Hydrogen bonds (H-bonds) play a significant role in liquid water, aqueous solutions, and bio-systems and, therefore, a deep understanding of H-bonds in liquid phases is of great importance. Although many theoretical and experimental studies have been devoted to understand their properties at ambient conditions, our knowledge of H-bonds under extreme conditions, such as high hydrostatic pressure (HHP) conditions in the kilo-bar regime, is not enough. Especially, the pressure response of aqueous trimethylamine N-oxide (TMAO) is of great interest because it works as a protecting osmolyte against high pressure denaturation. Vibrational spectroscopy is one of the most powerful methods to elucidate structure and dynamics of H-bonds. Concerning the theoretical side, quantitative analysis on IR spectra of H-bonding systems requires not only atomistic trajectories but also accurate dipole moments because the effects of induced dipole moments and charge transfer are not negligible in these cases. In this study, by using ab initio MD simulations, we analyzed IR spectra of TMAO(aq) under HHP conditions. The solvation structure of TMAO-oxygen changes dramatically under HHP conditions; threefold H-bonding structure is dominant at 1 bar, whereas three and fourfold H-bonding structures are almost equally populated at 10 kbar [1,2]. We found that the increase in the number of H-bonds can be observed as a skewness of the N-O stretch spectra since three and fourfold H-bonding structures have slightly different resonance frequencies [3]. 

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