Effects of Hydrophobic Substituents on Intermolecular Interactions and Terahertz Spectroscopy of Aqueous Solutions of...

We have investigated the terahertz (THz) absorption spectra of aqueous solutions of urea and its dimethyl derivatives (1,1-dimethylurea and 1,3-dimethylurea) through molecular dynamics simulations using polarizable AMOEBA force fields for both water and the solutes. We calculated the total THz absorption spectra for these systems, including the difference absorption spectra by subtracting the contribution of pure water from the respective total absorption spectra of the solutions. The difference absorption spectrum was dissected into solute-solute, water-water, and solute-water contributions. The solute-solute term was further dissected into self- and cross-correlation components. The water-only contribution shows negative features in the THz spectrum due to the disruption of the hydrogen bond network of water, and the effects are more pronounced for the aqueous solution of 1,1-DMU. In the THz spectra arising from solute-water interactions, the aqueous solutions of 1,1-DMU and 1,3-DMU exhibit lower intensities than that of urea, which shows that the dimethyl substitution of urea decreases the interaction of the solutes with water. Additionally, we also calculated the vibrational density of states (VDOS) of the center of mass of the solutes in the solutions and also atom-specific VDOS, which provided further insights into how individual solute atoms contribute to the observed spectral features. Aqueous solutions of the three solutes, namely, urea, 1,1-DMU, and 1,3-DMU, which are considered here, are found to exhibit distinct spectral behaviors originating from dimethyl substitution in the urea molecule. Our analysis revealed that intramolecular vibrational modes of the solute molecules also appear in the terahertz region. The motion of solute molecules as a whole surrounded by hydration water is found to contribute to the spectral modes below 100 cm-1.