The vibrational spectra of the smallest ammonium–halide salt cations, (NH4X)NH4+, and their trimeric aggregates (NH4X)3NH4+ are studied in the spectral range from 700 to 4200 cm-1 using cryogenic ion trap vibrational spectroscopy . Here, we compare the gas phase infrared spectra of the ammonium salts series: ammonium chloride, ammonium bromide, and ammonium iodide, to investigate the ionic strength of the ammonium-halide interactions. This qualitative comparison is made by analyzing the red-shifted NH stretching mode involved in the cation-anion bond observed in infrared photodissociation (IRPD) measurements as well as ab initio computations provides new molecular insights into the fundamental conecpts of acid-base chemistry. Anharmonic vibrational frequencies were computed to assign a number of features observed in the experiment were otherwise unassignable. The (NH4X)3NH4+ cations form a three-dimensional structure which can be directly compared to the structure of other molecular and atomic salt clusters. While this work is a simple model for investigating the early stage of aerosol growth of ionic species in Earth’s atmosphere, these results provide an alternative approach to measuring lattice energies of CsCl-type salts.

Spectral fingerprint of ammonium halide complexes by cryogenic ion trap vibrational spectroscopy

2019 Spring National Meeting

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