Spectroscopic fingerprints in the low frequency spectrum of ice (Ih), clathrate hydrates, supercooled water, and hydrophobic hydration reveal similarities in the hydrogen bond network motifs

Abstract

Solid phases of water, such as ice (Ih) and clathrate hydrates, form characteristic hydrogen bond network motifs, such as hexagonal ice, pentagons, and dodecahedrons. The same motifs might be present in supercooled water and in the hydration structure around hydrophobes. Here, we present the characteristic low frequency fingerprints of ice (Ih), tetrahydrofuran (THF) clathrate hydrates, and tetrabutyl-ammonium bromide (TBAB) semiclathrate close to their melting point, as well as supercooled water at 266.6 K and aqueous alcohol solutions. Interestingly, we find in all these cases two characteristic resonances in the THz frequency range: at least, one intensive band in the frequency range between 190 cm−1 and 220 cm−1 which is a characteristic of a tetrahedral hydrogen bond network configuration and a second band in the frequency range between 140 cm−1 and 170 cm−1, indicating a component with weaker hydrogen bonds. For solvated alcohols, we find spectroscopic fingerprints of a clathratelike structure at 164 cm−1 as well as a tetrahedral network structure at 194 cm−1, which is close to one of ice (Ih) at 192 cm−1. We propose that in the hydration shell of hydrophobes, both structural motifs are present. In the case of supercooled water—unlike ice—only one peak was found in the frequency range between 190 cm−1 and 220 cm−1. Interestingly, the latter peak center-frequency (204 cm−1) corresponds to the average of those of the two peaks observed for ice Ih (191 cm−1 and 215 cm−1). This indicates a homogeneous intermediate hydrogen bonding, providing no evidence for any heterogeneity in two high-density and low-density phases.

An isolated water droplet in the aqueous solution of a supramolecular tetrahedral cage

“…2A is very different from the sharp absorption features observed for gas phase water clusters (43). Although the THz absorption spectroscopy is not a direct probe of the structure of a system, these low-frequency spectral signatures are specific fingerprints of the HB network of the isolated water cluster in the [Ga 4 L 6 ] 12− host compared to other water systems (34,38). In previous structural studies on supramolecular hosts in the solid state, encapsulated hydrogen-bonded (H 2 O) 8-10 water clusters were identified to be similar to the smallest subunit of cubic ice (Ic) (26)(27)(28).…”

“…Here, we use terahertz (THz) absorption spectroscopy and ab initio molecular dynamics (AIMD) to characterize low-frequency vibrations and structural organization of water in the nanoconfined environment. THz is ideally suited to probe the intermolecular collective dynamics of the water hydrogen bond (HB) network with extremely high sensitivity, as illustrated for different phases of water (33)(34)(35)(36)(37)(38), and for aqueous solutions of salts, osmolytes, alcohols, and amino acids (36,(39)(40)(41)(42). The THz spectra of the water inside the nanocage has been quantitatively reproduced with AIMD, allowing us to confidently characterize the water network in the cage in order to provide a more complete dynamical, structural, and thermodynamic picture.…”

Probing Local Electrostatics of Glycine in Aqueous Solution by THz Spectroscopy

“…To separate the spectral contributions of the different protonation states of Gly, as well as those due to the co‐solvents HCl and NaOH, we employed a principal component analysis (PCA) approach. PCA allows us to dissect the spectra into their individual components with respect to one variable, here the co‐solvent concentration [27–29] . A more detailed description of the titration studies of glycine as a function of NaOH and HCl concentrations is presented in the SI.…”

The water molecule arrangement over the side chain of some aliphatic amino acids: A quantum chemical and bottom‐up investigation

“…On the other hand, there are evidences that the hydrophobe imposes some geometrical constraints to water molecules in the immediate neighbors that reduce the configurational space with respect to water molecules in the bulk . Very recently, THz/FIR absorption spectroscopy studies suggested that hydration water layer around hydrophobes might contain clathrate‐like structural motifs …”

“…Hydrophobicity is one of the most important and one of the most discussed issues of structural biology . In spite of the enormous experimental and modeling efforts, many microscopic aspects related to the nature of forces and action mechanism are still not well understood.…”