The geometries of transition states and adducts have been optimized in MP2/
6-311++G** calculations on both the singlet and triplet potential energy su
rfaces for the reaction of NO3 with OH. Single point calculations have been
made at the G2MP2 level. Reaction is shown to be most likely to occur on t
he singlet surface via an adduct whose structure is very similar to that of
pernitric acid, HO2NO2. The formation of the adduct is exothermic, and the
calculated enthalpy of activation is -8 kJ mol(-1). This adduct can dissoc
iate to HO2 + NO2 directly or via rotational isomers. Reaction on the tripl
et surface is unlikely to be important because the formation of a triplet a
dduct has a large enthalpy of activation.