Possibility of Proton Motion Through Buckminsterfullerene

24 December 1999

Chemical Physics Letters 315Ž. 1999 181±186 www.elsevier.nlrlocatercplett

Possibility of proton motion through buckminsterfullerene

S. Maheshwari, Debashis Chakraborty, N. Sathyamurthy ) Department of Chemistry, Indian Institute of Technology, Kanpur 208 016 India Received 20 April 1999; in final form 1 October 1999

Abstract

The possibility of a proton entering a fullerene cage without breaking any C±C bond and undergoing oscillations through the cage is considered. A detailed understanding of the interaction of a proton with the five- and six-membered rings is obtained by examining proton±corannulene interaction, as a prototype. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction broken, the rare gas enters through the resulting window and the cage closes by itself. Jimenez- Following the synthesis of fullerenes and the de- Â Vazquez et al.wx 10 have found evidence for trapping velopment of fullerene chemistrywx 1±5 , considerable Â of tritium inside the C cage, when the latter was effort has gone into the production of endohedral 60 exposed to moderated tritium coming from a nuclear fullerenes ± represented as M@C , where M is an 60 reaction. But, to the best of our knowledge, the atomrion trapped inside the cage of C , for exam- 60 interaction of protons with fullerenes has not been ple. This has opened up new ways of making materi- examined so far, either theoretically or experimen- als with specific properties at the molecular level. tally. Therefore, we have undertaken a study of One way to make metallo-endohedral fullerenes is to proton±fullerene interactions and found that a proton vaporize graphite doped with the appropriate metal can easily enter the fullerene cage through either the oxide or saltwx 6,7 . An alternative and more specific five-or six-membered ring and that it can oscillate way is to repeatedly expose monolayers of C to an 60 through the cage and also `rattle' inside the cage. intense beam of alkali ions at an energy such that Recently we have pointed outwx 11 the possibility they penetrate the cage but not destroy itwx 8 . Saun- of a proton oscillating through the benzene ring and ders et al.wx 9 have been able to produce Rg@C 60 also through other aromatic rings such as naphtha- Ž.RgsHe, Ne, Ar, Kr and Xe by heating C to 60 lene and anthracene. Therefore, it is not unreason- 6508C under a high pressureŽ. 3500 atm of the rare able to expect such a possibility to also exist in the gas for a few hours. These workers have proposed a case of fullerenes. `window' mechanism for the formation of endohe- Traditionally, it is believed that protons form a dral fullerenes. It is envisaged that one C±C bond is s-complex with aromatic hydrocarbons like benzene Ž.Bz but Mason et al.wx 12 have found mass spectral q ) Corresponding author. Fax: q91-512-597436; e-mail: evidence for the formation of a Bz±H p-complex. [email protected] Ab initio calculations performed in our laboratory

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0009-2614Ž. 99 01229-4 182 S. Maheshwari et al.rChemical Physics Letters 315() 1999 181±186 showedwx 11 that there is a potential minimum, corre- Although Glukhovtsev et al.wx 14 have pointed out sponding to the p-complex formation, on either side that the p-complex corresponds to a second-order of the benzene ring and that the proton can actually saddle point, 2.06 eV higher in energy than the go through the ring without facing any barrier. More s-complex, there is no reason why the p-complex refined calculationswx 13 have reiterated our findings. cannot be formed and the proton cannot oscillate

Fig. 1.Ž. a Frontal and Ž. b side views of the corannulene structure. C55 indicates the position of the C axis. The numbers 1±3 refer to the particular sites referred to in the text and in Fig. 2b. The s-complex and p-complex formations are also indicated schematically. The distance of the proton approaching the hexagonrpentagon along a perpendicular direction is labelled Z, with Zs0 referring to the centre of r q the hexagon pentagon.Ž. c The ground state interaction potential, V, for H ±C20 H 10 as a function of the approach coordinate, Z, with the q zero of energy corresponding to the asymptotically separated H and C20 H 10 in its equilibrium geometry. The solid line corresponds to motion through the centre of the hexagon, and the dashed line, motion through the centre of the pentagon. The break in the curves implies that the calculations have been carried out only for a limited range of Z values and an extrapolation is made to the asymptote. S. Maheshwari et al.rChemical Physics Letters 315() 1999 181±186 183 through the ring, if the incoming proton has suffi- We have examined the tendency of the proton to cient momentum in the direction orthogonal to the bind with one of the carbon atomsŽ. labelled 1 in plane of the ring. Fig. 1a of the five-membered ring and so form a Before investigating the interaction of a proton s-complex by plotting the interaction potential as a with C60 , we studied the interaction potential be- function of the approach coordinate Z Ždistance along tween a proton and corannulene which has a bowl the line pointing towards the carbon atom and per- shapeŽ. see Fig. 1a and b and for which the carbon pendicular to the plane of the ring. in the form of a framework constitutes the polar cap of buckminster- dashed line, as shown in Fig. 2a. It is clear that the fullerene and the results are summarized below. s-complex at the 1-position is indeed more stable

than the interaction along the C5 axis and that the exohedral C±H bond would be stronger than the 2. Results and discussion endohedral bond.

Keeping corannuleneŽ. C20 H 10 in its equilibrium The results shown in Fig. 1c represent the ground geometryŽ.ŽŽ see Fig. 1a r along the pentagon .state adiabatic potential energy curves and therefore C±C11 s1.41 A,ÊÊr Ž.along the periphery s1.45 A, imply that a slow moving proton brought towards the C±C23 r s1.36 AÊÊÊ, r s1.37A, r s1.07 A. we pentagon would tend to form an exohedral C±H C 125 CC 335 CC±H varied the distance Z of Hq from the centre of a s-complex. However, a fast moving proton directed hexagonal ring and for each value of Z, we compute along the C5 axis can easily pass through the centre the ground state interaction potential using the 6- of the pentagon and preferentially form an endohe- )) 31G basis set and the GAUSSIAN 94 set of pro- dral complex. The barrier to penetration is lowest wx grams 15 . It is clear from the potential-energy along the C5 axis and increases as the proton drifts curve shown in the form of a solid line in Fig. 1c towards one of the carbon atoms labelled 1 in Fig. that there are two minima: one corresponding to an 1a. exohedral positionŽ. convex side and another to an An examination of the interaction potential for the endohedral positionŽ. concave side . It is also clear proton approaching any of the peripheral carbon that the double-well potential is asymmetric, favor- atomsŽ. labelled 2 in Fig. 1b , in the form of a dashed ing the Hq to be on the endohedral side of corannu- line marked s in Fig. 2b reveals a minimum corre- lene. It should be emphasized that the barrier separat- sponding to an exohedral C±H s-complex. The pro- ing the two minima is well below the zero of energy ton can also form a stable p-complex with any of the corresponding to the asymptotically separated proton 1±2 or 3±3 C5C bonds as shown by the dashed and corannulene. This means that proton motion lines labelled p1±2and p 3±3, respectively in Figs. through the centre of any one of the hexagonal rings 1b and 2b. However, this does not preclude the faces no barrier. possibility of a proton passing through the center of Proton interaction with the pentagonal ringŽ along the hexagon and forming an endohedral complex, as the C5 axis.Ž see Fig. 1b . is qualitatively similar to can be seen from the potential plotted in the form of that with any of the hexagonal rings as can be seen a solid line in Fig. 2b. from the results shown in the form of a dashed line Since a single point calculation using the 6-31G )) q in Fig. 1c. Although the barrier separating the two basis set Ž.GAUSSIAN 94 for C60 -H interactions takes minima is larger in the case of the pentagon, it is still nearly two days on our workstation, we have gener- below the zero of energy, suggesting a free motion ated potential energy curves for the fullerene±proton through the pentagon as well. In addition, it can be interaction using the 4-21G basis set. In any case, seen that the proton interaction with the pentagon is our studies on Hq±corannulene interaction have stronger than that with the hexagon, at the endohe- shown that calculations using the 4-21G basis set dral as well as the exohedral side of corannulene. yield qualitatively the same result as do the ones We have reported the potential energy curves for using 6-31G )). only a limited range of Z as one has to go beyond We keep fullerene in its equilibrium geometry s s Hartree±Fock level calculations to get asymptoti- Ž.rC±C1.45 A,ÊÊr C5 C 1.36 A and vary the dis- cally correct results. tance Z from the centre of a hexagonal ring as 184 S. Maheshwari et al.rChemical Physics Letters 315() 1999 181±186

Fig. 2.Ž. a The ground state interaction potential for a proton approaching one of the carbon atoms labelled 1 of the pentagon in C20 H 10 Ž.Fig. 1a is plotted in the form of a dashed line. The interaction potential along the C5 axis is included as a solid line for comparison.Ž. b The dashed line marked s shows the interaction potential for the proton approaching carbon atoms labelled 2. The dashed lines marked p1±2and p 3±3 represent the interaction potential for proton approaching the 1±2 and 3±3 p-bonds, respectively. The solid line represents the interaction potential for the proton approaching through the centre of a hexagonal ring, for comparison.

shown in Fig. 3a. For each value of Z we computed means that as a proton approaches C60 , it can easily PPP q the ground state C60 H interaction potential. It go through any one of the hexagonal rings. As a becomes clear from the results shown in the form of matter of fact, it can easily come out on the other a soild line in Fig. 3b that there are two minima: one side of the cage as well. If it loses some of its energy corresponding to Zs1.3 AÊ and another to Zsy0.9 to the cage, it can result in oscillations through the A,Ê implying the formation of an exohedralŽ outside cage. It is quiet possible that a slow moving proton the cage.Ž. and an endohedral inside the cage com- can form an exohedral s-complex with any of the plex, respectively. carbon atoms but we would like to emphasize that a Although there is a barrier separating the exo- and fast moving proton can easily pass through the cage endohedral complexes, the maximum of the barrier is and form an endohedral complex. still below the zero of energy corresponding to the Results for proton motion through the pentagonal proton asymptotically separated from C60 . This ring are qualitatively similar, as can be seen from the S. Maheshwari et al.rChemical Physics Letters 315() 1999 181±186 185

dashed line in Fig. 3b. Understandably, the barrier separating the exo- and endohedral complexes is larger for the pentagonal face than for the hexagon. q Our calculations show that the C60 ±H interaction potential has a local minimum at the centre of the cage, that is higher in energy than the endohedral and exohedral minima. We have difficulty in getting convergence at some of the in-between positions. Therefore, we represent the interaction potential schematically as a function of the radial distance, r through the pentagonal ring, in Fig. 3c. Considering the anisotropy of the fullerene cage, one can antici- pate that there might be significant hindered oscillations and rattling of the proton inside the cage. Although one could argue in favor of more exten- sive calculations using larger basis sets for determin- q ing accurately the C60 ±H interaction, it is very unlikely that the qualitative findings of our work will change. Interestingly, our ab initio calculationswx 16 for the q interaction of Li and He with C66 H show that there is a large barrier to penetrationŽ larger than the C±C bond dissociation energy. thus making the penetration route accessible only to proton. Within the local density approximation, Dunlap et al.wx 17 found q Li @C60 to be stable. A similar conclusion was s arrived at for M@C60 Ž. M Li, K and O by Li and TomanekÂ wx 18 . It has also been found that the guest atomrion does not prefer the central position in the cage and it `rattles' around, giving rise to a large number of peaks in the infrared spectrumwx 19 . Buckingham and Readwx 20 , using a minimal basis set calculation, have explained how the eccentric

position of Li in Li@C60 is stabilized. Hauser et al. wx21 have shown, using semiempirical and density functional theory calculations that the interior of the

Fig. 3.Ž. a Fullerene structure, with a proton approaching along a C60 cage is inert and that the exterior is reactive with r s direction perpendicular to the hexagon pentagon, with Z 0 respect to H, F, and CH3 radicals. representing the centre of the hexagonrpentagon.Ž. b The ground When it comes to the C ±Hq interaction, ener- q 60 state interaction potential, V, for H ±C60 as a function of the getic considerationsŽ ionization potentials for H and approach coordinate, Z, with the zero of energy corresponding to wx q C60 are 13.6 and 7.54 eV, respectively 22. suggest the asymptotically separated H and C60 in its equilibrium geometry. The solid line represents the interaction potential for that motion through the centre of a hexagon, and the dashed line is for qq ™ q q q motion through the centre of a pentagon. The break in the curves H C60H C 60 6.06 eV . implies that the calculations have been carried out only for a limited range of Z values and an extrapolation is made to the q This would mean that a lot of energy could asymptote.Ž. c Schematic representation of H ±fullerene interac- q tion along the radial distance r of the fullerene cage, with r s0 potentially be released as H approaches C60 and representing the centre of the fullerene cage. the proton may no longer remain a bare proton. It 186 S. Maheshwari et al.rChemical Physics Letters 315() 1999 181±186 could easily be considered to be a hydrogen atom wx5 R.E. Smalley, Rev. Mod. Phys. 69Ž. 1997 723. wx while in the vicinity of the positively charged cage 6 Y. Chai, T. Guo, C. Jin, R.E. Haufler, L.P.F. Chibante, J. and it can still come out as a proton at the other end. Fure, L. Wang, J.M. Alford, R.E. Smalley, J. Phys. Chem. 95 Ž.1991 7564. A similar mechanism has been invoked to explain wx7 D.S. Bethune, R.D. Johnson, J.R. Salem, M.S. de Vries, C.S. the results of experiments involving high-energy Yanoni, NatureŽ.Ž. London 366 1993 123. Ž.20±60 eV projectiles of Heq colliding with HCl wx8 R. Tellgmann, N. Krawez, S.-H. Lin, I.V. Hertel, E.E.B. wx Campbell, NatureŽ.Ž. London 382 1996 407. targets 23 . We only wish to point out that a proton wx Ž.or a hydrogen atom could easily be trapped inside 9 M. Saunders, R.J. Cross, H.A. Jimenez-Vazquez,ÂÂ R. Shimshi, A. Khong, Science 271Ž. 1996 1693. or held outside the cage and that it can oscillate wx10 H.A. Jimenez-Vazquez,ÂÂ R.J. Cross, M. Saunders, R.J. Poreda, between the two minima. Furthermore, it should be Chem. Phys. Lett. 229Ž. 1994 111. pointed out that because of the differences in the wx11 R.S. Shresth, R. Manickavasagam, S. Mahapatra, N. Sathya- q murthy, Curr. Sci. 71Ž. 1996 49. interaction of H with the hexagonal and pentagonal wx rings and the in-between positions, the protonŽ or the 12 R.S. Mason, C.M. Williams, P.D.J. Anderson, J. Chem. Soc., Chem. Commun.Ž. 1995 1027. hydrogen atom. would be expected not only to oscil- wx13 S. Maheshwari, unpublished data. late radially, but also `rattle' around anisotropically wx14 M.N. Glukhovtsev, A. Pross, A. Nicolaides, L. Radom, J. Ž.see above . Chem. Soc., Chem. Commun.Ž. 1995 2347 Ž and references q therein. . It would be interesting to isolate C60 H and other wx protonated fullerenes in an ion trapwx 24 , for exam- 15 M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Peters- ple, and study their spectroscopy. son, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, Acknowledgements P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Re- plogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. The authors are grateful to Dr. S. 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