Early Steps in the O₂ Scavenger Process in the Aqueous Phase: Hydrazine vs DEHA

We investigate the first direct proton abstraction reactions from reducing agents (RAHs) hydrazine and diethyl hydroxylamine (DEHA), toward dioxygen (O₂) in the aqueous phase, spanning ambient to high-temperature conditions. Quantum chemistry methods and molecular dynamics simulations are employed in this study. Quantum chemistry methods are used to analyze the quasi-equilibrium between a reactive conformation and a transition state in the [RAH,O₂] cluster. On the other hand, molecular dynamics simulations estimate the probability of observing a reactive conformation of the [RAH,O₂] cluster in the solution. In this study, we assume that the energy barrier of the quasi-equilibrium is sufficiently high for the RAH/O₂ association process to be at equilibrium. Our findings indicate that the first proton abstraction process from a reactive conformation cluster by DEHA is energetically favored compared to hydrazine. Conversely, the association process of hydrazine and O₂ in solution is more favorable than that of DEHA. Consequently, the rate constant for the first proton abstraction process is similar for both hydrazine and DEHA, particularly at high temperatures, with activation energies of approximately 21.5 ± 1.5 kcal mol^-1 for both compounds. These results align with recent experiments investigating the complete O₂ scavenger process in liquid water with hydrazine and DEHA. Therefore, our findings support the assumption that first proton abstraction reactions are the rate-determining steps in O₂ scavenger processes in the aqueous phase.