Fig. 1. The large Debye-Scherrer camera at BL02B2. STRUCTURE OF IPR-VIOLATED FULLERENE, Sc 2 @ C 66 Isolated-pentagon rule ( IPR ) [1,2 ], stating that t h e m o s t s t a b l e f u l l e r e n e s a r e t h o s e i n w h i c h a l l p e n t a g o n s a r e s u r r o u n d e d b y f i v e h e x a g o n s , h a s be en c on si de re d as t he m os t im po rt an t an d es se nt ia l r u l e i n g o v e r n i n g t h e g e o m e t r y o f f u l l e r e n e s . I n f a c t , a l l t h e f u l l e r e n e s p r o d u c e d , i s o l a t e d a n d structurally characterized to date have been known to sa ti sf y IP R [3 -5 ] . Th er e ar e no IP R fu ll er en es p o s s i b l e b e t w e e n C 6 0 a n d C 7 0 , a n d s o t h e obse rvat ion of any full eren es in that rang e mean s the violation of the IPR. Recently, Shinohara et al. h a v e r e p o r t e d t h e f i r s t p r o d u c t i o n , i s o l a t i o n o f a n I P R - v i o l a t i n g m e t a l l o f u l l e r e n e , S c 2 @ C 6 6 [ 6 ] . Immediately after the first production and isolation o f S c 2 @ C 6 6 , t h e I P R - v i o l a t e d s t r u c t u r e o f t h e fullerene was determined for the first time [6] using synchrotron radiation powder data by the Maximum E n t r o p y M e t h o d ( M E M ) c o m b i n e d w i t h R i e t v e l d analysis, MEM/ Rietveld Method [5,7] . The purity (99.8%) of the material was confirmed by la se r- de so rp ti on ti me -o f- fl ig ht ma ss sp ec tr om et ry . S c 2 @ C 6 6 p o w d e r s a m p l e g r o w n f r o m t o l u e n e so lv en t wa s se al ed in a si li ca gl as s ca pi ll ar y (0 .3 m m i n s i d e d i a m e t e r ) . X - r a y p o w d e r p a t t e r n w i t h g o o d c o u n t i n g s t a t i s t i c s w a s m e a s u r e d b y t h e sy nc hr ot ro n ra di at io n ( SR ) X- ra y po wd er ex pe ri me nt w i t h i m a g i n g p l a t e ( I P ) a s d e t e c t o r s a t b e a m l i n e BL02B2 ( Fig. 1 ). The exposure time on IP was 2 hours. The wavelength of incident X-rays was 0.75 Å . T h e X - r a y p o w d e r p a t t e r n o f S c 2 @ C 6 6 w a s obtained with a 0.02 step up to 20.3 in 2 θ , which co rr es po nd s to 2. 0 Å re so lu ti on in d -s pa ci ng . By pre-Rietveld analysis of the MEM/ Rietveld Method, the Sc 2 @C 66 crystal structure is determined as that o f s p a c e g r o u p P m n 2 1 ( N o . 3 1 ) ; a = 1 0 . 5 5 2 ( 2 ) Å , b=1 4.1 98( 2) Å, c=1 0.5 53( 1) Å. The res ult of the pre-Rietveld fitting is shown in Fig. 2 . The reliable factors of the pre-Rietveld fitting were R wp = 2.4% and R I = 13.1%. Although there are a number of ways to violate I P R , t h e m o s t s t r a i g h t f o r w a r d w a y b e i n g t o g e n e r a t e t h e s o - c a l l e d “ f u s e d - p e n t a g o n ” w h e r e Sample X-ray Shutter Collimator Ion Chamber 2 θ ω Fig. 2. Fitting result of pre-Rietveld analysis for the Sc 2 @ C 66 crystal. p e n t a g o n s a r e a d j a c e n t w i t h e a c h o t h e r . F o r 6 6 - atom carbon cages with hexagonal and pentagonal f a c e s , t h e r e a r e i n t o t a l 4 4 7 8 p o s s i b l e ( n o n - I P R ) s t r u c t u r a l i s o m e r s w i t h 2 × D 3 , 1 × C 3 v , 1 8 × C 2 v , 1 1 2 × C s , 2 1 1 × C 2 a n d 4 1 3 4 × C 1 s y m m e t r y [ 8 ] . Co ns id er in g th e ob se rv ed 19 -l in es (5 × 2; 14 × 4) in th e hi gh re so lu ti on 13 C NM R sp ec tr um of Sc 2 @ C 66 , only 8 structural isomers of C 66 with C 2v symmetry are compatible with this 13 C NMR pattern [6] . Th e ME M 3- D el ec tr on de ns it y di st ri bu ti on of Sc 2 @C 66 is presented in Fig. 3(a) together with the S c 2 @ C 6 6 g e o m e t r y o p t i m i z e d b y t h e n o n - l o c a l d e n s i t y f u n c t i o n B 3 L Y P / B a s i s s e t [ S c ( L a n L 2 D Z ) ; C ( 3 - 2 1 G ) ] c a l c u l a t i o n s ( F i g . 3 ( b ) ) . T h e M E M c h a r g e d e n s i t i e s , w h i c h h a s r e l i a b l e f a c t o r o f R F = 5 . 4 % , c l e a r l y e x h i b i t a p a i r o f t w o - f o l d f u s e d - pe nt ag on s on a C 66 -C 2v ca ge th at en ca ps ul at e a Sc 2 dimer. The present Sc 2 @ C 66 structure has the least number and degree of fused-pentagons out of the 4478 possible isomers. Sc 2 @ C 66 shown in Fig. 3(b) contains two pairs o f t o w - f o l d f u s e d p e n t a g o n s t o w h i c h t h e t w o S c a t o m s a r e c l o s e l y s i t u a t e d . T h e o b s e r v e d S c - S c distance is 2.87(9) Å, indicating the formation of a S c 2 d i m e r i n t h e C 6 6 c a g e . T h e i n t r a f u l l e r e n e e l e c tr o n tr a n s fe r s i n e n d o h e d r a l m e ta l l o fu l l e r e n e s have been known to play crucial roles in stabilizing t h e m e t a l l o f u l l e r e n e s [ 3 - 5 , 9 , 1 0 ] . T h e n u m b e r o f e l e c t r o n s i n t h e a r e a c o r r e s p o n d i n g t o S c 2 d i m e r from the MEM charge density is 40.0(2) e , which is very close to that of ( Sc 2 ) 2+ with 40 e . The ab initio c a l c u l a t i o n a l s o i n d i c a t e s t h a t t h e S c 2 d i m e r donates two electrons to the C 66 cage providing a formal electronic state of ( Sc 2 ) 2+ @ C 66 2- . It is this cha rge -tr ans fer int era cti on bet wee n the Sc 2 dim er an d th e fu se d pe nt ag on s th at si gn if ic an tl y de cr ea se s t h e s t r a i n e n e r g i e s c a u s e d b y t h e p a i r o f f u s e d pe nt ag on s an d th us st ab il iz es th e fu ll er en e ca ge . IPR is not necessarily a test for the stable geometry of endohedral metallofullerenes [6,9] . Intensity 2 θ θ (degree) 5 10 15 20 10000 5000 R wp = 2.4% R I = 13.1% Fig. 3. (a) The X-ray structure of the IPR-violating Sc 2 @C 66 fullerene showing a top view along the C 2 axis and a side view. The equi-contour (1.4 e Å -3 ) surface of the final MEM electron charge density. The Sc 2 dimer is colored in red. The two pairs of fused- pentagons are clearly seen. (b) The calculated Sc 2 @ C 66 structures. Masaki Takata, Eiji Nishibori and Makoto Sakata Nagoya University E-mail: a41024a @ nucc.cc.nagoya-u.ac.jp References [1] H. Kroto, Nature 329 (1987) 529. [ 2 ] T . G . S c h m a l z e t a l . , J . A m . C h e m . S o c . 1 1 0 (1988) 1113. [3] H. Shinohara et al. , Nature 357 (1992) 52. [4] C.S. Yannoni et al. , Science 256 (1992) 1191. [5 ] M. Ta ka ta et al . , Na tu re 37 7 (1 99 5) 46 ; M. Takata et al. , Phys. Rev. Lett. 83 (1999) 2214. [6] C-R. Wang et al. , Nature 408 (2000) 426. [ 7 ] M . T a k a t a , E . N i s h i b o r i a n d M . S a k a t a , Z . Kristallogr. 216 (2001) 71. [8] P.W. Fowler and D.E. Manolopoulos, An Atlas of Fullerenes ( Clarendon, Oxford, 1995) 27. [ 9 ] K . K o b a y a s h i e t a l . , J . A m . C h e m . S o c . 1 1 9 (1997) 12693. [ 1 0 ] H . C . D o r n e t a l . , i n F u l l e r e n e s : R e c e n t A d v a n c e s i n t h e C h e m i s t r y a n d P h y s i c s o f F u l l e r e n e s a n d Related Materials ( eds. K.M. Kadish & R.S. Ruoff ) 9 9 0 . ( T h e E l e c t r o c h e m i c a l S o c i e t y , P e n n i n g t o n , 1998). (a) (b) Side View Top View Sc dimer Fused Pentagon
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