Unexpected dynamical effects change the lambda-doublet propensity in the tunneling region for the O( 3 P) + H 2 reaction.

One of the most relevant features of the O(P-3) + H-2 reaction is that it occurs on two different potential energy surfaces (PESs) of symmetries A ' and A " that correlate reactants and products. The respective saddle points, which correspond to a collinear arrangement, are the same for both PESs, whilst the barrier height rises more abruptly on the (3)A ' PES than on the (3)A " PES. Accordingly, the reactivity on the (3)A " PES should be always higher than on the (3)A ' PES. In this work, we present accurate quantum-scattering calculations showing that this is not always the case for rotationless reactants, where dynamical factors near the reaction threshold cause the (3)A ' PES to dominate at energies around the barrier. Further calculation of cross sections and ?-doublet populations has allowed us to establish how the reaction mechanism changes from the deep tunneling regime to hyperthermal energies.