A comparison of the C‒H bond dissociation enthalpies of sulfur-containing fused heterocyclic compounds to the C‒H bond dissociation enthalpies in other heterocycles

Abstract

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.

Is Vitamin A an Antioxidant or a Pro-oxidant?

“…0.5 kcal/mol, 21 +0.3 kcal/mol for C−C bond 22 and +1.8 kcal/mol for C−H bond of N, O, and S-containing monoheterocyclic compounds. 23 The three most common antioxidant mechanisms including HAT, SETPT, and SPLET which have been widely accepted were investigated in terms of their characteristic thermochemical properties: 24−26 Hydrogen atom transfer (HAT)…”

“…For example, BMK could reliably predict the C–H BDEs with a mean deviation of ca. 0.5 kcal/mol, +0.3 kcal/mol for C–C bond and +1.8 kcal/mol for C–H bond of N, O, and S-containing monoheterocyclic compounds …”