On the thermodynamics of homolytic C—H bond cleavage in linear and branched alkanes: Comparison of DFT and composite G4 and G4(MP2) methods

This work provides a systematic theoretical study on gas-phase bond dissociation enthalpies for homolytic C—H bond cleavage in linear and branched alkanes. Quantum chemical calculations were performed using the density functional theory (DFT) and ab initio composite (G4(MP2), G4) methods. In case of DFT calculations, the M06-2X and B3LYP functionals combined with 6-311++G**, aug-cc-pVDZ, aug-cc-pVTZ, and aug-cc-pVQZ basis sets were covered. In linear molecules, the best agreement with experiment was observed for B3LYP/aug-cc-pVQZ. In both composite approaches, as the number of C atoms increases, the BDE values are affected by the systematic error of the method. For example, G4 BDE of terminal —CH3 group of hexane is 422.6 kJ·mol⁻¹ while for undecane it is 415.5 kJ·mol⁻¹. On the contrary, the composite methods are in better agreement with experiments compared to the DFT approaches for simple branched hydrocarbons.