卤代甲烷的键离解能与取代基效应

将芳环上取代基的电子效应参数引入卤代甲烷, 以卤代甲烷分子Y-CH_nX_{3－n (n＝0～3; Y＝H, F, Cl, Br, I; X＝F, Cl, Br, I)中Y-C键的标准键焓 与中心C原子相键连原子的场/诱导效应之和ΣFi、共轭效应之和ΣRi以及诱导偶极之和Σ(α×F)为参数, 建立了一个定量估算卤代甲烷分子中Y-C键离解能(BDE)的通用模型, BDE(Y-C)＝57.5460＋0.8855 －101.0780ΣRi－64.8390ΣFi－10.1034Σ(α×F). 对35个C-H, C-F, C-Cl, C-Br和C-I键回归分析结果表明, 估算Y-C键离解能的精度在实验误差范围内. 对外部数据集的预测结果表明, 该模型具有较高的预测精度, 可用于预测还没被实验测定的卤甲烷中Y-C键离解能. 还对卤代甲烷中104个C-Y键的键离解能进行了预测. 将芳环上取代基效应用于研究饱和体系化学键性能, 有利于深入理解取代基效应对化学键性能的影响.}

Bond Dissociation Enthalpy of Halomethanes and the Substituent Effect

The electrostatic effect constants for the substituents in an aromatic system were used firstly to study the bond dissociation enthalpy (BDE) of a halomethane system Y-CH_nX_3-n (n＝0, 1, 2; 3; Y＝H, F, Cl, Br and I; X＝F, Cl, Br and I). Four parameters, the average bond energy of bond Y—C in the halomethane molecules, the sums of the field/induction effect ΣF_i and the conjugated effect ΣR_i of the groups (or atoms) binding with the carbon atom of the radical •CH_nX_3－n, and the sum of the induced dipole Σ(α×F), were employed, and a general model was obtained for estimating BDE of Y—C bond in the molecule Y-CH_nX_3－n: BDE_(Y—C)＝57.5460＋0.8855 －101.0780ΣR_i－64.8390ΣF_i－10.1034Σ(α×F). For the 35 Y—C bonds, the estimating error using the above model is within the experimental error bars. The predictive ability of the model was tested by the external data sets, and the predicted results showed that the model had a good prediction accuracy (the root square error was 6.77 kJ/mol). Thus, the model can be employed with confidence to predict the BDE for the Y—C bonds in Y-CH_nX_3－n whose experimental values are unavailable yet. The BDE values of other 104 Y—C bonds were predicted and tabulated. The present method provides a new avenue to study the BDE of the saturated system with the electrostatic effect constants for the substituents in an aromatic system, and it is helpful to understand the effect of substituent electrostatic effects on the property of a chemical bond.