Orientation of Endohedral H2, CO, andLiH Inside Heptagon-ContainingC58 and C58H18

LILI SUN, YINGFEI CHANG, SHUWEI TANG, RONGSHUN WANGInstitute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University,Changchun, Jilin 130024, China

Received 4 December 2008; accepted 22 December 2008Published online 6 July 2009 in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/qua.22070

ABSTRACT: Three H2@C58Hx, six CO@C58Hx, and six LiH@C58Hx (x � 0 and 18)complexes were optimized using B3LYP/6-31G* method. The results show that both C58

and C58H18 destabilize nonpolar H2 and weakly polar CO, and stabilize strongly polarLiH inside their cages. Three H2@C58Hx (x � 0 and 18) complexes are nearly equivalentin energy, and CO orients the longest direction of cage because of spatial repulsionbetween them in the most stable CO@C58Hx (x � 0 and 18) isomers. Orientation of LiHinside C58Hx (x � 0 and 18) cages is determined by dipole-induced dipole attractiveinteraction between them, and this attraction is especially significant in LiH@C58H18

complexes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem 110: 1080–1085, 2010

Key words: heptagon-containing; C58; C58H18; endohedral complexes; orientation

Introduction

S ince C60 [1] fullerene structure was discoveredin 1985, the possibility of encapsulating atoms

inside the hollow cages has been considered. In thesame year, Heath et al. presented the evidence forthe formation of a stable La@C60 [2], and since then,the ability of fullerenes to trap atoms and smallmolecules inside themselves to generate endohe-

dral complexes has attracted plentiful attention inboth theoretical and experimental points of view. Inthe past years, numerous attempts have been car-ried out to seek stable endohedral complexes, andimmense progress has been achieved.

Cavity of fullerene cage is an important factorinfluencing encapsulation. C20 is the smallestfullerene, and can only encapsulate small atoms. In1999, He@C20H20 [3] was successfully prepared.The succeeding theoretical studies on a series ofC20H20 derivatives [4, 5] show that the inclusionwill lengthen the COC bonds, and exohedral com-plexes are preferable to their endohedral isomers inenergy. As cavity of the fullerene cage increases,size of species can be enclosed also increases. En-

Correspondence to: R. Wang; e-mail: wangrs@nenu.edu.cnContract grant sponsor: National Natural Science Foundation

of China.Contract grant number: 20773021.

International Journal of Quantum Chemistry, Vol 110, 1080–1085 (2010)© 2009 Wiley Periodicals, Inc.

dohedral complexes such as M@C28 (M � U, Hf,Zr), U@C36, U@C50, and U2@C50 [6] were early ob-served in gas state, and therefore, C28, C36, and C50fullerenes are expected to trap metals, and manytheoretical studies have been done [7–11]. Diameterof C60 is 7.1 Å, and previous studies revealed that itcan encapsulate small molecules and dimetallicclusters, such as H2 [12], CO [13], NH3 [14], andM2@C60 (M � Cr, Mo, W, U) [15, 16], etc. In 2008,Komatsu and coworkers have reported the synthe-sis of an open-cage C70 derivative, which is able toencapsulate two H2 molecules [17]. The trimetallicnitride endohedral fullerenes have been attractinggreat interest since they were discovered by Dornand coworkers [18]. C68 is the smallest fullerenecage encapsulating Sc3N clusters reported up tonow [19–21], and Sc3N is forced to be pyramidalinside C68. However, Sc3N is planar inside largefullerene cage such as C80 [18, 22], which can en-capsulate clusters as large as Tb3N [23]. While Eche-goyen et al. demonstrated that trimetallic nitrideclusters M3N (M � Gd,, Nd, Pr, and Ce) with largesize preferred to be encapsulated into largefullerene cage C2n (2n � 88–96) [24].

The possibility for fullerenes to have heptagonwas first proposed in 1992 by Taylor [25], and aheptagon-containing C62, or C58, can be obtained byadding, or removing, a C2 unit from Ih C60, respec-tively. The heptagon-containing C62 is more stablethan all classical isomers [26, 27], and its derivativeshave been systemically studied [28]. The heptagon-containing C58 is calculated to be only 2.5 kcal/molless stable than the most stable classical fullereneisomer [29–34]. In 2005, C58F18 and C58F17CF3 [35]were successfully synthesized, and their heptagon-containing structures have been unambiguouslycharacterized by mass spectrometry and 19F NMRspectroscopy. Other possible exohedral derivativesof heptagon-containing C58 were studied soon after[36]. The heptagon-containing C58 is also expectedto have ability to encapsulate small molecules, suchas H2 and CO, and Hu demonstrated that orienta-tion of nonpolar H2 or weakly polar CO inside C58cage depends on size of endohedral molecule [37],i.e., spatial repulsion in endohedral complex. Pre-vious studies [38] revealed that the performance ofC60 and C60H60 on H2 or CO is same with C58, andorientation of strongly polar LiH inside C60 andC60H60 is determined by dipole-induced dipole at-traction between LiH and the cage. However, per-formance of C58 on LiH is unreported up to now.Which is the factor determining orientation of thestrongly polar molecule inside heptagon-containing

C58, spatial repulsion or dipole-induced dipole at-tractive interaction? How about the orientation ofweakly polar CO inside strong polar fullerene cage?To answer these questions, we investigate the struc-tures and relative stabilities of C58 and C58H18 cagesencapsulating nonpolar H2, weakly polar CO, andstrongly polar LiH with different initial orientationusing B3LYP method and 6-31G* basis set in thisarticle.

Calculated Methods

The heptagon-containing C58 will be called C58 forshort. Endohedral molecules were initially placed onthe x, y, and z axis, respectively, and consider-ing symmetry of molecules, three H2@C58Hx, sixCO@C58Hx, and six LiH@C58Hx (x � 0 and 18) wereoptimized using B3LYP/6-31G* method in all. Allcalculations were carried out using Gaussian 03package [39].

We prefixed the axis where endohedral mole-cules were initially placed to the isomers for distin-guishing them, and for instance, x-H2@C58Hx,y-H2@C58Hx, and z-H2@C58Hx (x � 0 and 18) standfor H2 was initially placed on the x, y, and z axis,respectively. For CO@C58Hx and Li@C58Hx (x � 0and 18) complexes, when O atom in CO, or Li atomin LiH, was initially placed on the positive axis,dipole of endohedral molecule had same orienta-tion with the axis, and the isomer was marked witha subscript A, or else marked with a subscript B. Forinstance, xA-CO@C58Hx (x � 0 and 18) means thatCO was initially placed on the x axis with the Oatom on the positive axis, while xB-CO@C58Hx (x �0 and 18) means the C atom was initially placed onthe positive x axis. The most stable isomer of eachendohedral complex was presented in Figure 1.

To investigate orientation of endohedral mole-cules inside C58Hx (x � 0 and 18) cages after opti-mization, � and � were defined. As seen in Figure 2,endohedral molecule was parallel shifted with Catom in CO, H atom in LiH, or either H atom in H2,placing on the center of the cage. Angle betweenendohedral molecule and the z axis is called �, andangle between x axis and projection of endohedralmolecule on the x-y plane is called �.

Results and Discussion

Hu and Ruckenstein [37] demonstrated that C58destabilizes the nonpolar H2 and weakly polar CO,

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and the inclusion energies are 3.3 and 18.6 kcal/mol, respectively. As seen in Table I, our results onH2@C58 and CO@C58 accord with the conclusions ofHu, but destabilization energies are smaller. Table Ialso presents that C58 cage stabilizes strongly polarLiH, and the performance of C58H18 cage is samewith C58.

Dipole-induced dipole attractive interaction be-tween nonpolar H2 molecule and fullerene cage isnegligible. Previous studies suggest that H2 shouldbe normal to the face of heptagon in C58 [37]; how-ever, our calculations in Table I demonstrate energydifferences between H2@C58Hx (x � 0 and 18) isomersare negligible. As seen, although z-H2@C58H18, inwhich spatial repulsion is minimized, is the moststable one among three isomers, it is only 0.62 kcal/mol more stable than the most labile one. Moreover,distortion of H2 from the initial place in H2@C58Hx

(x � 0 and 18) is negligible except for y-H2@C58H18.Figure 1 has presented structures of the most stableH2@C58Hx (x � 0 and 18) isomers, and as seen, theendohedral H2 molecules are nearly located in theirinitial place. In all, spatial repulsion in H2@C58Hx (x �0 and 18) complexes is not significant, and hardlyaffects their relative stabilities and structures.

Considering the symmetry of CO, we have opti-mized six CO@C58 isomers. As seen in Table I, ourresults show two z-CO@C58 isomers, in which COorients the longest direction of the cage, are morestable than other isomers, which agrees with con-clusions of Hu and Ruckenstein [37]. Moreover,two x-CO@C58 isomers, or two y-CO@C58 isomers,are nearly equivalent in energy. Table I also pre-sents that the endohedral CO is significantly dis-torted from the initial place due to dipole-induced

FIGURE 1. Structures of the most stable isomer for H2@C58Hx, CO@C58Hx, and LiH@C58Hx (x � 0 and 18) com-plexes. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

FIGURE 2. Sketch map of � and �.

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dipole attractive interaction in zB-CO@C58, which isslightly more stable than zA-CO@C58. ForCO@C58H18 complexes, two z-CO@C58H18 isomersare the most stable isomers, and we only presentzB-CO@C58H18 in Figure 1. Moreover, as polarity offullerene cage increase, energy difference betweentwo x-CO@C58H18 isomers, or two y-CO@C58H18isomers, also increase. On the other hand, values of� and � demonstrate that distortion of CO from theinitial place in CO@C58H18 is more significant thanthat in CO@C58. Therefore, we conclude that orien-tation of CO molecule inside C58Hx (x � 0 and 18)is mainly determined by size of CO, even if thefullerene cages are strongly polar. Dipole-induceddipole attractive interaction in CO@C58H18 canslightly influence relative stability of isomers.

Six LiH@C58 isomers were also computed. LiH islarger than CO in molecular size, and therefore,spatial repulsion in LiH@C58Hx (x � 0 and 18)should be more significant than that in CO@C58Hx

(x � 0 and 18). However, as seen in Table I, themost stable isomer is xA-LiH@C58 instead of eitherof z-LiH@C58 isomers, in which LiH orients thelongest direction of C58. This can be explained byexamining the dipole of C58, which is shown inFigure 3(a). As seen, orientation of C58’s dipole isnearly opposite to the x axis, and therefore, xA-LiH@C58 has the most significant dipole-induced

dipole attractive interaction among six isomers, andit is the most stable isomer accordingly. Twoz-LiH@C58 isomers are the second most stable ones,and only 0.59 kcal/mol higher than xA-LiH@C58 inenergy. Table I also shows that LiH is significantly

TABLE I ______________________________________________________________________________________________Relative energies, inclusion energies, � and � of H2@C58Hx, CO@C58Hx, and LiH@C58Hx (x � 0 and 18)complexes.

Endohedralcomplex

n � 0 n � 18

Erel Einca � � Erel Einc

a � �

x-H2@C58Hn 0.00 2.27 90.00 �0.02 0.62 3.74 89.93 �0.05y-H2@C58Hn 0.01 2.28 90.00 90.02 0.26 3.38 90.00 255.50z-H2@C58Hn 0.05 2.32 0.00 0.00 0.00 3.12 0.00 0.00xA-CO@C58Hn 4.79 13.86 90.00 �0.20 3.55 17.50 90.00 �1.66xB-CO@C58Hn 4.73 13.80 89.99 180.07 2.39 16.34 90.00 163.04yA-CO@C58Hn 7.24 16.31 90.00 87.85 0.83 14.77 90.00 123.63yB-CO@C58Hn 7.31 16.38 90.00 269.71 4.04 17.99 90.00 41.10zA-CO@C58Hn 0.24 9.31 0.02 90.00 0.00 13.94 0.30 �70.88zB-CO@C58Hn 0.00 9.07 172.40 127.53 0.00 13.94 179.72 �70.77xA-LiH@C58Hn 0.00 �8.03 90.02 �11.64 5.51 �1.27 90.00 �2.381xB-LiH@C58Hn 2.09 �5.92 90.00 176.75 0.01 �6.77 90.02 172.26yA-LiH@C58Hn 0.84 �7.19 89.63 48.76 0.02 �6.76 90.00 189.62yB-LiH@C58Hn 1.27 �6.76 89.99 �94.21 0.00 �6.78 89.89 194.69zA-LiH@C58Hn 0.59 �7.44 12.45 24.29 0.01 �6.77 90.04 189.97zB-LiH@C58Hn 0.59 �7.44 167.80 24.90 0.01 �6.77 89.98 189.95

a Einc � E(EC) – E(E) – E(C), where E(EC), E(E), and E(C) are total energies of endohedral complex, C58Hx (x � 0 and 18), and the freemolecule, respectively.

FIGURE 3. Dipole of C58 (a) and C58H18 (b). [Colorfigure can be viewed in the online issue, which is avail-able at www.interscience.wiley.com.]

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distorted from the initial place in all isomers. There-fore, structures of LiH@C58 isomers are mainly de-termined by dipole-induced dipole attractive inter-action between them, and spatial repulsion is thesecond important factor. For LiH@C58H18 com-plexes, values of � and � in Table I reveal thatexcept for xA-LiH@C58H18, the other five isomershave similar structures. Relative energies confirmthat xA-LiH@C58H18 is the most labile one amongsix isomers, and energies of other five isomers arenearly equivalent. Figure 1 has present structure ofmost stable yB-LiH@C58H18, and as seen, due to thestrong dipole-induced dipole attractive interactionbetween LiH and C58H18 cage, LiH does not orient thelongest direction of the cage. Moreover, values of �and � suggest that LiH must distort with large angleto reach the equilibrium geometry in some cases, forexample, in y-LiH@C58H18 and z-LiH@C58H18. In all,in LiH@C58H18 complexes, influence of dipole-in-duced dipole attractive interaction dramatically in-creases due to the large polarity of C58H18 cage, andspatial repulsion become insignificant.

Conclusions

We have optimized three H2@C58Hx, sixCO@C58Hx, and six LiH@C58Hx (x � 0 and 18)complexes, and the results show that C58Hx (x � 0and 18) cages destabilize nonpolar H2, weakly polarCO, and stabilize strongly polar LiH. Energy differ-ences between three H2@C58Hx (x � 0 and 18) arevery small, and therefore, spatial repulsion inH2@C58Hx (x � 0 and 18) complexes is not signifi-cant. In CO@C58Hx (x � 0 and 18) complexes, size ofCO is the main factor determining its orientationinside the cages. Orientation of LiH inside C58Hx

(x � 0 and 18) cages is determined by dipole-in-duced dipole attractive interaction between them,and this interaction is especially significant inLiH@C58H18 complexes.

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