Hydrogen Bonding in Liquid Ammonia

Ab Initio Molecular Dynamics Simulations of Amino Acids and Their Ammonia-Based Analogues in Ammonia

α-Amino acids are the fundamental building blocks for complex molecular structures within the water-based biochemistry of Earth. In a hypothetical ammonia-based biochemistry, α-amino amidines may serve an equivalent role. In this study, we explore the basic properties of α-amino amidines in comparison to α-amino acids solvated in ammonia, utilizing ab initio molecular dynamics simulations. The investigation of the time-resolved molecular dipole moment reveals, in intricate detail, the relationship among the conformation, state, and magnitude of the dipole moment. Moreover, it allows for the tracking of proton-transfer reactions. In ammonia, α-amino acids tend to adopt an anionic state, with the zwitterionic state still being accessible. In contrast, the α-amino amidines remain neutral. Grotthuss diffusion is induced by the deprotonation of zwitterionic alanine. The charge transferred upon solvation serves as an indicator for the interaction strength between the solute and solvent. It is much stronger for α-amino acids, while, on average, the α-amino amidines exchange no charge with ammonia. The analyses reveal that in terms of structure, anionic α-amino acids behave very similarly to neutral α-amino amidines.

Size-Dependent Effects of Electron Solvation on the Kinetics of Ammonia Revealed on the Molecular Scale

The high relevance of electron solvation in several branches of physics, chemistry, and environmental science arises from its efficient electron transfer mechanism. The effect of solvated electrons on solvent structure has been considered local and transient due to a lack of real-space studies. An experiment was designed to study the impact of solvated electrons on the ammonia structure while adsorbed to Cu(110) using low-temperature scanning tunneling microscopy with an adjoined femtosecond laser. The enhanced molecular kinetics induced by the solvated electrons are explained using density functional theory and first-principles molecular dynamics. The electrons have a substantially different impact on the kinetics of ammonia within clusters below and above a cluster size threshold, reflecting hydrogen bond rearrangement (mass transport) and hydrogen bond cleavage (desorption), respectively. This size-dependent effect has implications on the efficiency of processes that involve solvated electrons. Altering the solvent structure more than transiently demands the subsequent solvation of two electrons.

Encapsulating Proton Inside C60 Fullerene: A Density Functional Theory Study on the Electronic Properties of Cationic X+@C60 (X+ = H+, H3O+ and NH4+)

Confining protons into an enclosed carbon cage is expected to give rise to unique electronic properties for both the inner proton and the outer cage. In this work, we systematically investigated the geometric and electronic structures of cationic X+@C60 (X+ = H+, H3O+, and NH4+), and their corresponding neutral species (X = H2O, NH3), by quantum chemical density functional theory calculations. We show that C60 can trap H2O, NH3, H3O+ and NH4+ at the cage center and only slightly influence their geometries. The single proton clings to the inner wall of C60, forming a C-H chemical bond. The encapsulated neutral species almost do not change the electronic structure of the C60, while the internal cations have obvious effects. The charge transfer effect from the inner species to the C60 cage was found for all X@C60 (X = H2O, NH3) (about 0.0 e), X+@C60 (X+ = H3O+, NH4+) (about 0.5 e) and H+@C60 (about 1.0 e) systems. Encapsulating different forms of protons also regulates the fundamental physico-chemical properties of the hollow C60, such as the HOMO-LUMO gaps, infrared spectra, and electrostatic potential, etc., which are discussed in detail. These findings provide a theoretical insight into protons' applications, especially in energy.

The incorporation of chitosan nanoparticles enhances the barrier properties and antifungal activity of chitosan-based nanocomposite coating films

The potential alternative of exploring the development of nanocomposites through a single-molecule approach, such as combining chitosan nanoparticles (ChiNP) with chitosan (Chi), remains to be investigated. To maintain the insolubility of the ChiNP filler in the system, the protonation of weakly basic amino groups necessitates the pH of the coating solution above the pKa (6-6.5). This study aimed to evaluate the biofunctional properties improvements of Chi coatings incorporated with ChiNP as filler agents. The coating film forming solution comprised of 0.8 % Chi combined with varying concentrations (0 %, 0.1 %, 0.5 %, and 1 %) of ChiNP. The morphology of ChiNP was characterized via atomic force spectroscopy (AFM). Incorporating the ChiNP (1 %) significantly enhanced antifungal efficacy, i.e., an 88.28 % reduction in fungal activity compared with the control group, and a 65 % reduction compared with pure Chi against Botrytis cinerea. The incorporation of ChiNP improved the ultraviolet and visible light wavelengths, water vapor permeability, hydrophobicity, and thermal properties. Scanning electron microscopy and AFM were performed to assess the surface and internal microstructures of the coating. The findings of this study suggested that the nanocomposite coatings herein presented is potential for use in active packaging, especially in the context of preserving fresh fruit products.

The structure of water-ammonia mixtures from classical and ab initio molecular dynamics

The structure of aqueous ammonia solutions is investigated through classical molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulations. We have preliminarily compared three well-known classical force fields for liquid water (SPC, SPC/E, and TIP4P) in order to identify the most accurate one in reproducing AIMD results obtained at the Generalized Gradient Approximation (GGA) and meta-GGA levels of theory. Liquid ammonia has been simulated by implementing an optimized force field recently developed by Chettiyankandy et al. [Fluid Phase Equilib. 511, 112507 (2020)]. Analysis of the radial distribution functions for different ammonia concentrations reveals that the three water force fields provide comparable estimates of the mixture structure, with the SPC/E performing slightly better. Although a fairly good agreement between MD and AIMD is observed for conditions close to the equimolarity, at lower ammonia concentrations, important discrepancies arise, with classical force fields underestimating the number and strength of H-bonds between water molecules and between water and ammonia moieties. Here, we prove that these drawbacks are rooted in a poor sampling of the configurational space spanned by the hydrogen atoms lying in the H-bonds of H2O⋯H2O and, more critically, H2O⋯NH3 neighbors due to the lack of polarization and charge transfer terms. This way, non-polarizable classical force fields underestimate the proton affinity of the nitrogen atom of ammonia in aqueous solutions, which plays a key role under realistic dilute ammonia conditions. Our results witness the need for developing more suited polarizable models that are able to take into account these effects properly.

Quantum cluster equilibrium theory applied to liquid ammonia

Through this paper, the authors propose using the quantum cluster equilibrium (QCE) theory to reinvestigate ammonia clusters in the liquid phase. The ammonia clusters from size monomer to hexadecamer were considered to simulate the liquid ammonia in this approach. The clusterset used to model the liquid ammonia is an ensemble of different structures of ammonia clusters. After studious research of the representative configurations of ammonia clusters through the cluster research program ABCluster, the configurations have been optimized at the MN15/6-31++G(d,p) level of theory. These optimizations lead to geometries and frequencies as inputs for the Peacemaker code. The QCE study of this molecular system permits us to get the liquid phase populations in a temperature range of 190-260 K, covering the temperatures from the melting point to the boiling point. The results show that the population of liquid ammonia comprises mainly the ammonia hexadecamer followed by pentadecamer, tetradecamer, and tridecamer. We noted that the small-sized ammonia clusters do not contribute to the population of liquid ammonia. In addition, the thermodynamic properties, such as heat of vaporization, heat capacity, entropy, enthalpy, and free energies, obtained by the QCE theory have been compared to the experiment given some relatively good agreements in the gas phase and show considerable discrepancies in liquid phase except the density. Finally, based on the predicted population, we calculated the infrared spectrum of liquid ammonia at 215 K temperature. It comes out that the calculated infrared spectrum qualitatively agrees with the experiment.

Impact of Sacrificial Hydrogen Bonds on the Structure and Properties of Rubber Materials: Insights from All-Atom Molecular Dynamics Simulations

The inclusion of sacrificial hydrogen bonds is crucial for advancing high-performance rubber materials. However, the molecular mechanisms governing the impact of these bonds on material properties remain unclear, hindering progress in advanced rubber material research. This study employed all-atom molecular dynamics simulations to thoroughly investigate how hydrogen bonds affect the structure, dynamics, mechanics, and linear viscoelasticity of rubber materials. As the modified repeating unit ratio (β) increased, both interchain and intrachain hydrogen bond content rose, with interchain bonds playing a predominant role. This physical cross-linking network formed through interchain hydrogen bonds restricts molecular chain movement and relaxation and raises the glass transition temperature of rubber. Within a certain content of hydrogen bonds, the mechanical strength increases with increasing β. However, further increasing β leads to a subsequent decrease in the mechanical performance. Optimal mechanical properties were observed at β = 6%. On the other hand, a higher β value yields an elevated stress relaxation modulus and an extended stress relaxation plateau, signifying a more complex hydrogen-bond cross-linking network. Additionally, higher β increases the storage modulus, loss modulus, and complex viscosity while reducing the loss factor. In summary, this study successfully established the relationship between the structure and properties of natural rubber containing hydrogen bonds, providing a scientific foundation for the design of high-performance rubber materials.

Neutron scattering and neural-network quantum molecular dynamics investigation of the vibrations of ammonia along the solid-to-liquid transition

Vibrational spectroscopy allows us to understand complex physical and chemical interactions of molecular crystals and liquids such as ammonia, which has recently emerged as a strong hydrogen fuel candidate to support a sustainable society. We report inelastic neutron scattering measurement of vibrational properties of ammonia along the solid-to-liquid phase transition with high enough resolution for direct comparisons to ab-initio simulations. Theoretical analysis reveals the essential role of nuclear quantum effects (NQEs) for correctly describing the intermolecular spectrum as well as high energy intramolecular N-H stretching modes. This is achieved by training neural network models using ab-initio path-integral molecular dynamics (PIMD) simulations, thereby encompassing large spatiotemporal trajectories required to resolve low energy dynamics while retaining NQEs. Our results not only establish the role of NQEs in ammonia but also provide general computational frameworks to study complex molecular systems with NQEs.

Emerging trends in research and development on earth abundant materials for ammonia degradation coupled with H2 generation

Ammonia, as an essential and economical fuel, is a key intermediate for the production of innumerable nitrogen-based compounds. Such compounds have found vast applications in the agricultural world, biological world (amino acids, proteins, and DNA), and various other chemical transformations. However, unlike other compounds, the decomposition of ammonia is widely recognized as an important step towards a safe and sustainable environment. Ammonia has been popularly recommended as a viable candidate for chemical storage because of its high hydrogen content. Although ruthenium (Ru) is considered an excellent catalyst for ammonia oxidation; however, its high cost and low abundance demand the utilization of cheaper, robust, and earth abundant catalyst. The present review article underlines the various ammonia decomposition methods with emphasis on the use of non-noble metals, such as iron, nickel, cobalt, molybdenum, and several other carbides as well as nitride species. In this review, we have highlighted various advances in ammonia decomposition catalysts. The major challenges that persist in designing such catalysts and the future developments in the production of efficient materials for ammonia decomposition are also discussed.