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The local environment of rubidium ion in aqueous rubidium tetraborate solutions was investigated using extended X-ray absorption fine structure. The Rb–O distance and coordination number in the first hydration shell of rubidium ion are 2.92 A and 8.0, respectively. The distribution map of borate anions was calculated using the Newton iteration method, and further verified using nuclear magnetic resonance spectroscopy (NMR). This shows that the dominant species are $${\text{B}}_{4} {\text{O}}_{5} \left( {{\text{OH}}} \right)_{4}^{2 - }$$ and $${\text{B}}_{3} {\text{O}}_{3} \left( {{\text{OH}}} \right)_{4}^{ - }$$ in aqueous rubidium tetraborate solutions at 298.15 K. The dominant species change to $${\text{B}}_{3} {\text{O}}_{3} \left( {{\text{OH}}} \right)_{4}^{ - }$$ and B(OH)3 at 333.15 K. Hydrolysis and polymerization coexist in aqueous rubidium tetraborate solutions with the changes of concentration and temperature.

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.

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 concentrati...

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 A, 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 A 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 A between the two parallel layers, which is a crucial species for the crystallization of H3BO3 from aqueous solution.

In this study, microstructure such as intra-molecule interaction B O,O O in polyborate anions (B3O3(OH)4-, B4O5(OH)4-) and inter-molecule (B O(W),M(Rb/Cs) O, M(Rb/Cs) B and O O, M-M i.e.) interaction have been taken into consideration by model calculation with the method of X-ray scattering. The models of B3O3(OH)4-, B3O3(OH)52− and B4O5(OH)42− were optimized by DFT (density functional theory) calculation which have a good agreement with the model calculation. During the process of data analyzing, Raman spectrum, pH, density, stoichiometric volume, atomic mass absorption coefficient have also been carried out. The coordination distance and number of Cs O(wI)(∼0.336 nm, ∼7.8), Cs O(wII)(∼0.526 nm, ∼8.4), O O(W)(∼0.278 nm, ∼2.15) and Cs B(∼0.458 nm, ∼0.9) are concerned to Cs2B4O7 solution. For aqueous Rb2B4O7 samples, the structure parameters of Rb O(wI)(∼0.292 nm, ∼7.0), Rb O(wII)(∼0.467 nm, ∼8.0), O O(W)(∼0.278 nm, ∼2.22), Rb B(∼0.377 nm, ∼0.9) are also listed in the discussion part. And also concluded that cesium ion forms bidentate chelate complexes with tetraborate ion in the solution. Moreover, the influence of the samples, concentration on RDFs (radial distribution function curves) and the difference between cesium tetraborate and rubidium tetraborate also illustrated in paper.

In this paper, the factors determining the spectroscopic characteristics of vsym–B(OH)4− band including coupling effect, hydrogen bonding effect, and direct contact effect in Li[B(OH)4] solutions are investigated by using ab initio calculation. The coupling effect between the liberations of water and [B(OH)4−] has a larger effect on vsym–B(OH)4− in solvent-shared ion pair (SIP) and monodentate contact ion pair (MCIP), but the smaller effect in bidentate contact ion pair (BCIP). Water molecule tends to hydrate to the middle position between the first sphere of B(OH)4− and outer–sphere of [Li(H2O)4+] and has a different effect on vsym–B(OH)4− in ion pairs. The direct contact effect and polarization effect lead to 19.7 cm−1 red shift of vsym–B(OH)4− in MCIP, and 0.4 cm−1 blue shift in BCIP. The association process in Li[B(OH)4] solution was also introduced by using Raman spectral evolution of vsym –B(OH)4− in the dehydration process.

Structure of alkaline aqueous NaBH4 solutions at NaBH4 concentrations of 1.0 to 9.1 mol·dm−3 (almost saturated) as well as a 1.3 mol·dm−3 NaOH solution was studied by X-ray diffraction and Empirical Potential Structure Refinement (EPSR) modelling. In the 1.0 mol dm−3 alkaline NaBH4 solution, Na+ is surrounded by around six water molecules with octahedral geometry. With an increase in NaBH4 concentration, the hydration number of Na+ decreases from 4.8 ± 1.1 at 2.25 mol·dm−3 solution to 2.9 ± 1.3 at 9.1 mol·dm−3 with an almost constant Na+-O (W: H2O) distance of 2.34 A. The decrease in hydration number for Na+ is compensated by the formation of ion pairs between Na+ and BH4− to ensure an octahedral hydrated geometry for Na+. About 6.0 ± 1.6 water molecules are likely to bond to BH4− via tetrahedral edges or tetrahedral corners without a very specific hydration geometry, giving rise to each hydrogen atom of BH4− bound to 2.3 ± 1.0 water molecules through dihydrogen bonds. The hydration number around BH4− decreases with increasing concentration. BH4− tends to form contact ion pairs with Na+ at the Na-B distances of 3.24 A and 2.82 A in tetrahedral-corner-shared and tetrahedral-edge-shared fashions, respectively; the number of Na-B interactions increases from 0.3 ± 0.5 in 1.0 mol·dm−3 (almost negligible) to 2.1 ± 0.9 in 9.0 mol·dm−3. Tetrahedral-edge-shared bidentate ion pair is likely to be the preferred and dominant ion cluster over the wide concentration range. Ion association between the Na+ and BH4− was crosschecked by density function theory (DFT) calculations on [NaBH4(H2O)6] clusters.

In this work, the structures of micro-species in [CaCl2(H2O)n = 0–12] were systematically studied by density functional theory. The distances between Ca2+ and the two Cl− ions (rCa–Cl) are all less than 4.2 A when n = 1–4, which shows that the main species is contact ion pair. When n = 5, the main species is solvent separated ion-pair [CaCl−(H2O)5···Cl−] (SIP/s) with one Cl− dissociated from [CaCl2]. When n = 6–9, the main species is changed into solvent separated ion-pair (SIP/d) with two Cl− dissociated from [CaCl2]. Six water molecules are located in the inner shell and other water molecules hydrate in the outer sphere in the form of hydrogen bonding. When n = 10–12, the main structure is still SIP/d. Moreover, the hydration number of calcium with the most stable structure in the first hydration shell is 7. The inner hydration distance remains almost unchanged and the outer water molecules have little effect on the inner ones. The vibrational frequencies of the water molecules in [CaCl2(H2O) n ] were also studied and discussed in detail.

Quantum chemistry calculations were used to study the structure and energy of strontium (Sr) ion hydrated clusters [Sr(H2O)1−25]2+. The saturated hydration number of the first hydration layer of Sr...

In this work, the structural details and Raman spectra of the Ca(NO3)2(H2O) n=0–10 clusters were studied by using ab initio method. The results show that the main species in the cluster is the contact ion pair (CIP) when n = 1–7. When n = 8–10, the main species changes into solvent shared ion pair (SIP) CaNO3(H2O) n …NO3 − in the bidentate form. One of the r Ca–N distances remains unchanged at ~2.95 A, while the other one increases to more than 4.8 A. The hydration distance r Ca–O remains at 2.42 A. The contact between Ca2+ and NO3 leads to a red shift of the v 1–NO3 − band while the polarization of water by Ca2+ leads to a blue shift. The vibrational frequency of water molecules remains unchanged for the same types of water molecules. Hydrogen bonds are the main reason for the red shift of vibrational frequency of water molecules.

Potassium tetrahydroxyborate solution is a significant material in the borate solution family, but there is limited knowledge about hydration structures and interactions of K+, [B(OH)4−], and water. In this study, the X-ray diffraction measurements of potassium tetrahydroxyborate solutions have been made. The experimental structure factors are subjected to Empirical Potential Structure Refinement (EPSR) modeling to reveal the details of ion hydration and association in the aqueous solutions. This study shows that the O(W)–O(W) distance of water in the studied solutions ranges from 2.82 to 2.76 A with a coordination number that ranges from 4.7 ± 1.4 to 3.1 ± 1.3 when the value of the water–salt molar ratio (WSR) is decreased from 30 to 6. The addition of ions slightly affects the tetrahedral structure of water even when the concentration of ions is high. The first hydration distance of K+ remained at ∼2.67 A, whereas the value of the coordination number (CN) decreased from 5.4 ± 1.3 to 3.9 ± 1.5 when the concentration of the borate solution was increased. The hydration ability of K+ was weak and almost did not have a fixed local hydration structure. The pair distribution function (PDF) of gB–O(W)(r) shows that [B(OH)4−] has a broad hydration distance from 2.9 to 5.4 A because of the complex interactive relationship between K+, [B(OH)4−] and water. There is a competitive hydration between K+ and [B(OH)4−]. Both the X-ray diffraction and DFT-based calculations confirm that the main species is monodentate contact ion pairs when WSR = 30, bidentate contact ion pairs when WSR = 14, and triple contact ion pairs when WSR = 6. These results will provide a new understanding about potassium tetrahydroxyborate solution.

Recognition of the reaction mechanism of hydrogen generation by borohydrides hydrolysis can provide effective help for optimizing the reaction condition and catalyst design. In the present work, the underlying mechanisms of BH 4 − spontaneous hydrolysis under water-deficiency (2 water molecules) and water-rich (4 water molecules) condition were elaborated by the second-order Moller-Plesset perturbation theory (MP2) at MP2/6-311++G(3df, 3pd) level of theory. The present theoretical calculations predicted that dihydrogen bonding plays an extremely important role in the process of BH 4 − hydrolysis, and H2 is formed by a hydridic B-H hydrogen and a protic O H hydrogen. The hydrogen generation mechanism of BH 4 − hydrolysis in water-rich condition may experience BH 4 − → BH 3 ( OH ) − , BH 3 ( OH ) − → BH 2 ( OH ) 2 − , BH 2 ( OH ) 2 − → BH ( OH ) 3 − and BH ( OH ) 3 − → B ( OH ) 4 − four steps. BH 4 − hydrolysis mechanism in water-deficiency condition includes four steps in two possible reaction pathways as the water molecules participate with the reaction in different steps. All those reaction pathways with the same first step ( BH 4 − → BH 3 ( OH ) − ) and it's the Rate Determining Step (RDS) of the whole reaction. BH 4 − spontaneous hydrolysis at neutral or weak alkaline should be in the acid-catalyzed mechanism.

Based on the measured pH data and reported equilibrium constants, the main polyborate species formation existing in solution was investigated using the Newton iteration method at 298 and 333 K, and checked by the method of 11B nuclear magnetic resonance (NMR) spectroscopy. The neighboring structure of rubidium ion was studied at room temperature, which the structural parameters, including the coordination number, interatomic distance of Rb-O and Debye-Waller factor, were determined by the synchrotron radiation extended X-ray absorption fine structure (EXAFS). The results show that the average interatomic distance of Rb-O is 2.93 ± 0.004 A and coordination number is 7.7 ± 0.9 in the first hydration shell around rubidium ion.

The structure of aqueous lithium tetraborate solutions was investigated by species distribution calculation and synchrotron X-ray scattering. It shows that the dominant species in supersaturated solution at 298.15 K is B4O5(OH) 4 2− and the minor species are B3O3(OH) 5 2− , B3O3(OH) 4 − and B(OH)3. The ‘intramolecular’ structural parameters of B4O5(OH) 4 2− , such as bond length and coordination number, were gives out using density function theory calculation. X-ray scattering study shows that the distance Li–O(H2O)I of [Li(H2O)4]+ is about 0.1983 nm with the coordination number(CN) 4 in tetrahedral configuration. The B–O(H2O) distance in hydrated anion B4O5(OH)4(OH2) 8 2− is 0.3662 nm with the CN 12. The Li+–B distance is about 0.3364 nm with a coordination number ~1.0. The temperature effect on solution structure was also discussed.

Molecular dynamics (MD) simulation was employed to explore the structural and thermodynamic characteristics of 18-crown-6 ether selectivity to alkali metal ions in aqueous solution. The radial distribution functions (RDFs), potentials of mean force (PMFs) and binding free energies between Li+, Na+, K+, Rb+, Cs+ and 18-crown-6 ether with SPC/E and TIP4P water models in aqueous solutions at 278.15 K, 298.15 K and 318.15 K were obtained. It shows that K+ is selected over other alkali metal ions by 18-crown-6 ether in aqueous solution at all water models and temperatures we considered, and increasing temperature is beneficial for the binding of 18-crown-6 ether to alkali metal ions. The calculated selective sequence (Li+ ≪ Na+ Rb+ > Cs+) in aqueous solution is close to the experimental results. The gas-phase binding energies of 18-crown-6 to alkali metal ions and the minimum potential energies with corresponding minimum positions of alkali ions complexes were got by density functional theory (DFT). The results reveal that the selectivity of 18-crown-6 to alkali metal ions in gas-phase follows the sequence, Li+ > Na+ > K+ > Rb+ > Cs+, and follows their ionic potentials. The effect of solvent on the selectivity of 18-crown-6 ether to alkali ions is confirmed by comparing the selective sequences in gas-phase and aqueous solution. The noncovalent interactions sites and types between alkali ions and 18-crown-6 ether were determined by electrostatic potential (ESP) and the average reduced density gradient (aRDG), which shows that the interactions between alkali ions and 18-crown-6 are electrostatic dominated.

In this study, ion pairs in a single sodium tetrahydroxyborate [Na [B(OH)4] droplet were analyzed using an “in-situ strategy” in which a sample-droplet of nanogram mass was deposited on a hydrophobic substrate and droplet was forced to enter into a supersaturated state by decreasing the relative humidity (RH) of the environment. The structure of the solvated [B(OH)4−] ionic moiety with various molar water-to-solute ratios (WSR) was analyzed using Raman spectroscopy. To confirm the structural changes in the droplet, electronic structure calculations were carried out using density functional theory (DFT). The frequencies calculated for the totally symmetric BO stretching vibration (vsym(BO)) of the [B(OH)4−] moiety were compared with those of the fundamental bands observed in the Raman spectra recorded of the droplets. The following results have been obtained: (i) when WSR is reduced from 9 to 0.1, the frequency of the band that corresponds to vsym(BO) shifts from 745 to 746 cm−1, and its full-width at half-maximum value increases from 19.7 to 20.5 cm−1; (ii) when WSR ≥7, the solvent-shared ion pair (SIP) is predominantly present in the solution, whereas in the case of WSR

In this study, microscopic Raman spectroscopy and Ab initio quantum chemical calculation were used to determine the structural details of ion pairs and their transformation in concentrated K[B(OH)4] droplets. The Raman experiment shows that the vsym-B(OH)4− undergoes a downward shift with the decrease of WSR. The contact ion pairs (CIPs) change to solvent shared ion pairs when the molar water-to-solute ratio (WSR) is bigger than 6; CIPs are the dominant species when 1.33

DFT-based calculations show that B5O6(OH)4- forms B5O5(OH)6− intermediate during the first step of hydrolysis. Then, the hydrolysis pathways are divided to principal and secondary reactions. In the principal reaction, hydrolysis occurs at the bridge position between the six-membered ring and branch BO3 units to form B(OH)3 and B3O3(OH)4−. B3O3(OH)4− hydrolyzes in dilute solution to B(OH)3 and B(OH)4−. In the secondary reaction, hydrolysis of B5O5(OH)6− occurs at the bridge position in the ring to form a “network-chain” structure B5O4(OH)8−. Then, hydrolysis occurs at the bridge atoms, each BO3 unit is converted to B(OH)3, and BO4 unit is finally converted to B(OH)4−.

The hydration of K+(H2O) n has been widely studied and believe to be important for understanding solvent properties in biological and chemical systems. However, understanding the structure and the spectrum information K+(H2O) n with changing n is limited. Here, we investigated the clusters K+(H2O) n=1–16 and further studied the IR spectrums of the most stable clusters with density functional theory. The configuration, bond length, vibration frequency were given out. It shows that K+(H2O)8(H2O) n , a distorted square antiprism in inner layer, is the main configuration with hydration distance r K - OI 0.296 nm when the hydration number n is bigger than 8. The saturated hydration number is 8 in the first hydration layer and the water molecules of the second hydration sphere have little effect on the inner ones when n> 8. A detailed classification about the hydrated water molecules was made according to the role of acceptor or donor hydrogen bonding in clusters. The vibration frequency of the different kinds of water molecules were also detailly identified. The results are valuable for further determination of the K+(H2O) n clusters in aqueous solutions.

In the present work, a detailed investigation of Ni(II) hydration in water solutions was carried out using density functional theory (DFT) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The hydrated characteristics of [Ni(H2O)n](2+) clusters, such as energy parameters, atomic charge distributions, and bond parameters, were explored using DFT with Becke's three-parameter exchange potential and the Lee-Yang-Parr correlation functional (B3LYP). DFT calculations indicated that the preferred structure of the first hydration shell of Ni(II) generally has a coordination number of six and is almost unaffected by the water molecules in the outer solvation shell, whereas the structure of the second solvation shell varies as the hydration proceeds. EXAFS measurements are reported for aqueous NiSO4 and Ni(NO3)2 solutions and the Ni(NO3)2·6H2O crystal. Analysis of the EXAFS spectra of these three systems using a multiparameter fitting procedure showed that, in each case, the first coordination shell consists of six water molecules with a Ni-O coordination distance of 2.04 Å, and that there is no Ni-S or Ni-N coordination in the first shell. There was no evidence of outer-shell SO4(2-) or NO3(-) ions substituting inner-sphere water molecules in NiSO4 and Ni(NO3)2. The characteristics of Ni(II) hydration obtained from DFT calculations agreed well with those obtained experimentally using EXAFS.