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NEAR INFRARED STUDY OF HYDRATION OF AQUEOUS HYDROXIDE **

Abstract

The near infrared (NIR) and visible spectrum of the hydroxide anion (OH-) has been measured in aqueous solutions. We observe broad spectroscopic bands due to hydrogen-bonded water along with relatively narrow transitions due to hydroxide. All of the observed spectroscopic transitions are due to excitation of multiple quanta of O-H stretching vibrations. We have observed hydroxide stretching vibration transitions from 0 - 3 up to 0 - 6. Including data from previous infrared and Raman experiments yields the first determination of the spectroscopic constants, we, wexe and weye wfor the aqueous OH- anion. In careful measurements in the region of the 0-3 transition of aqueous OH-, we observe a new spectral transition that we attribute to the overtone spectrum of water molecules that are strongly hydrogen-bonded to the OH- anion. In further experiments, we have observed changes in this spectrum as a function of the OH- concentration, and we attribute the change in the spectrum to be due to changes in the number of water molecules that hydrogen bond to OH-, suggesting that the hydration number (n) of OH⁻(H₂O)ₙ is not constant but concentration-dependent. These results indicate that the hydration of OH⁻ ions in aqueous solutions are more complex than previously assumed, with implications for understanding solvation and ion-water interactions.

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