Article pubs.acs.org/JPCA
Probing Dative and Dihydrogen Bonding in Ammonia Borane with Electronic Structure Computations and Raman under Nitrogen Spectroscopy Katelyn M. Dreux,† Louis E. McNamara,† John T. Kelly,†,‡ Ashley M. Wright,† Nathan I. Hammer,† and Gregory S. Tschumper*,† †
Department of Chemistry and Biochemistry, University of Mississippi, University, Mississippi 38677-1848, United States S Supporting Information *
ABSTRACT: Although ammonia borane is isoelectronic with ethane and they have similar structures, BH3NH3 exhibits rather atypical bonding compared to that in CH3CH3. The central bond in ammonia borane is actually a coordinate covalent or dative bond rather than the conventional covalent C−C bond in ethane where each atom donates one electron. In addition, strong intermolecular dihydrogen bonds can form between two or more ammonia borane molecules compared to the relatively weak dispersion forces between ethane molecules. As a result, ammonia borane’s physical properties are very sensitive to the environment. For example, gas-phase and solid-state ammonia borane have very different BN bond lengths and BN stretching frequencies, which led to much debate in the literature. It has been demonstrated that the use of cluster models based on experimental crystal structures led to better agreement between theory and experiment. Here, we employ a variety of cluster models to track how the interaction energies, bond lengths, and vibrational normal modes evolve with the size and structural characteristics of the clusters. The M06-2X/6-311++G(2df,2pd) level of theory was selected for this analysis on the basis of favorable comparison with CCSD(T)/aug-cc-pVTZ data for the ammonia borane monomer and dimer. Fourteen unique fully optimized molecular cluster geometries, (BH3NH3)n≤12, and nine crystal models, (BH3NH3)n≤19, were used to elucidate how the local environment impacts ammonia borane’s physical properties. Computational results for the BN stretching frequencies are also compared directly to the Raman spectrum of solid ammonia borane at 77 K using Raman under liquid nitrogen spectroscopy (RUNS). A strong linear correlation was found to exist between the BN bond length and stretching frequency, from an isolated monomer to the most distorted BH3NH3 unit in a cluster or crystal structure model. Excellent agreement was seen between the frequencies computed for the largest crystal model and the RUNS experimental spectra (typically within a few wavenumbers).
1. INTRODUCTION Since it was first synthesized in 19551 ammonia borane has generated a large amount of interest both experimentally2−20 and computationally.21−47 The coordinate covalent, often called dative, bond between boron and nitrogen is an intramolecular bond with an interaction energy of −43.91 kcal mol−1 at the estimated CCSD(T) complete basis set (CBS) limit that arises from nitrogen donating a pair of electrons into the empty porbital on boron.46 The presence of both hydridic and protonic hydrogens in BH3NH3 allows for the formation of dihydrogen bonds (BHδ−···Hδ+N) between adjacent molecules. These interactions give rise to short intermolecular H−H distances in the crystal structure ( 0.98) between the two properties regardless of the which data set is considered. Figure 5b shows the closely related correlation between changes in BN bond lengths (ΔRBN) and BN stretching frequency shifts (ΔωBN) upon complexation for all the isolated clusters as well as the optimized BH3NH3 unit of the crystal structures in Figure 3. The equations in Figure 5b have intercepts approaching 0 which merely indicates that there is no frequency shift if the bond length does not change. Figure 5c shows the increase in electronic binding energy as the number of monomers increases in the isolated clusters. The
Table 5. Low Energy Intermolecular Lattice Modes (cm−1) of Solid Ammonia Borane
9-Cs
ref 18
ref 14
ref 12
this work Raman 77 K
16C1
19C1
IR 10 K
Raman 15 K
INS 30 K
359 337 319 256 235 209 175
337 337
399 334
221 212
202
210
185
171
156
141 128
159 150 116
150
87
100
94
BH3 twist
371
397 390
352
NH3 twist
198
236 219
193
BH3 rock
162
170
NH3 rock
152
Rotation
127
Translation
111
Translation
78
170 156 147 138 136 131 115 113 90 75
158 117 116 104
G
DOI: 10.1021/acs.jpca.7b03509 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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Figure 5. Plots of the correlation between various properties of ammonia borane clusters and crystal models. The table inserts for (a) and (b) contain the equations and R2 values for sets of data, including the isolated clusters (C), the crystal models (X), and the BH3NH3 monomer (M). The table inserts for (c) contain data from the isolated clusters, divided into three classes symmetry equivalent (Symm Equiv), centrally coordinated (Centrally Coord), and all isolated clusters (All). See section 3.6 for additional details.
ammonia borane and provides a more resolved spectrum, particularly in the BH and NH stretching regions. A rigorous comparison of methods and basis sets on the monomer and the C2h dimer was carried out (Tables S1 and S2 in the Supporting Information), which determined that the M06-2X/6-311+ +G(2df,2pd) level of theory provides good agreement between computational demand and an accurate description of the unusual inter- and intramolecular interactions of (BH3NH3)n. Although previous Hartree−Fock computations revealed that the BN stretching frequency in ammonia borane is exquisitely sensitive to its environment,32 the computations presented here reveal that Hartree−Fock dramatically overestimates the effect. This systematic analysis of both isolated clusters and crystal models of (BH3NH3)n with the M06-2X DFT method has demonstrated that system size (i.e., the magnitude of n) is not the main factor influencing the BN stretching frequency. Instead, the relative orientations of the BH3NH3 fragments plays a large role in determining the magnitude of the BN stretching frequency. For example, the central BH3NH3 in the
least-squares regression for different categories of isolated clusters are displayed in Figure 5c where “Symm Equiv” denotes data associated with the symmetry equivalent clusters, “Centrally Coord” denotes data associated with the centrally coordinated clusters, and “All” denotes the data for all the isolated clusters. The symmetry equivalent clusters grow in energy slightly faster than their centrally coordinated counterparts. As expected, the intercepts of the linear equations in Figure 5c approach 0 as n → 1 because the electronic binding energies is effectively 0 for a single molecule. There is no clear linear correlation between the BN frequency shifts and the number of fragments (n), the electronic binding energies or the normalized electronic binding energies of the isolated clusters. (See figures in the Supporting Information.)
4. CONCLUSIONS Raman under liquid nitrogen spectroscopy (RUNS) shows good agreement with previous low-temperature spectroscopic studies performed on the orthorhombic polymorph of H
DOI: 10.1021/acs.jpca.7b03509 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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small isolated cluster 3-Cs where the fragments adopt an antiparallel orientation experiences the same +109 cm−1 shift relative to the isolated monomer as the central fragment in the largest crystal model (19-C1) where the dipole moments of the fragments are more closely aligned. When the antiparallel motif in the isolated clusters is extended, the shift can actually grow as large as +176 cm−1 in 12-C3v. The sensitivity of the BN stretching frequency to its environment can also been observed within a given centrally coordinated isolated cluster of (BH3NH3)n where the BN stretch of the central fragment is consistently at least 100 cm−1 higher than that of the peripheral BH3NH3 fragments. In contrast, the surrounding environment does not appear to affect the crystal models as strongly as the isolated clusters. For example, the BN stretching frequency associated with crystal model 9-Cs changes by less than 10 cm−1 for crystal model 16-C1 in which two 9-Cs models are effectively merged to allow two BH3NH3 subunits to relax rather than just one. The 19-C1 crystal model has a BN stretching frequency in remarkably good agreement (within 2 cm−1) with the experimental RUNS data. For the intermolecular and lattice modes of solid ammonia borane, 16-C1 presents the best agreement with experiment likely due to the optimization of two central fragments rather than just one.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b03509. Monomer and 2-C2h dimer calibration information performed in this study (Tables S1 and S2), leastsquares regression plots of several properties (Figure S1a−S1c), optimized geometries, Cartesian coordinates, harmonic vibrational frequencies, IR intensities, and Raman activities are available for all the isolated clusters and crystal models (PDF)
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AUTHOR INFORMATION
Corresponding Author
*G. S. Tschumper. E-mail:
[email protected]. Phone: +1 662 915 7301. Fax: +1 662 915 7300. ORCID
Nathan I. Hammer: 0000-0002-6221-2709 Gregory S. Tschumper: 0000-0002-3933-2200 Present Address ‡
Wilhelm-Ostwald-Institut für Physikalische and Theoretische Chemie Universität Leipzig, Linnéstrasse 2, 04103, Leipzig (Germany). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by the National Science Foundation (CHE-0955550, CHE-1338056, OIA-1430364, OIA-1539035, CHE-1532079). The Mississippi Center for Supercomputing Research (MCSR) is also thanked for a generous allocation of time on their computational resources.
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