Aug 2026· Crystal Growth & Design· 0 citations· 38 references
Abstract
Hydrates are common solid forms that can significantly affect a compound’s stability, physicochemical properties, and commercial viability. Despite their importance in pharmaceutical development, hydrate structures, their relationship to material properties, and their propensity for formation remain poorly understood. In 2003, Gillon et al. introduced a simple yet powerful framework for the structural classification of hydrates based on the hydrogen-bonding environment of crystalline water molecules. This framework categorizes water molecules according to the number of hydrogen-bond donor (D) and acceptor (A) interactions in which they participate, giving rise to eight distinct environments. In the original study, the DDA environment, where water donates two hydrogen bonds and accepts one, was reported as the most common. The statistical distribution of these environments in the Cambridge Structural Database (CSD) has since provided a useful benchmark for the qualitative assessment of hydrate structures. Here, we revisit Gillon’s water environment classification using a substantially expanded data set comprising 13,881 hydrate entries from the CSD and provide updated statistics on hydrate structures. Our analysis confirms that the DDA environment remains the most prevalent, followed by DDAA and DD, while the remaining environments occur less frequently. Extending beyond the original work, we quantify the energetics associated with each environment and demonstrate that the four-hydrogen-bond DDAA environment is energetically the most favorable, followed by environments involving three, two, and one hydrogen bonds, respectively. We further show that water-mediated intermolecular interactions contribute up to 40% of the total lattice interaction energy in the hydrate structures, highlighting the surprisingly large influence of this small solvent molecule on crystal stability. Despite its superior energetic stability, the DDAA environment is not the most frequently observed experimentally. This apparent discrepancy arises because the hydrogen-bonding capability and topology of the main component in the hydrate constrains the maximum hydrogen-bonding environment that water can achieve. Overall, this work provides a comprehensive analysis of hydrate structures and energetics across the CSD. The resulting insights offer a valuable framework for the qualitative assessment of newly discovered hydrate forms and for evaluating whether their structures conform to established crystallographic trends.
Hydrophobic hydration underpins processes central to chemistry, biology, and technology, from protein folding and molecular self-assembly to catalysis and nanomaterial dispersion. However, direct structural information on water surrounding large hydrophobic molecules remains scarce due to their very low solubility. Here, we overcome this limitation by using low-temperature vapor codeposition to form amorphous mixtures of water and anthracene, a hydrophobic “mini-graphene”. We then resolve the structure of anthracene’s first hydration shell by neutron diffraction. The linear polycyclic aromatic hydrocarbon induces distinct structural motifs in its hydration shell. Water molecules above and below the outer aromatic rings exhibit one broken water–water hydrogen bond enabling weak O–H···π interactions with the solute, while a pronounced “dewetting” effect emerges at the central ring. These findings provide experimentally constrained structural insights into the hydration around a large hydrophobe under conditions where conventional liquid-state experiments are not feasible.
Gustavo A. Madrigal, Christos Vasilopanagos, T. Headen et al.· Journal of Physical Chemistr...· 0 citations
In recent decades, old ideas of liquid water as a mixture of two distinct states, even at room temperature, resurged fueled by x-ray results that can be interpreted as fluctuating patches of high-density and low-density liquids. Isosbestic points in vibrational spectra indicate more strongly and more weakly hydrogen-bonded molecules, not necessarily patches. Despite mutual agreement among the fractions of the two states obtained from these experiments and with thermodynamic (two-state) and molecular models, chemical (example: water auto-dissociation) and interfacial (example: water surface tension) properties varying with temperature and salt content currently show opposite trends with the postulated fractions, which can therefore not be reconciled with a simple mixing model of the two states, whereas some physical properties (example: water static dielectric constant) can. Consequently, if the tale of two liquids were to be ultimately accepted and thermodynamic models for aqueous electrolyte solutions would need to be revised, then a simple mixing model would be insufficient with high probability.
Johannes Lützenkirchen· Colloids and Interfaces· 0 citations
Amino acid behavior in water-mediated processes is not well captured by hydrophobicity or side-chain class alone, because residues with similar hydropathy can differ in charge localization, geometry, surface exposure, and hydration response. The objective of this study is to determine whether integrating complementary molecular representations can reveal a chemically interpretable molecular coordinate that organizes amino acid behavior across distinct water-mediated processes. We fused electronic-, structural-, and solvation-level representations into a similarity network and embedded it spectrally to obtain a one-dimensional coordinate, Z F, which captures coupled variation in electrostatics-, geometry-, and hydration-related features. Across gas hydrate formation, ice recrystallization inhibition, and CaCO3 crystallization, Z F provides a consistent residue-level coordinate and captures interaction trends that differ in direction from those described by the Kyte–Doolittle hydropathy scale in the latter two systems.
Yusung Ok, Y. Park· Journal of Chemical Informat...· 0 citations
Water’s unique solvating properties and its interactions with hydrophobic surfaces play crucial roles in various chemical processes ranging from self-assembly to phase separation. This study investigates the structure and hydrogen bonding of water in ternary solvent systems composed of water, a cosolvent (acetone, dimethylformamide, or dimethyl sulfoxide), and an oil (2-hexanone or 2-pentanone) to reveal how solvent organization changes upon oil addition. Using Raman spectroscopy coupled with multivariate curve resolution, we probe changes in hydrogen bonding as a function of ternary composition. Counterintuitively, replacing the cosolvent with oil while maintaining constant water levels reduces the volume of perturbed water within hydrophobic hydration shells. We also find a higher population of dangling hydroxyl groups upon oil addition. We propose that these results are indicative of the formation of nanoscale, oil-rich aggregates (nanophases) dispersed in water. This work offers insights into the role of hydrophobic hydration in ternary solvents, contributing to our understanding of nanophase formation, solvent behavior, and interfacial phenomena in complex mixtures.
Binish Ashfaq, Chun-Ting Lin, Paul S. Cremer et al.· Journal of the American Chem...· 0 citations
Understanding the stability of small transition-metal clusters in solution is critical for their catalytic applications, yet the role of the solvent environment remains insufficiently resolved. In this study, classical molecular dynamics simulations are used to examine the solvation, structural organization, and agglomeration behavior of palladium clusters (Pd3 and Pd4) in water, N-methyl-2-pyrrolidone, and their solution. The results reveal two distinct stabilization mechanisms: water forms hydrogen-bonded solvation shells that are relatively diffuse and dynamic, facilitating rapid cluster agglomeration, whereas N-methyl-2-pyrrolidone stabilizes the clusters through strong coordination of its carbonyl oxygen, generating dense and well-defined solvation layers that inhibit aggregation. In their mixed solvents, N-methyl-2-pyrrolidone preferentially occupies the first solvation shell, displacing water to outer regions where it interacts primarily with N-methyl-2-pyrrolidone rather than the cluster. This redistribution produces a more stable solvation environment than the pure solvents. Cluster size further influences this behavior. Pd4 accommodates a denser coordinating shell and shows greater resistance to aggregation than Pd3. Solvent composition is shown to provide a means of tuning cluster evolution, with water promoting agglomeration and fragmentation processes and N-methyl-2-pyrrolidone retarding these effects, thereby enabling control over cluster size and stability. These findings provide molecular insight into the solvent-mediated stabilization of metal clusters and establish a framework for rational design of solvent environments to control their size.
Alok Ranjan, Kaushik Sachan, Raghav Shrimali et al.· Journal of Chemical Physics· 0 citations
The understanding of the properties of advanced materials requires a deep knowledge of chemical bonding beyond the classical chemical bonding picture of covalent, ionic, and metallic bonds. For this purpose, we need to include multicenter bonds into the equation. Here, we approach multicenter bonds in discrete molecular systems by resorting to experimental structural data of finite non-branched chain-like molecules and ions formed by main-group non-metallic elements (X) with s- and p-type valence electrons, as obtained from the Cambridge Structural Database. Using very basic concepts and methods, we demonstrate that multicenter bonds in Xn molecules with n ≥ 3 can be understood with the help of current knowledge of two-center bonds. We evidence that some of the studied chains exhibit covalent bonds, while others exhibit two types of multicenter bonds: electron-rich multicenter bonds (ERMBs), best known as three-center four-electron (3c–4e) bonds or hyperbonds, and electron-deficient multicenter bonds (EDMBs), whose better known example is the three-center-two-electron (3c–2e) bond. Interestingly, linear Xn molecules for n > 3 do not exhibit ERMBs, but “hybrid” multicenter bonds, which consist of a concatenation of ERMBs and EDMBs, where the ratio of EDMBs to ERMBs increases with n; i.e., the electron deficiency in the chain increases with n. Consequently, infinite linear atomic chains feature exclusively EDMBs (even for electron-rich elements). The above observations suggest that there is an inherent limitation to the formation of linear atomic molecules for n > 3 with only ERMBs. All these observations fully agree with the recently proposed unified theory of multicenter bonding, which suggests that multicenter bonds are the missing link between weak secondary bonds and strong primary bonds and promotes a unitary vision of chemical bonds (both in molecules and solids).
M. Savastano, C. Echeverría-Arrondo, S. C. Lemos et al.· Journal of Applied Physics· 0 citations
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