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A quantum Monte Carlo study of energy differences in C 4 H 3 and C 4 H 5 isomers *
Author(s) -
Krokidis Xénophon,
Moriarty Nigel W.,
Lester William A.,
Frenklach Michael
Publication year - 2001
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.10004
Subject(s) - chemistry , enthalpy , ab initio , radical , ring (chemistry) , computational chemistry , ab initio quantum chemistry methods , conjugated system , density functional theory , atom (system on chip) , crystallography , molecule , thermodynamics , physics , organic chemistry , polymer , computer science , embedded system
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 CC 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 CC 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