CO₂incorporation in hydroxide and hydroperoxide containing water clusters—a unifying mechanism for hydrolysis and protolysis

Abstract

The reactions of CO2 with anionic water clusters containing hydroxide, OH(-)(H2O)n, and hydroperoxide, HO2(-)(H2O)n, have been studied in the isolated state using a mass spectrometric technique. The OH(-)(H2O)n clusters were found to react faster for n = 2,3, while for n >3 the HO2(-)(H2O)n clusters are more reactive. Insights from quantum chemical calculations revealed a common mechanism in which the decisive bicarbonate-forming step starts from a pre-reaction complex where OH(-) and CO2 are separated by one water molecule. Proton transfer from the water molecule to OH(-) then effectively moves the hydroxide ion motif next to the CO2 molecule. A new covalent bond is formed between CO2 and the emerging OH(-) in concert with the proton transfer. For larger clusters, successive proton transfers from H2O molecules to neighbouring OH(-) are required to effectively bring about the formation of the pre-reaction complex, upon which bicarbonate formation is accomplished according to the concerted mechanism. In this manner, a general mechanism is suggested, also applicable to bulk water and thereby to CO2 uptake in oceans. Furthermore, this mechanism avoids the intermediate H2CO3 by combining the CO2 hydrolysis step and the protolysis step into one. The general mechanistic picture is consistent with low enthalpy barriers and that the limiting factors are largely of entropic nature.

Understanding the Formation and Growth of New Atmospheric Particles at the Molecular Level through Laboratory Molecular Beam Experiments

“…NO replaces water molecules into the cluster [259] and reacts to form ONOO − anion and nitrate NO 3 − anion. And ONOO − can be isomerized to NO 3 − , which is supported by theoretical calculations, and the reaction is energetically favorable due to exotherm [260–261] …”

Oxidation of NO˙ by small oxygen species HO2−and O2˙−: the role of negative charge, electronic spin and water solvation

“…Interestingly, these two anions, as well as their water clusters HO 2 À (H 2 O) n and O 2 À (H 2 O) n , are formed when pure water is electrosprayed at atmospheric pressure. 20 It is known that O 2…”

“…24 For HO 2 À (H 2 O) we also observe some degree of dehydration caused by either collision induced dissociation (CID) or by spontaneous evaporation. We expect the former process to be the main contributor to the loss of H 2 O, as this is typically the case for this kind of experiment, 20,35 thus we consider the reaction to be…”

“…This effect is well known in water cluster chemistry (see references mentioned by Ryding et al, in particular Bohme et al). 20,60 This general drop in reactivity with increasing cluster size, and thereby the increase in the degree of solvation of the core ion, is usually understood from a reaction model where the core ion needs to be activated by desolvation before reaction can occur. We find in Fig.…”

“…This is a consequence of the solvation shell around the superoxide ion preventing NO from contacting O 2 À and reacting, an effect that is often observed for water clusters. 20 The addition of NO to the cluster-ion without forming covalent bonds means that NO is susceptible to evaporation, thereby reforming the reactant cluster, especially if the binding energy of NO to the cluster is lower than that of the water molecules. A closer look at the number of water molecules lost in reaction (15) shows that when the superoxide reactant is hydrated by 3-4 water molecules, the typical number of 2H 2 O are lost (Fig.…”