Ammonia borane in an external electric field: structure, charge transfer, and chemical bonding

Abstract

It is well known that ammonia borane (BH₃NH₃) is one of the simplest donor–acceptor complexes. The donor–acceptor bond (B–N bond) is formed by sharing lone pair electrons between BH₃ and NH₃ groups. In the present work, different strengths of external electric fields (E_ext) are applied along the Z-axis direction to investigate the electric field induced effect on the BH₃NH₃ structure and properties. Interestingly, we have found that the lone pair electrons can be gradually moved in the “channel” between the BH₃ and NH₃ groups by modulating E_ext. The donor–acceptor bond (B–N bond) is gradually elongated until it breaks when the E_ext ranges from 0.0000 to 0.0321 au. The B–N bond is the shortest at E_ext = −0.0519 au. Interestingly, the negative charge on BH₃ groups sharply decreases from 0.161 to 0.005 (for NH₃ groups decreases from 0.161 to 0.005) and the electron cloud of HOMO−2 exhibits an obvious transformation at “broking bonding” E_ext ranging from 0.0320 to 0.0321 au, indicating the electron movement induced by the electric-field is the main reason to change the structure and stability of BH₃NH₃. Further, atoms in molecules (AIM) analysis shows that the B⋯N interactions are similar to that of hydrogen bonds at E_ext = −0.0767 au and the iconicity of B–N bond in BH₃NH₃ is confirmed by its low electron density for B–N ρ_(B–N) (0.103–0.073) in the region of E_ext (0–0.025 au).

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1. Introduction

Since the synthesis and isolation of famous ammonia borane (BH₃NH₃) was realized by Shore and Parry,¹ its structure,² phase transition,^2i,3 hydrogen storage^3d,4 and dehydrogenation⁵ have been investigated by numerous experimental or theoretical studies. It is well known that BH₃NH₃ is one of the simplest electron donor–acceptor complexes^2b which is formed by the dative bond between the electron rich NH₃ as the Lewis acid and electron deficient BH₃ as the Lewis base.^2g The variations in the geometrical and electronic characteristics of the isolated BH₃NH₃ molecule along the donor–acceptor distance (d_B–N) have been analyzed at the MP2(fc)/6-311+G(2df,p) level.^2c The boron–nitrogen dative bond (B–N bond) is formed by electron transfer from the lone nitrogen pairs to the vacant orbital on boron^2e,f and B–N bond distance is 1.658 Å for the isolated BH₃NH₃ molecule which is inferred from a microwave spectrum.^2b,h However, for the crystal structure, the B–N bond distance is 1.597 Å which is reported by single crystal X-ray spectroscopy at 298 K.^2h,i The shortening of B–N distance can be attributed to the dipole–dipole interaction between “middle” BH₃NH₃ molecule and surrounding other BH₃NH₃ units.⁶ As a white crystalline solid at room temperature, BH₃NH₃ can be transformed into ordered orthorhombic structure at low-temperature and a transition to tetragonal structure accurse around 255 K.^3b The investigation of bonding in low-temperature orthorhombic phase of BH₃NH₃ finds the existence of a strong covalent bond between B–H atoms.^2d Significantly, hydrogen is a high chemical stable and hydrogen content (19.6 wt%)^3d,7 compound which is considered as an important hydrogen storage material. In addition, hydrogen release from BH₃NH₃ and its dimers can be controlled by modulated temperature.^2h,4b,8 Therefore, it is has been discovered as potential materials in fuel cell application.^4a,5a,7b,9 The synthesis of the compound bis-BN cyclohexane which is an unusually kinetically stable chemical hydrogen storage material with H₂ storage capacity of 4.7 wt% have been developed.¹⁰

On the other hand, the effect of an electric field is widely applied in the field of controlling and modulating the electric and magnetic properties of organic conductors,¹¹ hydrogen bonding complexes,¹² nanotubes,¹³ and graphene,¹⁴ and so on. Li group found that the degree of proton transfer for HN₃–HCI and H₂O–HCl can be controlled by modulating the strength of E_ext.¹² The application of a static external electric field have resulted in the enhancement of third-order nonlinear optical (NLO) response in symmetric diradicals.¹⁵ In addition, our group has investigated the second-order NLO response of HArF system optimized after different external electric field.¹⁶

From the above discussion, we can find that the formation of B–N bond may be ascribed to donation of the ammonia lone-pair electrons (see Scheme 1), and external electric field can be applied to control structures and properties of a certain matter. In present work, the E_ext is applied along Z-axis direction of BH₃NH₃ to control its electron movement, molecular structure and chemical bonding characteristic. It is our hope that this work may provide a new method to modulate the stability of BH₃NH₃ by controlling structure of donor–acceptor complex BH₃NH₃.

2. Computational details

The MP2 method have been applied to investigate the structure and properties of BH₃NH₃, and the equilibrium geometrical parameters at the MP2/6-311+G(2df,p) level are close to experimental geometry.^2c,g Therefore, the optimized geometric structures of BH₃NH₃ with all real frequency in a series of external electric fields −0.0767 to 0.0321 au (1 au = 5.142 × 10⁹ V cm⁻¹) were obtained at the level of MP2/6-311+G(2df,p). The external electric field was applied along Z-axis direction which is parallel to B–N bond. The natural bond orbital (NBO) analyses were performed at the QCISD/aug-cc-pVTZ level and the NBO population of BH₃NH₃ in/after different external electric fields has been listed in Table S2.† The topological properties of the electron density distribution were studied by atoms in molecules (AIM)¹⁷ theory at MP2/6-311+G(2df,p) level. In addition, the optimized structural parameters of (BH₃NH₃)₂ dimers in a series of external electric fields (−0.0200 to 0.0200 au) were also obtained at the level of MP2/6-311+G(2df,p). The external electric field direction is parallel to B–N bond. All of the calculations were performed with Gaussian 09 software package.¹⁸

3. Results and discussion

3.1 The structures of BH₃NH₃ in external electric field

The selected structural parameters of BH₃NH₃ in external electric fields have been listed in Table 1, and others are listed in Table S1 of the ESI.† When E_ext along Z-axis negative direction is applied to BH₃NH₃ and gradually increase from 0.0000 to 0.0519 au, the B–N bond is slowly shrinked from 1.649 to 1.606 Å (see Fig. 1 and Table 1). This decrease of B–N distance suggests that the interaction between B atom and N atom slowly gets stronger and the stability of BH₃NH₃ can be increased. The increased stability further indicates we can modulate the degree of hydrogen storage by controlling strength of E_ext along Z-axis negative direction. However, when E_ext further increases from 0.0519 to 0.0767 au, the B–N bond is gradually elongated by 0.047 Å. This change indicates that the interaction between B and N atom slowly becomes weak, and the BH₃ groups and NH₃ groups tend to be separated from each other. On the other hand, the external electric field is applied along Z-axis positive direction and E_ext increases from 0.0000 to 0.0321 au. From Fig. 1 and Table 1, under the region of E_ext (0.0000–0.0320 au), it can be obviously found that B–N distance is gradually elongated from 1.649 to 1.923 Å and the stability of BH₃NH₃ is gradually weaker and weaker. This variation of stability suggests that our work may provide a new method to modulate the degree of hydrogen release. After this, when E_ext further increases from 0.0320 to 0.0321 au, an interesting phenomenon can be found: B–N distance is elongated suddenly by 1.305 Å which is 5 times larger than the increment 0.253 Å (E_ext is from 0.0000 to 0.0320 au). This sudden crucial sharp increase of B–N distance apparently indicates that the B–N bond is suddenly broken at E_ext = 0.0321 au. The adjustable stability of BH₃NH₃ shows that our work may provide a potential method to control the degree of hydrogen release or storage.

Table 1 The optimized structural parameters and dipole moment (μ) of BH₃NH₃ determined at the MP2/6-311+G(2df,p) level in some important external electric fields^a

Eext × 10^−4 au	r(B–N)	r(B–H)	r(N–H)	H–B–H	H–N–H	H1–B–H2–H3	H′1–N–H′2–H′3	μ1	μ2
a The bond distance are in angstroms, angles; in degrees; dipole moment, in Debye.
−767	1.653	1.263	1.045	106.7	101.0	113.9	103.7	13.33	6.30
−519	1.606	1.239	1.031	108.1	102.8	116.8	106.5	10.16	6.14
0	1.649	1.208	1.017	113.6	107.9	131.9	116.3	5.44	5.44
320	1.902	1.197	1.009	118.3	114.9	154.7	136.8	1.80	3.97
321	3.207	1.192	1.021	119.7	102.3	168.3	105.7	4.20	2.01

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Fig. 1 Variations of B–N distance with the increase of E_ext.

In order to further clearly show structural variations of BH₃NH₃, the B–H bond distance and dihedral angle H₁–B–H₂–H₃ of BH₃ groups (N–H bond distance and dihedral angle H′₁–N–H′₂–H′₃ of NH₃ groups) in different external electric fields have been analyzed, respectively. The trends of the dihedral angle H₁–B–H₂–H₃ and H′₁–N–H′₂–H′₃ are shown in Fig. 2. For NH₃ groups (see Fig. 2 and Table 1), in the range of E_ext (0.0000–0.0320 au), the dihedral angle H′₁–N–H′₂–H′₃ gradually increases from 116.3 to 136.8° which indicates that structure of NH₃ groups gradually tend to be coplanar. However, when E_ext further increases to 0.0321 au, dihedral angle H′₁–N–H′₂–H′₃ suddenly decreases to 105.7° which indicates that NH₃ groups still has a trigonal pyramidal shape at E_ext = 0.0321 au. For BH₃ groups (see Fig. 2 and Table 1), when the E_ext gradually increase from 0.0000 to 0.0321 au, the B–H bond is shrinked from 1.208 to 1.192 Å and the values of dihedral angle H₁–B–H₂–H₃ increase from 131.9 to 168.3°. As we all known, the structure of borane (BH₃) is trigonal planar (D₃h molecular symmetry) with experimentally determined by the B–H bond length of 119 pm.¹⁹ Hence, it can be obviously found that the shape of BH₃ groups gradually tends to be coplanar with the increasing of E_ext along Z-axis positive direction.

Fig. 2 Variation of dihedral angle H₁–B–H₂–H₃ and H′₁–N–H′₂–H′₃ with the increase of E_ext.

The optimized structural parameters of (BH₃NH₃)₂ dimers in different external electric fields have been shown in Table S4 of the ESI.† From Table S4,† we can find that the B–N bond of BH₃NH₃ monomer is longer than (BH₃NH₃)₂ dimmers, indicating that the electron correlation have effected B–N bond length. In the region of E_ext (0.0000–0.0100 au), the B–N bond is gradually elongated and the B–N bond length are all shorter than 1.649 Å. This change suggests that the effects of electron correlations are stronger than the E_ext. However, when the E_ext increases to 0.0150 au, the B–N bond length is longer than 1.649 Å, indicating that the effect of E_ext on B–N bond is more obvious than electron correlations. To explain the variations of the structure of BH₃NH₃ in different external electric fields, we further analyze the charge transfer between BH₃ and NH₃ groups.

3.2 The charge transfer analysis

A The valence molecular orbital of BH₃NH₃. The formation of B–N bond has been explained by the interaction of occupied molecular orbital of NH₃ and unoccupied molecular orbital of BH₃.²⁰ The boron atom having three valence is subject to sp-orbital hybridization with an empty orbital to interact with lone nitrogen pairs through electrostatic interaction. The valence molecular orbital of BH₃NH₃ in some important E_ext has been shown in Fig. 3. From Fig. 3, we can easily find that the linear combinations of the lowest unoccupied molecular orbital (LUMO) of BH₃ groups and the highest occupied molecular orbital (HOMO) of NH₃ groups constitute the bonding orbital (HOMO−2) and the antibonding orbital (LUMO+3) of BH₃NH₃. When the external electric field is applied along the Z-axis positive direction and E_ext gradually increases from 0.0000 to 0.0320 au, the electron cloud of HOMO−2 is not obviously shifted. However, when E_ext further increases from 0.0320 to 0.0321 au, the electron cloud of HOMO−2 exhibits a dramatic transformation. With the increase of E_ext, the electric cloud of HOMO−2 is more and more concentrated on NH₃ groups, which is in agreement with the trend of B–N distance and dihedral angle H₁–B–H₂–H₃ (or H′₁–N–H′₂–H′₃). It is shown that the lone nitrogen pairs of NH₃ groups can be moved along electric field direction by modulating E_ext. Then, the electrostatic interaction of B–N bond are modulated, which result in the elongation of B–N distance and the change of dihedral angle dihedral angle H₁–B–H₂–H₃ (or H′₁–N–H′₂–H′₃).

Fig. 3 The molecular orbitals of BH₃NH₃ in external electric field.

B Dipole moment. The dipole moment of BH₃NH₃ at the MP2/6-311+G(2df,p) level are listed in Table 1 and Table S1.† The variations of the dipole moment (μ₁) of BH₃NH₃ in external electric field and the dipole moment (μ₂) of BH₃NH₃ after external electric field have been shown in Fig. 4. From Fig. 4, it is obvious that the values of μ₁ and μ₂ all gradually decrease when the E_ext is from −0.0767 to 0.0320 au. Most importantly, the values of μ₁ are different from μ₂ in the same E_ext, and the difference of μ₁ and μ₂ values in the same E_ext are gradually increase when the E_ext increase from 0 to −0.0767 au. It suggests that the movement of electrons caused by the external electric field results in the change of dipole moment.

Fig. 4 Variations of dipole moment (μ₁) of BH₃NH₃ in external electric fields and dipole moment (μ₂) of BH₃NH₃ after external electric fields.

C Natural bond orbital (NBO) analyses. The natural bond orbital (NBO) analyses of BH₃NH₃ at the MP2/6-311+G(2df,p) level are listed in Table 2 and Table S2.† The variations of NBO analyses of BH₃NH₃ in/after external electric field have been shown in Fig. 5. From Fig. 5 and Table 2, we can obviously find that the charge locates at BH₃ and NH₃ groups of BH₃NH₃ after external electric field have no obvious change when the E_ext is from −0.0767 to 0.0320 au. However, for BH₃NH₃ in external electric field along Z-axis positive direction, the positive charge on NH₃ groups gradually decreases from 0.316 (E_ext = 0 au) to 0.161 (E_ext = 0.0320 au) and the negative charge on BH₃ groups decreases from 0.316 (E_ext = 0 au) to 0.161 (E_ext = 0.0320 au). After this, when E_ext increases from 0.0320 au to 0.0321 au (see Fig. 5), the negative charge on BH₃ groups suddenly decreases from 0.161 to 0.005 (for NH₃ groups is from 0.161 to 0.005). Significantly, the variations of NBO charge population are exactly accordance with the elongation of B–N bond length and the change of dihedral angle H₁–B–H₂–H₃ (or H₁′–N–H′₂–H′₃). It is shown that the electron movement from BH₃ groups to NH₃ groups results in the change of electrostatic interaction. And then, the B–N bond is gradually elongated and the shapes of BH₃ groups and NH₃ groups show tends to be coplanarity, respectively. When the BH₃ groups and NH₃ groups separated from each other (E_ext = 0.0321 au), the gradually decreased interaction lead to the shape of BH₃ groups is nearly coplanar and NH₃ groups reform its trigonal pyramidal shape. On the other hand, by adding external electric field along Z-axis negative direction, charge on NH₃ groups gradually increases from 0.316 (E_ext = 0 au) to 0.706 (E_ext = 0.0767 au) and the negative charge on BH₃ groups increase from 0.316 (E_ext = 0 au) to 0.706 (E_ext = 0.0320 au). Obviously, lone nitrogen pairs of NH₃ groups are gradually moved to electron-deficient BH₃ groups to form stable octet configuration. Due to the existence of the repelling interaction, this electron movement results in slow enhance of interaction between BH₃ groups and NH₃ groups. Therefore, the B–N bond is slowly shrinked when the E_ext along Z-axis negative direction increases in appropriate range. In addition, the partial negative charges on N atom show very special trend which increase from 0.911 (E_ext = −0.0767 au) to 1.18 (E_ext = 0.0321 au) and the charge on B atom increase from −0.186 to 0.432. How do these interesting electron movement affect the chemical bonding in BH₃NH₃?

Table 2 NBO charge q₁ of BH₃NH₃ in external electric fields and NBO charge q₂ of BH₃NH₃ after external electric fields

Eext × 10^−4 au	q1(NH3)	q1(BH3)	q1(N)	q1(B)	q2(NH3)	q2(BH3)
−767	0.706	−0.706	−0.911	−0.186	0.318	−0.318
−519	0.449	−0.449	−0.997	0.044	0.329	−0.329
0	0.316	−0.316	−0.935	0.021	0.313	−0.313
320	0.161	−0.161	−1.033	0.162	0.237	−0.237
321	0.005	−0.005	−1.180	0.432	0.018	−0.017

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Fig. 5 The NBO charge of BH₃ groups and NH₃ groups in BH₃NH₃.

3.3 The chemical bonding analysis

To analyze the chemical bonding of BH₃NH₃ under a series of E_ext, atoms in molecules (AIM) analysis is performed by using the calculation results at MP2/6-311+G(2df,p) level. The results of the electron density ρ and the Laplacian ∇²ρ_(r) at the bond critical points (BCP) have been shown in Tables 3 and S2.† The Laplacian of the electron density and electron density ρ of the bond critical points (BCP) have been used to analyze the types of chemical bonds.^2a,21 For electrostatic interaction (ionic bond, hydrogen bonds, and van der Waals interactions) the Laplacian of the electron density ∇²ρ_(r) is positive, while the ∇²ρ_(r) is negative for the covalent bond. The ∇²ρ_(r) value of hydrogen bond lies in the proposed range of 0.014–0.139 au. From Table 3, in the range of −0.0767 to 0.0250 au, we obviously find that the ∇²ρ_(r) for B–N is from 0.124 to 0.436 indicating that the B–N bond is electrostatic interaction. When the E_ext is from 0 to 0.0250 au, the iconicity of B–N bond in BH₃NH₃ is confirmed by its low ρ_(B–N) (0.103–0.073).^2a In addition, it is obvious that the value (0.134) of ∇²ρ_(r) for B–N lies in the range of 0.014–0.139 au when E_ext is 0.0767 au. This interesting phenomenon indicates that the B⋯N interaction are similar to that of hydrogen bonds at E_ext = −0.0767 au. It is well known that covalent bonding is observed between B–H atom (N–H atom) in BH₃NH₃.^2d From Table 3, without external electric field, the negative values of ∇²ρ_(r) for B–H and ∇²ρ_(r) for N–H just demonstrate that covalent bond exists between B–H atom (N–H atom). Furthermore, in the region of −0.0767 to 0.0321 au, the negative values of ∇²ρ_(r) for B–H and ∇²ρ_(r) for N–H confirm that the B–H bond and N–H bond are always covalent bond when different external electric fields are applied to BH₃NH₃. In addition, the detailed fragment contributions to the bonding orbital (HOMO−2) of the BH₃NH₃ have been analyzed and shown in Fig. 6.

Table 3 The Laplacian of the electron density (∇²ρ_(r)) and electron density (ρ) of the bond critical points (BCPs)

Eext × 10^−4 au	∇^2ρ(B–N)	∇^2ρ(B–H)	∇^2ρ(N–H)	ρ(B–N)	ρ(B–H)	ρ(N–H)
−767	0.124	−0.053	−1.809	0.134	0.140	0.310
−519	0.314	−0.063	−1.828	0.136	0.151	0.314
0	0.494	−0.128	−1.775	0.103	0.169	0.342
50	0.494	−0.137	−0.137	0.099	0.170	0.343
100	0.495	−0.146	−0.146	0.094	0.172	0.343
150	0.487	−0.155	−0.155	0.088	0.173	0.344
200	0.469	−0.167	−0.167	0.081	0.174	0.344
250	0.436	−0.180	−0.180	0.073	0.176	0.344
300		−0.200	−0.200		0.178	0.344
320		−0.217	−1.775		0.180	0.344
321		−0.288	−1.621		0.186	0.335

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Fig. 6 The detailed fragment contribution to the bonding orbital HOMO−2 of BH₃NH₃.

In order to further reveal the relation between the electric structure and geometric structure for BH₃NH₃, the localized orbital locator²² (LOL) and electron location function²³ (ELF) analyses over BH₃NH₃ were carried out. Fig. 7 shows the LOL and ELF schemes of BH₃NH₃ in some important external electric fields. For BH₃NH₃, both the LOL and ELF maps indicate that the significant coordination electron density localization between N atom and B atom get weaker and weaker when the E_ext increases from 0 au to 0.321 au. Most importantly, electron density localization is not observed between N atom and B atom, indicating bond breaking when the external electric field increases to 0.0321 au. These results clearly explain the increase of B–N distance when the external electric field along Z-axis positive is applied to BH₃NH₃.

Fig. 7 The ELF and LOL models of BH₃NH₃ in different external electric fields.

Scheme 1 Chemical structure of BH₃NH₃.

4. Conclusions

In present work, we added a series of different strength of external electric fields along Z-axis direction of BH₃NH₃ to investigate the electric field induced effect on electron transfer, structure and properties of BH₃NH₃. Interestingly, when the E_ext along Z-axis positive direction increase from 0 to 0.0321 au, the B–N bond is gradually elongated until broken at E_ext = 0.0321 au. The change of the B–N bond distance indicates the stability can be modulated by adding E_ext. With the increase of E_ext along Z-axis positive direction, the electric cloud of HOMO−2 is more and more concentrated on NH₃ groups and the positive charge on NH₃ groups decreases from 0.316 (E_ext = 0.0320 au) to 0.005 (E_ext = 0.0321 au) and the negative charge BH₃ groups decreases from 0.316 (E_ext = 0.0320 au) to 0.005 (E_ext = 0.0321 au). The variations of NBO charge are in agreement with the trend of B–N distance, which indicates the electrostatic interaction of B–N bond are modulated by controlling electron movement induced by electric-field. From the atoms in AIM analysis, in the range of E_ext (0–0.0250 au), the iconicity of B–N bond in BH₃NH₃ is confirmed by its low ρ_(B–N) (0.103–0.073). In addition, the B⋯N interaction are similar to that of hydrogen bonds at E_ext = −0.0767 au due to the value (0.134) for ∇²ρ_(r) for B–N lies in the range of 0.014–0.139 au. As such, our work may put forward a new method to control the structure and chemical bonding characteristic for donor–acceptor complex BH₃NH₃.

Acknowledgements

The authors gratefully acknowledge financial support from National Science Foundation of China (NSFC) (21473026), the Science and Technology Development Planning of Jilin Province (20140101046JC), and H.-L. X. acknowledges support from the Hong Kong Scholars Program XJ2013007. And Project funded by China Postdoctoral Science Foundation (2014M560227).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra10156e

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