DOI: 10.1002/chem.201203245

C5Li7+ and O2Li5

+ as Noble-Gas-Trapping Agents

Sudip Pan,[a] Maryel Contreras,[c] Jonathan Romero,[d] Andres Reyes,[d]

Pratim K. Chattaraj,*[a] and Gabriel Merino*[b]

Introduction

The chemical fact that appears in the mind instantaneouslywith the name of noble gases is their chemical inertness.Ample justification for this strong myth is provided by thefact of complete silence in the history of chemistry aboutthe chemical reactivity of these elements, members of theextreme right-hand column of the periodic table. The theoryof chemical bonding provides the reason for the lack of pre-history regarding noble-gas-based compounds, in spite ofthe advent of it well over a century ago. This is due to theircompletely filled outer-shell configurations, owing to whichthey cannot share their valence electrons to make a newbond. For several decades after their discovery, it became acommon belief that the chemical inertness of noble gaseswas absolute in all senses. In that spirit, the famous quote,

“The unreactivity of the noble gas elements belongs to thesurest of all experimental results”, made by Paneth[1] shouldalso be mentioned here. In 1933, Pauling[2] argued thatchemical bonding might be possible for the bottom membersof Group 18 in the periodic table because their outermostelectronic shells are less strongly bound due to their rela-tively greater distance from the nuclei. After a series of fail-ures by many scientists, Bartlett and co-workers[3,4] achievedan amazing breakthrough by experimentally preparingxenon hexafluoroplatinate, Xe+ ACHTUNGTRENNUNG[PtF6]

� , thereby rupturingthe existing myth regarding the chemical inertness of noblegases. This advent again stimulated curiosity and frenzy inthis field throughout the worldwide scientific community. Asa result, not only rich numbers of krypton and xenon com-pounds[5–8] were successfully synthesized experimentallywithin a short duration, but also a significant qualitative per-ception regarding their bonding nature became possible toachieve with the help of modern theoretical tools.[9]

The gallery of noble-gas-containing compounds has beenrapidly enriched as a result of the outputs of several effortsmade by scientists during the last several years. The pioneer-ing works of R�s�nen and co-workers,[10–17] the successfulsynthesis of a series of compounds of type HXY (X=Ar,Kr, Xe; Y=electron-withdrawing group) by means of pho-todissociation of HY in a noble-gas matrix, are worth men-tioning. The articles entitled “A Renaissance in Noble GasChemistry” by Christe[18] and “New Chemistry of the NobleGas Elements: Novel Molecules, Polymers and Crystals” byGerber[19] portraying some of the recent achievements inthis field should also be cited. A couple of papers exploringthe structure and bonding of xenon hydrides and radon hy-drides by Merino�s group[20, 21] and a series of works onnoble-gas compounds by Frenking et al.[22–28] are also wellappreciated in this regard. However, in addition to thesechemically bound noble-gas compounds, extensive researchhas been performed both computationally and experimen-tally exploring the stability of van der Waals complexes ofnoble gases. Small Ne–NH3,

[29] He–N2,[30] and C2–Ne[31] van

Keywords: ab initio calculations ·cluster compounds · density func-tional calculations · lithium · noblegases

Abstract: The noble-gas-trapping ability of the star-shaped C5Li7+ cluster and

O2Li5+ super-alkali cluster is studied by using ab initio and density functional

theory (DFT) at the MP2 and M05-2X levels with 6-311 +GACHTUNGTRENNUNG(d,p) and 6-311 +G(d)basis sets. These clusters are shown to be effective noble-gas-trapping agents. Thestability of noble-gas-loaded clusters is analyzed in terms of dissociation energies,reaction enthalpies, and conceptual DFT-based reactivity descriptors. The presenceof an external electric field improves the dissociation energy.

[a] S. Pan, Prof. P. K. ChattarajDepartment of Chemistry and Center for Theoretical StudiesIndian Institute of TechnologyKharagpur 721302 (India)E-mail : pkc@chem.iitkgp.ernet.in

[b] Prof. G. MerinoDepartamento de F�sica AplicadaCentro de Investigaci�n y de Estudios Avanzados Unidad M�ridakm 6 Antigua carretera a Progreso. Apdo. Postal 73Cordemex, 97310 M�rida, Yuc. (M�xico)E-mail : gmerino@mda.cinvestav.mx

[c] M. ContrerasDepartamento de Qu�micaDivisi�n de Ciencias Naturales y ExactasUniversidad de GuanajuatoCol. Noria Alta s/n C.P. 36050, Guanajuato, Gto. (M�xico)

[d] J. Romero, Prof. A. ReyesDepartamento de Qu�micaUniversidad Nacional de Colombiasede Bogot� (Colombia)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201203245.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 2322 – 23292322

der Waals complexes are some of the examples in this class.Moreover, the viability of cagelike structures having endo-hedrally trapped noble-gas atoms has also been studied boththeoretically and experimentally.[32–35] But the main problemwith these weak van der Waals complexes is that their sta-bilities are only supported by weak dispersion forces. ThenPauzat et al.[36–38] reported that the H3

+ system can trapnoble-gas atoms with interaction energy, which is in betweenthat of the above-mentioned two extremes. The experimen-tal thermochemical data and binding energies[39] for ArnH3

+

(n=1–3) were already described in the literature. The re-ported binding energies[39] of Ar with H3

+ are 6.69, 4.56, and4.28 kcal mol�1 for n=1–3, respectively. By ab initio anddensity functional studies, Pauzat et al.[38] showed that H3

+

can trap up to three noble-gas atoms forming so-called ion-neutral complexes. They also concluded through their elec-tron localization function (ELF) analysis[38] that the interac-tion between the noble-gas atom and H3

+ is mostly electro-static in nature. A net charge transfer from the noble-gasatom to the H3

+ fragment (ionic donation) is the key factorfor the stability of such types of complex. Inspired by thenoble-gas-trapping ability of H3

+ , Chakraborty et al.[40] havestudied this for the cyclic Li3

+ species and the result is moreor less the same as that of H3

+ . Each Li center was found tobind with one noble-gas atom at a binding energy is in be-tween that of physisorption and chemisorption.

Li decorating systems are very well known for their hy-drogen trapping ability.[41–48] By virtue of the net positivecharge gained within the molecule, Li centers interact withmolecular hydrogen through dipole–induced-dipole anddipole–quadrupole types of interactions and the correspond-ing dissociation energy ranges within the intermediate scalebetween the two extremes, namely that of physisorption andchemisorption. Herein, we have explored by means of abinitio and density functional studies the notion that, in spiteof their closed-shell configuration and inertness, noble gasesshow substantial activity towards Li centers with significantpositive charge, similar to molecular hydrogen. We havechosen two reported global minima structures, C5Li7

+ ,[49] astarlike cluster, and O2Li5

+ ,[50] a super-alkali ion,[51,52] for athorough study and a series of Li-containing systems fortrial. Significantly, the hydrogen-trapping capabilities of thestudied systems are already available in the litera-ture.[44, 47,53, 54] The outcome of this study is quite astonishing.C5Li7

+ [49] has been found to trap seven He atoms and 12noble gas (Ng) atoms (Ng=Ne, Ar, Kr), whereas O2Li5

+ [50]

can trap seven Ng atoms (Ng=He, Ne, Ar, Kr) with suffi-cient dissociation energy and reaction enthalpy values. Inaddition, Ng atoms have also been found to interact quiteeffectively with the Li centers of all the hydrogen-storingsystems studied herein.

Another well-known key factor, which improves the hy-drogen-binding ability significantly, is the presence of an ex-ternal electric field.[44,55, 56] Zhou et al.[55] reported that thehydrogen-binding properties of easily polarizable substratescan be improved by applying an external electric field,whereas the same approach was successfully applied to mes-

oporous metal oxides by Sun et al.[56] both experimentallyand theoretically. Now the question is: Can we apply thesame strategy to improve the dissociation energy for noble-gas trapping? Our theoretical computations reveal that fornoble-gas trapping, the external electric field also plays amajor role in modifying the corresponding dissociationenergy, similar to that in the case of hydrogen gas storage.

Computational Details

The structures of Ng···C5Li7+ and Ng···O2Li5

+ (Ng =He, Ne, Ar, Kr), in-cluding the bare clusters, were optimized by using the Truhlar metahybrid exchange-correlation functional, M05-2X,[57] and the second-orderMøller–Plesset perturbation theory method, MP2,[58] with the 6-311+G-ACHTUNGTRENNUNG(d,p) basis set for the systems containing He atoms and 6-311+ G(d) forthe rest by using the Gaussian 03 program package.[59] Harmonic vibra-tional frequency analysis was also performed for all the structures at theM05-2X level to assess their existence as minima on the potential energysurface. All dissociation energies reported herein are zero-point energy(ZPE) corrected (D0). To understand how the noble gas interacts withboth clusters, we also carried out optimizations and frequency computa-tions for the clusters with one noble gas interacting in either the axial orequatorial positions at the MP2 level of theory with the same basis sets.Both methodologies were used to compare results between them andensure that they behaved similarly (Table 1). From now on, we will con-tinue discussing just the M05-2X results for the rest of the analysis. Addi-tional data at the MP2 level can be consulted in the Supporting Informa-tion (Tables S1–S8).

The natural population analysis (NPA) scheme was adopted to computeatomic charges (QNPA). The Wiberg bond index (WBI)[60] was also com-puted to assess the bond order. Conceptual density functional theory(CDFT)[61, 62]-based global reactivity descriptors, such as electronegativity(c),[63] hardness (h),[64] and electrophilicity (w),[65] were used as tools foranalyzing the stability and reactivity of the studied systems.

Table 1. The dissociation energy of the Ng···cluster (D0, kcal mol�1), thereaction enthalpy for Ng+cluster!Ng···cluster (DH, kcal mol�1, at298 K), and the Li�Ng bond length (rLi�Ng, ) at the MP2 and M05-2Xlevels with basis sets 6-311+G ACHTUNGTRENNUNG(d,p) and 6-311 +G(d).

Clusters MP2 M05-2X MP2 M05-2XD0 DH D0 DH rLi�Ng rLi�Ng

He···C5Li7+ (ax) 0.5 �0.6 0.8 �0.8 1.994 1.996

He···C5Li7+ (eq) 0.2 �0.2 0.6 �0.6 2.089 2.042

He···O2Li5+ (ax) 0.5 �0.4 0.6 �0.6 2.062 2.042

He···O2Li5+ (eq) 0.2 �0.2 0.5 �0.5 2.129 2.071

Ne···C5Li7+ (ax) 2.0 �1.9 1.4 �1.3 2.141 2.198

Ne···C5Li7+ (eq) 1.6 �1.5 1.2 �1.1 2.189 2.211

Ne···O2Li5+ (ax) 1.7 �1.6 1.1 �1.1 2.180 2.204

Ne···O2Li5+ (eq) 1.4 �1.4 1.1 �1.0 2.196 2.298

Ar···C5Li7+ (ax) 4.5 �4.4 2.0 �1.8 2.393 2.575

Ar···C5Li7+ (eq) 3.6 �3.6 1.8 �1.6 2.422 2.591

Ar···O2Li5+ (ax) 3.9 �2.4 2.1 �2.0 2.423 2.546

Ar···O2Li5+ (eq) 3.1 �1.6 1.5 �1.4 2.454 2.626

Kr···C5Li7+ (ax) 3.9 �3.8 2.5 �3.5 2.627 2.704

Kr···C5Li7+ (eq) 2.9 �2.8 2.3 �3.3 2.689 2.713

Kr···O2Li5+ (ax) 3.4 �3.3 2.8 �2.7 2.667 2.546

Kr···O2Li5+ (eq) 2.4 �2.3 2.0 �1.9 2.735 2.626

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FULL PAPER

Results and Discussion

He-loaded C5Li7+ and O2Li5

+ : It has been found that bothC5Li7

+ and O2Li5+ clusters can capture up to seven He

atoms (see Figures 1 and 2). In the case of C5Li7+ , both

axial and equatorial Li centers can trap one He atom percenter. This cluster can also trap one Ng atom (Ng = Ne,Ar, Kr) per axial Li center and two per equatorial center.For the O2Li5

+ cluster, each axial Li center can bind to twoHe atoms and each equatorial Li center can bind to one Heatom (see Figure 2).

Note that DH gets stronger as the number of interactingNg atoms increases, thereby conferring these systems with ahigher stability (Tables 2 and 3). A comparison of dissocia-

tion energy (D0) and reaction enthalpy for the first He-loaded analogues at axial and equatorial positions of bothC5Li7

+ and O2Li5+ clusters reveals that axial Li centers have

a better capability to capture He atoms than the equatorialones. For the axial case, in one He-loaded C5Li7

+ andO2Li5

+ cluster, the Li�He distances are 1.996 and 2.042 ,respectively, whereas for the equatorial case these valuesare longer (2.042 and 2.071 , respectively). For7He···C5Li7

+ the axial and equatorial Li�He distancesremain the same as those for one He atom, whereas, for7He···O2Li5

+ , the axial and equatorial distances slightly in-crease to 2.058 and 2.067–2.081 , respectively. All sevenHe atoms interacting with the C5Li7

+ cluster have a dissocia-tion energy of 1.0 kcal mol�1 and with the O2Li5

+ cluster of0.9 kcal mol�1 per He atom (Tables 2 and 3). The reactionenthalpy terms show the exothermicity of the reaction fortrapping the He atoms on to both studied clusters.

Scrutiny of Tables 2 and 3 reveals that, upon gradual Heloading, the electronegativity (c) has a decreasing trend,

Figure 1. Optimized geometries of the C5Li7+ cluster, and the seven He-

trapped and highest Ng-trapped analogues (Ng= Ne, Ar, Kr) studied atthe M05-2X level with the 6-311+ G ACHTUNGTRENNUNG(d,p) basis set for the He-trappingcase and 6-311 +G(d) for the rest.

Figure 2. Optimized geometries of the O2Li5+ cluster and its highest Ng-

trapped analogues (Ng =He, Ne, Ar, Kr) studied at the M05-2X levelwith the 6-311 +G ACHTUNGTRENNUNG(d,p) basis set for the He-trapping case and 6-311+

G(d) for the rest.

Table 2. The dissociation energy of nHe···C5Li7+ per He atom (D0, kcal

mol�1), the total reaction enthalpy for nHe +C5Li7+!nHe···C5Li7

+ (DH,kcal mol�1, at 298 K), and the Li�He bond length (rLi�He, ), electronega-tivity (c, eV), hardness (h, eV), and electrophilicity (w, eV) of the studiedclusters at the M05-2X/6-311+G ACHTUNGTRENNUNG(d,p) level.

Clusters D0 DH rLi�He c h w

C5Li7+ – – – 6.13 6.17 3.05

1He···C5Li7+ (ax) 0.8 �0.8 1.996 6.11 6.19 3.01

1He···C5Li7+ (eq) 0.6 �0.6 2.042 6.09 6.21 2.99

2He···C5Li7+ 0.7 �1.5 1.996 6.08 6.23 2.97

3He···C5Li7+ 0.7 �2.1 1.997 (ax)

2.042 (eq)6.04 6.27 2.91

4He···C5Li7+ 0.6 �2.6 1.997 (ax)

2.039 (eq)5.99 6.33 2.83

5He···C5Li7+ 0.6 �3.8 1.997 (ax)

2.038 (eq)5.94 6.40 2.75

6He···C5Li7+ 0.6 �3.8 1.997 (ax)

2.039 (eq)5.88 6.47 2.68

7He···C5Li7+ 0.6 �4.3 1.997 (ax)

2.042 (eq)5.81 6.60 2.55

Table 3. The dissociation energy of nHe···O2Li5+ per He atom (D0, kcal

mol�1), the total reaction enthalpy for nHe +O2Li5+!nHe···O2Li5

+ (DH,kcal mol�1, at 298 K), and the Li�He bond length (rLi�He, ), electronega-tivity (c, eV), hardness (h, eV), and electrophilicity (w, eV) of the studiedclusters at the M05-2X/6-311+G ACHTUNGTRENNUNG(d,p) level.

Clusters D0 DH rLi�He c h w

O2Li5+ – – – 7.33 8.15 3.29

1He···O2Li5+ (ax) 0.6 �0.6 2.042 7.29 8.18 3.25

1He···O2Li5+ (eq) 0.5 �0.5 2.071 7.28 8.21 3.23

2He···O2Li5+ 0.6 �1.8 2.038 7.25 8.24 3.19

3He···O2Li5+ 0.6 �1.6 2.038 (ax)

2.073 (eq)7.19 8.30 3.12

4He···O2Li5+ 0.5 �2.7 2.039 (ax)

2.075 (eq)7.13 8.39 3.03

5He···O2Li5+ 0.5 �3.2 2.044–2.065 (ax)

2.071 (eq)7.06 8.49 2.94

6He···O2Li5+ 0.5 �3.7 2.040–2.072 (ax)

2.067–2.076 (eq)7.02 8.53 2.88

7He···O2Li5+ 0.5 �3.7 2.058 (ax)

2.067–2.081 (eq)6.95 8.62 2.81

www.chemeurj.org � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 2322 – 23292324

P. K. Chattaraj, G. Merino et al.

which shows the reluctance to attract further electrons intothe system. The hardness (h) and electrophilicity (w) have agradual increasing and decreasing trend, respectively, thusimplying the increased stability with gradual He binding ac-cording to the maximum hardness principle (MHP)[66] andminimum electrophilicity principle (MEP).[67] An NPAshows that a net electron transfer of 0.06 and 0.04 e� fromHe to Li centers takes place for axial and equatorial casesof the C5Li7

+ cluster, respectively. For the O2Li5+ cluster,

the net electron transfer from He to the Li center is 0.03 e�

for both axial and equatorial cases (Tables S9 and S10 in theSupporting Information). Wiberg bond index (WBI) analysisshows a very low value of WBILi�He [7He···C5Li7

+ : 0.071(eq), 0.109 (ax); 7He···O2Li5

+ : 0.052 (eq), 0.038 (ax)].

Ne-, Ar-, and Kr-loaded C5Li7+ and O2Li5

+ : Unlike the He-trapping case, the C5Li7

+ cluster can trap up to 12 Ng atoms(Ng=Ne, Ar, Kr; Figure 1), but the O2Li5

+ cluster followsthe same trend as the He case in which up to seven Ngatoms can be trapped (Figure 2). The C5Li7

+ cluster bindsthrough equatorial Li centers with two Ng atoms andthrough axial Li centers with just one. In contrast, theO2Li5

+ cluster binds through equatorial Li centers with justone Ng atom and through axial Li centers with two. For sim-plicity, we only show the results of the krypton complexes.The results for Ne and Ar show a similar behavior (see Ta-bles S11–S14 in the Supporting Information). The axial Licenters of every cluster bind more strongly to the Ng atomsthan the equatorial ones. The reaction enthalpies and disso-ciation energies increase following the trend He<Ne<Ar<Kr for both clusters and all the trapping processes are exo-thermic. The 12 Ng atoms bind with the C5Li7

+ cluster withdissociation energies per Ng atom of 1.2 (Ne), 1.6 (Ar), and2.1 kcal mol�1 (Kr). Interestingly, the separation of all Kratoms from the 12Kr···C5Li7

+ compound requires 25.8 kcalmol�1. The axial and equatorial Li center distances to theNg atom, respectively, are 2.197 and 2.239–2.322 for Ne,2.589 and 2.639–2.819 for Ar, and 2.710–2.731 and 2.799–2.931 for Kr (Table 4 for Kr and Tables S11 and S13 inthe Supporting Information for Ne and Ar, respectively).

For the O2Li5+ cluster binding to the seven Ng atoms, the

dissociation energy per Ng atom is 1.1 (Ne), 1.7 (Ar), and2.2 kcal mol�1 (Kr). The axial and equatorial Li center dis-tances to the Ng atom, respectively, are 2.217–2.229 and2.226–2.236 for Ne, 2.581 and 2.584–2.601 for Ar, and2.585–2.642 and 2.616–2.642 for Kr (Table 5 for Kr andTables S12 and S14 in the Supporting Information for Neand Ar, respectively).

The electronegativity (c), hardness (h), and electrophilici-ty (w) were evaluated as well, for these three noble gaseswith both clusters. As the number of Ng atoms increases,the trends are as folllow: For Ne with both clusters, the elec-tronegativity and electrophilicity decrease, whereas thehardness increases, thereby showing an increased stabilityaccording to MHP[66] and MEP.[67] For Ar with both clusters,the electrophilicity decreases, which increases the stabilityaccording to MEP. The hardness for the C5Li7

+ cluster first

slightly decreases up to three trapped Ar atoms, then itgradually increases. In contrast, for the O2Li5

+ cluster, thehardness has a more or less decreasing trend. Finally, for Krin both clusters, the electrophilicity, electronegativity, andhardness decrease. Kr-trapped analogues become slightlysofter than the bare cluster, presumably due to the largersize.

The NPA showed that for the C5Li7+ clusters, the net

charge transfer from Ng atom to axial (equatorial) Li cen-

Table 4. The dissociation energy of nKr···C5Li7+ per Kr atom (D0, kcal

mol�1), the total reaction enthalpy for nKr+ C5Li7+!nKr···C5Li7

+ (DH,kcal mol�1, at 298 K), and the Li�Kr bond length (rLi�Kr, ), electronega-tivity (c, eV), hardness (h, eV), and electrophilicity (w, eV) of the studiedclusters at the M05-2X/6-311+G(d) level.

Clusters D0 DH rLi�Kr c h w

C5Li7+ – – – 6.13 6.17 3.05

1Kr···C5Li7+ (ax) 2.5 �3.5 2.704 6.07 6.15 2.99

1Kr···C5Li7+ (eq) 2.3 �3.3 2.713 6.05 6.16 2.97

2Kr···C5Li7+ 2.4 �5.2 2.705 6.01 6.13 2.94

3Kr···C5Li7+ 2.4 �6.7 2.718 (ax)

2.723 (eq)5.92 6.12 2.87

4Kr···C5Li7+ 2.3 �9.4 2.715 (ax)

2.725–2.736 (eq)5.84 6.12 2.79

5Kr···C5Li7+ 2.3 �12.0 2.712–2.727 (ax)

2.731–2.736 (eq)5.76 6.12 2.71

6Kr···C5Li7+ 2.3 �13.3 2.717–2.720 (ax)

2.728–2.745 (eq)5.68 6.14 2.63

7Kr···C5Li7+ 2.3 �17.6 2.711–2.713 (ax)

2.723–2.734 (eq)5.59 6.16 2.54

8Kr···C5Li7+ 2.2 �16.5 2.713–2.716 (ax)

2.742–2.797 (eq)5.56 6.14 2.52

9Kr···C5Li7+ 2.1 �18.2 2.719–2.732 (ax)

2.745–2.870 (eq)5.52 6.13 2.49

10Kr···C5Li7+ 2.1 �20.1 2.709–2.717 (ax)

2.735–2.869 (eq)5.48 6.13 2.45

11Kr···C5Li7+ 2.1 �22.9 2.719–2.734 (ax)

2.737–2.868 (eq)5.45 6.11 2.43

12Kr···C5Li7+ 2.1 �25.8 2.710–2.731 (ax)

2.799–2.931 (eq)5.41 6.11 2.40

Table 5. The dissociation energy of nKr···O2Li5+ per Kr atom (D0, kcal

mol�1), the total reaction enthalpy for nKr +O2Li5+!nKr···O2Li5

+ (DH,kcal mol�1, at 298 K), and the Li�Kr bond length (rLi�Kr, ), electronega-tivity (c, eV), hardness (h, eV), and electrophilicity (w, eV) of the studiedclusters at the M05-2X/6-311+G(d) level.

Clusters D0 DH rLi�Kr c h w

O2Li5+ – – – 7.33 8.15 3.29

1Kr···O2Li5+ (ax) 2.8 �2.7 2.546 7.22 8.12 3.21

1Kr···O2Li5+ (eq) 2.0 �1.9 2.626 7.23 8.13 3.22

2Kr···O2Li5+ 2.8 �6.0 2.542 7.12 8.11 3.12

3Kr···O2Li5+ 2.5 �7.2 2.541 (ax)

2.626 (eq)7.03 8.09 3.05

4Kr···O2Li5+ 2.3 �9.0 2.542 (ax)

2.612 (eq)6.95 8.08 2.99

5Kr···O2Li5+ 2.3 �11.3 2.546–2.549 (ax)

2.607–2.623 (eq)6.87 8.06 2.92

6Kr···O2Li5+ 2.2 �13.3 2.548–2.604 (ax)

2.608–2.622 (eq)6.80 8.05 2.87

7Kr···O2Li5+ 2.2 �15.1 2.585–2.642 (ax)

2.616–2.642 (eq)6.74 8.05 2.82

Chem. Eur. J. 2013, 19, 2322 – 2329 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 2325

FULL PAPERNoble-Gas-Trapping Agents

ters is 0.04 (0.05) e� for Ne, 0.11 (0.08) e� for Ar, and 0.13(0.109) e� for Kr. The net charge transfer to the O2Li5

+ clus-ter is almost half that of the carbon analogue. Note thatthere is no correlation between the charge transfer and thedissociation energy; thus, perhaps the interaction is not elec-trostatic at all. On the other hand, the WBI also showedvery small values for Ng–Li interactions.

One remaining question is the possible interaction be-tween two Ng atoms. The average bond lengths betweentwo Ng atoms interacting with the same Li centers are2.906 in 12Ne···C5Li7

+ and 2.904 in 7Ne···O2Li5+ clus-

ters (more or less the same as in Ne2, 2.899 at the M05-2X/6-311+G(d) level), 3.790 and 3.729 in 12Ar···C5Li7

+

and 7Ar···O2Li5+ clusters, respectively (slightly shorter than

that in Ar2, 3.947 at the M05-2X/6-311+ G(d) level), and4.016 and 4.057 in 12Kr···C5Li7

+ and 7Kr···O2Li5+ clusters,

respectively (almost the same as in Kr2, 3.997 at the M05-2X/6-311+G(d) level). However, there is a decrease in thedissociation energy, which could be related to the repulsionof the Ng atoms.

Energy decomposition analysis of Ng···C5Li7+ and

Ng···O2Li5+ : Let us analyze the nature of the Ng···cluster in-

teraction by using the energy decomposition analysis (EDA)suggested by Su and Li.[68] In this scheme, the interactionenergy (DEint) is split into five terms: electrostatic (DVelstat),exchange (DEex), repulsion (DErep), polarization and chargetransfer (DEpol+ct), and dispersion (DEdisp). The final term isequal to the post-Hartree–Fock correction to the interactionenergy, applicable, in our case, to the MP2 computations.

Table 6 provides the EDA results for Ng···C5Li7+ and

Ng···O2Li5+ clusters. The chosen fragments were the Ng

atoms and the bare clusters. Clearly, DEex, DEpol+ ct, DVelstat,and DEdisp are the attractive terms, with DEpol+ ct being themost important contribution. Considering that the transfer-

red net charge from Ng atoms to Li centers is small, as ex-plained above, it can be inferred that most of the DEpol+ct

term arises from the polarization interaction, thereby show-ing that this effect is the key factor in the formation of thesecomplexes. DEdisp is also relevant, as expected in weaklybound systems. All the interaction terms increase in theorder He<Ne<Ar<Kr, following the periodic trend.

A careful inspection of Table 6 reveals that a major differ-ence between axial and equatorial centers in the Ng-loadedC5Li7

+ clusters arises from the DEdisp term, which is 2–3 times larger for axial ones and is the stabilizing factor thatcontributes to a bigger DEint. Similarly, DEint values for theO2Li5

+ clusters are larger for axial cases. Nevertheless, thisbehavior has a different origin. First, although DVelstat issmall, and usually positive for axial centers, this term has anon-negligible and stabilizing effect for equatorial centersbinding with Ne, Ar, and Kr. In contrast to Ng···C5Li7

+ clus-ters, DEex and DEdisp values are larger for equatorial centersthan for axial ones, which favors the former interaction.However, for axial Li centers DEpol+ct is around 50 % largerthan for equatorial ones, and also DErep is smaller. As aresult, axial centers have larger DEint, the polarizationenergy being the main contribution that favors this prefer-ence.

In other words, even though both C5Li7+ and O2Li5

+ clus-ters have a great capability to capture Ng atoms, the type ofinteraction responsible for that preference is different foreach cluster. Although in C5Li7

+ the preference for axialcenters is mainly a consequence of the dispersion interac-tion, in the O2Li5

+ cation this preference is favored by po-larization. Li�O bonds polarize more than Li�C bonds,given the higher electronegativity of the oxygen atom.

Ng loading in selected other clusters : To test the Ng-trap-ping ability, we chose some other clusters that have alreadybeen reported in the literature as effective hydrogen-storingcandidates. Recently, we have explored the hydrogen-trap-ping ability of B3ACHTUNGTRENNUNG(m-Li)3H3

+ , B3ACHTUNGTRENNUNG(m-Li)2H3,[47] Si5Li7

+ ,[69] Si4Li4,Ge4Li4, and super-alkali clusters: FLi2

+ , OLi3+ , N2Li7

+ , andBLi6

+ .[44] The Li-decorated BHNH sheet and graphenesheets are also very well accepted as hydrogen-storage sys-tems.[53,54] Herein, we studied the Ng-trapping ability of theabove-mentioned clusters and the repeating fragments ofBHNH and graphene sheets as a representative of them.For these systems, we do not go into detail about their maxi-mum number of trapped Ng atoms, but rather try to exploretheir efficacy to interact with Ng atoms in terms of dissocia-tion energy, by taking adsorption of one Ng atom as a refer-ence. The optimized geometries of the studied clusters andtheir Ng-trapped analogues are displayed in Figure 3, andthe results regarding their dissociation energy and Li�Ngbond length are given in Table 7.

The dissociation-energy values for all cases are strongenough to justify their efficiency to interact with Ng atoms.However, the dissociation-energy values are a maximum forAr-trapped analogues and follow the order DEAr>DEKr>

DENe>DEHe for all cases at the MP2 level.

Table 6. Decomposition analysis by Su and Li[68] of the electron interac-tion energy (DE) between Ng atoms and the C5Li7

+ and O2Li5+ clusters

computed at the MP2/6-311 +G ACHTUNGTRENNUNG(d,p) level for the He-trapping case andat the MP2/6-311 +G(d) level for other Ng-trapping cases. Energies inkcal mol�1.

Clusters DVelstat DEex DErep DEpol+ct DEdisp DEint

He···C5Li7+ (ax) �0.01 �0.52 1.10 �0.73 �0.37 �0.53

He···C5Li7+ (eq) �0.01 �0.53 1.15 �0.76 �0.10 �0.26

Ne···C5Li7+ (ax) �0.13 �0.69 1.49 �0.71 �0.39 �0.43

Ne···C5Li7+ (eq) �0.16 �0.68 1.49 �0.79 �0.14 �0.28

Ar···C5Li7+ (ax) �0.12 �1.45 3.01 �1.76 �1.34 �1.66

Ar···C5Li7+ (eq) �0.16 �1.32 2.87 �1.97 �0.70 �1.30

Kr···C5Li7+ (ax) �0.29 �2.32 4.63 �2.35 �1.94 �2.28

Kr···C5Li7+ (eq) �0.21 �1.75 3.75 �2.63 �0.96 �1.80

He···O2Li5+ (ax) 0.04 �0.30 0.77 �0.90 �0.09 �0.49

He···O2Li5+ (eq) �0.01 �0.47 1.00 �0.60 �0.12 �0.20

Ne···O2Li5+ (ax) �0.05 �0.38 0.97 �0.97 �0.15 �0.58

Ne···O2Li5+ (eq) �0.16 �0.60 1.30 �0.63 �0.14 �0.24

Ar···O2Li5+ (ax) 0.02 �0.87 2.30 �2.79 �0.50 �1.83

Ar···O2Li5+ (eq) �0.11 �1.17 2.49 �1.59 �0.62 �1.00

Kr···O2Li5+ (ax) 0.02 �1.13 3.00 �3.76 �0.55 �2.42

Kr···O2Li5+ (eq) �0.17 �1.59 3.30 �2.12 �0.85 �1.42

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P. K. Chattaraj, G. Merino et al.

Effect of applied electric field : The effect of an applied elec-tric field on Ng trapping was studied by taking the adsorp-tion of one Ng atom onto C5Li7

+ and O2Li5+ clusters as a

reference. To study the variation of the dissociation energyDe (without ZPE correction at the MP2 level) with an ap-plied electric field, the electric field was tuned from 0.001 to0.006 a.u. with an interval of 0.001 a.u. along the direction ofthe bound Ng atom. It was found that, very similar to hydro-gen-gas trapping,[44,53, 54] the dissociation-energy value for Ng

binding was gradually improved with the applied electricfield (Table S15 in the Supporting Information andFigure 4).

For example, the De per He atom increases from 0.9 kcalmol�1 in the field-free case to 2.0 kcal mol�1 in the presenceof an electric field of 0.003 a.u. along the Z direction. How-ever, it becomes 4.5 kcal mol�1 when the strength of the ap-plied electric field increases to 0.006 a.u. (Table S15 in theSupporting Information). The rate of improvement in thedissociation energy with the applied electric field is the high-est for Kr trapping and follows the order: DEKr>DEAr>

DENe>DEHe. This is presumably due to the atomic polariza-bility order of DKr>DAr>DNe>DHe. Therefore, the sametechnique to improve the dissociation energy as that in thecase of hydrogen storage can also be successfully applied forNg trapping.

Table 7. The dissociation energy of the 1Ng···cluster (De,[a] kcal mol�1)

and Li�Ng bond length (rLi�Ng, ) of the studied clusters at the MP2level with the 6-311+ GACHTUNGTRENNUNG(d,p) and 6-311 +G(d) basis sets.

Clusters De rLi�Ng Clusters De rLi�Ng

FLi2+ BLi6

+

1He···FLi2+ 1.0 2.029 1He···BLi6

+ 0.2 2.3841Ne···FLi2

+ 2.2 2.166 1Ne···BLi6+ 1.4 2.220

1Ar···FLi2+ 6.8 2.387 1Ar···BLi6

+ 3.5 2.4351Kr···FLi2

+ 5.0 2.610 1Kr···BLi6+ 3.1 2.690

OLi3+ Si4Li4

1He···OLi3+ 0.8 2.048 1He···Si4Li4 0.6 2.054

1Ne···OLi3+ 2.0 2.178 1Ne···Si4Li4 1.8 2.155

1Ar···OLi3+ 6.0 2.411 1Ar···Si4Li4 3.9 2.414

1Kr···OLi3+ 4.0 2.645 1Kr···Si4Li4 3.2 2.661

N2Li7+ Ge4Li4

1He···N2Li7+ 0.6 2.064 1He···Ge4Li4 0.4 2.106

1Ne···N2Li7+ 1.8 2.186 1Ne···Ge4Li4 1.5 2.185

1Ar···N2Li7+ 4.0 2.429 1Ar···Ge4Li4 3.2 2.425

1Kr···N2Li7+ 3.3 2.677 1Kr···Ge4Li4 2.7 2.681

B3 ACHTUNGTRENNUNG(m-Li)3H3+ Li@BHNH

1He···B3ACHTUNGTRENNUNG(m-Li)3H3+ 0.9 2.012 1He···ACHTUNGTRENNUNG[Li@BHNH] 0.4 2.163

1Ne···B3 ACHTUNGTRENNUNG(m-Li)3H3+ 2.1 2.141 1Ne··· ACHTUNGTRENNUNG[Li@BHNH] 1.7 2.207

1Ar···B3ACHTUNGTRENNUNG(m-Li)3H3+ 4.9 2.376 1Ar··· ACHTUNGTRENNUNG[Li@BHNH] 3.7 2.449

1Kr···B3ACHTUNGTRENNUNG(m-Li)3H3+ 4.5 2.611 1Kr···ACHTUNGTRENNUNG[Li@BHNH] 2.8 2.717

B3 ACHTUNGTRENNUNG(m-Li)2H3 Li@C8H8

1He···B3ACHTUNGTRENNUNG(m-Li)2H3 0.6 2.073 1He···ACHTUNGTRENNUNG[Li@C8H8] 0.9 1.9751Ne···B3 ACHTUNGTRENNUNG(m-Li)2H3 1.3 2.160 1Ne··· ACHTUNGTRENNUNG[Li@C8H8] 2.3 2.1091Ar···B3ACHTUNGTRENNUNG(m-Li)2H3 3.4 2.430 1Ar··· ACHTUNGTRENNUNG[Li@C8H8] 4.8 2.3581Kr···B3ACHTUNGTRENNUNG(m-Li)2H3 2.5 2.695 1Kr···ACHTUNGTRENNUNG[Li@C8H8] 4.0 2.587

Si5Li7+

1He···Si5Li7+ (ax) 0.6 2.075

1He···Si5Li7+ (eq) 0.6 2.052

1Ne···Si5Li7+ (ax) 2.0 2.184

1Ne···Si5Li7+ (eq) 1.9 2.157

1Ar···Si5Li7+ (ax) 4.9 2.412

1Ar···Si5Li7+ (eq) 4.4 2.398

1Kr···Si5Li7+ (ax) 4.5 2.643

1Kr···Si5Li7+ (eq) 3.9 2.634

[a] De is the dissociation energy per Ng atom without ZPE correction.

Figure 3. Optimized geometries of the studied clusters at the MP2/6-311+ GACHTUNGTRENNUNG(d,p) and 6-311 +G(d) levels.

Chem. Eur. J. 2013, 19, 2322 – 2329 � 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 2327

FULL PAPERNoble-Gas-Trapping Agents

Conclusion

Density functional and ab initio studies have been per-formed to understand the noble gas (He to Kr) trappingability of C5Li7

+ and O2Li5+ clusters. The C5Li7

+ cluster cantrap up to seven He atoms and 12 Ng atoms (Ng= Ne, Ar,Kr), whereas the O2Li5

+ cluster binds to seven Ng atoms(Ng=He, Ne, Ar, Kr). The dissociation-energy (D0) valuesper Ng atom follow the order DEKr>DEAr>DENe>DEHe atthe M05-2X level. There is no direct orbital overlap betweenLi and Ng atoms, so it appears that the effective positivecharge on each Li center polarizes the electron density of

Ng atoms, resulting in a net charge transfer from Ng atomsto Li centers. All probable Ng-atom-trapping processes onto C5Li7

+ and O2Li5+ clusters are exothermic in nature.

Therefore, the Li centers, which have a sufficient amount ofpositive charge, can interact not only with a hydrogen mole-cule, but also with Ng atoms having dissociation energy inbetween that of physisorption and chemisorption. BothC5Li7

+ and O2Li5+ clusters are capable of capturing Ng

atoms, but the type of interaction responsible for that pref-erence is different for each cluster. In C5Li7

+ , the preferencefor axial centers is mainly a consequence of the dispersioninteraction, whereas for the O2Li5

+ cluster this preference isgiven by polarization Li�O bonds polarize more than Li�Cbonds, given the higher electronegativity of the oxygenatom. CDFT-based descriptors such as electronegativity (c),hardness (h), and electrophilicity (w), coupled with electron-ic structure principles such as MHP and MEP, provide addi-tional insights into the stability of these clusters upon Ngtrapping. In addition, a series of hydrogen-gas-trapping sys-tems have been considered at the MP2 level to study theirpotential towards binding to Ng atoms. The dissociation-energy values per Ng atom showed enough stability in everycase and follow the trend: DEAr>DEKr>DENe>DEHe. Theeffect of applied electric field has been studied by taking thesingle Ng atom adsorption case as a reference, and it isfound that an applied electric field improves the correspond-ing dissociation-energy values for Ng trapping quite signifi-cantly, similar to hydrogen binding. However, the dissocia-tion energy improves most for the Kr-trapping case and fol-lows the order: DEKr>DEAr>DENe>DEHe. The atomic po-larizability values seem to be responsible for this trend.

Acknowledgements

Financial assistance from CSIR, New Delhi and the Indo-EU (HYPO-MAP) project is gratefully acknowledged. P.K.C. would like to thankDST, New Delhi for the J. C. Bose National Fellowship. M.C. acknowl-edges Conacyt for the PhD fellowship. We acknowledge the General Co-ordination of Information and Communications Technologies (CGSTIC)at CINVESTAV for providing HPC resources on the Hybrid Cluster Su-percomputer “Xiuhcoatl”, which have contributed to the research resultsreported herein. The work in Mexico and Colombia was supported byConacyt and Colciencias via a Bilateral project.

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Received: September 11, 2012Published online: January 7, 2013

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