Electron and Hydrogen Transfer in Small Hydrogen Fluoride Anion

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Electron and Hydrogen Transfer in Small Hydrogen Fluoride Anion Clusters Xin Bai, Ming Ning, and Richard E. Brown* Department of Chemistry, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States ABSTRACT: A new stable structure has been found for the anion clusters of hydrogen fluoride. The ab initio method was used to optimize the structures of the (HF)3 , (HF)4 , (HF)5 , and (HF)6 anion clusters with an excess “solvated” electron. Instead of the wellknown “zig-zag” (HF)n structure, a new form, (HF)n 1F 3 3 3 H, was found with lower energy. In this new form, the terminal hydrogen atom in the (HF)n chain is separated from the other part of the cluster and the inner hydrogens transfer along the hydrogen bonds toward the outside fluoride. The negative charge also transfers from the terminal HF molecule of the chain to the center fluoride atoms. The (HF)n clusters for n = 4, 5, and 6 have not yet been observed experimentally. These results should assist in the search for these systems and also provide a possible way to study the proton and electron transfer in some large hydrogen bonding systems.

’ INTRODUCTION Small hydrogen fluoride clusters can host an excess electron at their positive polar ends, forming anions in the “zig-zag” structure,1 3 as shown in Figure 1. Generally, these “zig-zag” chains are considered to be the most stable structure for the small hydrogen fluoride anion clusters.3 5 Bowen and his co-workers have observed the existence of the anions (HF)2 and (HF)3 experimentally in the late 1990s.6,7 However, no experimental evidence for the theoretically more stable (HF)4 , (HF)5 , and (HF)6 anions have been reported in the past decade. Such anion clusters for n = 2 6 have been studied at length by other authors. Works by Boldyrev, Gutowski, Skurski, Jordan, and Simons have shown that the now familiar zigzag structures to be the most stable.1 8 They also studied at length the symmetric structures where the electron is trapped in a cavity surrounded by HF monomers with their positive ends orientated toward the cavity center. However, this research shows that besides these structures, the (HF)n (n = 3 6) clusters can lose their end hydrogen atom and re-form as the corresponding (HF)n 1F 3 3 3 H clusters. Some of these systems have lower energies than the corresponging zigzag structures. Moreover, the transfer of the proton and the charge occurs along the hydrogen bond chain. This suggests a possible way to study hydrogen and electron transfer in some large biological systems. A recent review article gives a thorough background on the experimental and theoretical work that has been published on anion clusters including the systems discussed here.9 ’ COMPUTATIONAL METHOD The Gaussian 98/03 packages were used for this study. All the geometries were optimized at the MP2 level10 with the 6-311G++(d,p) basis set.11 The QST3 method12 was used to find the transition state, and all of the single point energy calculations were carried out at the CCSD(T) level with the aug-cc-pvdz basis r 2011 American Chemical Society

set and additional diffuse functions7,13 on the end hydrogen atoms.

’ RESULTS AND DISCUSSION The structures of the (HF)3 and (HF)2F 3 3 3 H anions and their transition state are shown in Figure 2. Their bond and hydrogen bond lengths and charge distributions are given in Table 1. In the optimized (HF)2F 3 3 3 H structure, the end hydrogen atom H1 is separated from F1 by 2.86 Å and the H2 atom moves toward F1, forming a new H F bond with a 1.01 Å bond length, whereas the old H2 F2 bond becomes a hydrogen bond with a length of 1.36 Å. The F2 atom loses its hydrogen to F1 and becomes anionic, and the H3 atom moves to F2, forming a shorter hydrogen bond (1.37 Å compared to the original value of 1.69 Å) and a longer H F bond (1.01 Å compared to the previous value of 0.94 Å). The negative electron charge also transfers from the first hydrogen fluoride molecule to the center F2 atom. So the structure is FH 3 3 3 F 3 3 3 HF anion plus a separated single hydrogen atom, as illustrated in Figure 2. The electron charge transfer also indicates that the hydrogen exchange between the F atoms involves the process where the hydrogen atom loses its own electron and transfers to the adjacent F atom as H+. So it is a proton transfer along with an electron transfer. This is a very important factor in many biological processes.14,15 The total energies for these two anion structures are 300.8180 and 300.8126 au, respectively. The (HF)3 cluster has about 3.39 kcal/mol lower energy than the corresponding (HF)2F 3 3 3 H cluster, and the neutral (HF)3 cluster with a free electron has 300.8161 au total energy, which is slightly higher than the (HF)3 cluster but lower than the (HF)2F 3 3 3 H cluster. Received: March 30, 2011 Revised: July 23, 2011 Published: August 19, 2011 10596

dx.doi.org/10.1021/jp202968n | J. Phys. Chem. A 2011, 115, 10596–10599

The Journal of Physical Chemistry A

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Figure 1. Zigzag structures for the small (HF)n anion clusters.

Figure 2. Conformations for (HF)3 , (HF)2F 3 3 3 H, and their transition state clusters.

Table 1. Bond Lengths and Atomic Charges on (HF)3 and (HF)2F 3 3 3 Ha distance (Å) (HF)3

a

(HF)2F 3 3 3 H charge (HF)3

(HF)2F 3 3 3 H

H1F1

0.95

2.86

H1

0.47

0.01

F1H2

1.63

1.01

F1

0.51

0.60

H2F2 F2H3

0.95 1.69

1.36 1.37

H2 F2

0.43 0.49

0.60 0.99

H3F3

0.94

1.01

H3

0.47

0.58

F3

0.43

0.58

The bold numbers indicate where the excess electron lies.

Figure 3. Energy barrier for the (HF)3 f (HF)2F 3 3 3 H transition state.

This result supports the experimental observation of (HF)3 as a metastable anion in the gas phase whereas the (HF)2F 3 3 3 H cluster is not observed. We also performed a transition state

search for (HF)3 f (HF)2F 3 3 3 H using the QST3 method in the Gaussian03 package (Figure 3). The conformation of the transition state is also included in Figure 2. In this transition state, the H1 atom has moved away from F1 to a distance of 1.277 Å and the H2 atom has moved toward F1 along the hydrogen bond and lies between F1 and F2 at distances of 1.270 and 1.035 Å, respectively. The energy of this transition state is 300.7985 au, which is about 12.24 kcal/mol higher than the zigzag (HF)3 cluster and 8.85 kcal/mol higher than that for the (HF)2F 3 3 3 H anion cluster. The structure of the (HF)4 cluster and the corresponding (HF)3F 3 3 3 H cluster are shown in Figure 4. The end H1 atom is separated from F1 at a large distance of 2.91 Å. As a consequence, the H2 atom transferred to F1 and the H3 moves to F2 as well. The F2 and F3 atoms share H3 in middle at distances of 1.13 Å for both to form a F H F center in the zigzag chain, and the H4 atom moves toward F3, forming a shorter hydrogen bond (1.46 Å compared to the original value of 1.69 Å) and a longer H F bond (0.96 Å compared to the original value of 0.93 Å). The negative electron charge transfers from the first hydrogen fluoride molecule to the F2 H3 F3 center, as shown in Table 2. The energies for the (HF)3F 3 3 3 H, (HF)4 and neutral (HF)4 clusters are 401.1078, 401.0967, and 401.0994 au, respectively, as shown in Table 5. Unlike the (HF)3 and the (HF)2F 3 3 3 H anion clusters, the (HF)4 anion has a higher energy than both the (HF)3F 3 3 3 H anion and the neutral (HF)4 with a free electron. The (HF)3F 3 3 3 H cluster has the lowest energy, which is about 6.97 kcal/mol lower than the (HF)4 cluster and 5.27 kcal/mol lower than the neutral (HF)4 cluster with free electron. So it is the most stable structure for the (HF)4 cluster binding the excess electron and should be experimentally observable. Similar proton and charge transfers are also found in the (HF)5 and (HF)6 clusters, as shown in Figure 5. The H1 F1 bonds are broken, and the H1 atoms move away from the clusters. The inner H2 and H3 atoms transfer along their respective hydrogen bonds to the adjacent F atoms, and the negative charge transfers from the polar end to the center of the zigzag chains. Both the (HF)4F 3 3 3 H and (HF)5F 3 3 3 H clusters have lower energies than the corresponding (HF)5 and (HF)6 clusters by 13.55 and 17.82 kcal/mol, respectively, whereas the neutral (HF)5 and (HF)6 clusters with free electrons also have 2.32 and 1.69 kcal/mol lower energy than the corresponding (HF)5 and (HF)6 anions (Tables 3 and 4). This means that the (HF)4F 3 3 3 H and (HF)5F 3 3 3 H structures, which hold the excess electron in the middle of the chains, are more stable than the zigzag (HF)5 and (HF)6 structures, which hold the excess electron at their poplar ends. The lack of 10597

dx.doi.org/10.1021/jp202968n |J. Phys. Chem. A 2011, 115, 10596–10599


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Figure 4. Conformations for the (HF)4 and (HF)3F 3 3 3 H clusters.

Table 2. Bond Lengths and Atomic Charges on (HF)4 and (HF)3F 3 3 3 Ha distance (Å)

a

(HF)3F 3 3 3 H

(HF)4

charge

(HF)4

(HF)3F 3 3 3 H

H1F1

0.98

2.91

H1

0.40

0.01

F1H2

1.49

0.96

F1

0.57

0.52

H2F2

0.96

1.46

H2

0.49

0.53

F2H3

1.53

1.13

F2

0.55

-0.87

H3F3

0.95

1.13

H3

0.55

0.73

F3H4

1.69

1.46

F3

0.52

0.87

H4F4

0.93

0.96

H4

0.42

0.51

F4

0.41

0.50


Figure 5. Conformations for the (HF)5 , (HF)6 , (HF)4F 3 3 3 H, and (HF)5F 3 3 3 H clusters.

Table 3. Bond Lengths and Atomic Charges on (HF)5 and (HF)4F 3 3 3 Ha distance (Å) (HF)5

a


Table 4. Bond Lengths and Atomic Charges on (HF)6 and (HF)5F 3 3 3 Ha

(HF)4F 3 3 3 H

distance (Å) (HF)6


(HF)5F 3 3 3 H

H1F1

0.95

2.89

H1

0.39

0.01

H1F1

0.98

3.52

H1

0.38

0.01

F1H2

1.55

0.95

F1

-0.58

0.47

F1H2

1.44

0.94

F1

0.49

0.45

H2F2

0.96

1.54

H2

0.52

0.47

H2F2

0.98

1.60

H2

0.53

0.45

F2H3

1.53

1.03

F2

0.55

0.72

F2H3

1.45

0.99

F2

0.53

0.63

H3F3

0.96

1.26

H3

0.51

0.68

H3F3

0.96

1.35

H3

0.59

0.62

F3H4

1.57

1.26

F3

0.50

0.95

F3H4

1.52

1.13

F3

0.58

0.87

H4F4 F4H5

0.95 1.69

1.03 1.55

H4 F4

0.41 0.41

0.69 0.71

H4F4 F4H5

0.95 1.59

1.13 1.35

H4 F4

0.54 0.58

0.73 0.87

H5F5

0.94

0.95

H5

0.57

0.47

H5F5

0.94

0.99

H5

0.50

0.63

F5

0.58

0.46

F5H6

1.73

1.60

F5

0.58

0.62

H6F6

0.93

0.94

The bold numbers indicate where the excess electron lies. a

any experimental observation for the (HF)5 and (HF)6 anions may be linked to the fact that they are energetically less preferred than both the corresponding (HF)n 1F 3 3 3 H anion and the neutral (HF)n cluster with a free electron. . These results are supported by several other published results. Recent work by Freza and Skurski show that the anionic (HF)nF structures can

H6

0.40

0.45

F6

0.40

0.44


have vertical detachment energies of nearly 14 eV.16 Although these structures have a formula similar to those discussed here (without the detached hydrogen atom), the Freza Skurski structures show a much higher degree of symmetry. Additionally, 10598



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Table 5. Total Energies for (HF)n 1F 3 3 3 H, (HF)n , and (HF)n + e n=3

n=4

n=5

n=6

Etotal((HF)n 1F 3 3 3 H) (au) Etotal((HF)n )) (au)

300.8126

401.1078

501.3973

601.6826

300.8180

401.0967

501.3757

601.6542

Etotal((HF)n + e) (au)

300.8161

401.0994

501.3794

601.6569

these comments are reinforced by the fact that the HF2 anion has the strongest known hydrogen bond and the HF2 system has an electron affinity of nearly 5 eV.6,17,18

’ CONCLUSIONS In summary, the (HF)n 1F 3 3 3 H clusters have significantly lower energy than both the (HF)n anion clusters and the neutral (HF)n clusters for n = 4 6, as shown in Table 5. This means that after binding the excess electron, the large (HF)n zigzag anion clusters could lose their end hydrogen and become the more stable (HF)n 1F 3 3 3 H anion clusters. Moreover, except for (HF)3 , the zigzag (HF)4 , (HF)5 , and (HF)6 anion clusters have higher energies than their corresponding neutral clusters with free electrons. This could in part explain why the zigzag (HF)3 was observed as a metastable structure for the “solvated electron” whereas the larger (HF)4 , (HF)5 , and (HF)6 anion clusters have not been observed experimentally. Instead of being bound at the polar end of the (HF)n anions, the excess electron transfers to the central fluorine atoms in the (HF)n 1F 3 3 3 H clusters. It is noteworthy that as each proton transfers from its bonded fluorine atom to the adjacent fluorine atom, the basic zigzag profile is maintained. This is, however, a more stable anion structure that can better “solvate” the excess electron.

(9) Simons, J. J. Phys. Chem. A 2008, 112 (29), 6401. (10) Møller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618. (11) Krishnan, R; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (12) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. J. Comput. Chem. 1996, 17, 49. (13) Skurski, P.; Gutowski, M.; Simons, J. Int. J. Quantum Chem. 2000, 80, 1024. (14) Hodgkiss, J. M.; Damrauer, N. H.; Presse, S.; Rosenthal, J.; Nocera, D. G. J. Phys. Chem. A 2006, 110, 18853. (15) Damrauer, N. H.; Hodgkiss, J. M.; Rosenthal, J.; Nocera, D. G. J. Phys. Chem. B 2004, 108, 6315. (16) Freza, S.; Skurski, P. Chem. Phys. Lett. 2010, 497, 19. (17) Larson, J. W.; McMahon., T. B. J. Am. Chem. Soc. 1983, 105, 2944. (18) Wenthold, P. G.; Squires, R. R. J. Phys. Chem. 1995, 99, 2002.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Science Foundation under the following NSF program: Partnerships for Advanced Computational Infrastructure, Distributed Terascale Facility (DTF), and Terascale Extensions: Enhancements to the Extensible Terascale Facility (grant number TG-CHE040036N). ’ REFERENCES (1) Jordan, K. D.; Luken, W. J. Chem. Phys. 1976, 64, 2760. (2) Simons, J.; Jordan, K. D. Chem. Rev. 1987, 87, 535. (3) Gutowski, M.; Skurski, P. J. Chem. Phys. 1997, 107, 2968. (4) Wang, F.; Jordan, K. D. J. Chem. Phys. 2001, 114, 10717. (5) Gutowski, M.; Skurski, P. J. Phys. Chem. B 1997, 101, 9143. (6) Hendricks, J. H.; de Clercq, H. L.; Lyapustina, S. A.; Bowen, K. H. J. Chem. Phys. 1997, 107, 2962. (7) Gutowski, M.; Hall, C. S.; Adamowicz, L.; Hendricks, J. H.; de Clercq, H. L.; Lyapustina, S. A.; Nilles, J. M.; Xu, S.-J.; Bowen, K. H. Phys. Rev. Lett. 2002, 88, 143001. (8) Gutowski, M.; Skurski, P.; Boldyrev, A. I.; Simons, J.; Jordan, K. D. Phys. Rev. A 1996, 54, 1906. Gutowski, M.; Skurski, P.; Simons, J.; Jordan, K. D. Int. J. Quantum Chem. 1997, 64, 183. Skurski, P.; Simons, J. J. Chem. Phys. 2002, 116, 6118. 10599


Electron and Hydrogen Transfer in Small Hydrogen Fluoride Anion

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