A quantum Monte Carlo study of energy differences in C4H3 and C4H5 isomers

Quantum Monte Carlo and a series of other ab initio as well as density func tional theory calculations were performed for the enthalpy of formation of C4H3 and C4H5 radicals. The computed Delta H-f(298)0 values, in kcal/mol, a re 126.0 for n-C4H3, 119.4 for i-C4H3, 83.4 for n-C4H5, and 76.2 for i-C4H5 , all with one standard deviation of 0.6 kcal/mol.The enthalpy differences between the n and i isomers of C4H3 and C4H5 are predicted to be substantia lly lower than those obtained in recent theoretical studies. The nature of the middle C-C bond in these radicals was examined using the electron local ization function topological analysis performed by bonding evolution theory for partitioning the molecular space into regions with clear chemical mean ing. This analysis shows that the n isomers are represented by a unique Lew is structure while the i isomers are represented by a resonance description , For the latter systems, the middle C-C bond is only mildly conjugated and the corresponding degree of conjugation is calculated. These results signi fy higher prominence of the even-carbon-atom reaction pathways in the forma tion of the first aromatic ring. in hydrocarbon pyrolysis and oxidation, co nsistent with the past kinetic modeling and recent experimental studies. (C ) 2001 John Wiley & Sons, Inc.*