Crossed-beam reactive scattering of F2 plus C6H6: Heat of formation of ipso-fluorocyclohexadienyl radical

Angular distributions and relative total cross sections were measured for products of the collisions of F2 with C6 H6 at relative collision energies of 14 to 26 kcal mol-1 . The most conspicuous product is the ipso-fluorocyclohexadienyl radical i-C6 H6 Fts, which displays a rather narrow peak very near the center-of-mass angle at all collision energies studied. Product C6 H5 F is masked at Me = 96 by dissociative ionization of C6 H6 Fts near the center-of-mass angle, but becomes observable at smaller angles to which little or no C6 H6 Fts is scattered. This means that at least one reaction C6 H6 +F2 →C6 H5 F+HF occurs, and does so in such a way that the products acquire high translational energies. The dependence on collision energy of the total cross section for the production of C6 H6 Fts was probably measured, and displays a well-marked onset at 13.9 +- 0.3 kcal mol-1. The center-of-mass system means final state translational energy as a function of collision energy was obtained from the angular distributions and goes to zero at 13.9 +- 0.5 kcal mol-1 . This probably means that the onset is a true threshold, from which the heat of formation of the ipso-fluorocyclohexadienyl radical is calculated to be 14.8 +- 0.4 kcal mol-1. The mean final state translational energy asymptotically approaches 3.7 kcal mol-1 as the collision energies are increased, which is so large that only a fraction of the vibrational modes of C6 H6 Fts can be involved in the energy partitioning