Crossed molecular beam studies of the reactions of methyl radicals with iodoalkanes

Abstract

The I atom exchange reactions, CH3 +RI→CH3 I+R[R=CF3 , (CH3 )3 C], were investigated at a collision energy of ≈13 kcal/mol using the crossed molecular beams technique. The supersonic beam of methyl radicals was formed by pyrolyzing a mixture of ≈1% di-tert-butyl peroxide in helium in a quartz nozzle. A large fraction of the total energy available to the products from these reactions is channeled into relative translation [≈50% for R=(CH3 )3 C and ≈70% for R=CF3 ] suggesting that the dominant interaction among the products is repulsive. The CH3 I product from both reactions was observed to be entirely backward scattered with respect to the incident radical beam indicating that a roughly collinear C–I–C transition state geometry is favored. The present results are compared to those of earlier crossed beam studies of the CH3 +IY→CH3 I+Y(Y=Cl, Br, I) reactions; the differences observed among these reactions are explained with reference to the CH3 I–Y and CH3 I–R interaction potentials.