20
J . Phys. Chem. 1993,97, 20-22
High-Energy Collisions of Carbon Cluster Cations with Helium: Experimental Support for the Existence of Imperfect Fullerene Structures John H. Callahan and Mark M. Ross' Chemistry DivisionlCode 61 13, Naval Research Laboratory, Washington, D.C. 20375-5000
Thomas Weiske and Helmut Schwarz* Institut fur Organische Chemie der Technischen Universitat Berlin, Strasse des 17, Juni 135, W-1000 Berlin 12, F.R.C. Received: October 5. 1992; In Final Form: November 18, 1992
High-energy (kilovolt) collisions of carbon cluster (fullerene) cations with helium a r e used to differentiate between species that d o and d o not satisfy the isolated pentagon rule (IPR). T h e initial studies of collisions of c60'+ and C70*+with helium, which showed incorporation of the helium atom in the fullerene cage and inelastic scattering, have been extended to other C,+ with even x. W e report here that, unlike c60'+ and c70'+, kilovolt collisions Of C58+and c68' with helium d o not yield a detectable helium/parent ion endohedral complex, (He@Css)+or (He@C6g)+, nor is an inelastic scattering 'foot" in the kinetic energy distribution of the precursor ion detected. These two observations are taken as evidence of structures that a r e less stable than those that satisfy the IPR. However, adducts of helium with carbon cluster fragment ions, (C,He)+ with x C 58 or x < 68, are observed from c58'+and c68" collisions with He, indicating that the helium is taken u p by the parent ion yielding a relatively unstable product ion.
In contrast to icosahedral c 6 0 and c 7 0 clusters (as well as those C, with even x > 70), which obey the isolated pentagon rule (IPR),' theoretical work2 clearly indicates that all fullerenes C, with even x in the ranges of x < 60 and 60 < x < 70 must contain several adjacent pentagon rings. The relative destabilization of these local pentalene structures has been evaluated recently by performing theoretical calculations. It was reported3 that the energy contributions of adjacent pentagons are roughly additive, witheach pair making thestructurelessstable by0.7-1 evrelative tosystems following the IPR. This hugedestabilizing effect holds true for both neutral and cationic fullerenes.2aqc So far there are no reports on a relationship between chemical reactivity and 'defect" fullerene structures. It has been stated that it would be difficult to detect fullerenes and distinguish them from IPRobeying isomers using, for example, photoemission or neutron scattering.2c-dIn the present Letter we report experimental results pertinent to this problem. For fullerenes obeying the IPR, several laboratories have recently s ~ c c e e d e d ~in- performing ~ unprecedented experiments consisting of high-energy collisions between fullerene cations and noble gas target atoms (He, Ne, Ar). Some of these collisions result in injection of the noble gas atom, M, into the hollow space of c60'+ and C70'+ yielding stable endohedral complex product ions, (M@IC,).+. Both theoretical and experimental evidence-in particular the observation of sizable activation barriers for the noble gas capture and4c-5b-7 the sequential injection of two helium atoms in c60'+ 4g as well as the formation of a neutral He@C60 species4' in beam-type experiments-leave no doubt that the noble gases are indeed trapped inside of the fullerene cage and prevented by high barriers from a facile e ~ ~ a p e . ~A ~typical . ~ ~ example , ~ , ~ ~ is illustrated in Figure 1 which shows the mass-analyzed ion kinetic energy (MIKE) spectrum resulting from 8-keV C70'+/He using theBEqQ hybrid massspectrometerat NRLasde~cribedear1ier.I~ Virtually the same spectrum is obtained for C60'+.5a In addition, common to all endohedral fullerene complexes, (M@C,)'+, where M is a metal or noble gas atom? is the loss of C2 or multiple units of C2 upon excitation with retention of the encapsulated atom ('shrink-wrap" mechanismL2). When mass-selected C58'+ and C68'+ (generated from c 6 0 and c 7 0 by electron impact induced C2 loss) are subjected to 8-keV
-z
10
Y
P !5
70), which obey the IPR, exhibit in the high-energy ion beam experiment an endohedral complex product ion and the inelastic scattering 'foot", as observed with c60" and C70". For example, Figure 3 shows the MIKE spectra for C76'+ and C98'+, which are similar for all higher fullerenes C,'+ studied thus far with even x in the range 70 < x < 100. The only exceptions are C72'+ and C74*+,for which the endohedral complex and inelastic scattering 'foot" are not observed. The origin of this observation remains to be clarified. The observation that C,'+ clusters (x = 5 8 , 68) do not form stable endohedral complexes (He@C,)+ does by no means imply that in the collision with helium penetration of the carbon cage
0022-3654/58/2097-0020%04.00/0 0 1993 American Chemical Society
Letters 5-
c5;
(a)
The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 21
-
I
4-
(a) CYI*'IHe
3-
21-
840
ion Energy (ESA volts)
Fipre2. The&keV(a) CJB'+/Heand (b) Ct,8'+/HeCID/MIKEspectra with the parent ions off scale but showing no detectable endohedral complexes or inelastic scattering.
Figure 4. C I D mass spectra of C,'+/He (a, x = 58; b, x = 68); E1.b = 5 keV; linked scan with B(2)/E(2) = constant. The parent ions C& and
C68"
are not included in these spectra.
demonstrate that the helium atom is retained in some of the fullerene-fragment ions after elimination of C, units. As the loss of C, is very energy demanding,2a,t6it is conceivable that in the course of the fragmentation1*the incipient 'hot" complexes of (He@CSs)+and (He@Cbs)+cool down until the dissociations yield He/C;+ product ions with a low enough energy content to survive and be observed on the experimental time scale.1' In addition, the fragment carbon cluster-helium adduct ions, (C,He)'+, were confirmed by mass-resolved MIKE spectra in the N R L BEqQ instrument, and these ions appear at kinetic energies consistent with a precursor parent endohedral complex, (He@C&+ and (He@C6s)*+,that took up the full center-ofmass collision en erg^.^,^
Cg
