Microhydration of BH4â: Dihydrogen Bonds, Structure, Stability, and

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Micro-Hydration of BH : Dihydrogen Bonds, Structure, Stability, and Raman Spectra Yongquan Zhou, Koji Yoshida, Toshio Yamaguchi, Hong-Yan Liu, Chunhui Fang, and Yan Fang J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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The Journal of Physical Chemistry

Micro−Hydration of BH4−: Dihydrogen Bonds, Structure, Stability, and Raman Spectra Yongquan Zhou,1,* Koji Yoshida,2 Toshio Yamaguchi, 2 Hongyan Liu,1 Chunhui Fang1, and Yan Fang1 1

Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Qinghai

Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China 2

Department of Chemistry, Faculty of Science, Fukuoka University, 8−19−1

Nanakuma, Jonan, Fukuoka 814−0180, Japan

ABSTRACT: 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 the density functional theory (DFT) at the wB97XD/6−311++g(3df,3pd) basis set level. The successive micro-hydration 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 twelve 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.

*

E−mail: [email protected] 1

ACS Paragon Plus Environment


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1. INTRODUCTION As an electron-rich ligand, borohydride (BH4−) forms various dihydrogen−bonded complexes with protic donors that possess diverse chemical properties.1,2 This unconventional hydridic−to−protonic interaction, which is also called proton−hydride bonding, H···H hydrogen bonding, or hydrogen−hydrogen bonding, is comparable in strength and directionality to conventional hydrogen bonding. Consequently, it influences the structure, reactivity, and selectivity both in solution and in the solid state, and has great potential utility in catalysis, crystal engineering, and materials chemistry.1−8 The outstanding performance of NaBH4 in terms of hydrogen content and the large number of hydrogen released upon hydrolysis make it one of the most promising chemical hydrides for H2 storage.9−11 In addition, alkaline NaBH4 solutions are safe to transport, and the final products (B(OH)4−) of BH4− oxidation and hydration are environmentally safe. Dihydrogen bonding has been shown to play an important role in the stabilisation of this hydrogen storage system.1 Hydrolysis is an important pathway to liberate H2 from NaBH4, and dihydrogen bonds also contribute notably to NaBH4 hydrolysis.1,12−15 Direct borohydride fuel cell (DBFC) technology is being actively investigated by a number of research groups.16, 17 Further progress has suffered from a lack of research on the poor understanding of the complex electrode mechanism and electrode kinetics of the BH4− oxidation. Previous studies show that the solvation structure of the electrolyte is critical to solute electrode oxidation in solution.18, 19 A comprehensive, fundamental understanding of molecular−level BH4− hydration, and

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especially, the dihydrogen bonding therein, may shed new light on DBFC engineering. Computational chemistry suggest that polyhydrides can form bifurcated or chelate structures with angles significantly smaller 180°.8 For example, ab initio calculations on borohydrides evidenced a linear X−H···H bond, but a bent H···H−B bond.20 Dihydrogen bonding in NaBH4·2H2O was confirmed by Filinchuk et al21 through single−crystal X−ray diffraction and vibrational spectroscopy. Yamawaki et al.22 studied the amount of moisture adsorbed by LiBH4 under 5% relative humidity, identified LiBH4·H2O formation and confirmed the existence of dihydrogen bonds in LiBH4·H2O via powder X−ray diffraction and Rietveld analysis. Researchers have long focused on dihydrogen bonds in solution,2, 23 but specific investigations on borohydride in aqueous solution are rare. Shirk et al.24 collected Raman spectral data in the B−H stretching region for tetrahydroborate in aqueous NaBH4 solutions, but found no striking evidence for H···H hydrogen bonding. Conversely, Strauss et al.25 studied borohydride salts in various solvents using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, and suggested that BH4− ions are strongly solvated; they proposed a tetrahedral face hydration with the water protons pointing toward the faces of the BH4− tetrahedron. Duffin et al.26 examined the hydration of sodium borohydride by X−ray absorption spectroscopy (XAFS). The authors ascribed the uncharacteristically narrow absorption feature, which is shifted to lower energy to the formation of dihydrogen bonds between borohydride and water. However, XAFS can be used to probe scattering pairs in the vicinity of an excited atom only, which indicates that the boron K−edge XAFS spectra of aqueous boron oxides is insensitive to hydrogen

3



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bonds in aqueous solution.27 Giammanco et al.28 examined the dynamics of dihydrogen bonds between borohydride and water by ultrafast 2D−IR vibrational echo spectroscopy and polarisation selective IR pump−probe experiments. They found that dihydrogen bonds are very similar in nature to that of hydrogen bonds of water molecules, but somewhat weaker than the average water hydrogen bond. Duffin et al.26 examined the NaBH4 hydration by first principle QM/MM simulation, indicating that water preferentially associates with borohydride at the tetrahedral corners and edges. There is an urgent need for a detailed dihydrogen bonds between BH4− and water molecules. In the present work, the hydration structure, stability, and Raman spectra of aqua−BH4− clusters were systematically studied by density functional theory (DFT), Raman spectrum, and Car−Parrinello molecular dynamics (CPMD) simulation. The detailed micro-hydration between BH4− and water molecules were illustrated.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Samples Preparation and Raman Spectra Commercially available NaBH4 and NaOH (G R; Sinopharm Chemical Reagent Co., Ltd) were used without further purification. NaBH4·2H2O was synthesised according to the ternary phase diagram of NaBH4−NaOH−H2O.29 The sample solutions were prepared by mass using double−distilled water. Since borohydride hydrolyses slightly in neutral aqueous solution, 5% (by weight) NaOH was used to slow down the reduction of water and subsequent liberation of hydrogen gas30,

31

so that the

concentrations in solution remained constant and no new species were produced over

4


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the course of the experiment. Raman spectra of solid and liquid samples were recorded in the range 400−4000 cm−1 on a Nicolet Almega Dispersive Raman Spectrometer (laser: 514.5 nm, exposure time: 8 s). The solid samples were placed on a microscope slide and subjected to one exposure. The liquid samples were held in a quartz glass tube and subjected to 16 exposures.

2.2. Density Function Theory Calculations DFT energy includes an exchange term and a contribution to the electron correlation energy, and is well suited to describe molecular clusters.32 In the present work, the structure, stability, and Raman shift of [BH4(H2O)n]−(n=1−8,10,12,14,16) were investigated using the wB97XD method proposed by Head−Gordon and coworkers,33 which involves dispersion and features excellent geometry optimisation performance.34 The largest standard Pople basis set, 6−311++g(3df,3pd), was employed for all the atoms.35 The single−point polarised continuum model36, 37 was employed so that the long−range electrostatic effect of solvent could be considered to calculate the hydration energy for all configurations. The numerous stationary points on a shallow potential energy surface make it extremely difficult to locate the exact global energy minimum.

Therefore,

possible

configurations

were

performed

at

the

wB97XD/6−31G(d) level first, and the lowest energy structures were used in further geometry optimisations and Raman calculations at the wB97XD/6−311++g(3df,3pd) level. Vibrational frequency calculations were performed at the same level to ascertain the nature of the stationary points; there was no virtual frequency. BSSE correction involved partitioning the cluster into two fragments i.e. BH4− and (H2O)n were

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considered. DFT calculation Details can be found elsewhere.38 All geometry optimisations and frequency analyses were performed with the Gaussian 09 software package.39 2.3. Stabilization Energy Calculations The interaction between BH4− and the water cluster (H2O)n is known as the stabilisation energy (ESE) are and is defined as eq.( 1) ∆𝐸SE = 𝐸[BH4(H2O)n ]− − 𝐸BH−4 − 𝐸(H2 O)n

(1)

where the energy parameters 𝐸[BH4(H2O)n]− , 𝐸(H2 O)n , and 𝐸BH−4 refer to the energy of the cluster [BH4(H2O)n] −, (H2O)n and BH4−, respectively. The definition suggests that the stabilization energy refers to the net interaction of solute BH4- ion and the solvent water molecules which preclusive the inter-water interactions. Thermodynamics values for Gsolv were obtained using PCM via ∆𝐺solv = G[BH4 (H2O)n]− − 𝐺BH−4 − 𝐺(H2 O)n

(2)

Gsolv is calculated as below: Gsolv = Ggas + Gss + Gscrf

(3)

where the self-consistent reaction field energy (Gscrf) of a species is expressed as Gscrf= Gelectrostatic + Gnonelectrostatic = Gelectrostatic + Gcavition + Gdispersion + Grepulsion

(4)

and the correction for the standard state (Gss) change was calculated via Gss=RTln(pw/p0)/n

(5)

where pw is the pressure of liquid water assuming that it is an ideal gas, p0 is the pressure of the gas−phase standard state, R is the universal gas constant, T is the absolute temperature in Kelvin, and n is the number of water molecules in the cluster.

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2.4. Car−Parrinello Molecular Dynamics (CPMD) Simulation CPMD simulations40 were performed using CPMD code.41 The simulated alkaline aqueous NaBH4 solution consisted of 1 BH4−, 1 OH−, 2 Na+ ions, and 56 H2O molecules in a periodically repeated cubic box of length 12.417 Å, which corresponds to ~1.0 mol/L NaBH4 stabilized by 1.0 mol/L NaOH. The pure water system consisted of 64 H2O molecules in a periodically repeated cubic box of length 12.430Å. A generalized gradient approximation involving the Perdew−Burke−Ernzerhof (re−PBE) function was used for the exchange−correlation energy42 Martins Troullier norm-conserving potentials43 were employed to describe the core electrons. The electronic orbitals were expanded in a plane wave basis set using an 80 Ry electron density cut−off. A fictitious electron mass was set to 400 au and the time step was 4 au (~0.1 fs). The simulations were carried out under periodic boundary conditions and involved a 2 ps equilibration period and followed a 20.0 ps production simulation in the canonical ensemble. The final 100,000-step calculations were used for trajectory data collection in 10−step interval. During the simulation, the temperature was maintained at 300 K with a Nosé−Hoover thermostat44,45.

3. RESULTS AND DISCUSSION 3.1. DFT Calculations A geometry-optimised isolated BH4− ion at wB97XD/6−311++g(3df,3pd) in the gas phase with Td symmetry is shown in Figure 1. The B−H bond length in BH4− is 1.239 Å, and the∠HBH is 109.47°, which is consistent with the previous studies by Zhou et al.15 at MP2/6−311++G(3df, 3pd) (rB−H=1.233 Å), Mo et al.46 at

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CCSD/6−311+G(d,p) (rB−H=1.241 Å) and Zhang et.al.47 at B3PW91/6−311++G(2df, 2p) (rB−H=1.236 Å). The H2O is in a C2V point group and has an O−H distance of 0.957 Å and a ∠HOH of 105.112°. The charge distributions indicate that the hydrogen atom in BH4− has a negative Mulliken charge of -0.277 e, and the hydrogen atom in H2O has a positive Mulliken charge of 0.438 e; this indicates that the water molecules may undergo hydridic−to−protonic interactions with BH4−.

(a)

(b)

Figure 1. Electrostatic potentials mapped on the surfaces of fully optimised (a) BH4− and (b) H2O at the theoretical wB97XD/6−311++g(3df,3pd) level, showing the Mulliken charge distributions for all atoms. Positive charge is shown in blue; negative charge is in red.

3.1.2. Stabilization Energy The calculated interaction energy for [BH4(H2O)n]– (n=1−8,10,12,14,16) clusters is shown in Figure 2 and summarized in Table 1. The detailed energy parameter can be found in Table S1 in the supplementary materials.

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0

E

-20

Energy (kcal/mol)

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0 Gsolv

-40 -60 -80 -100 -120 -140 -160

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

th m H2O Figure 2. Stabilization energy of BH4− hydration with increasing [BH4(H2O)n]− (n=1−8,10,12,14,16) size. Table 1. Bond length and energy parameters of [BH4(H2O)n]–(n=0−8,10,12,14,16) in the gaseous and aqueous phase at the wB97XD/6−311++g(3df,3pd) level. Bond parameters/Å Configurations

Gaseous

Aqueous

CN

intra−B−H

H···H

B−O(W)

∆E0

∆Gsolv

BH4-

0

1.239

/

/

/

/

BHW1A

1

1.236

2.021

3.296

−11.638

−68.800

BHW2A

2

1.236

2.043

3.290

−24.571

−76.167

BHW2B

2

1.234

1.930

3.312

-23.545

-74.339

BHW3A

3

1.232

1.995

3.242

−35.434

−85.578

BHW3B

3

1.234

1.926

3.282

−34.895

−82.592

BHW3C

3

1.232

1.795

3.377

−33.249

−79.270

BHW4A

4

1.232

2.032

3.288

−44.885

-95.517

BHW4B

4

1.231

1.983

3.355

-44.576

−90.426

BHW4C

4

1.231

2.009

3.372

−43.540

−88.045

BHW5A

5

1.230

2.040

3.356

−53.348

−103.903

BHW5B

5

1.230

1.996

3.425

−52.213

-100.670

BHW5C

5

1.229

2.035

3.392

−52.031

−96.554

BHW6A

6

1.228

2.060

3.399

−60.860

−113.214

BHW6B

5

1.228

2.022

3.331

−60.632

−110.906

9



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BHW6C

6

1.228

2.008

3.460

-59.557

-106.603

BHW7A

6

1.228

1.900

3.516

-69.909

-117.284

BHW7B

5

1.228

2.021

3.495

-67.200

-117.658

BHW7C

7

1.228

1.997

3.323

-66.690

-115.127

BHW8A

8

1.228

2.152

3.495

-74.385

-126.594

BHW8B

7

1.227

1.959

3.468

-74.168

-127.321

BHW8C

8

1.228

2.137

3.520

-73.615

-125.992

BHW10A

7

1.226

2.029

3.491

-76.761

-127.720

BHW12A

7

1.225

2.068

3.480

-83.274

-135.334

BHW14A

8

1.224

2.039

3.492

-92.591

-145.869

BHW16A

8

1.221

2.009

3.463

-95.658

-143.044

CN is the hydration number i.e. the number of water molecules that bond to BH4− directly. ∆E0 is the zero−point corrected electronic energy in the gaseous phase, ∆Gsolv is the hydration free energy in the aqueous phase. All energies are in kcal/mol at a temperature of 298 K and a pressure of 1 atm.

3.1.2. BH4− successive hydration Discrete water molecules were added successively to BH4− through a number of possible pathways based on the chemical intuition. Then, full geometry optimisations were performed to locate equilibrium structures at the wB97XD/6−311++g(3df,3pd) basis level; several local minimum energy structures may be obtained for each of the discrete hydrated clusters. Mono−hydrate clusters: [BH4(H2O)1]−. There are four plausible configurations of BH4− hydration according to chemical intuition (Figure 3). QM/MM simulations by Andrew et al.26 show that water molecules hydrate BH4− through the tetrahedral corner (Figure 3(a)), the tetrahedral edge (Figure 3(b)) or the tetrahedral face (Figure 3(c)); the tetrahedral corner and the tetrahedral edge forms are preferred.

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(a)

(b)

(c)

(d)

Figure 3. Plausible water−tetrahydroborate interactions48 along the (a) tetrahedral corner, (b) tetrahedral edge, (c) tetrahedral face, (d) and symmetric double dihydrogen bonding.

All four idealized hydration configurations shown in Figure 3 are optimised at the wB97XD/6−311++g(3df, 3pd) basis level. Only one stable configuration, which has a relative stability of -12.226 kcal/mol is obtained, as shown in Figure 4. Other plausible idealized configurations, such as that featuring symmetric double hydration (which has proven stable in other polyatomic ions, including NO3−,49 SO42−,50 and B(OH)4- 51) appear unstable for BH4−. The ion−dipole complex in Figure 4(a) does not contain classical hydrogen bonds, but features instead two unsymmetrical dihydrogen bonds are found; this unsymmetrical dihydrogen bonding draws attention to the definition to bifurcated hydrogen bonding. 52 The H···H distances are substantially shorter than the 2.4 Å distance corresponding to twice the van der Waals radius of a hydrogen atom. One bond features a stronger interaction in an approximately linear form ( ∠ H(B) ) −H(B−O(W)=172.96°

with an H···H distance of 1.738 Å. The other bond is weaker and

has an H···H distance of 2.303 Å. Mulliken charge distributions can also reflect this feature to some extent; the hydration process disorders the charge distribution on the H atoms in BH4−, where the stronger H···H bond gets more charge from the water molecule. The calculated H···H distance is consistent with experimental data from crystalline NaBH4·2H2O.21 The calculated hydration distance (B−O(W)) in this configuration is 3.296 Å. 11



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(a)

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(b)

Figure 4. Electrostatic potentials mapped on the surface of fully optimised (a) mono−hydrate BH4− and (b) water dimer at the theoretical wB97XD/6−311++g(3df,3pd) level showing the distribution of Mulliken charge over all atoms. Positive charge is shown in blue; negative charge is in red.

A number of plausible water dimers were taken into consideration. The most stable dimer exists at the wB97XD/6−311++g(3df, 3pd) level, as shown in Figure 4(b), and has much lower stabilization energy of -2.692 kcal/mol (after adjusting the zero−point corrected electronic energy and BSSE 53, 54), which indicates that the dihydrogen bond between BH4− and H2O is more stable than the traditional hydrogen bond between two water molecules.

BHW2A

BHW2B

BHW3A

BHW3B

BHW3C

BHW4A

BHW4B

BHW4C

12


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BHW5A

BHW5B

BHW5C

BHW6A

BHW6B

BHW6C

BHW7A

BHW7B

BHW7C

BHW8A

BHW8B

BHW8C

BHW10A

BHW12A

BHW14A

BHW16A

Figure 5. Optimized lowest−energy structures of hydrated borohydride clusters [BH4(H2O)n]− (n=28,10,12,14,16) at the theoretical wB97XD/6−311++g(3df,3pd) level.

Di−hydrate clusters: [BH4(H2O)2]−. Two water molecules may hydrate the BH4- ion, as shown in BHW2A and BHW2B in Figure 5. In the most stable dihydrate cluster (BHW2A), two water molecules hydrate the BH4− ion in an unsymmetrical edge form with a relative stability of -24.571 kcal/mol. The average intra−B−H, H···H and B−O(W)

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distances in this cluster are 1.236, 2.043 and 3.290 Å, respectively. In BHW2B, one water molecule hydrates BH4− in an edge configuration, while the other one hydrates BH4− in a tetrahedral corner configuration and forms a conventional hydrogen bond with another water molecule. BHW2A is 1.026 and 1.466 kcal/mol more stable than BHW2B in gaseous and aqueous phases, respectively. There is one additional weak hydrogen bond interaction in BHW2A in comparison with BHW2B, which indicates that even the weak dihydrogen bond between BH4− and water molecule is more stable than a conventional hydrogen bond. Tri−hydrate clusters: [BH4(H2O)3]−. The three water molecules hydrate BH4− in BHW3A, BHW3B and BHW3C in Figure 5. In the most stable configuration (BHW3A), one molecule hydrates BH4− in the tetrahedral edge form, while the other two take the tetrahedral corner form. The intra−B−H, H···H and B−O(W) are 1.232, 1.995 and 3.242 Å, respectively. BHW3B has the same basic structure unit as BHW2B but features a more hydrated tetrahedral corner. The BHW3B and BHW3A configurations are similar in energy, with a difference in relative stability of

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