Two-dimensional terahertz/IR spectroscopy: a unique probe of ultrafast hydrogen-bond dynamics of liquid water and model systems

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Hydrogen bonds are the key to understanding aqueous chemistry and nearly all of biochemistry. Of course, hydrogen bonds determine the structure of pure water and give it many of its bizarre properties such as its density maximum at 4 degrees C and its expansion on freezing. Hydrogen bonds between amino-acid residues and cooperative effects determine the structure of peptides and proteins. Finally yet importantly, the surfaces of proteins and enzyme binding pockets are often solvated by water and there are strong indications that the protein changes the structure of the surrounding water while the water changes the dynamics of the protein. Protein-bound water appears to have many properties like that of crystalline or perhaps glassy water and often even shows up in X-ray crystal structures. It is therefore vital to study the hydrogen-bond structure and dynamics of supercooled water and especially glassy water, which can be made through nano-confinement. It is also essential to study the structure and dynamics of peptide model systems in order to be able to isolate the effects of hydrogen-bond dynamics and cooperativity.Hydrogen-bond dynamics takes on many forms. Equilibrium dynamics ranges from hydrogen-bond bend and stretch modes with periods of ~0.2-0.5 ps to diffusive translational and rotational relaxation as slow as 8 ps. Non-equilibrium dynamics (such as after the excitation of an OH-stretch mode) involve the breaking of hydrogen bonds on a ~1-2 ps timescale. These processes are difficult to study with one technique. Infrared and dielectric techniques suffer from limited frequency ranges. Raman scattering techniques suffer from weak signals and turn out not to be sensitive to rotational motion in water.Here we propose the development of the little-used ultrafast spectroscopy technique of terahertz-field-induced second-harmonic generation (TFISH). It is known to yield large signals in water and covers the entire range from rotational diffusion to hydrogen-bond bends and stretches. Moreover, it lends itself to be expanded into a multi-dimensional spectroscopy (2D-TFISH) that can be used to measure non-equilibrium dynamics. This would give us the unique opportunity to study the effects of non-equilibrium relaxation dynamics (such as the relaxation of OH-stretch and bend modes) on hydrogen bonds directly. The techniques will be used on supercooled water (bulk or confined in silica nanopores) to determine the presence of a liquid-liquid phase transition at ~220K and for the first time the effects of the phase transition on the structure and dynamics of water. They will also be used on peptide model systems such as N-methylacetamide and analogues to study cooperativity and non-equilibrium relaxation dynamics.