techniques were used to record the IR spectrum of the peroxynitrate (c-C{sub 6}H{sub 11})O{sub 2}NO{sub 2}. determine that the reaction between (c-C{sub 6}H{sub 11})O{sub 2}{sup {sm_bullet}} radicals and NO produces a (16 {+-} 4)% yield of the nitrate (c-C{sub 6}H{sub 11})ONO{sub 2}, and study the atmospheric fate of cyclohexoxy radicals. Decomposition via C-C bond scission and reaction with O{sub 2} are competing fates of the cyclohexoxy radical. In 700--750 Torr total pressure at 296 {+-} 2K, the rate constant ratio {kappa}{sub decomp}/{kappa}{sub O{sub 2}} = (8.1 {+-} 1.5) {times} 10{sup 18} molecule cm{sup {minus}3}. At 296 K in 1 atm of air, 61% of cyclohexoxy radicals decompose and 39% react with O{sub 2}.« less
  • The kinetics of the reactions of CH{sub 3}, C{sub 2}H{sub 5}, i-C{sub 3}H{sub 7}, s-C{sub 4}H{sub 9}, and t-C{sub 4}H{sub 9} with HI were studied in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature (typically between 295 and 648 K) to determine Arrhenius parameters. These results were combined with determinations of the rate constants of the reverse reactions (I + hydrocarbon) determined previously by others to obtain equilibrium constants for the following reaction: R + HI {leftrightarrow} R-H + I. Second and Third Law based analyses using these equilibrium constantsmore » yielded heats of formation for the five alkyl radicals whose R + HI reactions were studied. The Third Law heats of formation (obtained using calculated entropies) are extremely accurate, within {plus minus} 2 kJ mol{sup {minus}1} of the current best values. The cause of the long-standing disparity that has existed between the heats of formation of the alkyl radicals derived from studies of R + HI {leftrightarrow} R-H + I equilibria and those obtained from investigation of dissociation-recombination equilibria has been identified. It is the difference between the assumed generic activation energy of R + HI rate constants (4 kJ mol{sup {minus}1}) that had been used in all prior thermochemical calculations and the actual values of these activation energies. A complex mechanism for R + HI reactions that is consistent with the observed kinetic behavior of these reactions is discussed.« less
  • The 1,2 aryl migration and fragmentation reactions of 1-indanylmethyl (1), 2-tetralyl (2), 2-indanylmethyl (3), 1-tetralyl (4), 2-methyl-1-indanyl (5), and 1-methyl-2-indanyl (6) radicals were studied by flash vacuum pyrolysis of the tert-butyl perester precursors at 327 to 627/sup 0/C and 10/sup -2/ torr. Radicals 1 and 2 are interconverted via a 1,2 aryl migration which is readily reversible at all temperatures. This equilibrium is depleted by ..beta.. scission of 1 and recyclization to 4 and by ..beta.. scission of 2 followed by recyclization to 2 or to 3 in modest yields. The reverse neophyl-like rearrangement of 2 to 1 occurs withmore » a lower activation barrier than ..beta.. scission of 1 to form a 2-(o-vinylphenyl)ethyl radical. Enthalpies, entropies, and free energies of reaction were calculated for the above reactions from group additivity parameters, and activation energies were estimated from values reported for simple alkyl radicals. It is shown that the ..beta.. scission of 4 and recyclization to 1 is important only at very high temperatures (> 500/sup 0/C) as a mechanism for the isomerization of tetralin and related hydroaromatic structures to alkylindans and that the reverse neophyl-like rearrangement of 2 to 1 is the favored pathway for isomerizations observed during dissolution of coal in hydroaromatic media at elevated temperatures.« less
  • The ionization potentials of the transient species CH[sub 3]CH[sub 2]O, CH[sub 3]CHOH, and CH[sub 2]CH[sub 2]OH (generated by the F+ethanol reactions) are measured by photoionization mass spectrometry: I.P.(CD[sub 3]CD[sub 2]O)=10.29[plus minus]0.08 eV (tentative), I.P.(CH[sub 3]CHOH)[lt]6.85 eV, and I.P.(CD[sub 2]CH[sub 2]OH) [le]8.35[plus minus]0.06 eV. The latter results in a cation of uncertain structure. These reactions also generate vinyl alcohol (adiabatic I.P.=9.33[plus minus]0.01 eV) and acetaldehyde. A redetermined appearance potential of CH[sub 3]CHOH[sup +] from ethanol enables one to infer the proton affinity of acetaldehyde to be [ge]183.8[plus minus]0.2 kcal/mol and an [alpha] (C--H) bond energy in ethanol [gt]91.1 kcal/mol (0 K).more » The appearance potential of [ital m]/[ital e]=45 ion from bromoethanol is interpreted as formation of a C[sub 2]H[sub 5]O[sup +] isomer having the oxirane structure, with [Delta][ital H][sub [ital f][sub 0]][sup 0] of [similar to]173.9 kcal/mol, consistent with earlier alternative measurements. A second increase in the [ital m]/[ital e]=45 ion yield curve from ethanol is interpreted as formation of this same isomer. This interpretation, and an alternative cycle, lead to a [beta] (C--H) bond energy in ethanol of 98[plus minus]2 kcal/mol. The implication of the current results to the dynamics of dissociation of ethanol cations is discussed. [copyright] [ital 1994][ital American] [ital Institute] [ital of] [ital Physics].« less