The C−H bond dissociation enthalpies (BDEs) of 26 N, O, S-containing mono-heterocyclic compounds were assessed by the composite high-level ab initio methods G4 and CBS-QB3. In addition, the C−H BDEs of 32 heterocyclic compounds were evaluated by 13 density functional theory methods. The BMK method showed the lowest root mean square error of 7.2 kJ/mol and the correlation coefficient (R 2 ) was 0.9874 after being compared with the experimental values. Subsequently, we used this method to study the C−H BDE values of the different positions as well as the substituent effects on benzene and heterocycles in sulfur-containing fused heterocyclic compounds. Both the natural charge distributions of benzo[b]thiophene and benzothiazole and natural spin densities of benzo[b]thiophene derivative radicals were conducted by NBO analysis for further understanding the essence of the C−H bond. 0 5 10 15 20 25 30 35 -80 -60 -40 -20 0 20 40 60 80 BMK KMLYP MPW1KCIS O3LYP PBE1PBE TPSS1KCIS M05 MPWB1K M06-L MPWLYPW WB97X B3LYP M06-2X BDE(Theor.)-BDE(Exp.)(kJ/mol) C-H bond S CN Br 2 (3 equiv),solvent r.t.,18h S CN Br + S CN Br 4 6 S CN + S CN major Br 5 Br 7 minor Scheme 1. Regioselectivity studies on bromination.
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