Spectra vis-à-vis Molecular Motion. Non-linear Vibrational and X-ray Spectroscopic Studies of Interfacial and Liquid Water Using Molecular Simulations

Liquid water's intricate hydrogen-bonding network produces complex vibrational and electronic signatures that challenge experimental interpretation, necessitating molecular simulations to resolve microscopic structure and dynamics. Establishing reliable computational protocols remains difficult due to functional dependencies, dispersion errors, and the delicate sensitivity of free O-H groups at heterogeneous interfaces. Concurrently, interpreting ultrafast time-resolved X-ray scattering experiments requires robust theoretical frameworks to connect scattering observables directly to non-equilibrium molecular motion. This thesis benchmarks density functional approximations and dispersion corrections for the air–water interface and formulates a theoretical scheme for time-resolved nonlinear X-ray spectroscopy. Using ab initio molecular dynamics, generalized gradient approximations (GGA), meta-GGA, and hybrid functionals—paired with empirical and nonlocal van der Waals (vdW) corrections were evaluated against surface tension, interfacial water structure, and sum frequency generation (SFG) spectra. Base functionals primarily dictate accuracy: revPBE-D3(0) and revPBE0-D3(0) significantly outperform BLYP and PBE in reproducing experimental SFG profiles, with empirical dispersion performing comparably or superiorly to nonlocal schemes. In parallel, an X-ray Raman pump-X-ray diffraction probe framework was constructed to track charge migration and solvation dynamics around an excited chromophore using classical non-equilibrium molecular dynamics. The inner solvation shell response is dominant within first ~100 fs, whereas bulk solvent restructuring evolves past 1 ps and dominates the total scattering signal sixfold over solute-solvent contributions. Overall, these findings establish validated computational protocols for interfacial water and provide an operative predictive methodology for ultrafast X-ray liquidography.

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