Charge transfer reactions between gas-phase hydrated electrons, molecular oxygen and carbon dioxide at temperatures of 80–300 K

Abstract

The recombination reactions of gas-phase hydrated electrons (H₂O)_n˙⁻ with CO₂ and O₂, as well as the charge exchange reaction of CO₂˙⁻(H₂O)_n with O₂, were studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry in the temperature range T = 80–300 K. Comparison of the rate constants with collision models shows that CO₂ reacts with 50% collision efficiency, while O₂ reacts considerably slower. Nanocalorimetry yields internally consistent results for the three reactions. Converted to room temperature condensed phase, this yields hydration enthalpies of CO₂˙⁻ and O₂˙⁻, ΔH_hyd(CO₂˙⁻) = −334 ± 44 kJ mol⁻¹ and ΔH_hyd(O₂˙⁻) = −404 ± 28 kJ mol⁻¹. Quantum chemical calculations show that the charge exchange reaction proceeds via a CO₄˙⁻ intermediate, which is consistent with a fully ergodic reaction and also with the small efficiency. Ab initio molecular dynamics simulations corroborate this picture and indicate that the CO₄˙⁻ intermediate has a lifetime significantly above the ps regime.