J. Phys. Chem. B, Just Accepted Manuscript
DOI: 10.1021/acs.jpcb.5b04152
Publication Date (Web): July 27, 2015
Copyright © 2015 American Chemical Society
http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.5b04152
We have studied the hydration dynamics of Trimethylamine N-oxide (TMAO) in aqueous solution using a combination of concentration dependent THz/far-infrared and Raman spectroscopic techniques. Terahertz/FIR absorption was measured using narrow band (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 non-linear contributions of the solvated solute extinction. We attribute the linear part to the average extinction/Raman scattering of TMAO-water aggregates with approximately 3–4 water strongly hydrogen-bonded to TMAO. An additional non-linear 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 absorption bands at 0 cm-1, 176 cm-1, 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.
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