Hyperthermal collisions of ground-state atomic oxygen with H 2O have been investigated, with special attention paid to the H-atom elimination reaction, O( 3P) + H 2O(X 1A 1) → HO 2( 2A′) + H( 2S). This reaction was observed in a crossed-beams experiment, and the relative excitation function in the region around its energy threshold (50-80 kcal mol -1) was measured. Direct dynamics calculations were also performed at two levels of theory, B3LYP/6-31G(d,p) and MP2/6-31G(d,p). The shape of the B3LYP excitation function closely matches that of the experiment. The calculations provided a detailed description of the dynamics and revealed a striking dependence of the reaction mechanism on collision energy, where the cross section rises from a threshold near 60 kcal mol -1 to a peak at ∼115 kcal mol -1 and then decreases at higher energies as secondary dissociation of the internally excited HO 2 product becomes dominant. The calculations show that the cross section for H-atom elimination (O + H 2O → HO 2 + H) is about 10-25% that of the H-atom abstraction (O + H 2O → OH + OH) cross section for collision energies in the 70-160 kcal mol -1 range.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry