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Contra‐thermodynamic Behavior in Intermolecular Hydrogen Transfer of Alkylperoxy Radicals
Author(s) -
Pfaendtner Jim,
Broadbelt Linda J.
Publication year - 2007
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200700161
Subject(s) - chemistry , radical , exothermic reaction , reactivity (psychology) , isobutane , enthalpy , intermolecular force , alkane , propane , hydrogen atom abstraction , hydrogen , methane , computational chemistry , thermodynamics , photochemistry , hydrocarbon , molecule , organic chemistry , catalysis , medicine , physics , alternative medicine , pathology
Quantum chemical investigation of bimolecular hydrogen transfer involving alkylperoxy radicals, a key reaction family in the free‐radical oxidation of hydrocarbons, was performed to establish structure–reactivity relationships. Eight different reactions were investigated featuring four different alkane substrates (methane, ethane, propane and isobutane) and two different alkylperoxy radicals (methylperoxy and iso‐ propylperoxy). Including forward and reverse pairs, sixteen different activation energies and enthalpies of reaction were used to formulate structure–reactivity relationships to describe this chemistry. We observed that the enthalpy of formation of loosely bound intermediate states has a strong inverse correlation with the overall heat of reaction and that this results in unique contra‐thermodynamic behavior such that more exothermic reactions have higher activation barriers. A new structure–reactivity relationship was proposed that fits the calculated data extremely well: E A = E o + α Δ H rxn where α =−0.10 for Δ H rxn <0, and α=1.10 for Δ H rxn >0 and E o =3.05 kcal mol −1 .