Theoretical Study of the Stability and Electronic Structure of Al(BH4)n

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Theoretical Study of the Stability and Electronic Structure of Al(BH4)n=1f4 and Al(BF4)n=1f4 and Their Hyperhalogen Behavior C. Paduani,*,†,‡ M. M. Wu,§,† M. Willis,† and P. Jena† †

Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States ABSTRACT: Using density functional theory (DFT), we have systematically calculated the equilibrium geometries, electronic structure, and electron detachment energies of Al(BH4)n=1f4 and Al(BF4)n=1f4 at the B3LYP/6-311+G(2d,p) level of theory. The electron affinities of Al(BH4)n not only exhibit odd even alternation, just as seen in (BH4)n, but also, for n = 3 and 4, show a remarkable behavior: whereas the electron affinities of BH3 and BH4 are, respectively, 0.06 and 3.17 eV, those of Al(BH4)3 and Al(BH4)4 are 0.71 and 5.56 eV. Results where H is replaced by F are also very different. The electron affinities of BF3 and BF4 are, respectively, 0.44 and +6.86 eV, and those of Al(BF4)3 and Al(BF4)4 are 1.82 and 8.86 eV. The results demonstrate not only marked difference when H is replaced by F but also substantially enhanced electron affinities by almost 2 eV when BH4 and BF4 units are allowed decorate a metal atom, confirming the recently observed hyperhalogen behavior of superhalogen building blocks.

’ INTRODUCTION The halogens are a series of very similar nonmetal elements comprising fluorine (F), chlorine (Cl), bromine (Br), iodine(I), and astatine (At), which are known for their high reactivity. This property arises due to the atoms’ high electronegativity that provides a measure of the attraction of an atom for the electrons of another atom or molecule when they are brought to close proximity. The term “halogen” means “salt-former” because they react with most metals to form salts, although they can also form other types of compounds. Because halogens are highly reactive, they are all toxic elements, and as such can be harmful or lethal to biological organisms when used in sufficient quantities. This property is put to use to kill bacteria and other potentially harmful microorganisms through a process known as sterilization. Fluorine has the highest electronegativity of any element in the periodic table and is a corrosive and highly toxic gas. It is one of the most reactive elements attacking otherwise inert materials such as glass and forming compounds with heavier noble gases such as Xe. Except for astatine, the halogens are also known as diatomic molecules. They become less reactive down the group and all have 7 electrons in their outer shells, giving them an oxidation number of 1. Thus, they are very strong oxidizing agents meaning that when they form compounds, they accept electrons from the other atom to form a bond. Because of these properties, there has been constant search for molecules that can have electron affinities even larger than those of halogen atoms. This search led to the discovery of a class of molecules, known as superhalogens. Initially, these consisted of a metal atom at the core surrounded by halogen atoms. For this class of superhalogens a simple formula MXm+1 was proposed by Gutsev and Boldyrev.1 5 Here M is a main group or transition metal atom, X is a halogen atom, and m is the maximal formal valence of r 2011 American Chemical Society

the atom M. When the number of the halogen atoms exceeds the maximal valence of the metal atom, the molecule possesses electron affinities that are much larger than that of the halogen atoms. Recently, another class of electronegative molecules has been found whose electron affinities are even larger than that of the superhalogen moieties.6,7 These species termed hyperhalogens consist of a metal atom at the core surrounded by superhalogen moieties. Examples of hyperhalogens thus far include Au(BO2)n and Cu(BO2)n clusters.6,7 Other ways for increasing electron affinity were suggested earlier3 by increasing the number of halogen atoms such as in M2F11 (M being a metal atom with valence 5). These were also referred to as “hyperhalogens”. In this study we explore the possibility of creating new hyperhalogens by using BH4 and BF 4 as building blocks. Although both H and F require one electron to close their electronic shells, they have very different chemistry. The energy gain in adding an electron to BH4 is 3.17 eV whereas it is 6.86 eV in the case of BF4. Thus, although BF4 is classified as a superhalogen, BH4 is not. We have used Al(BH4)n=1f4 and Al(BF4)n=1f4 as examples. Our objective is to see if the electron affinities of these complexes can exceed that of their building blocks. Calculations based on density functional theory not only confirm this expectation but also illustrate interesting physics and chemistry when a systematic comparison is made between results of BHn and BFn versus Al(BH4)n and Al(BF4)n. We note that the aluminum borohydride Al(BH4)3 is a well-known covalent compound and is a volatile liquid that is used as rocket fuel or jet fuel. Received: July 5, 2011 Revised: August 10, 2011 Published: August 11, 2011 10237

dx.doi.org/10.1021/jp206330d | J. Phys. Chem. A 2011, 115, 10237–10243

The Journal of Physical Chemistry A

Figure 1. Bond lengths (Å) and natural bond orbital (NBO) charges of the BHn species corresponding to their equilibrium geometries. Red (white) spheres means B (H) atoms.

’ COMPUTATIONAL PROCEDURE Calculations were carried out using density functional theory and the Becke’s three-parameter hybrid exchange functional and the Lee Yang Par correlation functional (B3LYP) for the exchange correlation potential.8,9 The all-electron 6-311 +G(2d,p) basis set implemented in the GAUSSIAN 03 package10 was used for all atoms. The geometries of neutral and anionic clusters were optimized starting with several initial configurations and no symmetry constraints. The energies and forces at every atom site were converged to 10 6 eV and 10 2 eV/Å, respectively. Harmonic vibrational frequencies using the optimized geometry were also calculated to ensure that all calculated lowest-energy structures resided at local minima on the potential energy surface. The zero-point vibrational energy Z is estimated using the harmonic approximation. The nature of bonding in the studied species was investigated by calculating the atomic charges using the natural bond orbital (NBO) analysis. The electron affinity EA was calculated by taking the difference between the

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Figure 2. Bond lengths (Å) and natural bond orbital (NBO) charges (e) of the BFn species corresponding to their equilibrium geometries. Red (yellow) spheres means B (F) atoms.

Table 1. Symmetry, Electron Affinity (EA), Vertical Electron Detachment Energy (VDE), and Adiabatic Electron Affinity (AEA) Obtained at the B3LYP/6-311+G(2d,p) Level cluster

symmetry

BH BH BH2 BH2 BH3 BH3 BH4 BH4 BF BF BF2 BF2 BF3 BF3 BF4 BF4

C∞v C∞v C2v C2v D3h D3h C2v Td C∞v C∞v C2v C2v D3h C3v Cs Td

10238

EA (eV)

VDE (eV)

0.16

AEA (eV) 0.17

0.16 0.21

0.26 0.50

0.06

0.13 0.06

3.17

3.19 4.42

0.59

0.61 0.21

1.14

1.19 2.02

0.44

0.55 1.85

6.86

6.84 7.50

dx.doi.org/10.1021/jp206330d |J. Phys. Chem. A 2011, 115, 10237–10243


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Figure 3. Calculated equilibrium geometries of the Al(BH4)n clusters. Red spheres are B atoms, blue spheres are Al atoms, and white spheres are H atoms. Representative charges and bond lengths (Å) are also given.

total energy of the ground state of the anion and its corresponding neutral precursor, also in its ground state. The vertical detachment energy (VDE), on the other hand, is the energy required to detach an electron from the anionic cluster to its neutral, both calculated at the optimized structure of the anion. These results can be readily compared with photoelectron spectroscopy (PES) experiments when available.

’ RESULTS AND DISCUSSION A. BHn and BFn (n = 1 f 4). We begin our discussions with the geometries and energetics of BHn and BFn (n = 1 f 4) clusters. Although these studies have been done in the past,11 13 we have

repeated these calculations not only to validate our results by comparing with previous calculations but also to make the comparison with results on Al(BH4)n and Al(BF4)n free from ambiguity that can arise from the choice of methodology. The equilibrium geometries of neutral and anionic BHn and BFn (n = 1 f 4) clusters are shown in Figures 1 and 2, respectively. The calculated equilibrium geometries agree well with previous calculations.11 The bond lengths (Å) and NBO charges (e) are indicated in the figures. Upon electron attachment the average bond length stretches slightly in most anionic clusters. Furthermore, the geometries of the neutral precursors are similar to those of their corresponding anions. As can be seen from Figure 1, the geometries of BHn (n < 3) are planar. However, the geometry 10239



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Figure 4. Equilibrium geometries of the Al(BF4)n clusters. Red spheres are B atoms, blue spheres are Al atoms, and yellow spheres are F atoms. Representative charges and bond lengths are also indicated.

of BF3 is a trigonal pyramid (Figure 2), which agrees with results of earlier calculations.12,13 In the B H species the bond lengths for the neutrals decrease when H atoms are successively added to B, whereas for the anions this is only observed in BH3 . The charge on the B atom in the neutral BH3 cluster is about +0.33 whereas it is 0.21 in BH4. In the anion cluster the extra charge resides on the electron deficient B atom and the charge on the H atom decreases steadily as the number of H atoms in BHn increases. The corresponding charge distributions in BFn clusters are clearly different. With the exception of the anionic BF molecule the charges on the B atom are positive irrespective of whether BFn clusters are neutral or anionic. This is because F is far more electronegative than H and the added electron in the

anion prefers to go to the F atoms rather than electron deficient B atom. For the BF4 neutral the fourth F atom is bound very weakly, which is evidenced not only from the long B F bond but also from the nearly charge neutral state of this F atom. This situation is elevated in BF4 as the extra electron is now uniformly distributed over the four F atoms. We will see later that this charge distribution has significant impact on the electron affinities of BFn compared to that of BHn. We now discuss the relative stability of neutral and anion clusters. In Table 1 we see that BHn (n e 3) anions are slightly more stable than their neutrals, but this dramatically changes in BH4 where the anion is 3.17 eV more stable than the corresponding neutral. The difference in the total energies of the anion and 10240



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Table 2. Symmetry, Electron Affinity (EA), Vertical Electron Detachment Energy (VDE), and Adiabatic Electron Affinity (AEA) Obtained at the B3LYP/6-311+G(2d,p) Level cluster

symmetry

EA (eV) 0.22

Al(BH4)

C3v

Al(BH4)

C2v

Al(BH4)2

C2v C2

Al(BH4)3 Al(BH4)3

D3 C1

0.71

Al(BH4)4

C2

5.56

Al(BH4)

C2

Al(BF4)

C2v Cs

AEA (eV) 0.26

0.70

Al(BH4)2

Al(BF4)

VDE (eV)

2.28

2.38 3.55 0.93 2.67 5.43 7.18

0.69

0.70 1.50

Al(BF4)2

C2

Al(BF4)2

C1

Al(BF4)3 Al(BF4)3

D3 C1

1.82

Al(BF4)4

C2

8.86

Al(BF4)4

C1

3.42

3.44 5.45 1.88 5.61 8.86 9.96

neutral ground states yields the electron affinity (EA) and provides a measure of the energy gain due to the attachment of an additional electron. The electron affinities of BHn molecules are given in Table 1. The calculated electron affinity (EA) for BH, 0.16 eV, agrees well within the accuracy of our theoretical method, with the experimental value of 0.30 eV.14 Note that for BH and BH2 these values are small and nearly vanish for BH3, which is a closed shell molecule. The electron affinity of BH4, 3.17 eV, shows a sudden rise over the value of BH3 by nearly a factor of 20. Although this value does not make BH4 a superhalogen, this anomalously large electron affinity results from the distribution of the added electron equally over four H atoms. Because the electron affinity of BH4 is close to that of F, it can be classified as a pseudohalogen. It is also interesting to note that there is a significant difference between the EA and VDE of BH4. This is because in neutral BH4 one of the H atoms is very weakly bound whereas this bonding increases substantially in BH4 . The corresponding results for BFn clusters are very different. Here the electron affinity of BF and BF3 are negative, implying that the anions are less stable than the neutrals. Opposite is the case with BF2. This is because both BF and BF3 are closed shell systems whereas BF2 is an open shell system. These results agree well with earlier calculations.12 In particular, the metastability of BF is well-known.15,16 For the BFn molecules the calculated EA’s are much higher than those for the BHn species and the EA exceeds the value of F by almost a factor of 2 in BF4. With an electron affinity of 6.86 eV BF4 is a superhalogen, as has been described before. B. Al(BH4)n and Al(BF4)n (n = 1 f 4). As pointed out before, a new class of molecules with electron affinities even higher than superhalogens is possible when a metal atom is surrounded with superhalogen moieties. Even though BH4 is not a superhalogen, we wanted to see if the electron affinity of Al(BH4)4 can exceed the electron affinity of BH4 and be a superhalogen. Similarly, is Al(BF4)4 a hyperhalogen? To study this in a systematic way, we calculated the equilibrium geometries and total energies of Al(BX4)n clusters with X = H and F and n = 1 f 4. We begin with the geometries of these clusters in both neutral and anionic forms.

The equilibrium geometries of the Al(BH4)n=1f4 clusters are shown in Figure 3. The various bond lengths and NBO charges are also shown. These geometrical parameters of Al(BH4)3 agree well with those obtained from its crystal phase.17 23 Several observations can be made: (1) In all cases the structural unit of BH4 moiety is maintained, although there are some minor changes in the B H bond lengths given in Figure 1. (2) The charges on the Al atom in all neutral and anionic clusters, with the only exception of Al(BH4) , are positive. (3) The charge on the Al atom in neutral Al(BH4)4 is nearly same as that in anionic Al(BH4)4 . This implies that the extra charge on the anionic cluster is distributed almost entirely among the four BH4 units. Because the electron affinity of BH4 is similar to that of a halogen atom, neutral Al(BH4)4 behaves as a superhalogen with an electron affinity of 5.56 eV. We further note that the Al atoms carry positive charge, whereas the B atoms carry a negative charge, with approximately the same magnitude in Al(BH4)n=2f4. In the neutral molecules the bond lengths between B Al atoms changes from 2.13 to 2.19, 2.15, and 2.16 Å, as the number of BH4 radicals increases from 1 to 4. The trend is the opposite for the anions as the B Al bond lengths decrease from 2.44 to 2.41, 2.34, and 2.28 Å. The extra electron is distributed primarily over the B atoms in these molecules. With this, the charge on the Al atoms is observed to increase from 0.17 e in Al(BH4) up to +1.31 e in Al(BH4)4. The equilibrium geometries of Al(BF4)n=1f4 clusters shown in Figure 4 are similar for both the neutral and its corresponding anion, although the structures for n = 2 and 3 show much greater deviation than those seen in Figure 3. Nevertheless, the BF4 units retain their structural identity as seen in Figure 2. The charge transfers from Al to the BF4 moieties are also much larger than those to BH4 moieties shown in Figure 3. For Al(BF4)n clusters for n e 3, the extra charge in the anion is distributed among the Al atom as well as among the BF4 moieties. However, the charge on the Al atom in Al(BF4)4 is the same whether the cluster is a neutral or an anion. This means that the extra charge is distributed among the four BF4 moieties. This, as we will discuss later, is responsible for the large electron affinity of Al(BF4)4. The vertical detachment energy (VDE) is calculated as the energy difference between the ground state of the anion and its neutral counterpart at the anion geometry. The computed VDE values for the studied clusters are listed in Tables 1 and 2. The EA is always lower than the VDE, as the former corresponds to the energy difference between the ground states of neutral and anion. In most cases these values are close to each other. However, there are situations where the differences can be very large. This is what we see in the cases we have studied. This happens where the ground state geometries of the neutral and anion are very different. We see from Tables 1 and 2 that these differences can be as high as 3.79 eV. The adiabatic electron affinity (AEA) is defined as the EA but corrected for the corresponding zero-point vibrational energies. Within the Born Oppenheimer approximation, one can thus define AEA = EA + ΔZ, where ΔZ is the difference between the zero-point vibrational energy of the ground state for the neutral and anion. We note that these values are in very close agreement with the EA values, implying that the zero-point energy contributions are negligible. Next we examine the stability of the clusters against fragmentation into selected channels. This is done by calculating the differences in the total energies of the parent and daughters. The results are collected in Table 3. We note that energy gain in 10241



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Table 3. Bond Dissociation Energies of Neutral and Anionic BXn, Al(BX4)n (n = 1 f 4), X = H, F Calculated at the B3LYP/6-311+ G(2d,p) Level neutral channel

anion eV

channel

eV

BH2 f BH + H

4.17

BH2 f BH + H

3.94

BH3 f BH2 + H

5.10

BH3 f BH2 + H

4.87

BH4 f BH3 + H Al(BH4) f Al + BH4

0.96 4.34

BH4 f BH3 + H Al(BH4) f Al + BH4

3.93 1.39

Al(BH4)2 f Al(BH4) + BH4

2.17

Al(BH4)2 f Al(BH4) + BH4

1.28

Al(BH4)3 f Al(BH4)2 + BH4

4.20


1.74


0.44


1.95

BF2 f BF + F

4.98

BF2 f BF + F

2.63

BF3 f BF2 + F

7.24

BF3 f BF2 + F

3.32

BF4 f BF3 + F

0.04

BF4 f BF3 + F

3.42

Al(BF4) f Al + BF4 Al(BF4)2 f Al(BF4) + BF4

6.96 4.79

Al(BF4) f Al + BF4 Al(BF4)2 f Al(BF4) + BF4

0.79 1.35

Al(BF4)3 f Al(BF4)2 + BF4

7.10


2.05


0.17


0.17

Figure 6. Equilibrium geometries for B2F8, B2F8 , AlB2F8, and AlB2F8 . Representative charges and bond lengths are given.

Figure 5. Equilibrium geometries for B2H8, B2H8 , AlB2H8, and AlB2H8 . Representative charges and bond lengths are given.

binding a H(F) atom to BH2(F2) is larger than any other in the series. This is consistent with the valence of B being 3. In addition, we also note that BF4 is much less stable against ejection of a F atom than BH4 is against the ejection of a H atom. These trends continue when H(F) is replaced by BHn (Fn) units. We have also considered the possibility that BH4 and BF4 moieties may bind to each other to form B2H8 and B2F8 units that can then bind to the Al atom. If that were possible, the VDE and EA values of Al(BH4)n and Al(BF4)n can be very different from what is discussed. To examine these possibilities, we have first optimized the geometry of B2H8 and B2F8 clusters. In Figure 5a we show the optimized structures of neutral and anionic B2H8 clusters. In the neutral cluster two of the H atoms

form a molecular complex very weakly bound to the B2H6 unit. This is understandable, as diborane B2H6 is a well-known stable molecule. The anion is less stable than the neutral. In Figure 5b we also show the geometry of Al(B2H8) neutral and anionic isomers. These structures are 1.77 and 2.34 eV higher in energy than those shown in Figure 3. Corresponding results for B2F8 and Al(B2F8) are shown in Figure 6. In the neutral the bond length of the dimerized F F complex is 1.4 Å, which increases to 1.95 Å in the anion. In neutral AlB2F8 we find Al binds to two BF4 units. This structure is 0.01 eV higher in energy than that in Figure 4. The anion structure is same as that in Figure 4. These results indicate that BH4 and BF4 units will bind individually to the metal atom as shown in Figures 3 and 4.

’ CONCLUSIONS In summary, we have carried out a systematic study of the equilibrium geometries, electronic structure, relative stability, vertical detachment energy, and electron affinity of BHn, BFn, Al(BH4)n, and Al(BF4)n (n = 1 f 4) clusters. We conclude that 10242


The Journal of Physical Chemistry A although H and F are both monovalent atoms, their interaction with B is rather different. The electron affinity of BF4 is nearly twice that of BH4. This trend continues in Al(BH4)n and Al(BF4)n clusters as n increases from 1 to 4. In particular, the electron affinities of Al(BH4)4 and Al(BF4)4 clusters are significantly larger than those of BH4 and BF4. This confirms the earlier observation that a new class of highly electronegative molecules can be formed when a metal atom is decorated with superhalogen moieties. We also note that there is significant difference between the electron affinity and vertical detachment energies that originate from very different ground state geometries of neutral and anion species. Thus, one can see a clear distinction between adiabatic detachment energy, which corresponds to the transition from the anions’ ground state to the structurally similar neutral isomer lying near to the anion in the neutral potential energy surface, and the electron affinity, which results from the energy difference between the anion and neutral ground states. We hope that this work will stimulate experimental studies of these interesting highly electronegative species.

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(15) Zhao, X.-L.; Litherland, A. E.; Doupe, J. P.; Kieser, W. E. Nucl. Instrum. Methods B 2004, 223, 199–204. (16) Dreuw, A.; Sommerfield, T.; Cederbaum, L. S. J. Chem. Phys. 2002, 116, 6039–6044. (17) Al-Kahtani, A.; Williams, D. L.; Nibler, J. W.; Sharpe, S. W. J. Phys. Chem. A 1998, 102, 537–544. (18) Page, F. M.; Goode, G. C. Negative Ions and the Magnetron; Wiley: New York, 1969. (19) Schlesinger, H. I.; Brown, H. C.; Burg, A. B. J. Am. Chem. Soc. 1940, 62, 3421. (20) Beach, T. Y.; Bauer, S. H. J. Am. Chem. Soc. 1940, 62, 3440. (21) Ogg, R. A.; Ray, J. D. Discuss. Faraday Soc. 1955, 19, 215. (22) Maybury, P. C.; Ahnell, J. E. Inorg. Chem. 1967, 6, 1286. (23) Price, W. C. J. Chem. Phys. 1949, 17, 1044.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: paduani@fisica.ufsc.br. Present Addresses ‡

Departamento de Física, UFSC, Florian opolis, CEP 88040-900, SC, Brazil. § Department of Advanced Materials and Nanotechnology, and Center for Applied Physics and Technology, Peking University, Beijing 100871, China.

’ ACKNOWLEDGMENT This research was supported by grants from the Department of Energy and from the Brazilian Conselho Nacional de Desenvolvimento Cient ifico e Tecnol ogico (CNPq) and used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. ’ REFERENCES (1) Gutsev, G. L.; Boldyrev, A. I. Chem. Phys. 1981, 56, 277–283. (2) Gutsev, G.; Les, A.; Adamowicz, L. J. Chem. Phys. 1994, 100, 8925. (3) Gutsev, G. L.; Boldyrev, A. I. Chem. Phys. Lett. 1984, 108, 250. (4) Boldyrev, A. I.; Simons, J. J. Chem. Phys. 1993, 99, 4628. (5) Boldyrev, A. I.; von Niessen, W. Chem. Phys. 1991, 155, 71. (6) Willis, M.; Gotz, M.; Kandalam, A. K.; Gantefor, G; Jena, P. Angew. Chem., Int. Ed. 2010, 49, 8966. (7) Feng, Y.; Xu, H-Guang; Zheng, W.; Zhao, H.; Kandalam, A. K.; Jena, P. J. Chem. Phys. 2011, 134, 094309. (8) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (9) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377. Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (10) Frisch, M. J. et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (11) VanZee, R. J.; Williams, A. P.; Weltner, W., Jr. J. Chem. Phys. 1997, 107, 4756–4759. (12) Gutsev, G. L.; Jena, P.; Bartlett, R. J. Chem. Phys. Lett. 1998, 292, 289–294. (13) Ball, D. W. J. Mol. Struct. (THEOCHEM) 1995, 358, 95–98. (14) Reid, C. J. Int. J. Mass Spectrom. Ion Processes 1993, 127, 147–160. 10243


Theoretical Study of the Stability and Electronic Structure of Al(BH4)n

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