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

Enthalpies of formation of 19 free radicals at 298 K are calculated by ab initio methods. The equilibrium reactions of radical +HBr reversible arrow 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.