The Thermodynamic Properties Study On C-H Bond Activation Of Fused-heterocycle Compounds

In recent years, the fused-heterocyclic compounds have been widely used in medicine, organic electroluminescent materials, dyes, pesticides and other fields. Many organic chemical reactions are involved in the C-H bond activities of fused heterocyclic compounds. The knowledge of bond dissociation enthalpy(BDE), which is one of the thermodynamic properties, is fundmental to understand chemical reactions. Therefore, it has been attracted by a lot of chemists.In this paper, the C-H bond BDEs in a large number of heterocyclic compounds were calculated using quantum chemistry methods. The C-H bond experimental BDEs for the 26 kinds of single heterocyclic molecules were evaluated by the composite ab initio methods including G4, G3, G3B3 and CBS-QB3. It was found that the root mean square error(RMSE) of G4 was the smallest one after comparing their RMSEs. The experimental values for the 23 kinds of single heterocyclic molecules were fairly accurate. Then the C-H BDEs for the 23 kinds of single heterocyclic molecules and 9 kinds of fused heterocyclic molecules were calculated using 16 kinds of density functional methods(DFTs). It was found that the RMSE of BMK was only 7.2 k J/mol, which was the best method. Its mean deviation(MD) and mean absolute deviation(MAD) were-1.1 k J/mol and 5.7 k J/mol respectively. The correlation coefficient square R2 between calculated values and experimental values was as high as 0.9874. Furthermore, the BMK method was used to study in the following three aspects:1. In the system of N-fused-heterocyclic compounds: the C-H BDEs of the N-fused-heterocyclic compounds at different positions were calculated using BMK method. It was found that there appeared obvious position effects on the C-H BDEs of quinoline and isoquinoline. The two linear relationships between the C-H BDEs of quinoline and isoquinoline with natural charges q C/e in molecules and with natural charges q C. /e in radicals were found by NBO analysis, and the corresponding correlation coefficients R2 s were 0.9097 and 0.8272 respectively. The reasons for the C(a)-H BDEs gap of quinoline and isoquinoline were conducted by the single occupied molecular orbitals(SOMO). In addition, the substituent effects on C-H BDEs of N-fused-heterocyclic compounds were also discussed by the BMK method. For the electron-donating groups and conjugative effect groups, the correlation coefficients R2 s between the C(α)-H BDEs of quinoline and isoquinoline and q C(α)/e were 0.8222 and 0.9285 respectively.2. In the system of O-fused-heterocyclic compounds: the C-H BDEs of the O-fused-heterocyclic compounds at different positions were investigated by BMK method. And the NBO analysis for the O-fused-heterocyclic compounds was also conducted using BMK method. In addition, the substituent effects on C-H BDEs of O-fused-heterocyclic compounds were discussed by BMK method. For the electron-withdrawing groups and conjugative effect groups, it was found that there were two linear relationships between C-H BDEs and q O/e. The corresponding correlation coefficients R2 s were 0.8551 and 0.9379 repectively. Furthermore, the natural charge distribution of benzofuran, benzdioxan and 7H-Furo[3,2-b]pyran derivatives were selected to explain the reasons of linear relationships.3. In the system of S-fused-heterocyclic compounds: the C-H BDEs of the S-fused-heterocyclic compounds at different positions were studied by BMK method. The substituent effects on the C-H BDEs of benzene rings and heterocyclic rings for the S-fused-heterocyclic compounds were discussed. The benzo[b]thiophene, benzothiazole and benzo[b]thiophene with-NH2 were conducted by NBO analysis.