[HiSOR x WPI-SKCM² Seminar] Prof. Lars G.M. Pettersson (Stockholm University): X-Ray Spectroscopy to Probe Water Structure
I will describe ongoing theoretical and experimental work using XAS, XES and EXAFS to elucidate the character of local structures in ambient liquid water. A consequence of a two-state picture of water with a coexistence line between high- (HDL) and low-density liquid (LDL) that terminates at a critical point is that ambient water can be viewed as a supercritical liquid, i.e. the single-phase region beyond a critical point with fluctuations related to HDL and LDL. Here, I will describe our efforts to use X-ray techniques to determine the local H-bonded structures of these species at ambient conditions and their possible consequences to chemistry and biology.
XAS: We have performed PIMD simulations of ambient water using the MB-pol force-field and constructed the resulting distributions of structural descriptors (LSI, tetrahedrality, asymmetry, G5, Tanaka z). We have built a genetic algorithm to select from the ~245,000 individual centroid environments in the simulation ~1,500 structures that reproduce the distributions from the full set. Computing XAS for these 1,500 (times 16 beads) 32-molecules clusters using TDDFT with our recently calibrated functional2, we find that we still need to modify the PIMD MB-pol distributions using SpecSwap-RMC in order to match XAS.
XES: We have performed full Kramers-Heisenberg calculations of XES on a 2D OH-stretch grid using water pentamers in tetrahedral (“LDL”) and asymmetric (“HDL”) geometry to analyze the importance of interference effects and various vibrational states in the intermediate core- and final valence-hole states. We find that the 3a1 state becomes near-degenerate with the 1b1 as the O-H is stretched, but its contribution in the 1b1 region disappears when vibrational interference is included.
EXAFS: We have performed EXAFS measurements of Argon in water at four different temperatures (4, 15, 25, and 45 °C). Combining EXAFS calculations using FEFF and SpecSwap-RMC to match experiment we determine the coordination shell around solvated Argon to ~20 molecules in a likely dodecahedral cage. The assumption is that Argon acts as a neutral probe of existing LDL-like environments, but alternative hypotheses are also considered.