Icelike Vibrational Properties of Strong Hydrogen Bonds in Hydrated Lithium Nitrate

The hydrogen bond network accounts for many of the extraordinary physical properties of liquid water and ice. Its vibrational dynamics are quite complex in their entirety but can be accessed in detail by investigating small groups of only a few water molecules. Here, aqueous salt hydrates turned out to be an exceptional model system for water molecules arranged in well-defined geometrical structures that can be accessed by means of femtosecond spectroscopy of the OH stretching vibration. In this study, we find striking resemblance between the vibrational properties of three water molecules connected via strong hydrogen bonds in the trihydrate of LiNO₃ and those of ordinary ice I_h. As in ice, the vibrations of the hydrate water molecules show ultrafast excited state dynamics that are strongly accelerated when proceeding from deuterated to neat H₂O samples. The latter is analyzed by means of an additional relaxation channel that is due to Fermi resonance between the OH stretching vibration and the bend overtone accompanied by delocalization of the vibration over neighboring water molecules in the H₂O species. Moreover, in the hydrate and ice samples severe spectral broadening is examined when comparing fundamental and excited state absorption bands. Here, proton delocalization along the strong hydrogen bonds is given as a possible underlying mechanism.