几种卤代烃分子的C-X（X：Cl，Br）键离解能和键长的计算

用HF／6—31G（d），B3LYP／6—31＋＋G＊和MP2／6—311＋＋G＊＊方法，对CH3CH2Cl，CH2CHCl，CH3CH2Br,CH2CHBr分子中的C—Cl，C—Br键离解能进行计算，通过比较研究可知，计算CH3CH2Cl，CH2CHCl，CH2CHBr分子中的C—X键离解能时，利用B3LYP／6—31＋＋G＊方法更可靠；而计算CH3CH2-Br分子的键离解能时，选择MP2／6—311＋＋G＊＊方法可信．用以上二种方法对本文的四种分子的平衡几何结构进行优化，求出所需的键长，通过比较可知，用MP2／6—311＋＋G＊＊方法优化的平衡几何结构更可信．用MP4／6—311＋＋G＊＊方法对上述分子进行了势能面扫描，研究了C—X键的断裂过程．

Calculations of Bond Dissociation Energies and Bond Lengths of C-X(X:Cl,Br)for Several Kinds of Halogenohydrocarbon Molecules

The bond dissociation energies of C-Cl, C-Br for four molecules of CH3CH2Cl, CH2CHCl, CH3CH2Br, CH2CHBr are computed using the levels of HF/6-31G （d） and B3LYP/6-31＋＋G＊ and MP2/6-311＋＋G＊＊. By com- parison, the B3LYP/6-31 ＋＋G＊ is found to give the best results of C-X bond dissociation energies for CH3CH2Cl, CH2CHCl and CH2CHBr molecules. And then the MP2/6-311 ＋＋G＊＊ is the most reliable method for the evaluation of the bond dissociation energies of CH3CH2-Br molecules. The equilibrium geometries of the four molecular systems are optimized to compute their bond lengths using the above two ways. We find that the equilibrium geometries optimized using MP2/6-311＋＋G＊＊ are more reliable. The C-X bond rupture process is explored by scanning a potential energy surface by the levels of MP4/6-311＋＋G＊＊.