Solvation Structures and Dynamics of Small Molecules: Experimental and Computational Studies Using Carbonyl Vibrational Modes as Probe

Solutions are ubiquitous in both the global environment and the human body, and play a significant role in scientific research and industrial production. The structures and dynamics of solutions have been studied for centuries. However, conventional experimental methods, whose intrinsic measuring time is on the order of nanoseconds to microseconds, could not detect the fast dynamics taking place in the solution on the timescale of femto- and pico-second. In this dissertation, the ultrafast two-dimensional infrared (2DIR) spectroscopy was applied to characterize the structure and dynamics in three different types of solutions on the sub-picosecond timescale. Linear Fourier transform infrared spectroscopy (FTIR) and computational calculations were performed to confirm the interpretation of the microscopic framework. Firstly, the ion effect on water structural dynamic properties at high concentration was studied by looking into the solvation shell of acetate ion in D2O and in 6M NaCl solution. The FTIR lineshape of the carboxylate asymmetric mode and a dynamics component extracted from 2DIR is not affected by the highly concentrated salts, which is proved to be a particularity of the acetate ion by a comparative study on the azide ion. Ab initio molecular dynamics (AIMD) simulations confirmed the experimental observations and linked the observed vibrational phenomenon to the thermal rotation of acetate methyl group (—CH3). Secondly, ion solvation structure and dynamics in organic solvents were investigated. By AIMD and ab initio umbrella sampling (AIUS), the structure and dynamics of the lithium solvation shell in cyclic and in linear carbonates were compared. The intercalation of entire molecules into the first solvation shell of lithium is found in cyclic carbonate, which leads to a more rigid and more organized solvation structure in cyclic carbonate. Finally, a conductive neutral molecular mixture of acetic acid (HAc) and N- vi methylimidazole (C1Im) was characterized by both experimental and theoretical methods. The broadband peaks in FTIR was assigned as delocalized proton stretching mode in HAc-C1Im complexes. AIMD simulation, DFT calculation, and NMR measurements confirmed the existence of proton delocalization in the hydrogen bond between HAc and C1Im