Spin non-conservation in low-energy molecular charge-transfer reactions

The reaction H₂O⁺(²B)+NO₂(²A) → H₂O(¹A) + NO₂⁺(¹Σ) occurs at near the collision rate constant 1.2 × 10^?9 cm³ s^?1, in spite of the fact that the reactants produce both a singlet and a triplet state and the products correlate only with the singlet state. This would be expected to yield a statistical weight factor of $\text{1}\text{4}$ to be multiplied by the collision rate constant to obtain the maximum charge-tranfer rate constant. The triplet products of the charge transfer are clearly endothermic. The singlet—triplet intersection has not been identified but the available information about the singlet and triplet states of the intermediate protonated nitric acid molecule is discussed. Four other examples of apparent “spin violation” charge-transfer reactions have been noted H₂O⁺ + NO, N₂O⁺ + NO.CO⁺ + NO and CH₄⁺ + O₂.