Calculations at the (14,10)CASSCF/6-31G** and the MR-(S)DCI/cc-pVTZ levels
are employed to answer the title question by studying three possible modes
of reaction between dioxygen and dihydrogen molecules at the ground triplet
state and excited singlet state of O-2. These reaction modes, which are an
alogous to well-established mechanisms for oxidants such as transition meta
l oxene cations and mono-oxygenase enzymes, are the following: (i) the conc
erted addition, (ii) the oxene-insertion, and (iii) the hydrogen abstractio
n followed by hydrogen rebound. The "rebound" mechanism is found to be the
most preferable of the three mechanisms. However, the barrier of the H-abst
raction step is substantial both for the triplet and the singlet states of
O-2, and the process is highly endothermic (>30 kcal/mol) and is unlikely t
o proceed at ambient conditions. The calculations revealed also that the lo
west singlet state of O-2 has very high barriers for reaction and therefore
cannot mediate a facile oxidation of H-2 in contrast to transition metal o
xenide cation catalysts and monooxygenase enzymes. This fundamental differe
nce is explained.