Ultrafast dynamics of water and aqueous ions studied by two dimensional infrared spectroscopy

Water, one of the classical elements of ancient Greece and China, is a ubiquitous molecule because of its duality—namely, its simplicity (three-atom composition) and simultaneous complexity (hydrogen bond networks). Its uniqueness has fascinated researchers and philosophers for millennia. From scientific efforts, we now know that water plays a vital and active role in many chemical reactions; for example, an enzyme\u27s functionality can be mediated by water, and water is thought to facilitate protein folding. However a full understanding of this molecule has yet to be attained, as questions remain regarding the cooperative behavior of water molecules, the differential dynamics of water in varied environments, and so on. Among these, insight into the ultrafast dynamics of water and water-ion interactions is of great importance both because of fundamental theoretical interest as well as its potential to bridge the gap between our understanding of bulk water interactions and the more complex water-macromolecules interactions. These issues are challenging since the dynamics of water occurs in the regime of tens to hundreds of femtoseconds. Over past decades, two dimensional infrared spectroscopy (2D IR), driven by the desire to have sharp dynamical and structural probes of fundamental and photo-biological systems has been proven a suitable and favorable technique for tackling ultrafast problems. We have successfully utilized this technique to broaden our understanding of water dynamics. Two main topics are cover in this work, in the first three chapters, a terse but self-consistent theory of 2D IR and experimental setups are presented for background knowledge. Subsequently, research on the water-ion dynamics of three different ions (CN-, N3-, [C(ND2)3]+ is presented. It is revealed that the cyanide vibrational mode undergoes spectral diffusion on ultrafast time scales (with a time constant of ∼ 470 fs) which indicates a highly flexible solvation shell. For azide ions in neat water, there is a significant positive correlation of the two frequency distributions (azide asymmetric stretch and OH stretch of water molecule) that decays on a 140 fs time scale which suggests that the vibrational excitation of bound water is decoupled from bulk water system. For guanidinium ions, vibrational energy redistribution has been found among two nearly degenerate modes and the experimental results suggest the formation of hetero-ion complexes in concentrated solution. In total, these findings contribute towards realizing a comprehensive knowledge of water-mediated ultrafast dynamics