Dissecting the THz spectrum of liquid water from first principles via correlations in time and space

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 107; no. 27; pp. 12068 - 12073
• Main Authors: Heyden, Matthias, Sun, Jian, Funkner, Stefan, Mathias, Gerald, Forbert, Harald, Havenith, Martina, Marx, Dominik, Klein, Michael L.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 06.07.2010; National Acad Sciences
• Subjects: Absorption; Absorption spectra; Algorithms; Chemical reactions; Computer Simulation; Contrapuntal motion; Frequencies; Heavy water; Hydrogen; hydrogen bonding; Hydrogen bonds; Infrared radiation; Kinetics; Liquids; Models, Chemical; Molecular structure; Molecules; Physical Sciences; proteins; Resonance; Solutes; Solutions - chemistry; Solvation; space and time; Spectral correlation; Spectrophotometry, Infrared - methods; Spectroscopy; Spectrum analysis; Tetrahedrons; Time Factors; Vibration; Water; Water - chemistry
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.0914885107

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≈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 ≈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.