possibility of proton motion through buckminsterfullerene

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24 December 1999 Ž . Chemical Physics Letters 315 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 Following the synthesis of fullerenes and the de- w x velopment of fullerene chemistry 1–5 , considerable effort has gone into the production of endohedral fullerenes – represented as M@C , where M is an 60 atomrion trapped inside the cage of C , for exam- 60 ple. This has opened up new ways of making materi- als with specific properties at the molecular level. One way to make metallo-endohedral fullerenes is to vaporize graphite doped with the appropriate metal w x oxide or salt 6,7 . An alternative and more specific way is to repeatedly expose monolayers of C to an 60 intense beam of alkali ions at an energy such that wx they penetrate the cage but not destroy it 8 . Saun- wx ders et al. 9 have been able to produce Rg@C 60 Ž . Rg s He, Ne, Ar, Kr and Xe by heating C to 60 Ž . 6508C under a high pressure 3500 atm of the rare gas for a few hours. These workers have proposed a ‘window’ mechanism for the formation of endohe- dral fullerenes. It is envisaged that one C–C bond is ) Corresponding author. Fax: q91-512-597436; e-mail: [email protected] broken, the rare gas enters through the resulting window and the cage closes by itself. Jimenez- ´ w x Vazquez et al. 10 have found evidence for trapping ´ of tritium inside the C cage, when the latter was 60 exposed to moderated tritium coming from a nuclear reaction. But, to the best of our knowledge, the interaction of protons with fullerenes has not been examined so far, either theoretically or experimen- tally. Therefore, we have undertaken a study of proton–fullerene interactions and found that a proton can easily enter the fullerene cage through either the five-or six-membered ring and that it can oscillate through the cage and also ‘rattle’ inside the cage. w x Recently we have pointed out 11 the possibility of a proton oscillating through the benzene ring and also through other aromatic rings such as naphtha- lene and anthracene. Therefore, it is not unreason- able to expect such a possibility to also exist in the case of fullerenes. Traditionally, it is believed that protons form a s-complex with aromatic hydrocarbons like benzene Ž . w x Bz but Mason et al. 12 have found mass spectral evidence for the formation of a Bz–H q p-complex. Ab initio calculations performed in our laboratory 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 01229-4

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Page 1: Possibility of proton motion through buckminsterfullerene

24 December 1999

Ž .Chemical Physics Letters 315 1999 181–186www.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 throughthe cage is considered. A detailed understanding of the interaction of a proton with the five- and six-membered rings isobtained by examining proton–corannulene interaction, as a prototype. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction

Following the synthesis of fullerenes and the de-w xvelopment of fullerene chemistry 1–5 , considerable

effort has gone into the production of endohedralfullerenes – represented as M@C , where M is an60

atomrion trapped inside the cage of C , for exam-60

ple. This has opened up new ways of making materi-als with specific properties at the molecular level.One way to make metallo-endohedral fullerenes is tovaporize graphite doped with the appropriate metal

w xoxide or salt 6,7 . An alternative and more specificway is to repeatedly expose monolayers of C to an60

intense beam of alkali ions at an energy such thatw xthey penetrate the cage but not destroy it 8 . Saun-

w xders et al. 9 have been able to produce Rg@C60Ž .RgsHe, Ne, Ar, Kr and Xe by heating C to60

Ž .6508C under a high pressure 3500 atm of the raregas for a few hours. These workers have proposed a‘window’ mechanism for the formation of endohe-dral fullerenes. It is envisaged that one C–C bond is

) Corresponding author. Fax: q91-512-597436; e-mail:[email protected]

broken, the rare gas enters through the resultingwindow and the cage closes by itself. Jimenez-´

w xVazquez et al. 10 have found evidence for trapping´of tritium inside the C cage, when the latter was60

exposed to moderated tritium coming from a nuclearreaction. But, to the best of our knowledge, theinteraction of protons with fullerenes has not beenexamined so far, either theoretically or experimen-tally. Therefore, we have undertaken a study ofproton–fullerene interactions and found that a protoncan easily enter the fullerene cage through either thefive-or six-membered ring and that it can oscillatethrough the cage and also ‘rattle’ inside the cage.

w xRecently we have pointed out 11 the possibilityof a proton oscillating through the benzene ring andalso through other aromatic rings such as naphtha-lene and anthracene. Therefore, it is not unreason-able to expect such a possibility to also exist in thecase of fullerenes.

Traditionally, it is believed that protons form as-complex with aromatic hydrocarbons like benzeneŽ . w xBz but Mason et al. 12 have found mass spectralevidence for the formation of a Bz–Hq

p-complex.Ab initio calculations performed in our laboratory

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2614 99 01229-4

Page 2: Possibility of proton motion through buckminsterfullerene

( )S. Maheshwari et al.rChemical Physics Letters 315 1999 181–186182

w xshowed 11 that there is a potential minimum, corre-sponding to the p-complex formation, on either sideof the benzene ring and that the proton can actuallygo through the ring without facing any barrier. More

w xrefined calculations 13 have reiterated our findings.

w xAlthough Glukhovtsev et al. 14 have pointed outthat the p-complex corresponds to a second-ordersaddle point, 2.06 eV higher in energy than thes-complex, there is no reason why the p-complexcannot be formed and the proton cannot oscillate

Ž . Ž .Fig. 1. a Frontal and b side views of the corannulene structure. C indicates the position of the C axis. The numbers 1–3 refer to the5 5

particular sites referred to in the text and in Fig. 2b. The s-complex and p-complex formations are also indicated schematically. Thedistance of the proton approaching the hexagonrpentagon along a perpendicular direction is labelled Z, with Zs0 referring to the centre of

Ž . qthe hexagonrpentagon. c The ground state interaction potential, V, for H –C H as a function of the approach coordinate, Z, with the20 10

zero of energy corresponding to the asymptotically separated Hq and C H in its equilibrium geometry. The solid line corresponds to20 10

motion through the centre of the hexagon, and the dashed line, motion through the centre of the pentagon. The break in the curves impliesthat the calculations have been carried out only for a limited range of Z values and an extrapolation is made to the asymptote.

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( )S. Maheshwari et al.rChemical Physics Letters 315 1999 181–186 183

through the ring, if the incoming proton has suffi-cient momentum in the direction orthogonal to theplane of the ring.

Before investigating the interaction of a protonwith C , we studied the interaction potential be-60

tween a proton and corannulene which has a bowlŽ .shape see Fig. 1a and b and for which the carbon

framework constitutes the polar cap of buckminster-fullerene and the results are summarized below.

2. Results and discussion

Ž .Keeping corannulene C H in its equilibrium20 10Ž . Ž Ž .geometry see Fig. 1a r along the pentagonC – C1 1˚ ˚Ž .s1.41 A, r along the periphery s1.45 A,C – C2 3˚ ˚ ˚ .r s1.36 A, r s1.37A, r s1.07 A weC 5 C C 5 C C – H1 2 3 3

varied the distance Z of Hq from the centre of ahexagonal ring and for each value of Z, we computethe ground state interaction potential using the 6-31G ) ) basis set and the GAUSSIAN 94 set of pro-

w xgrams 15 . It is clear from the potential-energycurve shown in the form of a solid line in Fig. 1cthat there are two minima: one corresponding to an

Ž .exohedral position convex side and another to anŽ .endohedral position concave side . It is also clear

that the double-well potential is asymmetric, favor-ing the Hq to be on the endohedral side of corannu-lene. It should be emphasized that the barrier separat-ing the two minima is well below the zero of energycorresponding to the asymptotically separated protonand corannulene. This means that proton motionthrough the centre of any one of the hexagonal ringsfaces no barrier.

ŽProton interaction with the pentagonal ring along. Ž .the C axis see Fig. 1b is qualitatively similar to5

that with any of the hexagonal rings as can be seenfrom the results shown in the form of a dashed linein Fig. 1c. Although the barrier separating the twominima is larger in the case of the pentagon, it is stillbelow the zero of energy, suggesting a free motionthrough the pentagon as well. In addition, it can beseen that the proton interaction with the pentagon isstronger than that with the hexagon, at the endohe-dral as well as the exohedral side of corannulene.

We have reported the potential energy curves foronly a limited range of Z as one has to go beyondHartree–Fock level calculations to get asymptoti-cally correct results.

We have examined the tendency of the proton toŽ .bind with one of the carbon atoms labelled 1 in

Fig. 1a of the five-membered ring and so form as-complex by plotting the interaction potential as a

Žfunction of the approach coordinate Z distance alongthe line pointing towards the carbon atom and per-

.pendicular to the plane of the ring in the form of adashed line, as shown in Fig. 2a. It is clear that thes-complex at the 1-position is indeed more stablethan the interaction along the C axis and that the5

exohedral C–H bond would be stronger than theendohedral bond.

The results shown in Fig. 1c represent the groundstate adiabatic potential energy curves and thereforeimply that a slow moving proton brought towards thepentagon would tend to form an exohedral C–Hs-complex. However, a fast moving proton directedalong the C axis can easily pass through the centre5

of the pentagon and preferentially form an endohe-dral complex. The barrier to penetration is lowestalong the C axis and increases as the proton drifts5

towards one of the carbon atoms labelled 1 in Fig.1a.

An examination of the interaction potential for theproton approaching any of the peripheral carbon

Ž .atoms labelled 2 in Fig. 1b , in the form of a dashedline marked s in Fig. 2b reveals a minimum corre-sponding to an exohedral C–H s-complex. The pro-ton can also form a stable p-complex with any of the1–2 or 3–3 C5C bonds as shown by the dashedlines labelled p and p , respectively in Figs.1 – 2 3 – 3

1b and 2b. However, this does not preclude thepossibility of a proton passing through the center ofthe hexagon and forming an endohedral complex, ascan be seen from the potential plotted in the form ofa solid line in Fig. 2b.

Since a single point calculation using the 6-31G ) )

Ž . qbasis set GAUSSIAN 94 for C -H interactions takes60

nearly two days on our workstation, we have gener-ated potential energy curves for the fullerene–protoninteraction using the 4-21G basis set. In any case,our studies on Hq–corannulene interaction haveshown that calculations using the 4-21G basis setyield qualitatively the same result as do the onesusing 6-31G ) ).

We keep fullerene in its equilibrium geometry˚ ˚Ž .r s1.45 A, r s1.36 A and vary the dis-C – C C 5 C

tance Z from the centre of a hexagonal ring as

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( )S. Maheshwari et al.rChemical Physics Letters 315 1999 181–186184

Ž .Fig. 2. a The ground state interaction potential for a proton approaching one of the carbon atoms labelled 1 of the pentagon in C H20 10Ž . Ž .Fig. 1a is plotted in the form of a dashed line. The interaction potential along the C axis is included as a solid line for comparison. b5

The dashed line marked s shows the interaction potential for the proton approaching carbon atoms labelled 2. The dashed lines markedp and p represent the interaction potential for proton approaching the 1–2 and 3–3 p-bonds, respectively. The solid line represents1 – 2 3 – 3

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 computedthe ground state C PPP Hq interaction potential. It60

becomes clear from the results shown in the form ofa soild line in Fig. 3b that there are two minima: one

˚corresponding to Zs1.3 A and another to Zsy0.9˚ ŽA, implying the formation of an exohedral outside

. Ž .the cage and an endohedral inside the cage com-plex, respectively.

Although there is a barrier separating the exo- andendohedral complexes, the maximum of the barrier isstill below the zero of energy corresponding to theproton asymptotically separated from C . This60

means that as a proton approaches C , it can easily60

go through any one of the hexagonal rings. As amatter of fact, it can easily come out on the otherside of the cage as well. If it loses some of its energyto the cage, it can result in oscillations through thecage. It is quiet possible that a slow moving protoncan form an exohedral s-complex with any of thecarbon atoms but we would like to emphasize that afast moving proton can easily pass through the cageand form an endohedral complex.

Results for proton motion through the pentagonalring are qualitatively similar, as can be seen from the

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( )S. Maheshwari et al.rChemical Physics Letters 315 1999 181–186 185

Ž .Fig. 3. a Fullerene structure, with a proton approaching along adirection perpendicular to the hexagonrpentagon, with Zs0

Ž .representing the centre of the hexagonrpentagon. b The groundstate interaction potential, V, for Hq –C as a function of the60

approach coordinate, Z, with the zero of energy corresponding tothe asymptotically separated Hq and C in its equilibrium60

geometry. The solid line represents the interaction potential formotion through the centre of a hexagon, and the dashed line is formotion through the centre of a pentagon. The break in the curvesimplies that the calculations have been carried out only for alimited range of Z values and an extrapolation is made to the

Ž . qasymptote. c Schematic representation of H –fullerene interac-tion along the radial distance r of the fullerene cage, with r s0representing the centre of the fullerene cage.

dashed line in Fig. 3b. Understandably, the barrierseparating the exo- and endohedral complexes islarger for the pentagonal face than for the hexagon.

Our calculations show that the C –Hq interac-60

tion potential has a local minimum at the centre ofthe cage, that is higher in energy than the endohedraland exohedral minima. We have difficulty in gettingconvergence at some of the in-between positions.Therefore, we represent the interaction potentialschematically as a function of the radial distance, rthrough the pentagonal ring, in Fig. 3c. Consideringthe anisotropy of the fullerene cage, one can antici-pate that there might be significant hindered oscilla-tions 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-ing accurately the C –Hq interaction, it is very60

unlikely that the qualitative findings of our work willchange.

w xInterestingly, our ab initio calculations 16 for theinteraction of Liq and He with C H show that6 6

Žthere is a large barrier to penetration larger than the.C–C bond dissociation energy thus making the pen-

etration route accessible only to proton. Within thew xlocal density approximation, Dunlap et al. 17 found

Liq@C to be stable. A similar conclusion was60Ž .arrived at for M@C MsLi, K and O by Li and60

w xTomanek 18 . It has also been found that the guest´atomrion does not prefer the central position in thecage and it ‘rattles’ around, giving rise to a large

w xnumber of peaks in the infrared spectrum 19 .w xBuckingham and Read 20 , using a minimal basis

set calculation, have explained how the eccentricposition of Li in Li@C is stabilized. Hauser et al.60w x21 have shown, using semiempirical and densityfunctional theory calculations that the interior of theC cage is inert and that the exterior is reactive with60

respect to H, F, and CH radicals.3

When it comes to the C –Hq interaction, ener-60Žgetic considerations ionization potentials for H and

w x.C are 13.6 and 7.54 eV, respectively 22 suggest60

that

HqqC ™ HqCq q6.06 eV .60 60

This would mean that a lot of energy couldpotentially be released as Hq approaches C and60

the proton may no longer remain a bare proton. It

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( )S. Maheshwari et al.rChemical Physics Letters 315 1999 181–186186

could easily be considered to be a hydrogen atomwhile in the vicinity of the positively charged cageand it can still come out as a proton at the other end.A similar mechanism has been invoked to explainthe results of experiments involving high-energyŽ . q20–60 eV projectiles of He colliding with HCl

w xtargets 23 . We only wish to point out that a protonŽ .or a hydrogen atom could easily be trapped insideor held outside the cage and that it can oscillatebetween the two minima. Furthermore, it should bepointed out that because of the differences in theinteraction of Hq with the hexagonal and pentagonal

Žrings and the in-between positions, the proton or the.hydrogen atom would be expected not only to oscil-

late radially, but also ‘rattle’ around anisotropicallyŽ .see above .

It would be interesting to isolate C Hq and other60w xprotonated fullerenes in an ion trap 24 , for exam-

ple, and study their spectroscopy.

Acknowledgements

The authors are grateful to Dr. S. Manogaran forhis assistance and guidance at various stages of thework and to Dr. J. Chandrasekhar for pointing outthe significance of our earlier results to fullerenechemistry. This study was supported in part by aSenior Research Fellowship to S.M. from the Coun-cil of Scientific and Industrial Research, New Delhi.

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