To better understand the kinetic mechanisms of nitrate photolysis, the effects of ionic strength in solution has on nitrate ion symmetry breaking is needed. Nitrate ions are observed in different solvation (symmetric and less symmetric) geometries in aqueous environments. FTIR spectra of aqueous nitrate ion solution were obtained at a series of temperatures with different total electrolyte concentrations. The different electric fields (arising from water molecules and other ions in solution) in a bulk solution of nitrate break the trigonal planar geometry of the nitrate ion. Two solvation geometries manifest as a temperature-dependent difference in intensities between the split-degeneracy asymmetric-stretching frequencies in the IR spectrum. The spectrum is fit to two Gaussian peaks; their areas are used as a proxy for the relative ratio of each solvation geometry. A Van’t Hoff plot is created to observe the differences in enthalpy of the two solvation geometries. The difference in enthalpy between the two geometries is determined to be linearly proportional to the ionic strength of the solution. As the ionic concentration increases, one geometry is favored over the other. Electronic structure calculations suggest that the more symmetric geometry is favored at all concentrations, but that higher-ionic-strength solutions increase this stability relative to the less-symmetric geometry. The ratio of the nitrate ion solvation geometries can help explain the ionic-strength dependence on photolysis yields which have been reported by other researchers, and may have other implications for atmospheric aerosols.

Temperature and electric field dependence of asymmetric stretching of nitrate ion

253rd National Meeting (2017)

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