Crossed-Beams and Theoretical Studies of the O(3P) H2O → HO2H Reaction Excitation Function.

Hyperthermal collisions of ground-state atomic oxygen with H2O have been investigated, with special attention paid to the H-atom elimination reaction, O(3P) H2O(X 1A1) → HO2(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-1to a peak at ∼115 kcal mol-1and then decreases at higher energies as secondary dissociation of the internally excited HO2product becomes dominant. The calculations show that the cross section for H-atom elimination (O H2O → HO2H) is about 10−25% that of the H-atom abstraction (O H2O → OH OH) cross section for collision energies in the 70−160 kcal mol-1range. [ABSTRACT FROM AUTHOR]