A Recipe for Designing Molecules with Ever-Increasing Electron

Article pubs.acs.org/JPCA

A Recipe for Designing Molecules with Ever-Increasing Electron Affinities C. Paduani*,† and P. Jena Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States ABSTRACT: Halogens possess among the highest electron affinities of elements in the periodic table. Superhalogen molecules with electron affinities higher than those of halogen atoms have been known to form when a metal atom is surrounded with halogen atoms. Recently, it was discovered that a new class of molecules called hyperhalogens with electron affinities higher than those of superhalogens can form when the latter serve as the building block. By use of density functional theory and B3LYP hybrid exchange-correlation functional we show that molecules with electron affinities even higher can be formed by using hyperhalogens as building blocks. We demonstrate this by using Na and Li as metal atoms and F, BF4, and Na(BF4)2 as halogen, superhalogen, and hyperhalogen building blocks. The predicted electron affinities of Na[Na(BF4)2]2 and Li[Li(BF4)2]2 are 9.18 and 9.01 eV, which are, respectively, 0.85 and 0.5 eV higher than those of their hyperhalogen [Na(BF4)2 and Li(BF4)2] counterparts.

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dimethoxyethane, and/or γ-butyrolactone for use as an electrolyte in lithium batteries. NaF is an inorganic chemical compound used for fluorination reactions and is a versatile reducing agent that can be used in aqueous solution. It has wide application in chemistry such as in bleaching wood pulp. Sodium fluoroborate, NaBF4 is used as flame retardant for cotton, catalyzer, electroplating, granule reagent, fluoridizer and chemical reagent. In section II we provide a description of our theoretical methods. The results are presented in section III and summarized in section IV.

INTRODUCTION Halogens, due to their high electron affinities (EA), comprise the most reactive group of nonmetals. Because of this halogens are found in the environment only in the form of negative ions or compounds. They have lower melting and boiling points compared to other nonmetals, and all halogens are poor conductors of electricity as well as poor thermal conductors. Organic halogen compounds have a wide range of applications and are used extensively as antiseptics, pesticides, anesthetics, fumigants, and solvents. Because of the importance of negative ions in the chemical industry there has been a constant search to find molecules that have ultrahigh electron affinities and can create unique salts or ionic liquids. This search has led to the discovery of a new class of compounds called superhalogens and hyperhalogens.1,2 Superhalogens consist of a metal atom at the core surrounded by halogens, while hyperhalogens consist of a metal atom at the core surrounded by superhalogen moieties. The electron affinities of superhalogens are higher than those of halogens while electron affinities of hyperhalogens are higher than those of superhalogens. Gutsev and Boldyrev3−7 proposed a simple formula for superhalogen namely, MXm+1, where M is a main group or transition metal atom with maximal formal valence m and X is a halogen atom. Similarly, hyperhalogens have the formula MYm+1, where Y is a superhalogen. In this paper we show that molecules with electron affinities even higher than the hyperhalogens can be achieved by decorating a metal atom with hyperhalogens as building blocks. The equivalent formula is MZm+1, where Z is a hyperhalogen. Their electron affinities continue to increase as the extra electron is delocalized over all the hyperhalogen moieties. We demonstrate this by using M = Na and Li as the metal atom and X as halogen, F; Y as superhalogen, BF4; Z as hyperhalogen, M(BF4)2. LiF is used for windows, prisms, and lenses in the vacuum UV, visible, and infrared where desired transmission is in the 0.104−7 μm range. LiBF4 can be dissolved in propylene carbonate, © 2012 American Chemical Society

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COMPUTATIONAL METHOD The equilibrium geometries and total energies of neutral and anionic MXn, M(BF4)n, and M[M(BF4)2]n species (M = Na and Li) were calculated using density functional theory (DFT) and Becke’s three-parameter hybrid exchange functional combined with the Lee−Yang−Par correlation functional B3LYP.8 The geometries were fully optimized without symmetry constraint. Normal-mode frequencies were calculated to ensure that the optimized structures belong to minima in the potential energy surface. Several initial geometries were tried to obtain the geometry belonging to the global minimum. All calculations were performed using the 6-311++G(3df) basis set and the Gaussian 03 package.9 The criteria for convergence was set such that total-energy differences are converged to better than 10−6 eV and the residual atomic forces no larger than 10−3 eV/Å. Natural bond orbital (NBO) analysis were performed to provide an insight into the bonding nature of these moieties. The vertical detachment energies (VDE) were calculated from the difference in total energy of the ground-state anion and its neutral at the anion geometry. The adiabatic detachment energy (ADE), on Received: December 6, 2011 Revised: January 3, 2012 Published: January 4, 2012 1469

dx.doi.org/10.1021/jp211736e | J. Phys. Chem. A 2012, 116, 1469−1474

The Journal of Physical Chemistry A

Article

Figure 1. Equilibrium ground-state geometries of neutral and anionic NaXn [X = F, BF4, and Na(BF4)2] species. Also given is the geometry of the neutral isomer obtained by starting with the anion ground-state geometry. Selected bond lengths (Å) and natural bond orbital (NBO) charges (e) are indicated for each species. Red spheres are B atoms and light blue spheres are Na atoms; yellow spheres are F atoms (color online).

note that in the photoelectron spectroscopy (PES) experiment the VDE and ADE are usually measured since the photoejection is a very fast process and any existing energy barriers may not allow the neutral to relax to its ground-state configuration after the extra electron is ejected.

the other hand, measures the energy difference between the ground-state anion and its neutral having geometry similar to that of the anion. This is obtained by starting with the groundstate anion geometry and relaxing it to its nearest equilibrium configuration after removing the extra electron. The EA is the energy difference between the ground-state geometries of the anion and its neutral. The ADE and EA are the same only if both the neutral geometries, following geometry optimization, lead to the same structure, irrespective of the starting point. We

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RESULTS AND DISCUSSION NaFn, Na(BF4)n, and Na[Na(BF4)2]n. The equilibrium geometries of neutral and anionic NaFn and Na(BF4)n (n = 1,2) 1470



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in good agreement with earlier calculations.18,19 With the addition of two BF4 units to the Na atom one obtains the equilibrium geometries depicted in Figure 1 for Na(BF4)2 and Na(BF4)2−. It can be seen that the BF4 units again appear distorted, and the NBO charge is distributed unequally among the F atoms; more charge is concentrated on the F atom that is closer to the Na atom. Interestingly, the NBO charge on the Na atom is nearly the same in NaF, NaF2, NaBF4, and Na(BF4)2. The extra electron in Na(BF4)2− is distributed among the F atoms. The F atoms closer to Na show longer B−F bonds and larger NBO charges than the further ones. Consequently, there is a substantial increase in both EA and VDE for Na(BF4)2, namely, 8.33 and 8.47 eV, respectively. These results confirm that Na(BF4)2 is indeed a hyperhalogen. We then wondered what would the results be if a Na atom is decorated with hyperhalogen units, such as Na(BF4)2? To study this problem we performed calculations on neutral and anionic Na[Na(BF4)2]n (n = 1 and 2) clusters. In Figure 1 we show the obtained equilibrium geometries. In all these clusters the BF4 units still preserve their tetrahedral form, despite small distortions. In the Na[Na(BF4)2]− anion, one Na atom resides near the geometrical center of the molecule and is bound to 4 F atoms, whereas the other Na is seen bound to only 2 F atoms. However, the ground-state geometry of neutral Na[Na(BF4)2] is very different. Here the two Na atoms form a bridge between the two BF4 units. Its higher energy isomer that is obtained from using the anion as the starting configuration lies about 1 eV higher in energy; it is metastable but belongs to a potential energy minimum as all the normal-mode frequencies are real. Note that

clusters with bond lengths (Å) and NBO charges (e) are summarized in Figure 1. We begin our discussion with NaF for which numerous earlier experimental and theoretical studies are available against which our computational methods can be benchmarked. The calculated B−F bond lengths for NaF and NaF− are, respectively, 1.93 and 2.01 Å, which are in good agreement with experimental (1.926 and 2.00 Å)10,11 and earlier theoretical results12−19 (1.934 and 2.00 Å). For NaF2 and NaF2− our results are in good agreement with results of CCSD(T) calculations (2.070 and 2.122 Å, respectively).20 In neutral NaF an electron is transferred from Na to F giving rise to an ionic bond. In NaF−, the extra electron goes to neutralize the positively charged Na atom, thus leaving the negative charge almost entirely on the F atom. However, in NaF2− the NBO charges are comparable (in absolute value) for all of them and the extra electron is distributed evenly among the two F atoms. The B−F bond lengths are enlarged over those in NaF but are seen to be unchanged between the neutral and its anion. The symmetries, EAs, VDEs, and ADEs along with the highest-occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gaps are reported in Table 1. Table 1. Symmetry, EA, VDE, ADE, and HOMO−LUMO Gap ΔHL Calculated at the 6-311++G(3df) Level cluster

symmetry

NaF NaF− NaF2 NaF2− Na(BF4) Na(BF4)− Na(BF4)2 Na(BF4)2− Na[Na(BF4)2] Na[Na(BF4)2]− Na[Na(BF4)2]2 Na[Na(BF4)2]2−

C∞v C∞v C2v C2v C2v Cs D2d C2 C2 C2 C1 C1

EA (eV)

VDE (eV)

ADE (eV)

0.69

0.67

5.10

5.09

1.12

1.01

8.47

8.33

1.90

1.71

9.19

8.68

0.67 5.19 1.15 8.33 0.66 9.18

ΔHL (eV) 4.82 1.10 9.70 4.70 7.35 1.39 10.72 8.53 8.56 1.70 12.44 8.55

The addition of two F atoms to Na leads to a significant increase in both EA and VDE values, as can be seen in Table 1 for NaF2 and NaF2−. Our results are in good agreement with earlier CCSD(T) calculations,20 which give 0.51 and 5.12 eV for the adiabatic EA of NaF and NaF2, respectively. The EA of NaF2 exceed that of Cl (3.6 eV), and as expected, NaF2 is a superhalogen. Now we investigate the effect of decorating the Na atom with superhalogen moieties. For this we chose BF4, which has EA = 6.86 eV and VDE = 7.50 eV, as obtained in our previous study.21 The equilibrium geometries of clusters formed with the addition of BF4 units to Na are shown in Figure 1. We recall that BF4− forms a perfect tetrahedron with BF bond length of 1.41 Å. The NBO charge on the B atom in BF4 is +1.34 e, while that on the Na atom is +0.98 e. The charge on the F atoms is −0.58 e. With the addition of the Na atom, the BF4 unit is slightly distorted. Two of the F atoms show increased B−F bond lengths (1.46 Å) as well as larger NBO charges (−0.63 e), while for the other two F atoms these are 1.36 Å and −0.54 e, respectively. The Na−B bond lengths in the neutral and anion are, respectively, 2.70 and 2.85 Å. The additional electron leads to almost complete depletion of the NBO charge at Na. Small EA and VDE values are seen in Table 1 for NaBF4 and NaBF4−, as expected for a closed-shell system. Our results on NaBF4 are

Figure 2. Plots of the HOMO of neutral and anionic NaXn (X = F, BF4, and Na(BF4)2) species (isovalue 0.02 Å−3) (Yellow (+) and green (−)). 1471



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Figure 3. Equilibrium ground-state geometries of neutral and anionic LiXn [X = F, BF4, and Li(BF4)2]] species. Also given is the geometry of the neutral isomer obtained by starting with the anion ground-state geometry. Selected bond lengths (Å) and natural bond orbital (NBO) charges (e) are indicated for each species. Red spheres are B atoms; light blue spheres are Li atoms; yellow spheres are F atoms.

is a saturation of the EA value rather than a systematic increase. Furthermore, the ionization potentials of the constituting fragments could be the natural limits for such growing electron binding energies. In Figure 2 we show 3D plots of the HOMO for these clusters. For both NaF− and Na(BF4)− one sees that the bonding orbital has most of its charge density located behind the electropositive (Na) atom. In addition, the HOMO of each of these species is a delocalized bonding orbital formed from the 2p atomic orbitals of both B and F atoms. This delocalized bonding system has two electrons satisfying the octet counting rule. Most of the negative charge density of the bonding orbitals is localized on the fluorine atoms whereas sodium has significant positive charge. This is consistent with the greater electronegativity of fluorine with respect to sodium, and thus we expect negative charge concentration on the former and negative charge depletion on the latter. For the new halogen moieties one sees in Figure 2 that the charge density of the bonding HOMO changes from the center in the neutral precursor to the extreme in the anionic molecule. LiFn, Li(BF4)n, and Li[Li(BF4)2]n. We show in Figure 3 the equilibrium geometries of neutral and anionic lithium clusters

all Na atoms have the same NBO charge as those observed earlier in the superhalogen and hyperhalogen moieties. The BF bond associated with the B atom that is also attached to Na is larger than the other BF bonds. Similar is the case with NBO charges. The calculated EA, ADE, and VDE values for these species are listed in Table 1. We note that the ADE and VDE values of Na[Na(BF4)2] are 0.66 and 1.71 eV. These small values are again characteristic of closed-shell systems. The significant difference between EA and ADE of Na[Na(BF4)2] is a direct reflection of the two very different geometries in Figure 1. The results are dramatically different for Na[Na(BF4)2]2. The electron affinity of Na[Na(BF4)2]2 is 8.52 eV higher than that of its closed-shell cousin Na[Na(BF4)2] and 0.85 eV higher than its hyperhalogen building block. Thus, Na[Na(BF4)2]2 cluster can be thought of as a megahalogen with Na(BF4)2 hyperhalogen as building blocks. From this, one is led to think it might be possible to increase systematically the EA of species by tailoring the building block. However, to check this hypothesis one has to investigate much more complex (larger) systems in a systematic procedure in order to verify whether, instead, there 1472



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building block. This confirms the trend discussed above for the Na-based system. Thus, a new class of molecules can be formed following the simple formula MZm+1, where Z is the corresponding hyperhalogen containing the metal M. The calculated HOMO−LUMO gaps are given in Table 2. The same trend is observed as for the Na-based clusters, namely, the ΔHL gap increases: the gap is smaller in the anionic clusters with open shell and is larger for the cluster with closed shell. As mentioned above, a large HOMO−LUMO gap can be taken as indicative of the stability of a cluster. The Li[Li(BF4)2]2 is, thus, expected to be a very stable moiety. The 3D plots of the HOMO for these clusters are shown in Figure 4. The positive contribution for the charge density of the

along with some selected bond lengths (Å) and natural bonding orbital (NBO) charges (e). The optimized structure for neutral LiF and LiBF4 are in good agreement with earlier reported results. Recent calculations13 give 1.564 Å for the Li−F bond length, whereas the experimental value is 1.563864 Å [10]. For LiF2 and LiF2− our results are also in good agreement with results based on CCSD(T) calculations (1.728 and 1.702 Å, respectively20). For LiBF4 our calculated result for the B−F bond length (1.35 Å) agrees well with the earlier reported result (1.34 Å) at the MP2/6-31G* level of theory.18,19 The bond length increases in the anionic form, where the extra electron enters to decrease the NBO charge on the Li atom, thus resulting in a longer bond length. When two BF4 units are attached to the Li atom one can see in Figure 3 nearly the same NBO charge on lithium, namely, +0.94 e. However, the NBO charges on the two B atoms as well as the B−Li bond lengths are reduced. In fact, the neutral molecule is completely symmetric with respect to the NBO charge distribution and bond lengths on the BF4 units. Nevertheless, in the anionic cluster this symmetry is broken and the BF4 units show different characteristics. In Table 2 we report the calculated EA and VDE values for these clusters. As one can see, EA increases remarkably from Table 2. Symmetry, EA, VDE, ADE, and HOMO−LUMO Gap ΔHL Calculated at the 6-311++G(3df) Level cluster

symmetry

EA (eV)

LiF LiF− LiF2 LiF2− Li(BF4) Li(BF4)− Li(BF4)2 Li(BF4)2− Li[Li(BF4)2] Li[Li(BF4)2]− Li[Li(BF4)2]2 Li[Li(BF4)2]2−

C∞v C∞v C2v C2v C2v Cs D2d C2 C2 C2 C1 C1

0.49

VDE (eV)

ADE (eV)

0.51

0.49

5.50

5.49

0.84

0.74

8.64

8.51

1.65

1.49

9.27

9.01

5.49 0.74 8.51 1.28 9.01

ΔHL (eV) 6.34 1.03 9.93 4.98 8.50 1.22 11.53 8.71 9.31 1.58 10.89 8.47

LiF to LiF2 and largely exceeds the EA of chlorine (3.6 eV), thus making LiF2 a superhalogen. These are to be compared with earlier CCSD(T) calculations that yielded EA = 0.36 eV for LiF and EA = 5.45 eV for LiF2.20 Calculations were repeated by replacing F with superhalogen moiety BF4. The geometries of LiBF4 and Li(BF4)2 as well as the VDE, ADE, and EA are given in Figure 3 and Table 2, respectively. We note that the geometry of anionic LiBF4 is rather similar to its neutral counterparts although the bond lengths are slightly different. The BF4 units maintain their tetrahedral structure. However, the geometry of the ground states of the anionic and neutral Li(BF4)2 are very different. Surprisingly, the EA and ADE only differ by 0.14 eV. The NBO charge on the Li atom is practically the same in all neutral clusters. When the Li atom is decorated with two Li(BF4)2 the resulting geometries of both the neutral and anions are linear chains with Li atoms sandwiched between BF4 units. Once again BF4 units maintain their tetrahedral structure due to the high stability of the BF4− superhalogen moiety. In Table 2 are also listed the calculated EA and VDE for these clusters. A remarkable feature is the significant increase in VDE for the Li[Li(BF4)2]2. One sees a gain of about 1.50 eV in the electron affinity as compared to to its hyperhalogen

Figure 4. Plots of the HOMO of neutral and anionic LiXn [X = F, BF4, and Li(BF4)2]] species (isovalue 0.02 Å−3) (yellow (+) and green (−)).

bonding orbital in the LiBF4 cluster is located between the Li atom and the F atoms at the base of the BF4 tetrahedral unit, whereas the negative contribution is almost entirely located in the diametrically opposed F atom. The directional character of the s−p σ bonding is clearly depicted in the plots of Figure 4, which is a distinctive characteristic of the Li−F clusters. The negative charge density of the bonding orbitals is seen mostly localized on the fluorine atoms. The charge density of the bonding HOMO shifts from the center of the neutral precursor to the edge unit in the anionic cluster. This feature is seen even more accentuated in the Li[Li(BF4)2]2 cluster, where the charge density of the bonding HOMO is seen particularly accumulated on the BF4 unit. 1473



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ACKNOWLEDGMENTS This research was supported by grants from the Department of Energy and from the Brazilian Conselho Nacional de Desenvolvimento Cientı ́fico e Tecnológico (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-AC0205CH11231.

In Table 3 we show results of calculations of the thermodynamic stability against fragmentation into selected channels Table 3. Bond Dissociation Energies of Neutral and Anionic Clusters Calculated at the B3LYP/6-311+G(3df) Level neutral

anion

channel

eV

channel

eV

NaF → Na+F NaF2 → NaF+F Na(BF4) → Na + BF4 Na(BF4)2 → Na(BF4) + BF4 Na[Na(BF4)2] → Na(BF4)2 + Na Na[Na(BF4)2]2 → Na[Na(BF4)2] + Na(BF4)2 LiF → Li + F LiF2→ LiF + F Li(BF4) → Li + BF4 Li(BF4)2 → Li(BF4) + BF4 Li[Li(BF4)2] → Li(BF4)2 + Li Li[Li(BF4)2]2 → Li[Li(BF4)2] + Li(BF4)2

4.835 0.988 6.654 0.771

NaF− → Na+F− NaF2− → NaF+F− Na(BF4)− → Na +BF4− Na(BF4)2− → Na(BF4) +BF4− Na[Na(BF4)2]− → Na(BF4)2 +Na− Na[Na(BF4)2]2− → Na[Na(BF4)2] + Na(BF4)2− LiF− → Li + F− LiF2− → LiF + F− Li(BF4)− → Li + BF4− Li(BF4)2− → Li(BF4) + BF4− Li[Li(BF4)2]− → Li(BF4)2 + Li− Li[Li(BF4)2]2− → Li[Li(BF4)2] + Li(BF4)2−

2.046 2.725 3.541 4.832

8.159 0.037 5.944 1.049 7.561 0.575 8.038 1.432

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REFERENCES

(1) Willis, M.; Gotz, M.; Kandalam, A. K.; Gantefor, G; Jena, P. Angew. Chem., Int. Ed. 2010, 49, 8966. (2) Feng, Y.; Xu, H-Guang; Zheng, W.; Zhao, H.; Kandalam, A. K.; Jena, P. J. Chem. Phys. 2011, 134, 094309. (3) Gutsev, G. L.; Boldyrev, A. I. Chem. Phys. 1981, 56, 277−283. (4) Gutsev, G.; Leś, A.; Adamowicz, L. J. Chem. Phys. 1994, 100, 8925. (5) Gutsev, G. L.; Boldyrev, A. I. Chem. Phys. Lett. 1984, 108, 250. (6) Boldyrev, A. I.; Simons, J. J. Chem. Phys. 1993, 99, 4628. (7) Boldyrev, A. I.; von Niessen, W. Chem. Phys. 1991, 155, 71. (8) Becke, A. D. J. Chem. Phys. 1993, 98, 1372−1377; J. Chem. Phys. 1993, 98, 5648−5652. (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (10) Huber, K. P.; Herzberg, G. Molecular spectra and molecular structure IV. Constants of diatomic molecules; Van Nostrand: Princeton, 1979. (11) Vezey, S. E.; Gordi, W. Phys. Rev. A 1965, 138, 1303. (12) Jordan, K. D.; Seeger, R. Chem. Phys. Lett. 1978, 54, 320. (13) Rusu, V. H.; Ramos, M. N.; Longo, R. L. J. Mol. Struct. 2011, 993, 86−90. (14) Pyykkö, P.; Diercksen, G. H. F.; Plathe, F. M.; Laaksonen, L. Chem. Phys. Lett. 1987, 141, 535. (15) Erikson, R. L.; Eary, L. E.; Hostetler, C. J. J. Chem. Phys. 1993, 99, 336. (16) Garcia-Cuesta, I.; Serrano-Andrés, L.; Sanchez de Merás, A.; Nebot-Gil, I. Chem. Phys. Lett. 1992, 199, 535. (17) Prascher, B. P.; Woon, D. E.; Peterson, K. A.; Dunning, T. H. Jr.; Wilson, A. K. Theor. Chem. Acc. 2011, 128, 69. (18) Ramondo, F.; Bencivenni, L.; Di Martino, V. Chem. Phys. 1991, 158, 41. (19) Spoliti, M.; Sanna, N.; Di Martino, V. THEOCHEM 1992, 258, 83−107. (20) Gutsev, G. L.; Bartlett, R. J.; Boldyrev, A. I.; Simons, J. J. Chem. Phys. 1997, 107, 3867. (21) Paduani, C.; Wu, M. M.; Willis, M.; Jena, P. J. Phys. Chem. A 2011, 115, 10237−10243. (22) Zakzhevskii, V. G.; Boldyrev, A. I.; Charkin, O. P. Zh. Neorg. Khim. 1980, 25, 1171−1175.

8.229 0.892 2.979 3.086 4.029 4.815 9.322 1.934

for the neutral and anionic systems. The largest stability of the closed-shell systems is clearly seen, which is consistent with the valence of Na and Li B being 1. The energy gain in binding a F atom to NaF is smaller to that one for LiF. Besides, we note that NaF2− is less stable against ejection of a F atom than LiF2−. The same trends are observed when F is replaced by BF4 units. In the anionic clusters as a general trend we observe that the energy gain in binding F− or BF4− is larger when the anion is attached to the open-shell system. However, as it can be seen in Table 3, by using hyperhalogen moieties as building blocks to decorate Na or Li atoms there is a significant increase in the electron affinity as compared to to its hyperhalogen building block. The higher bond dissociation energies thus are seen for Na[Na(BF4)2] and Li[Li(BF4)2].

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CONCLUSIONS Using density functional theory (DFT) we performed firstprinciples calculations of the structure, stability, and electron affinities of MXn (M = Na and Li and X = F, BF4, and M(BF4)2) at the B3LYP/6-311++G(3df) level. We found NaF2 to be a superhalogen with EA of 5.19 eV. When BF4 units decorate the Na atom the electron affinity increases from 1.15 eV in NaBF4 to 8.33 eV in Na(BF4)2, which characterizes it as a hyperhalogen. By using these latter as building blocks to decorate the Na atom the electron affinity achieves a value of 9.18 eV for Na[Na(BF4)2]2. Results are similar when Na is replaced by Li. Our recipe provides a systematic way for creating species with ever increasing EAs. The high oxidizing property of these molecules may have many applications in chemistry and biology.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

Departamento de Fı ́sica, UFSC, Florianópolis, CEP 88040−900, SC, Brazil. 1474


A Recipe for Designing Molecules with Ever-Increasing Electron

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