Ab initio barrier heights and branching ratios of isomerization reactions of a branched alkyl radical

The factors influencing the rate of isomerization of allyl radicals is investigated using ab initio calculations on the example of the 2-methylhexyl radical. The equilibrium geometries of the isomers and the transition structures of 16 isomerization channels connecting them are determined at the UHF/6-31G* level. The isomerization energies and barrier heights are calculated at the MP-SAC2/6-311G** level. The most stable isomer is the tertiary radical, less stable are the secondary isomers, and the least stable are the primary isomers of the 2-methylhexyl radical, the largest energy difference being about 3.5 kcal mol(-1). The heights of the barriers separating the isomers depend on the relative location of the radical center before and after the reaction. The barrier height for 1,2 as well as 1,3 H atom transfer is about 37-40 kcal mol(-1), that for the 1,4, 1,5, and 1,6 isomerizations is lower, about 20, 13, and 15 kcal mol(-1), respectively. The height of the barrier, and, accordingly, the activation energy vary by about 2 or 3 kcal mol(-1) depending on the substitution in the ring of the cyclic transition structure and the concomitant change of the reaction enthalpy. Our RRKM calculations show that the fastest isomerization reaction is the 1,5 H atom transfer taking place through a six-membered cyclic transition structure. The relative importance of 1,4 and 1,6 H atom transfers to that of 1,5 isomerization, however, being dependent on the pressure and temperature, may not be negligible, and they together may exceed 30%.