Structure and Stability of Endohedral X@ C60F, X@ C60F2 (X= N, H

Apr 13, 2010 - The structure and stability of endohedral X@C60Fn (X ) N, H; n ) 1, 2) and ... showed that other atoms, e.g., O,8b,f F,8b P,8f and S,8f...
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Structure and Stability of Endohedral X@C60F, X@C60F2 (X ) N, H), and (H@C60)2 Jianfeng Jia,† Hai-Shun Wu,*,† Xiao-Hong Xu,† Xian-Ming Zhang,† and Haijun Jiao*,‡ School of Chemistry and Materials Science, Shanxi Normal UniVersity, Linfen, 041004, China, and Leibniz-Institut fu¨r Katalyse e.V. an der UniVersita¨t Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany ReceiVed: October 13, 2009; ReVised Manuscript ReceiVed: February 16, 2010

The structure and stability of endohedral X@C60Fn (X ) N, H; n ) 1, 2) and (H@C60)2 are computed at the B3LYP level of density functional theory. The most stable N@C60F has one endo N-C bond, while N@C60F2 favors nitrogen atom at the cage center. Both H@C60F and H@C60F2 favor isomers with one endo C-H bond. The most stable dimer structure, (H@C60)2, has one intercage C-C bond and two endo C-H bonds, and is stable below 650 K, but dissociates above 650 K. Introduction One of the fascinating properties of fullerenes is their ability to encapsulate atoms and small molecules inside to generate endofullerenes, which have potential applications as superconductors, drug carriers, ferroelectronic materials, reactors, and nonlinear optical materials. Soon after the discovery of C60, Kroto, Smalley, and Curl reported the first evidence for endofullerene and believed that La atoms are encapsulated in a carbon cage in their experiment.1 Till now, endofullerenes with metal,2 noble gas,3 nitrogen atom,4 and metal nitride clusters5 have been reported in many experimental studies and also are concerned in many theoretical calculations.6-9 Among these endohedral fullerenes, N@C60 is the first endofullerene, confirmed both by experimental4a,b and by theoretical studies,8 in which the encapsulated nitrogen atom occupies the cage center with its open shell configuration. Theoretical calculations showed that other atoms, e.g., O,8b,f F,8b P,8f and S,8f also can keep their atomic electronic configuration and occupy the oncenter or nearly on-center position when being trapped into a C60 cage. More recently, communicating with encapsulated atoms or molecules in a carbon cage10 or controlling the characteristics (such as position, orientation) of the endo guests becomes a new interesting field of fullerene science because of its expected value in design of functional molecular devices with new electronic or magnetic properties.11 Theoretical calculations by Hu12 found that the external attachment of two hydrogen atoms to C60 can increase the stability of LiF inside its cage and determines the LiF orientation inside. Yamada et al. found that the free random motion of the encapsulated atoms in Y@C82, La@C82,13 and Ce2@C8014 can be fixed at specific positions by exohedral chemical functionalization. In this work, based on B3LYP density functional theory calculations, we used N@C60 and H@C60 as models to show how the external attachment of a fluorine atom or the dimerization of endofullerene monomers affects the behavior of the encapsulated atoms inside a C60 cage, which may provide an * To whom correspondence should be addressed. E-mail: wuhs@ dns.sxnu.edu.cn and [email protected]. † Shanxi Normal University. ‡ Leibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock.

idea to control the binding state and magnetic property of the endo atoms, and in turn to develop new molecular functional devices. Computational Methods Computations reported here are performed by the Becke’s15 three-parameter hybrid functional in conjunction with the exchange-correlation functional of Lee, Yang, and Parr (B3LYP).16 Calculations on a number of related open-shell systems, like the triplet state of the dianionic C60 dimer (C60)22-,17 and paramagnetic endofullerenes X@C60 (X ) 4N, 4 P, 3O, 3S),8f show that the B3LYP method provides consistent results. Free geometry optimization for X@C60F and X@C60F2 (X ) H, N) is performed without symmetry restriction at the start to allow free random motion of the endohedral atom at the B3LYP/6-31G(d,p) level. Frequency calculations are carried out at the same level to identify the optimized structures to be energy minima on the potential energy surface, and also to provide the zero-point energies (ZPE). Then, single point energy calculations on the corresponding structures are performed at B3LYP/6-311++G(d,p) to obtain more precise electronic energies. For the H@C60 dimers, geometry optimization and frequency calculation were done at the B3LYP/3-21G(d) level to get the ZPE, and then the structures are refined at the B3LYP/ 6-31G(d) levels. To check the possible spin contamination of the open-shell states, the total spin values 〈S2〉 for all reported structures are calculated, and as expected, no spin contamination was found. All calculations are carried out with the Gaussian 03 program.18 Results and Discussions a. N@C60F. Isomer 1 in Figure 1 represents the N@C60F isomer, in which the nitrogen atom is located at the cage center and does not bind to the fullerene shell like [email protected] Both ferromagnetic (multiplicity ) 5) and ferrimagnetic (multiplicity ) 3) states are considered, and they essentially have the same energy at B3LYP/6-31G(d,p), and no spin coupling between the N atom and the cage can be found (Supporting Information). Analysis of spin density on the ferromagnetic state in Figure 2 reveals that one unpaired electron locates on the carbon cage, and other three on the isolated N atom, i.e., the N atom retains its atomic configuration in 1. Therefore, 1 can be considered as the encapsulation of a nitrogen

10.1021/jp909808g  2010 American Chemical Society Published on Web 04/13/2010

Structure of Endohedral X@C60Fn and (H@C60)2

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Figure 2. Spin density of N@C60F, H@C60F and N@C60F2, H@C60F2 isomers and H@C60 (1 and 3, as well as 5 and 7 have similar spin density, respectively).

Figure 1. Structures and relative energies (in parentheses, at B3LYP/ 6-311++G(d,p) including B3LYP/6-31G(d,p) ZPE correction) of N@C60F (1, 2), H@C60F (3, 4), N@C60F2 (5, 6), and H@C60F2 (7, 8).

atom into the monofluorinated C60 cage, C60F, which is a neutral radical in nature. To obtain the most stable structure of N@C60F, all possible structures with N binding to each individual shell carbon atom are optimized at B3LYP/6-31G(d,p) with different spin states. Their energies are listed in the Supporting Information, and the most sable structure is isomer 2 as shown in Figure 1. Isomer 2 has Cs symmetry and two unpaired electrons; the endo N atom binds to a carbon atom on fullerene cage. One C-C bond shared by two six-membered rings is saturated by the exo F atom and the endo N atom. The endo C-N bond length is 1.529 Å, which is longer than the typical C-N single bond length of 1.47-1.48 Å.19 The longer endo bond is also observed for other atoms. For example, Bettinger et al. found the endo C-F bond on the carbon nanotube is 1.552 Å, which is 0.108 Å longer than the exo C-F bond on the same tube.20 Calculation for endofullerene H@C59N indicates the endo C-H bond to be about 0.02 Å longer than the exo ones.21 Spin density in Figure 2 reveals that two unpaired electrons are the 2p electrons on the N atom. 2 is more stable than 1 by 8.6 kJ/mol, and this small energy difference indicates they might exist in equilibrium depending on the change of the temperature.

b. H@C60F. Although there is no experimental evidence for H@C60 formation, ab initio quantum and molecular dynamics calculations by Smith et al. indicate that it is feasible to implant a H atom with a good probability into the fullerene cage.22 Buchachenko shows that the H atom can form an endo C-H bond when being encapsulated into the C59N fullerene,21 and Ramachandran suggests that H+ can form a stable H+@C60 complex.23 Estreicher shows that when a H atom is encapsulated in a fullerene cage, it will occupy the cage center and retain its atomic electron configuration.24 Isomer 3 in Cs symmetry is H@C60F with two unpaired electrons and the H atom is freely located at the C60F cage center. Both electronic states with parallel and unparallel alignments of spins are considered. They also have essentially the same energy as the situation for N@C60F. Spin density (ferromagnetic state) analysis shows that one of the unpaired electrons distributes on the H atom and the other on the carbon shell near the fluorinated carbon. Isomer 4 is the most stable H@C60F, in which th H atom binds to a carbon atom adjacent to the fluorinated carbon. Isomer 4 is more stable than 3 by about 150 kJ/mol, which is substantially greater than the energy differences between isomers 2 and 1 for N@C60F. The endo C-H bond length of 4 is 1.110 Å. That N and H binding to a carbon on the 6,6-bond is closely near the C-F bond instead of the other carbon atoms in isomer 2 and 4 can be understood easily on the basis of the singly occupied molecular orbital (SOMO) of C60F. The carbon atom binding to the N (or H) atom in isomer 2 (or 4) has the largest coefficient in the SOMO of C60F, which is a favorable site for a radical attack. c. N@C60F2. In general, fluorination of C60 takes place at a 6,6-bond with 1,2 addition,25 and our considered N@C60F2 isomers are based on this topology. Isomer 5 in C2V symmetry

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represents N@C60F2, in which the nitrogen atom is located at the cage center and does not bind to the carbon shell with three unpaired electrons. To find the most stable N@C60F2, all possible structures with N binding to each individual shell carbon atom are considered at B3LYP/6-31G(d,p) first. The most stable structure is isomer 6 in C1 symmetry, which has N bonded to two cage carbons (1.478 and 1.536 Å, respectively) and one unpaired electron. However, isomer 5 is computed to be 28.9 kJ/mol more stable than isomer 6; and this indicates that when N@C60 is fluorinated by two F atoms, the N atom retains its atomic configuration as in N@C60, and does not trend to bind to the carbon shell as in N@C60F. Therefore, 5 can be considered as the encapsulation of a nitrogen atom into the 1,2-C60F2 cage. d. H@C60F2. Isomer 7 represents the H@C60F2, in which the H atom freely locates at the cage center. Isomer 8 is the most stable H@C60F2 with H binding to carbon atom, obtained from 16 H@C60F2 isomers. Both isomers 7 and 8 have one unpaired electron; and in isomer 7 the unpaired electron is located on the H atom, while in 8, as shown in Figure 2, it mainly distributes on the carbon atom near the hydrogenated carbon atom. The length of the endo C-H bond of 8 is 1.116 Å. It should be noted that the stability order of two isomers of H@C60F2 is essentially different from that of N@C60F2 isomers. For N@C60F2, the nonbonded isomer 5 is more stable than the bonded isomer 6, while for H@C60F2, the bonded isomer 8 is about 41 kJ/mol more stable than the nonbonded isomer 7. This means that the endo hydrogen atom has a good probability of bonding to the carbon shell when the fullerene shell is attached by one or two fluorine atoms. Actually this energetic difference is directly associated with the electronic configuration and property of H atom and N atom and can be easily explained by their difference in electron affinity. Electron affinity (EA) is the energy associated with the addition of an electron to a gaseous atom;26 a negative EA corresponds to an attraction for an electron, while an unbound electron has an EA of zero. It is found that a hydrogen atom has a negative EA, while a nitrogen atom has an EA close to zero with its half-filled valence orbital, which represents a stable electronic configuration, and adding one electron to nitrogen destabilizes it because it loses this halffilled shell by adding an extra electron. This is why isomer 8 is the most stable isomer for H@C60F2, while isomer 3 is the most stable isomer for N@C60F2. e. H@C60 Dimers. Inspired by the structures of H@C60F and H@C60F2, it is interesting to explore the structure of dimerized H@C60. We are interested to know if the dimerization can trigger the binding state of the endo hydrogen atom with the fullerene shell. It is well-known that C60 fullerene can form a dimer linked by a cyclic C4 unit in a [2+2] cycloaddition pattern.27 Under high pressure and high temperatures C60 can polymerize easily into two-dimentional28 or three-dimentional polymers.29 For the electron-rich C59N, it was synthesized and isolated as its dimer (C59N)2, in which two C59N are linked by a single C-C bond formed by carbon atoms neighboring the nitrogen atom on each monomer.30 X-ray structure analysis of an electron-doped C60 dimer dianion phase also suggested a single C-C bonded structure, which is formed by direct covalent C-C binding between two fullerene monomers.31 Moreover, the singly bonded dimeric (C70)22+ and (C70)22- were also found.32 In our work both H@C60 dimers linked by one C-C bond (9 and 10) and by two C-C bonds (11 and 12) are studied as shown in Figure 3. For isomer 9, 11, and 12, both the electronic configurations of the ferromagnetic state (with parallel alignment

Jia et al.

Figure 3. Structures and relative energies of (H@C60)2 (the first energy term in parentheses is the relative energy to isomer 9, and the second term is dimerization energy relative to two H@C60; calculated at B3LYP/6-31G(d) including B3LYP/3-21G(d) ZPE corrections).

of spins) and the antiferromagnetic state (open-shell singlet) are considered first at the B3LYP/3-21G(d) level, and there is no strong interaction between spin electrons found (Supporting Information). Thus, only the ferromagnetic state is further optimized at the B3LYP/6-31G(d) level. Isomer 9 in C2 symmetry is a dimer with one C-C bond, in which two hydrogen atoms are freely located at the centers of two fullerenes. Isomer 9 has four unpaired electrons, two located on the H atoms and two on fullerene cages as shown in Figure 4. The intercage C-C bond length is 1.611 Å, which is much longer than that of other C-C bonds on the fullerene cages (ranging from 1.396 to 1.544 Å). Isomer 10 in C2 symmetry has a singlet electronic state; the two endo hydrogen atoms bind to the carbon cages with a C-H bond length of 1.110 Å. The intercage C-C bond length is 1.547 Å, which is much shorter than that in isomer 9, the same as a typical C(sp3)-C(sp3) bond length (1.54 Å).19 Isomers 11 and 12 are dimers linked by two C-C bonds, and both have Ci symmetry and are confirmed as local minima on the B3LYP/3-21G(d) potential energy surface. In 11, two hydrogen atoms freely reside in the fullerene cage with a slight deviation from the center of the cage. The intercage C-C bond

Structure of Endohedral X@C60Fn and (H@C60)2

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Figure 5. Dimerization Gibbs free energies for isomers 9-12 at different temperature.

free energies are not possible to form; and isomer 12 is only favorable at very low temperature (below 50 K). In contrast, isomer 10 is stable up to about 650 K, but will dissociate into monomer at temperatures over 650 K.35 Conclusions

Figure 4. Spin density of isomer 9, 11, and 12.

length is 1.591 Å, which is similar to the theoretically predicted value of the C60 dimer.33,34 In 12 endo hydrogen atoms bind to the fullerene cage. The length of the endo C-H bond is 1.117 Å, nearly the same as that of isomer 9. The length of the intercage C-C bonds is 1.580 Å, shorter than that in isomer 11. Both 11 and 12 have two unpaired electrons, which are located on the H atoms in 11 but on the carbon cage in 12. Isomer 10 is most stable, while isomer 9, 291.5 kJ/mol higher than 10 in energy, is least stable. Despite being connected by two C-C bonds, isomer 12 is about 151.7 kJ/mol higher in energy than 10. Isomer 11 with two broken endo C-H bonds is less stable than 12 by about 39.1 kJ/mol. The higher energy of 11 indicates that dimerization of H@C60 will cause the hydrogen atom binding to the fullerene cage to form endo C-H bonds. To estimate the stability of the dimer relative to the H@C60 monomer, we define the dimerization energy (Ed) according to eq 1.

Ed ) E(H@C60)2 - 2E(H@C60)

(1)

where 2H@C60 ) (H@C60)2. The Ed of 10 is exothermic by 161.4 kJ/mol, while the Ed of 9 is endothermic by 130.1 kJ/mol. With the formation of two C-C bonds, the Ed of 11 is endothermic by 29.4 kJ/mol, and the Ed of 12 is slightly exothermic by 9.7 kJ/mol. The positive dimerization energies of 9 and 11 reveal that they are not possible to form. The negative dimerization energies reveal that isomer 10 is most stable, followed by 12; and high pressure may induce this type of polymerization of H@C60 as C60.28,29 Apart from the dimerization energy, we have computed the change of Gibbs free energy at the B3LYP/3-21G(d) level (Figure 5). As expected, isomers 9 and 11 with positive Gibbs

The structure and stability of X@C60Fn (X ) N, H; n ) 1, 2) and (H@C60)2 have been computed at the B3LYP level of density functional theory. It is found that N@C60F favors isomer 2 with one endo N-C bond in the triple ground state, and the isomer with the N atom at the cage center (1) in the quartette ground state is 8.6 kJ/mol higher in energy. This rather small energy difference indicates the possible equilibrium on the change of temperature. For the 1,2-C60F2 cage, N@C60F2 (5) favors the nitrogen atom located at the cage center in the quartette ground state, in agreement with the electronic structure of N@C60. However, both H@C60F and H@C60F2 favor isomers 4 and 8 with an endo C-H bond in singlet and double ground state, respectively. The opposite energetic order for N@C60F2 and H@C60F2 is directly associated with the ground state electronic configuration of the hydrogen and nitrogen atoms. The most stable dimer ((H@C60)2, 10) structure favors two endo H-C bonds and one intercage C-C bond in singlet ground state, in agreement with the electron-rich structures of (C59N)2 and (C60)22-. The computed Gibbs free energy reveals that 10 is stable below 650 K, but dissociates into monomer above 650 K. Acknowledgment. This work was supported by the Natural Science Foundations of China (20673070 and 20871077). Supporting Information Available: Computed energetic data and Cartesian coordinates for all reported structures. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Heath, J. R.; O’Brien, S. C.; Zhang, Q.; Lui, Y.; Curl, R. F.; Kroto, H. W.; Tittel, F. K.; Smalley, R. E. J. Am. Chem. Soc. 1985, 107, 7779. (2) (a) Cox, D. X.; Trevor, D. J.; Reckmann, K. C.; Kaldor, A. J. Am. Chem. Soc. 1986, 108, 2457. (b) Elkind, F. D.; O’Brien, S. C.; Carl, R. F.; Smalley, R. E. J. Am. Chem. Soc. 1988, 110, 4464. (c) Bethune, D. S.; Johnson, R. D.; Salem, J. R.; der Vries, M. S.; Yannoni, C. S. Nature 1993, 366, 123, and references cited therein. (d) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843. (e) Nishibori, E.; Takata, M.; Sakata, M.; Tanaka, H.; Hasegawa, M.; Shinohara, H. Chem. Phys. Lett. 2000, 330, 497. (f) Klingeler, R.; Kann, G.; Wirth, I.; Eisebitt, S.; Bechthold, P. S.; Neeb, M.; Eberhardt, W. J. Chem. Phys. 2001, 115, 7215. (g) Kanbara, T.; Kubozono, Y.; Takabayashi, Y.; Fujiki, S.; Iida, S.; Haruyama, Y.; Kashino, S.; Emura, S.; Akasaka, T. Phys. ReV. B 2001, 64, 113403. (h) Shiotani, H.; Ito, T.;

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