Systematic study of the effect of small-molecules on the low-frequency Raman spectrum of water

Abstract

Low-frequency Raman, which is Raman spectroscopy focused onto the wavenumber region below 100 cm -1 , has gained considerable interest over the last decades due to better availability of instrumentation. However, so far it has mostly been restricted to the investigation of solid structures, where clear features are present. For dissolved, especially aqueous, samples, the use of low-frequency Raman is hindered by a lack of sharp features and strong background signal of water. Nevertheless, by influencing hydrogen bonding, dissolved species affect the low-frequency spectrum of water. To use these changes analytically, a systematic knowledge of the effects is necessary. Therefore, we present a systematic study of the effect of small molecules ranging from apolar to salts on the low-frequency spectrum of water. Changes to the hydrogen-bonding associated vibrations could be detected for all molecules that correlate well with their physicochemical properties. Furthermore, by employing two-dimensional correlation spectroscopy, it could be revealed that the changes to the hydrogen bonding environment often follow more complicated complex mechanisms, which helps to understand the mechanism behind hydrogen bonding in heterogenous systems. The present study is a first step to understand the low-frequency spectra of aqueous solutions.