Dynamics and Spectral Response of Water Molecules around Tetramethylammonium Cation.

Till now, there has been ambiguity about the structural heterogeneity inside a solute solvation shell and the dynamical response of the surrounding solvent molecules. To address the dynamics and spectral response of solvent molecules, we performed first-principles molecular dynamics simulations for the comprehensive study of water's hydroxyl stretch frequency evolution due to environmental variations (also called "spectral diffusion") in the vicinity of a hydrophobe, tetramethylammonium (TMA) cation. The N-Ow radial distribution function (RDF), spatial distribution function (SDF), and combined distribution function (CDF) were calculated to provide information about the arrangement of water molecules around TMA. In the probability distribution plot of the cosine of the angle (θ) between Ow-Hw and NTMA-Ow bond vectors, the hydrogen atoms are observed being oriented toward TMA and the oxygen atoms aligned away. The decaying dynamics of the orientation autocorrelation function (OACF) reveals the reorientation time is more inside the solvation shell (4.1 ps) as compared to bulk (2.8 ps), matching with the trend obtained from water's orientational dynamics in tetraalkylammonium salts. Wavelet transform of the obtained trajectory was used to calculate the time-dependent vibrational stretching frequencies of the OH modes of water molecules. The normalized frequency distribution in the aqueous solvation shell of TMA, tagging a particular water molecule within the N-Ow cutoff distance 5.5 Å, displays an intense peak at 3661 cm-1 representing non-hydrogen bonded or dangling or free OH modes. Simulations around aromatic solutes and Raman-MCR studies in the hydration shell of hydrophobic TBA reported a distinctive dangling OH band at 3660 cm-1 (range: 3661 ± 2 cm-1). Besides dangling water molecules in the first hydration layer of ammonium nitrogen, few OH modes are strongly hydrogen-bonded having an average frequency at 3300 cm-1. The predominance of dangling hydroxyl modes around apolar hydrophobic TMA was further explored by comparing the dangling lifetime (∼0.68 ps) with the lifespan of hydrogen bonded OH modes (∼0.48 ps). At our simulation temperature 330 K, a significant fraction of the water molecules in the vicinity of TMA ion are free or dangling, and a few of them form an ordered structure with enhanced hydrogen bonding. Structural analysis, orientation correlation, frequency fluctuations, dangling, and hole dynamics calculations provide the evidence of the existence of dangling OH modes dominating over highly ordered strong hydrogen bonded structure in the cationic TMA solvation shell at an elevated temperature.