Infrared Photodissociation Spectroscopy of Microhydrated Nitrate-Nitric Acid Clusters NO3-(HNO3)m(H2O)n

Infrared multiple photon dissociation (IRMPD) spectra of NO3-(HNO3)(m)(H2O) (H-2)(z) with m = 1-3, up to n = 8 and z >= 1, are measured in the fingerprint region (550-1880 cm(-1)), directly probing the NO-stretching modes, as well as bending and other lower frequency modes. The assignment of the spectra is aided by electronic structure calculations. The IRMPD spectrum of the m = 1, n = 0 cluster is distinctly different from all the other measured spectra as a result of strong hydrogen bonding, leading to an equally shared proton in between two nitrate moieties (O2NO-center dot center dot center dot H+center dot center dot center dot ONO2-). It exhibits a strong absorption at 877 cm(-1) and lacks the characteristic NO2-antisymmetric stretching/NOH-bending mode absorption close to 1650 cm(-1). Addition of at least one more nitric acid molecule or two more water molecules weakens the hydrogen bond network, breaking the symmetry of this arrangement and leading to localization of the proton near one of the nitrate cores, effectively forming HNO3 hydrogen-bonded to NO3-. Not all IR active modes are observed in the IRMPD spectra of the bare nitrate-nitric acid clusters. Addition of a water or a hydrogen molecule lowers the dissociation limit of the complexes and relaxes (H2O) or lifts (H-2) this IRMPD transparency.