An experimental and theoretical study of the reactions NaO+H2O(D2O)→NaOH(D)+OH(OD)

Abstract

The title reactions were studied by the pulsed laser photolysis of a mixture of Na vapour, H₂O (D₂O) and N₂O. NaO, formed from the reaction between Na and N₂O, was monitored by time-resolved chemiluminescence at 589 nm (Na(3²P_J–3²S_1/2)), generated from the reaction between atomic O and NaO. This yielded k(NaO+H₂O, T=260–716 K)=(4.4_-1.6^+2.4)×10^-10 exp(-(507±178)/T) and k(NaO+D₂O, T=257–725 K)=(4.2_-1.4^+2.1)×10^-10 exp(-(606±153)/T) cm³ molecule^-1 s^-1. Abinitio quantum calculations on the reaction potential energy surfaces were then performed using the complete basis set (CBS-Q) model of Petersson and co-workers (J. Chem. Phys., 1996, 104, 2598). These calculations show that: the reaction is essentially thermoneutral, ΔH₀^°(NaO+H₂O→NaOH+OH)=0.0±7.5 kJ mol^-1; the small kinetic isotope effect most likely arises from the deuterated reaction being about 2 kJ mol^-1 more endothermic; and both the ²A″ and ²A′ surfaces contain Na(OH)₂ adducts, the most strongly bound of which is 88.2 kJ mol^-1 below the reactants (including zero-point energies). An RRKM model incorporating the inverse Laplace transformation method of Pilling and co-workers (J. Phys. Chem. A, 1997, 101, 9681) was then developed to investigate the effect of these adducts on the kinetics. Finally, the role of these reactions in the chemistry of sodium in the mesosphere, and in the removal of N₂O in combustion systems, is briefly considered.