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Comparisons of CBS‐q and G2 calculations on thermodynamic properties, transition states, and kinetics of dimethyl‐ether + O 2 reaction system
Author(s) -
Yamada T.,
Bozzelli J. W.,
Lay T. H.
Publication year - 2000
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/(sici)1097-4601(2000)32:7<435::aid-kin6>3.0.co;2-4
Subject(s) - chemistry , isodesmic reaction , dimethyl ether , standard enthalpy of formation , radical , reaction rate constant , transition state , bond dissociation energy , kinetics , enthalpy , ab initio , ether , transition state theory , thermodynamics , dissociation (chemistry) , organic chemistry , catalysis , physics , quantum mechanics
Reaction pathways and kinetics are analyzed on the CH 3 OC·H 2 + O 2 reaction system using thermodynamic properties (Δ H ° f 298 , S ° 298 , and C p ( T )'s 300 ≤ T / K ≤ 1500) derived by two composite ab initio calculation methods, CBS‐q and G2. Thermodynamic properties are determined for reactants, intermediate radicals, and transition‐state (TS) species and kinetic parameters are determined. Enthalpies of formation (Δ H ° f 298 in kcal/mol) of reactant CH 3 OC·H 2 and two intermediate radicals CH 3 OCH 2 OO· and C·H 2 OCH 2 OOH are determined using isodesmic reactions, where zero‐point vibrational energies and thermal corrections to 298.15 K are taken into account. Entropy ( S ° 298 in cal/mol K) and heat capacities (C p ( T )'s, 300 ≤ T / K ≤ 1500, in cal/mol K) are determined using geometric parameters and vibrational frequencies obtained at the MP2(full)/6‐31G(d,p) level of theory for CBS‐9. Quantum Rice‐Ramsperger‐Kassel (QRRK) analysis is used to calculate energy‐dependent rate constants, k ( E ) and master equation is used to account for collisional stabilization of adduct and isomer. Overall reaction parameters are determined as a function of temperature and pressure. The dimethyl‐ether radical CH 3 OC·H 2 (Δ H ° f 298 (CBS‐q) = 0.1 kcal/mol and Δ H ° f 298 (G2) = −0.1 kcal/mol) adds to O 2 to form a peroxy radical CH 3 OCH 2 OO· (Δ H ° f 298 (CBS‐q) = −33.9 kcal/mol and Δ H ° f 298 (G2) = −34.1 kcal/mol). The peroxy radical can undergo dissociation back to reactants or isomerize via hydrogen shift (TS1) ( E a,rxn (CBS‐q) = 17.7 kcal/mol and E a,rxn (G2) = 20.1 kcal/mol) to form a hydroperoxy alkyl radical C·H 2 OCH 2 OOH (Δ H ° f 298 (CBS‐q) = −26.5 kcal/mol and Δ H ° f 298 (G2) = −25.9 kcal/mol). This alkyl radical can undergo β‐scission reaction to formaldehyde (CH 2 O) + hydroperoxy methyl radical (TS2) C·H 2 OOH ( E a,rxn (CBS‐q) = 24.7 kcal/mol and E a,rxn (G2) = 25.2 kcal/mol). The hydroperoxy methyl radical rapidly decomposes to a second CH 2 O + OH. The reaction barriers for CH 3 OCH 2 + O 2 to 2 CH 2 O + OH are less than the energy needed for reaction back to CH 3 OC·H 2 + O 2 and provide a low‐energy chain propagation path for dimethyl oxidation. i OH + CH 3 OCH 3 → CH 3 OC·H 2 + H 2 O ii CH 3 OC·H 2 + O 2 → 2 CH 2 O + OH i+ii CH 3 OCH 3 + O 2 → 2 CH 2 O +H 2 O Comparison of calculated fall‐off with experiment indicates that the CBS‐q and G2 calculated E a,rxn for the second transition state (β‐scission reaction to CH 2 O + C·H 2 OOH) needs to be lowered by 3.3 and 6.0 kcal/mol, respectively, in order to match the data of Sehested et al. Reaction of C·H 2 OCH 2 OOH to dioxetane + OH has a barrier 4.6 and 3.4–5.2 kcal/mol above reactant for CBS‐q and G2 calculation methods, respectively, and is not important at low temperature. Rate constants of important reactions based on CBS‐q results in the form ( k = A ( T / K ) n exp(− E a /RT), A in cm 3 /(mol s), E a in kcal/mol) are k 1 , (2.33 × 10 63 )( T / K ) −16.89 e −11.89/RT for CH 3 OC·H 2 + O 2 → CH 3 OCH 2 OO·; k 3 , (6.42 × 10 29 )( T / K ) −5.46 e −8.59/RT for CH 3 OC·H 2 + O 2 → CH 2 O + CH 2 O + OH. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 435–452, 2000