Breakdown of the Born-Oppenheimer approximation in the F+o-D2→DF+D reaction

The reaction of F with H-2 and its isotopomers is the paradigm for an exothermic triatomic abstraction reaction. In a crossed-beam scattering experiment, we determined relative integral and differential cross sections for reaction of the ground F(P-2(3/2)) and excited F*(P-2(1/2)) spin-orbit states with D-2 for collision energies of 0.25 to 1.2 kilocalorie/mole. At the lowest collision energy, F* is similar to 1.6 times more reactive than F, although reaction of F* is forbidden within the Born-Oppenheimer (BO) approximation. As the collision energy increases, the BO-allowed reaction rapidly dominates. We found excellent agreement between multistate, quantum reactive scattering calculations and both the measured energy dependence of the F*/F reactivity ratio and the differential cross sections. This agreement confirms the fundamental understanding of the factors controlling electronic nonadiabaticity in abstraction reactions.