Theoretical C−H Bond Dissociation Enthalpies for CH3Br and CH2ClBr

Abstract

The enthalpies of formation for the closed-shell molecules CH{sub 3}Br and CH{sub 2}ClBr, and the free radicals CH{sub 2}Br and CHClBr were estimated by ab initio molecular orbital computations using hydrogenation and isodesmic reactions as working chemical reactions. Four variants of theoretical approaches (levels) and three extended basis sets were applied in the calculations. The methods included fourth-order Moeller-Plesset perturbation theory (level 1), coupled-cluster theory (level 2), density functional theory (level 3), and Gaussian-2 (G2) theory (level 4). The standard enthalpies of formation values obtained at levels 1--4 for both the closed-shell and open-shell species agreed within {+-}1 kcal mol{sup {minus}1} irrespective of the working chemical reactions chosen. No basis set effects were observed. These invariances support the reliability of the computed data and are, for the most part, due to the cancellation of the computational errors as a result of the application of the working chemical reactions where the ab initio energies are combined with established experimental enthalpies of formation. The four methods provided enthalpies of formation values for CH{sub 3}Br, CH{sub 2}ClBr, and CH{sub 2}Br in good agreement with experimental values as well; the calculations supplied the first known enthalpy data for the ChClBr radical. The recommended standardmore » enthalpies of formation (at 298.15 K and 1.00 atm) are the unweighted averages of the results obtained at levels I-IV with the different hydrogenation and isodesmic reactions: CH{sub 3}Br, {minus}8.9 {+-} 0.8; CH{sub 2}ClBr, {minus}11.5 {+-} 1.1; CH{sub 2}Br, 40.7 {+-} 1.1; CHClBr, 35.1 {+-} 1.5 kcal mol{sup {minus}1}. The error limits designate the estimated maximal uncertainties. These enthalpies of formation values correspond to bond dissociation enthalpies of DH{sub 298}{degree}(BrH{sub 2}C-H) = 101.7 {+-} 1.4 kcal mol{sup {minus}1} and DH{sub 298}{degree}(BrClHC-H) = 98.7 {+-} kcal mol{sup {minus}1}.« less