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-met hylhexyl radical. The equilibrium geometries of the isomers and the tr ansition structures of 16 isomerization channels connecting them are d etermined at the UHF/6-31G level. The isomerization energies and barr ier heights are calculated at the MP-SAC2/6-311G* level. The most sta ble isomer is the tertiary radical, less stable are the secondary isom ers, 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 relat ive 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 k cal mol(-1), that for the 1,4, 1,5, and 1,6 isomerizations is lower, a bout 20, 13, and 15 kcal mol(-1), respectively. The height of the barr ier, and, accordingly, the activation energy vary by about 2 or 3 kcal mol(-1) depending on the substitution in the ring of the cyclic trans ition structure and the concomitant change of the reaction enthalpy. O ur RRKM calculations show that the fastest isomerization reaction is t he 1,5 H atom transfer taking place through a six-membered cyclic tran sition structure. The relative importance of 1,4 and 1,6 H atom transf ers to that of 1,5 isomerization, however, being dependent on the pres sure and temperature, may not be negligible, and they together may exc eed 30%.