Infrared spectroscopy of [H2O–Xn]+ (n = 1–3, X = N2, CO2, CO, and N2O) radical cation clusters: competition between hydrogen bond and hemibond formation of the water radical cation

Abstract

The water radical cation H₂O⁺ is an important intermediate in radiation chemistry and radiobiology, and its role in radical reactions has recently attracted much attention. However, knowledge of intermolecular interactions of H₂O⁺ remains very limited due to its high reactivity. We focus on structures of [H₂O–X]⁺, formed by H₂O⁺ with a counter molecule X, as a model for intermediates in reactions of H₂O⁺. Such structural information provides the basis for understanding reaction processes of H₂O⁺. Two structural motifs for [H₂O–X]⁺ have been known: hydrogen bond and hemibond, which are expected to have very different reactivities from each other. Due to the high acidity of H₂O⁺, the H-bonded form is mostly considered to be preferred. However, it has recently been reported that the hemibonded form is preferred in some cases. We perform infrared photodissociation spectroscopy and quantum chemical calculations on [H₂O–X_n]⁺ (n = 1–3, X = N₂, CO₂, CO, and N₂O) to determine their structural motifs. The competition between the hydrogen bond and hemibond formation is systematically examined based on the firm structure information. The competition is interpreted in terms of the proton affinity (PA) and the ionization potential (IP) of X. The rough ranges of PA and IP for the priority of the hemibond motif are determined. The impact of other factors on the competition is also discussed.