Oxygenate, oxyalkyl and alkoxycarbonyl thermochemistry and rates for hydrogen abstraction from oxygenates

Abstract

Ab initio molecular orbital theory calculations are performed at the complete basis set (CBS-Q) level to determine thermochemical parameters (D f H 298 , S 298 , C T p ) of alcohols, methyl ethers, formates, acetates, hydroperoxides, methyl alkylperoxides and the corresponding methoxy, primary and secondary alkoxy radicals derived from these molecular families. A consistent set of 12 Benson groups transferable among these six molecular families is derived from CBS-Q results via regression together with 20 hydrogen bond increment (HBI) group values for estimating the thermochemistry of oxymethyl ( CH 2 OX), oxyethyl (CH 3 CH OX), oxyisopropyl ((CH 3 ) 2 C OX) with varying substituent X ( ¼ H, CH 3 , C(O)H, C(O)CH 3 , OH, OCH 3 ) and alkoxycarbonyl (ROC(O) ) radicals. These new groups are useful in estimating the thermochemistry of large oxygenated molecules and radicals involved in complex chemical reaction mechanisms. The kinetics of nearly 50 elementary bimolecular hydrogen abstraction reactions belonging to eight different reaction families viz.,are studied using transition state theory coupled with CBS-Q energetics. The calculated barrier height within a given reaction family varies with the nature of the b substituents. The reliability of the calculated variations in the barrier height with varying b substituents is determined through comparison of the calculated rate constants with the scarcely available experimental data. Thermodynamically consistent generic rate rules have been derived in terms of group additivity. New transferable supergroup values corresponding to the thermochemistry of the reactive moiety in transition structures are developed. The effect of non-next neighbor substituents on the enthalpy of the supergroup is quantified using Evans-Polanyi relations.

Methyl formate oxidation: Speciation data, laminar burning velocities, ignition delay times, and a validated chemical kinetic model

“…We have made two more signiÞcant and a number of relatively minor modiÞcations from the adopted pathway of Fisher et al; the thermochemistry of species involved in the low-temperature MF submechanism has been reestimated so as to be consistent with those of the high-temperature MF mechanism, i.e., we incorporate the work of Sumanthi and Green [35].…”

Thermochemical Properties, Rotation Barriers, Bond Energies, and Group Additivity for Vinyl, Phenyl, Ethynyl, and Allyl Peroxides

“…The isodesmic reactions used to determine the enthalpy of CH 3 CH 2 OOCH 3 are listed in Table 3, and our recommended value is -39 kcal mol -1 ; the values estimated with group additivity (-39.3) are also in good agreement. A value of -37.0 kcal mol -1 is reported in ref 25. CH 3 OOCH 3 was also calculated with isodesmic reactions (see Table 3).…”

Enthalpies of Formation and Bond Dissociation Energies of Lower Alkyl Hydroperoxides and Related Hydroperoxy and Alkoxy Radicals

“…Methyl hydroperoxide, CH 3 OOH, has been the subject of numerous publications [20][21][22][23][24] since a comprehensive review by Dorofeeva et al in 2001. 25 They concluded that ∆H f (298.15K) ) -139.0 ( 5.0 kJ mol -1 represented the best estimate from values which ranged from -122 to -143 kJ mol -1 .…”