Vibrational Spectroscopic Study of Contact Ion Pairing between Zn++ and NO3− in Water

Raman and infrared spectra of zinc nitrate have been measured in the presence of various amounts of water. In the presence of NH4NO3, a liquid solution was studied which at room temperature contained but five H2O molecules per zinc ion. Spectral features characteristic of complexed nitrate and water were compared with those from various aqueous solutions of zinc nitrate, solid zinc nitrate hexahydrate, anhydrous zinc nitrate, ammonium nitrate, and pure water. The evidence obtained, both from spectral lines characteristic of complexed nitrate ions and those of hydrated zinc ions, shows the existence of contact ion pairs in the concentrated solutions. The symmetry of the complexed nitrate ion is lowered to C2v by association with a Zn++ ion through a single O atom, as shown by Raman line polarization data. Absence of Zn–O Raman frequencies indicates essentially ionic binding.