Dynamics of Dihydrogen Bonding in Aqueous Solutions of Sodium Borohydride

Effects of Nanoconfinement on Dynamics in Concentrated Aqueous Magnesium Chloride Solutions

Water behavior in various natural and manufactured settings is influenced by confinement in organic or inorganic frameworks and the presence of solutes. Here, the effects on dynamics from both confinement and the addition of solutes are examined. Specifically, water and ion dynamics in concentrated (2.5-4.2 m) aqueous magnesium chloride solutions confined in mesoporous silica (2.8 nm pore diameter) were investigated using polarization selective pump-probe and 2D infrared spectroscopies. Fitting the rotational and spectral diffusion dynamics measured by the vibrational probe, selenocyanate, with a previously developed two-state model revealed distinct behaviors at the interior of the silica pores (core state) and near the wall of the confining framework (shell state). The shell dynamics are noticeably slower than the bulk, or core, dynamics. The concentration-dependent slowing of the dynamics aligns with behavior in the bulk solutions, but the spectrally separated water-associated and Mg2+-associated forms of the selenocyanate probe exhibit different responses to confinement. The disparity in the complete reorientation times is larger upon confinement, but the spectral diffusion dynamics become more similar near the silica surface. The length scales that characterize the transition from surface-influenced to bulk-like behavior for the salt solutions in the pores are discussed and compared to those of pure water and an organic solvent confined in the same pores. These comparisons offer insights into how confinement modulates the properties of different liquids.

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.

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.

Bulk-like and Interfacial Water Dynamics in Nafion Fuel Cell Membranes Investigated with Ultrafast Nonlinear IR Spectroscopy

The water confined in the hydrophilic domains of Nafion fuel cell membranes is central to its primary function of ion transport. Water dynamics are intimately linked to proton transfer and are sensitive to the structural features and length scales of confinement. Here, ultrafast polarization-selective pump-probe and two-dimensional infrared vibrational echo (2D IR) experiments were performed on fully hydrated Nafion membranes with sodium counterions to explicate the water dynamics. Like aerosol-OT reverse micelles (AOT RMs), the water dynamics in Nafion are attributed to bulk-like core water in the central region of the hydrophilic domains and much slower interfacial water. Population and orientational dynamics of water in Nafion are slowed by polymer confinement. Comparison of the observed dynamics to those of AOT RMs helps identify local interactions between water and sulfonate anions at the interface and among water molecules in the core. This comparison also demonstrates that the well-known spherical cluster morphology of Nafion is not appropriate. Spectral diffusion of the interfacial water, which arises from structural dynamics, was obtained from the 2D IR experiments taking the core water to have dynamics similar to bulk water. Like the orientational dynamics, spectral diffusion was found to be much slower at the interface compared to bulk water. Together, the dynamics indicate slow reorganization of weakly hydrogen-bonded water molecules at the interface of Nafion. These results provide insights into proton transport mechanisms in fuel cell membranes, and more generally, water dynamics near the interface of confining systems.

Host-Guest Complexations of Amine Boranes and Isoelectronic/Isostructural Quaternary Alkylammonium Cations by Cucurbit[7]uril in Aqueous Solution

The host-guest complexation of six amine boranes (R3NBH3) by the macrocyclic host molecule cucurbit[7]uril (CB[7]) in aqueous solution has been investigated using 1H and 11B NMR spectroscopy. The limiting complexation-induced 1H and 11B chemical shift changes indicate that the amine boranes are included in the hydrophobic cavity of the host molecule. The host-guest stability constants for neutral  R3NBH3∙CB[7]  complexes (in the range of 105-107M-1) have been determined by 1H NMR competition experiments and are compared with the corresponding values for the isoelectronic/isostructural  R3NCH3∙CB[7] + complexes. Ammonia borane (H3NBH3) does not form a host-guest complex with CB[7]. The trends in the host-guest stability constant with the guest molar volume are examined, and the stability is ascribed to the hydrophobic effect (packing coefficient) and quadrupole-dipole interactions.

Vibration Spectral Dynamics of Weakly Coordinating Water Molecules near an Anion: FPMD Simulations of an Aqueous Solution of Tetrafluoroborate

The extent to which the ions affect the nearby water molecules will decide the structure-making or breaking nature of those ions in aqueous solutions. The effects of a weakly coordinating anion on the structure, dynamics, and vibrational properties of water molecules are not so significant as compared to an anion capable of making strong ion-water hydrogen bonds. The present work deals with the first-principles molecular dynamics study of an aqueous solution of such a weakly coordinating anion, tetrafluoroborate (BF₄^-), using dispersion-corrected DFT-based first-principles molecular dynamics (FPMD) simulations. Various structural, dynamical, and spectral properties, such as radial distribution functions (RDFs), rotational dynamics, vibrational density of states (VDOS), hydrogen bond as well as dangling OH autocorrelation functions, and residence dynamics, were calculated to investigate the effects of the anion on nearby water molecules. The process of spectral diffusion was assessed through a time series wavelet transformation of trajectories obtained from FPMD simulations. The first ion-water solvation shell extends up to 5.5 Å, containing around 20 water molecules. The lifetime of the ion-water hydrogen bond is found to be 1.19 ps, whereas the water-water hydrogen bond lifetime is found to be 1.13 ps. Inside the solvation shell, the persistence time of dangling OH chromophores and the average frequency of OH modes inside the solvation shell are found to be more compared to bulk. Three time scales are found for solvation shell OH modes from the frequency-frequency correlation function. A very short time scale is found for the intact ion-water interaction; the short time scale is for the ion-water hydrogen bond, and the long time scale is for escape dynamics of water molecules from the ion solvation shell. From the mean squared displacement, it is found that solvation water molecules diffuse slower than the bulk. However, solvation shell water molecules show faster relaxation from the analysis of rotational anisotropy. Within the longer time scale of spectral diffusion, this process (which is related to various dynamics of the molecules) is not yet complete, as compared to fast anisotropic decay. This fact is similar to the experimental finding of spectral diffusion and anisotropy time scales in the aqueous solution of borohydride anion. The calculated results are also compared with available experimental data wherever possible.

Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast IR Spectroscopy

Aqueous proton transport involves the ultrafast interconversion of hydrated proton species that are closely linked to the hydrogen bond dynamics of water, which has been a long-standing challenge to experiments. In this study, we use ultrafast IR spectroscopy to investigate the distinct vibrational transition centered at 1750 cm^-1 in strong acid solutions, which arises from bending vibrations of the hydrated proton complex. Broadband ultrafast two-dimensional IR spectroscopy and transient absorption are used to measure vibrational relaxation, spectral diffusion, and orientational relaxation dynamics. The hydrated proton bend displays fast vibrational relaxation and spectral diffusion timescales of 200-300 fs; however, the transient absorption anisotropy decays on a remarkably long 2.5 ps timescale, which matches the timescale for hydrogen bond reorganization in liquid water. These observations are indications that the bending vibration of the aqueous proton complex is relatively localized, with an orientation that is insensitive to fast hydrogen bonding fluctuations and dependent on collective structural relaxation of the liquid to reorient. We conclude that the orientational relaxation is a result of proton transfer between configurations that are well described by a Zundel-like proton shared between two flanking water molecules.

Dihydrogen vs. hydrogen bonding in the solvation of ammonia borane by tetrahydrofuran and liquid ammonia

The solvation structures of two systems rich in hydrogen and dihydrogen bonding interactions have been studied in detail experimentally through neutron diffraction with hydrogen/deuterium isotopic substitution.

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

Hydridic-to-protonic interactions in unconventional dihydrogen bonding influence the structure, reactivity, and selectivity in solution and in the solid state. In this study, the structure, stability, and Raman spectra of BH₄^- hydrated clusters, [BH₄(H₂O)_n]^- (n = 1-8, 10, 12, 14, 16) are systematically investigated using density functional theory (DFT) at the wB97XD/6-311++g(3df,3pd) basis set level. The successive microhydration process is described to illustrate in detail the changes in dihydrogen bonding with increasing hydration cluster size. The results of DFT calculations indicate that seven or eight water molecules hydrate BH₄^- with a total of 12 dihydrogen bonds in the tetrahedral edge or tetrahedral corner forms, and a maximum of six water molecules in the tetrahedral-edge form. Raman spectra of [BH₄(H₂O)_n]^- show a blue shift in the B-H stretching band due to hydration. Car-Parrinello molecular dynamics simulations verify strong BH₄^- water interactions. The hydration number of BH₄^- is 6.7, with a hydration B-O(W) distance of 3.40 Å, and each hydrogen in BH₄^- bonds with 2.66 hydrogen atoms from water.

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.