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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Theoretical C-H Bond Dissociation Enthalpies for CH_3Br and CH_2CIBr
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Theoretical C-H Bond Dissociation Enthalpies for CH_3Br and CH_2CIBr

机译：CH_3Br和CH_2CIBr的理论C-H键解离焓

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摘要

The enthalpies of formation for the closed-shell molecules CH_3Br and CH_2ClBr, and the free radicals CH_2Br 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 Moller-Plesset perturbation theory (level I), coupled-cluster theory (level II), density functional theory (level III), and Gaussian-2 (G2) theory (level IV). The standard enthalpies of formation values obtained at levels I-IV for both the closedshell and open-shell species agreed within +-1 kcal mol~(-1) irrespective of the working chemical reactions chosen. No basis set effects were observed. These invariances support the relability 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_3Br, CH_2ClBr, and CH_2Br in good agreement with experimental values as well; the calculations supplied the first known enthalpy data for the CHClBr radical. The recommended standard 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_3Br, -8.9 +- 0.8; CH_2ClBr, -11.5 +- 1.1; CH_2Br, 40.7 +_ 1.1; CHClBr, 35.1 +- 1.5 kcal mol~(-1). The error limits designate the estimated maximal uncertainties. These enthalpies of formation values correspond to bond dissociation enthalpies of DHdeg_(298)(BrH_2C-H)=101.7 +- 1.4 kcal mol~(-1) and DH deg_(298_-) (BrClHC-H)=98.7 +- 1.9 kcal mol~(-1).
机译：闭壳分子CH_3Br和CH_2ClBr以及自由基CH_2Br和CHClBr的形成焓是通过使用氢化和等渗反应作为工作化学反应的从头算分子轨道计算来估算的。理论方法（水平）的四个变体和三个扩展的基础集被应用在计算中。这些方法包括四阶Moller-Plesset微扰理论（I级），耦合聚类理论（II级），密度泛函理论（III级）和Gaussian-2（G2）理论（IV级）。无论选择哪种化学反应，在闭壳和开壳物种的I-IV级获得的标准形成焓值均在+ -1 kcal mol〜（-1）范围内。没有观察到基集效应。这些不变性支持所计算数据的可靠性，并且在很大程度上，这是由于由于应用了从头算起的能量与已建立的实验焓相结合的工作化学反应而消除了计算误差。四种方法提供的CH_3Br，CH_2ClBr和CH_2Br的形成焓与实验值吻合良好。计算结果提供了CHClBr自由基的第一个已知焓数据。推荐的标准生成焓（在298.15 K和1.00 atm时）是在I-IV级通过不同的氢化和等渗反应得到的结果的未加权平均值：CH_3Br，-8.9 +-0.8; CH_2ClBr，-11.5±1.1; CH_2Br，40.7±1.1; CHClBr，35.1±1.5kcal mol（-1）。误差极限表示估计的最大不确定性。这些形成值的焓对应于DHdeg_（298）（BrH_2C-H）= 101.7±1.4kcal mol〜（-1）和DHdeg_（298 _-）（BrClHC-H）= 98.7±1.9kcal的键解离焓摩尔〜（-1）。

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