We have studied the hydration dynamics of trimethylamine N-oxide (TMAO) in aqueous solution using a combination of concentration-dependent terahertz/far-infrared (THz/FIR) and Raman spectroscopic techniques. Terahertz/FIR absorption was measured using narrowband (76-93 cm(-1)) p-Ge laser and broad band (30-400 cm(-1)) Fourier transform spectroscopy. We used principal component analysis in combination with a semi-ideal chemical equilibrium model to dissect the spectra into linear and nonlinear contributions of the solvated solute extinction. We attribute the linear part to the average extinction and Raman scattering of TMAO-water aggregates with approximately 3-4 water strongly hydrogen bonded to TMAO. An additional nonlinear concentration dependence indicates a decrease of the number of attached water molecules with increasing TMAO concentrations due to a shift in association equilibria. The Raman spectra reveal a frequency shift of the (narrowband) intramolecular vibrations with decreasing dilution. Based on the results of a detailed analysis and isotopic substitution, the experimentally observed absorption bands at 0, 176, and 388 cm(-1) could be assigned to water relaxation modes, an intermolecular TMAO-H2O stretch, and the C-N-C bending mode, respectively. Our results provide evidence for a local modification of the water structure.

High-precision THz (30 to 360 cm −1 ) spectra of bulk liquid water are presented from ambient conditions up to hydrostatic pressures of 10 kbar. In concert with ab initio simulations, this allows us to characterize the molecular-level changes of the H−bond network under solvent stress conditions. Both the experimental and theoretical THz spectra reveal a blue shift in the intermolecular translational mode at 180 cm −1 by 40 cm −1 at 10 kbar and a blue shift together with an intensity increase in the relaxation mode. These changes can be traced back to a pressure-induced increase of the population of so-called short H−bond double donor configurations at the expense of those with longer such intermolecular bonds. Distinct electronic polarization effects are critical to capture the characteristic intensity changes of the THz line shape function. These advances in high-pressure THz spectroscopy open the door to investigate the pressure response of solvation shells and solute−solvent couplings.

Trimethylamine N-oxide (TMAO) is a well-known osmolyte in nature, which deep-sea fish use to stabilize proteins against High Hydrostatic Pressure (HHP). We present a combined ab initio molecular dynamics, force...

We present a novel setup to investigate the low frequency (THz/FIR) spectra of an aqueous solution under high hydrostatic pressure (HHP). By integration of a diamond anvil cell into a THz Fourier transform spectrometer, we are able to record the absorption of bulk water in the pressure range from 1 bar to 10 kbar. The difference in intensity can directly be compared to the difference in extinction coefficients. The spectroscopic data reveal a blue shift of the H-bond stretch vibration at 180 cm, which is evidence of changes in the H-bond network dynamics.