Microhydration of BH4-: Dihydrogen Bonds, Structure, Stability, and Raman Spectra

Unveiling the Borohydride Ion through Force-Field Development

The borohydride ion, BH4-, is an essential reducing agent in many technological processes, yet its full understanding has been elusive, because of at least two significant challenges. One challenge arises from its marginal stability in aqueous solutions outside of basic pH conditions, which considerably limits the experimental thermodynamic data. The other challenge comes from its unique and atypical hydration shell, stemming from the negative excess charge on its hydrogen atoms, which complicates the accurate modeling in classical atomistic simulations. In this study, we combine experimental and computer simulation techniques to devise a classical force field for NaBH4 and deepen our understanding of its characteristics. We report the first measurement of the ion's activity coefficient and extrapolate it to neutral pH conditions. Given the difficulties in directly measuring its solvation free energies, owing to its instability, we resort to quantum chemistry calculations. This combined strategy allows us to derive a set of nonpolarizable force-field parameters for the borohydride ion for classical molecular dynamics simulations. The derived force field simultaneously captures the solvation free energy, the hydration structure, as well as the activity coefficient of NaBH4 salt across a broad concentration range. The obtained insights into the hydration shell of the BH4- ion are crucial for accurately modeling and understanding its interactions with other molecules, ions, materials, and interfaces.

Understanding the Role of Poly(vinylpyrrolidone) in Stabilizing and Capping Colloidal Silver Nanocrystals

The ligands anchored to the surface of metal nanocrystals play an important role in controlling their colloidal synthesis for a broad spectrum of applications, but it remains a daunting challenge to investigate the ligand-surface and ligand-solvent interactions at the molecular level. Here, we report the use of surface-enhanced Raman scattering (SERS) to extract structural information about the binding of poly(vinylpyrrolidone) (PVP) to Ag nanocubes as well as its conformational changes in response to solvent quality. When a PVP chain binds to the surface of a Ag nanocube through some of its carbonyl groups, the segments between adjacent binding sites are expelled into the solvent as loops. As a result, the carbonyl peak (ν_C═O) resolved in the SERS spectrum includes the contributions from those anchored to the surface and those residing on the loops, with their frequencies located at ν_C═O(Ag) and ν_C═O(free), respectively. While ν_C═O(Ag) remains at a fixed frequency due to the coordination between the carbonyl groups with Ag surface, the spectral position of ν_C═O(free) is dependent on the solvent. As the strength of hydrogen bonding between PVP and solvent increases, the peak position of ν_C═O(free) shifts toward lower frequencies. When exposed to bad and good solvents in an alternating manner, the PVP loops undergo conformational changes between collapsed and extended states, altering the separation between the free carbonyl groups and the Ag surface and thereby the intensity of the ν_C═O peak.

Predicting the size of salt-containing aqueous Na-AOT reverse micellar water-in-oil microemulsions with consideration for specific ion effects

HYPOTHESIS

Reverse micellar solutions are thermodynamically stable systems in which surfactant molecules surround water droplets within a continuous organic phase. Among their many applications, they can be used for the synthesis of nanoparticles of controlled agglomeration. Here, we consider the role specific ion effects play in reverse micelle size reduction.

EXPERIMENTS

Dynamic light scattering measurements and the Gouy-Chapman electrical double layer model were combined to study water/AOT/isooctane reverse micellar systems (w_o = 10). Linear relationships between the solvodynamic diameter (D) of reverse micelles containing various concentrations of FeSO₄, Mg(NO₃)₂, CuCl₂, Al(NO₃)₃, Fe(NO₃)₃, Y(NO₃)₃, NaBH₄, ZrOCl₂, and NH₄OH, and their calculated Debye screening lengths, κ^-1, were observed with decreasing D and increasing salt concentration (c).

FINDINGS

By comparing the linear fits for reverse micelle size as a function of c^-1/2, we determined the size can be described as a function of the Debye screening length, cation valency (z), and specific anion hydrated radius (r_an), where D = 3.1z κ^-1 + b_i, and b_i is linearly related to r_an. Our model accurately predicts reverse micelle sizes with the addition of monovalent, divalent, and trivalent salts for which the primary hydrolyzed cation species has a charge that is equal to the cation valency.

The structural elucidation of aqueous H3BO3 solutions by DFT and neutron scattering studies

The micro-structure of aqueous boric acid (H3BO3) solutions is of broad interest in earth sciences, geochemistry, material science, as well as chemical engineering. In the present study, the structure of aqueous H3BO3 solutions was studied via neutron scattering with 2H and 11B isotope labelling combined with empirical potential structure refinement (EPSR) modelling. In aqueous H3BO3 solutions, B(OH)3 is the dominant borate species. Density function theory (DFT) calculations show that the boron hydroxyl has a lower electrostatic potential (ESP), which makes B(OH)3 a relatively weakly hydrated, compared with the bulk water. In the 0.95 mol L-1 H3BO3 solution at 298 K (saturated), ∼18 water molecules enter the hydration sphere of B(OH)3 with the hydration distance (B-O(W)) of 3.75 Å, while only 4.23 of them hydrate with H3BO3 as the hydrogen bond (H-bond) acceptor or H-bond donor. Both neutron scattering and DFT calculations for 2B(OH)3·6H2O clusters at the ωB97XD/6-311++g(3df,3pd) basis level show that B(OH)3 forms molecular clusters in bidentate contact molecular pairs (BCMP), mono-dentate molecular pairs (MCMP), solvent-shared molecular pairs (SMP), and parallel solvent-shared molecular pairs (PSMP) in aqueous solutions. Their relative contents are both concentration- and temperature-sensitive. BCMP with the B-B distance of ∼4.1 Å is the dominant molecular pair in the aqueous solutions. Relatively less content and van der Waals interactions stabilized PSMP, with a B-B distance of ∼3.6 Å between the two parallel layers, which is a crucial species for the crystallization of H3BO3 from aqueous solution.

Structure of aqueous sodium acetate solutions by X-Ray scattering and density functional theory

The structure of aq. sodium acetate solution (CH3COONa, NaOAc) was studied by X-ray scattering and density function theory (DFT). For the first hydrated layer of Na+, coordination number (CN) between Na+ and O(W, I) decreases from 5.02 ± 0.85 at 0.976 mol/L to 3.62 ± 1.21 at 4.453 mol/L. The hydration of carbonyl oxygen (OC) and hydroxyl oxygen (OOC) of CH3COO− were investigated separately and the OC shows a stronger hydration bonds comparing with OOC. With concentrations increasing, the hydration shell structures of CH3COO− are not affected by the presence of large number of ions, each CH3COO− group binds about 6.23 ± 2.01 to 7.35 ± 1.73 water molecules, which indicates a relatively strong interaction between CH3COO− and water molecules. The larger uncertainty of the CN of Na+ and OC(OOC) reflects the relative looseness of Na-OC and Na-OOC ion pairs in aq. NaOAc solutions, even at the highest concentration (4.453 mol/L), suggesting the lack of contact ion pair (CIP) formation. In aq. NaOAc solutions, the so called “structure breaking” property of Na+ and CH3COO− become effective only for the second hydration sphere of bulk water. The DFT calculations of CH3COONa (H2O)n=5–7 clusters suggest that the solvent-shared ion pair (SIP) structures appear at n = 6 and become dominant at n = 7, which is well consistent with the result from X-ray scattering.

Dihydrogen Bonds in Aqueous NaBD4 Solution by Neutron and X-ray Diffraction

Neutron diffraction, X-ray diffraction, and empirical potential structure refinement modeling were employed to study the structure of alkaline aqueous NaBD₄ solutions at different NaBD₄ concentrations and temperatures. In 1.0 mol·dm^-3 NaBD₄ aqueous solutions, about 5.6 ± 1.6 water molecules bond to BD₄^- via tetrahedral edges or tetrahedral corners without a very specific hydration geometry; that is, each hydrogen atom of BD₄^- bonds to 2.2 ± 1.0 water molecules through dihydrogen bonds with the D(B)···D(W) distance of 1.95 Å. The number of dihydrogen bonds decreases with increasing concentration and increases with temperature. Dihydrogen bonding is a predominantly electrostatic interaction which shows relatively lower directionality and saturability in comparison with the regular hydrogen bonds between water molecules. The water orientation around BD₄^- shows that the proportion of tetrahedral-edge dihydrogen bonds increases with temperature and decreases with concentration.

Maximum bonding fragment orbitals for deciphering complex chemical interactions

An optimal set of fragment orbitals is proposed as a simple and powerful tool for analyzing complex bonding interactions.