Article pubs.acs.org/JPCB
Solution Structure of Energy Stored System I: Aqua-B(OH)4−: A DFT, Car−Parrinello Molecular Dynamics, and Raman Study Yongquan Zhou,†,‡ Yan Fang,† Chunhui Fang,*,† Fayan Zhu,† Haiwen Ge,† and Qiaoling Chen†,‡ †
CAS Key Laboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, People’s Republic of China ‡ University of Chinese Academy of Science, Beijing 100049, People’s Republic of China S Supporting Information *
ABSTRACT: A systematic study on the structure, stability, and Raman spectra of the metaborate anion hydrated clusters, B(OH)4−(H2O)n, (n = 1−15) was carried out by DFT in both gaseous and aqueous phase at the B3LYP/aug-cc-pVDZ level; all of these stable configurations were described, and the most stable hydrated clusters were chosen. The hydrogen bonds in those hydrated clusters were described in three different items: symmetrical double hydrogen bonding (DHB), single hydrogen bonding (SHB), and interwater hydrogen bonding (WHB). The distance of SHB is shorter than that of DHB, and multiple SHBs are more stable than a single DHB. In small size clusters (n ≤ 5), a structure with more DHBs is more stable than other arrangements. With continued increase in size, more SHBs were found in the first hydration sphere: when n ≥ 9, only SHBs can be found, and when n ≥ 12, a full hydration structure is formed with 12 SHBs and a hydration number of 10−12. The Car−Parrinello molecular dynamics simulation shows that only the first hydration sphere can be found, and the hydration number of B(OH)4− is 9.2 and the hydration distance is 3.68. The total symmetrical stretching vibration of B(OH)4− in hydrated B(OH)4−(H2O)n is blue shifted with increasing cluster size. After consideration of hydration, the calculated characteristic frequencies are in accord with the experiment characteristic frequency of B(OH)4−. scattering.13 Unfortunately, the more studies on borate, the more controversial problems were found, and the solution structure, especially the details of B(OH)4− hydration, is still unclear. First principles based on theoretical calculations has become a powerful tool to obtain molecular level information on the interaction between solute and solvent molecules. A great deal of theoretical studies on monatomic metallic ions, such as alkali metals,14,15 alkaline earth,14,16 transition metal17−19 and even the radioactive elements are included.20 Positively charged ions with simpler hydrated structures compared to a negatively charged system as a cation is always identified as a ball and binds strongly with solvent water molecules. Plenty of experiments, theories, and simulation studies have been carried out to understand the structure and dynamic aspects of hydration at molecular level on simple negatively charged ions. The most familiar is the hydrated halide series, X (H2O)n (X = Cl, Br, I).21,22 The common polyatomic inorganic anions like NO3− (D3h symmetry),23,24 SO42− (Td symmetry),25,26 SeO3 (C3v symmetry),27 and O3 (C2v symmetry)28 were also reported. Theoretical studies on small clusters B(OH)4−(H2O)n and B(OH)4−(aq) are reported by Takao et al.29 and Rustad et al.,30 who mainly focused on the isotopic fractionation of B(OH)4− hydration. However, none report on
1. INTRODUCTION Sodium tetrahydroborate (NaBH4) is a versatile reducing agent in various organic and inorganic processes.1,2 The high percentage of hydrogen present and release in NaBH4 makes it the most attractive chemical hydride for H2 generation and storage in automotive fuel cell applications.3,4 At the same time, the extensive use of NaBH4 fuel would require the disposal of large quantities of the byproduct NaB(OH)4. The electrochemical reduction of B(OH)4− to BH4− was first proposed in the early 1960s,5 and a number of patents indicate the possibility of electroreduction of borate compounds to BH4− with 20 to 25% current efficiency and 20 to 80% yield on electrocatalytic hydrogenation cathodes.6,7 However, reproduction of these claims has faced a number of difficulties; some researchers have even concluded that direct electro-reduction of NaB(OH)4 into NaBH4 was impossible.8,9 Further progress suffered a hiatus due to lack of research on the understanding of the complex electrode mechanism and electrode kinetics of the B(OH)4− electro-reduction reaction. The microstructure of NaB(OH)4, especially the hydration of B(OH)4−, may play an important role in the B(OH)4− electro-reduction process. The distribution equilibria and relevant interactions among polyborates in aqueous borate solutions are effected by temperature, pH, total boron concentration, and coexisting cations.10 Many studies have been done in this field, including conductometric/potentiometric titration, 11B NMR, Fourier transform infrared (FT-IR), FT-Raman,11,12 and X-ray © 2013 American Chemical Society
Received: June 9, 2013 Published: August 19, 2013 11709
dx.doi.org/10.1021/jp405708e | J. Phys. Chem. B 2013, 117, 11709−11718
The Journal of Physical Chemistry B
Article
fictitious electron mass was set to 400 au, and the time step was set to 4 au (0.1 fs). The simulations were carried out under periodic boundary conditions and involved 2 ps equilibration, followed by 14.5 ps production in the microcanonical ensemble. During the equilibration, the temperature was maintained at 300 K using a Nosé−Hoover thermostat.
the systematical study on the structure, stability, and Raman spectra of aqua-B(OH)4− is available in the literature. The objective of this article is to report the microscopic structure and energy parameters of B(OH)4−(H2O)n clusters (n = 1−15) based on a rigorous and systematical study; Car−Parrinello molecular dynamics (MD) simulation and Raman spectra were also studied to confirm our study results.
3. RESULTS AND DISCUSSION 3.1. Ab Initio Simulation. In the geometry optimization of isolated metaborate anion in the gas phase, B(OH)4− produces a structure of S4 symmetry as the most stable structure by applying the B3LYP density functional at the Aug-cc-pVDZ level. The calculated B−O bond distance is 148.93 pm, and the optimized structure is showed in Figure 1. There are two types
2. EXPERIMENTAL AND THEORETICAL METHODS 2.1. Raman Spectrum Experiment. Commercially available NaBO2·4H2O, Sinopharm chemical reagent Co., Ltd. AR, was recrystallized twice from distilled water. The entire sample solutions were prepared by mass using double-distilled water (κ109.5°) and were indexed as α-type. In the DHB in BW1B, two hydrogen atoms of the water molecule bond to the two oxygen atoms of B(OH)4− with a ∠OBO angle of 106°(∠OBO
