Oxidation of alkali-metal atoms with nitrous oxide: Molecular mechanisms from first principles calculations

The reactions of nitrous oxide with alkali-metal atoms were studied theoretically by means of CASSCF(11/12)/MR-MP2 calculations. Critical points on the electronic ground-state (2)A' potential energy surface (PES) were determined and characterized by harmonic vibrational frequencies at the CASSCF(11/12) level. The molecular mechanism is rather consistent involving in each reaction two distinct entrance channels. Small barriers of 5.9 kcal/mol for Li + N2O, 1.6 kcal/mol for Na + N2O, and 1.2 kcal/mol for K + N2O are predicted for the lower energy reaction channel, which is mainly determined by the NNO bending vibrational mode nu(2). These barriers originate as a result of strong avoided crossings between the two lowest (2)A' PESs that gradually transform the character of the electronic ground state from a neutral to an ionic one in going from reactants to products. Along this channel, the ground-state PES is fairly isolated and the reaction is adiabatic. By way of contrast, the reaction along its higher energy channel occurs via nonadiabatic electron transfer from alkali metal to N2O in the region of the 1(2)A'/2(2)A' conical intersection and likely contributes to the observed non-Arrhenius behavior of the reaction rates. These findings provide clarifying insights into the nature of a number of previous kinetic and spectroscopic observations.