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

✍ Scribed by Xénophon Krokidis; Nigel W. Moriarty; William A. Lester Jr.; Michael Frenklach

• Publisher: John Wiley and Sons
• Year: 2001
• Tongue: English
• Weight: 525 KB
• Volume: 33
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.10004

✦ Synopsis

Abstract

Quantum Monte Carlo and a series of other ab initio as well as density functional theory calculations were performed for the enthalpy of formation of C~4~H~3~ and C~4~H~5~ radicals. The computed Δ~f~H~298~^0^ values, in kcal/mol, are 126.0 for n‐C~4~H~3~, 119.4 for i‐C~4~H~3~, 83.4 for n‐C~4~H~5~, and 76.2 for i‐C~4~H~5~, all with one standard deviation of 0.6 kcal/mol. The enthalpy differences between the n and i isomers of C~4~H~3~ and C~4~H~5~ are predicted to be substantially lower than those obtained in recent theoretical studies. The nature of the middle CC bond in these radicals was examined using the electron localization function topological analysis performed by bonding evolution theory for partitioning the molecular space into regions with clear chemical meaning. This analysis shows that the n isomers are represented by a unique Lewis 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 signify higher prominence of the even‐carbon‐atom reaction pathways in the formation of the first aromatic ring in hydrocarbon pyrolysis and oxidation, consistent with the past kinetic modeling and recent experimental studies. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 808–820, 2001