Encounters of H and D atoms with O2 in water: Relative diffusion and reaction rates.

摘要

Diffusive encounters of H and D atoms with O(sub 2) in water are investigated with the time-domain EPR free induction decay attenuation technique. Given the paramagnetic triplet ground state of the O(sub 2) molecule, it is expected that all H-O(sub 2) encounters will contribute to spin dephasing, regardless of whether reaction to form HO(sub 2) occurs. In H(sub 2)O the second-order spin-spin dephasing rate of H in the presence of O(sub 2) is 1.94 (times) 10(sup 10) M(sup (minus)1) sec(sup (minus)1) at 25(degrees)C, with an activation energy of 11.22 (+-) 0.33 kJ/mole between 8 and 60(degrees)C. In a mixture of 90% D(sub 2)O and 10% H(sub 2)O, H atom dephasing is marginally slower in the same temperature range. Dephasing of D atoms in 90% D(sub 2)O is ca. 5--10% slower than H, indicating that diffusion of D is slower than H. The results are analyzed in terms of other available data concerning H and O(sub 2) diffusion and the reaction rate in water. It seems clear that neither the Stokes Einstein hydrodynamic theory nor classical activated rate theory applies to the diffusion of light hydrophobic gases in water. The authors tentatively conclude that H-O(sub 2) spin exchange is slightly less efficient than can be predicted in the theoretical diffusion limit. (ERA citation 18:033318)