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Infrared spectra of clusters of protonated nitric acid and water exhibit a marked change with cluster size, indicating that an intracluster reaction occurs with sufficient solvation. In small clusters, H2O binds to a nitronium ion core, but at a critical cluster size the NO+2 reacts. A lower bound of 174 kcal/mol is found for the proton affinity of HNO3.
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Cao et al. (1993) studied this question.
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