Communication: Vibrational spectroscopy of atmospherically relevant acid cluster anions: Bisulfate versus nitrate core structures

Infrared multiple photon dissociation spectra for the smallest atmospherically relevant anions of sulfuric and nitric acid allow us to characterize structures and distinguish between clusters with a bisulfate or a nitrate core. We find that bisulfate is the main charge carrier for HSO4−·H2SO4·HNO3 but not for NO3−·H2SO4·HNO3. For the mixed dimer anion, we find evidence for the presence of two isomers: HSO4−·HNO3 and NO3−·H2SO4. Density functional calculations accompany the experimental results and provide support for these observations.