Dissecting the THz spectrum of liquid water from first principles via correlations in time and space.
Solvation of molecules in water is at the heart of a myriad of molecular phenomena and of crucial importance to understanding such diverse issues as chemical reactivity or biomolecular function. Complementing well-established approaches, it has been shown that laser spectroscopy in the THz frequency domain offers new insights into hydration from small solutes to proteins. Upon introducing spatially-resolved analyses of the absorption cross section by simulations, the sensitivity of THz spectroscopy is traced back to characteristic distance-dependent modulations of absorption intensities for bulk water. The prominent peak at approximately 200 cm(-1) is dominated by first-shell dynamics, whereas a concerted motion involving the second solvation shell contributes most significantly to the absorption at about 80 cm(-1) approximately 2.4 THz. The latter can be understood in terms of an umbrella-like motion of two hydrogen-bonded tetrahedra along the connecting hydrogen bond axis. Thus, a modification of the hydrogen bond network, e.g., due to the presence of a solute, is expected to affect vibrational motion and THz absorption intensity at least on a length scale that corresponds to two layers of solvating water molecules. This result provides a molecular mechanism explaining the experimentally determined sensitivity of absorption changes in the THz domain in terms of distinct, solute-induced dynamical properties in solvation shells of (bio)molecules--even in the absence of well-defined resonances.
Heyden
Matthias
M
Lehrstuhl für Physikalische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany. martina.havenith@rub.de
Sun
Jian
J
Funkner
Stefan
S
Mathias
Gerald
G
Forbert
Harald
H
Havenith
Martina
M
Marx
Dominik
D
D000465:Algorithms; D003198:Computer Simulation; D007700:Kinetics; D008956:Models, Chemical; D012996:Solutions / Q000737:chemistry*; D013055:Spectrophotometry, Infrared / Q000379:methods*; D013997:Time Factors; D014867:Water / Q000737:chemistry*
D012996:Solutions; D014867:Water