Ab initio study of the transition states for determining the enthalpies of formation of alkyl and halogenated alkyl free radicals

Abstract

Enthalpies of formation of 19 free radicals at 298 K are calculated by ab initio methods. The equilibrium reactions of radical +HBr⇄molecule+Br were studied by optimizing the transition states of the reactions at the MP2(fc)/6-31G(d,p) level of theory. The ab initio calculated threshold energies of the reverse reactions were combined with the experimentally determined activation energies of the forward reactions in a second-law method to determine the enthalpies of the reactions. The enthalpy of formation values are (in kJ mol⁻¹): 149.3±2.8 (CH₃), 172.5±3.7 (CH₂Br), 121.7±4.5 (CH₂Cl), 180±5 (CHBr₂), 145±8 (CHBrCl), 92.2±3.9 (CHCl₂), 195±5 (CBr₃), 163±8 (CBr₂Cl), 124±8 (CBrCl₂), 70.6±3.7 (CCl₃), 120.4±2.7 (CH₃CH₂), 81.1±2.9 (CH₃CHCl), 99.7±2.9 (CH₂ClCH₂), 46.0±3.2 (CH₃CCl₂), 90.1±3.2 (CHCl₂CH₂), 102.5±2.8 (n-C₃H₇), 83.6±2.8 (iso-C₃H₇), 29.9±3.0 (2-iso-C₃H₆Cl ), 80.7±2.8 (n-C₄H₉). The chemical nature of the transition states was studied by localizing them along the minimum energy path of the reaction.