Article pubs.acs.org/JPCC
First-Principles Calculations of Potassium Amidoborane KNH2BH3: Structure and 39K NMR Spectroscopy Keiji Shimoda,† Aki Yamane,‡ Takayuki Ichikawa,*,† and Yoshitsugu Kojima† †
Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan
‡
S Supporting Information *
ABSTRACT: We have studied the structural, electronic, and vibrational properties of potassium amidoborane (KAB, KNH2BH3) using density-functional theory (DFT) calculations. The optimized structural parameters of KAB were found to be in good agreement with the experimental data. The electronic structure calculations suggested the covalent characters of N−H, B−H, and N−B as well as the ionic character of K. Moreover, we computed the 39K NMR parameters of KAB and several K-containing materials by using the gauge-including projector augmented wave (GIPAW) approach, and found that the precise theoretical predictions and assignments of 39K NMR parameters were possible. The DFT-GIPAW calculations were successfully applied to assign the 39K MAS NMR signals of KAB to the crystallographically inequivalent K1 and K2 sites.
I. INTRODUCTION Ammonia borane (AB, NH3BH3) and its derivatives, metal amidoboranes (MAB, MNH2BH3), have been considered to be the most promising chemical hydrogen storage material.1 The hydrogen capacity of AB is very high (19.6 mass %), and its dehydrogenation by solid-state thermolysis starts from ∼100 °C.2−5 However, the kinetics of the H2 release is slow (induction period), and it is highly contaminated with volatile impurities such as borazine (NH 3 BH 3 ) 3 , aminoborane NH2BH2, diborane B2H6, and ammonia NH3.2,4−6 Although the hydrogen capacity decreases to 10.9 and 7.5 mass % for LiNH2BH3 (LiAB) and NaNH2BH3 (NaAB), respectively, these metal amidoboranes drastically suppress such impurities and release high purity H2 even at lower temperature of ∼90 °C.7,8 The kinetics of the dehydrogenation is also improved. However, some papers showed that they still desorbed a small amount of NH3.6,9,10 KNH2BH3 (KAB) has a gravimetric disadvantage of hydrogen capacity (6.5 mass %), but it was shown to release the highest purity H2 at ∼90 °C, with the complete suppression of NH3 within the detection limit of thermal gas analysis.6,11 Because KAB was successfully prepared and its crystal structure was determined in 2010,11 the detailed information about its structural and electronic properties has not been well-explored in the theoretical manner. This situation is in contrast with AB and other MAB.12−21 Nuclear magnetic resonance (NMR) spectroscopy has been widely used to identify the local structure of the specific element with a nuclear spin I. Potassium is an important element in the field of inorganic and biological chemistry. Although 39K has the natural abundance of 93.7%, the practical applications of 39K NMR have been scarce in the solid-state chemistry because it has a quadrupolar spin of I = 3/2 and a © 2012 American Chemical Society
low resonance frequency of 18.7 MHz at 9.4 T. Alternatively, first-principles calculations have been developed to compute very reliable NMR chemical shifts in solids.22−30 This theoretical approach is now proved to be applicable to distinguish between the crystallographically distinct sites seen in the spectra, which is usually difficult to be assigned experimentally.31,32 In this article, we calculated the electronic structure of KAB within the framework of density functional theory (DFT). The 39K NMR parameters were then computed for the first time for KAB and K-containing reference materials to prove the successful signal assignment (and prediction) to distinct K sites in 39K MAS NMR spectroscopy.
II. CALCULATION AND EXPERIMENTAL PROCEDURES First-principles calculations were performed using the planewave basis pseudopotential method, as implemented in the CASTEP code.33,34 The generalized gradient approximation (GGA) was employed with PBE functional form for the exchange-correlation term.35 An energy cutoff of 600 eV was used for the plane-wave basis expansion. The Vanderbilt-type ultrasoft pseudopotentials were internally generated within the CASTEP code (on-the-f ly generation) to model the core− valence interaction.36 The core radii (and the valence configurations) were 0.8 (1s1), 1.4 (2s22p1), 1.5 (2s22p3), and 1.8 au (3s23p64s1) for H, B, N, and K, respectively. The structural information of KAB was taken from Diyabalanage et al.11 In the geometry optimization, the atomic positions were allowed to vary with the lattice parameters fixed at the Received: February 29, 2012 Revised: August 21, 2012 Published: September 4, 2012 20666
dx.doi.org/10.1021/jp302018h | J. Phys. Chem. C 2012, 116, 20666−20672
The Journal of Physical Chemistry C
Article
experimental values at room temperature. The residual force and stress were smaller than 0.01 eV/Å and 0.02 GPa in the final configuration. For phonon calculations, norm-conserving pseudopotentials were applied with the cutoff of 700 eV. The phonon frequencies were determined by evaluating the dynamical matrix based on the linear response method. The 39 K NMR parameters (isotropic chemical shift δiso and the quadrupolar coupling constant Cq) of KAB and six reference materials (KH, KNH 2 , KBH 4 , K 2 SO 4 , K 2 CO 3 , and K2MoO4)37−42 were calculated in the gauge-including projector-augmented wave (GIPAW) algorithm implemented in the NMR CASTEP code,22 which enables accurate calculations of the NMR parameters for solids by allowing the reconstruction of the all-electron wave function in the framework of the DFT. The k points for the Brillouin zone integration were sampled by Monkhorst-Pack scheme,43 with the grid spacing of
