Infrared spectroscopy of [H 2 O-X n ] + ( n = 1-3, X = N 2 , CO 2 , CO, and N 2 O) radical cation clusters: competition between hydrogen bond and hemibond formation of the water radical cation.

The water radical cation H 2 O + 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 H 2 O + remains very limited due to its high reactivity. We focus on structures of [H 2 O-X] + , formed by H 2 O + with a counter molecule X, as a model for intermediates in reactions of H 2 O + . Such structural information provides the basis for understanding reaction processes of H 2 O + . Two structural motifs for [H 2 O-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 H 2 O + , 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 [H 2 O-X n ] + ( n = 1-3, X = N 2 , CO 2 , CO, and N 2 O) 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.