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C70 Is More Stable than C60 Experimental Determination of the Heat of Formation of C70.

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C70 Is More Stable than C6,,:
Experimental Determination of the
Heat of Formation of C7,**
Hans-Dieter Beckhaus, Sergej Verevkin,
Christoph Riichardt,* FranGois Diederich,*
Carlo Thilgen, Hans-Ulrich ter Meer, Holger Mohn,
and Wolfgang Miiller
Before the chemistry of C,, took the current fast development,"] the first two years following the landmark discovery of
how to prepare macroscopic amounts of fullerenes by
Kriitschmer and HuffmanI2] were characterized mainly by investigations of the physical properties of f u l l e r e n e ~ .However,
it took more than a year for a fundamental thermodynamic
quantity of C,,-its
heat of combustion -to be deterThis time lapse was due to problems with both the
production of highly pure C,, in sufficient quantities and the
accuracy of calorimetric measurements on small samples --a
procedure which is still far from trivial.
Both problems are reflected by large differences among the
values published in the
a fact that led us to reinvestigate the thermodynamic data in question. Reliable values
for the heat of formation of the carbon spheres are of great
importance not only because they provide an experimental criterion for the quality of theoretical calculations on fullerene structures, but also because they give an insight into the stability of
C,, and of highly strained molecules in general.
Even though C,, , the most strained molecule known so far,
is much higher in energy than the other two carbon allotropes
graphite and diamond, it exhibits a surprising kinetic stability.
With regard to the higher fullerenes C,, ( n 2 7 0 ) , the question
arises whether the incorporation of additional six-membered
rings into a carbon network with a fixed number (12) of fivemembered rings leads to a continuous increase in stability.
asymptotically approaching the value of graphite.'*' The determination of the heat of combustion of C7(,reported here pro-
vides the second experimental point on a curve describing the
thermodynamic stability of fullerenes as a function of their size.
A redetermination of the heats of formation of C,, and C,,
was made possible because sufficiently large quantities of highly
[*] Pi-of. Dr. C Ruchardt. Dr. H:D. Beckhaus. Dr. S Vcrevkiii
Institut fur Organische Cheinie und Biochemie der Universitit
Albertstrasse 21, D-79104 Freiburg (FRG)
Telefax: Int. code + (761)203-5987
Prof. Dr. F. Dicderich. Dr. C . Thilgen
Laboratorium fur Organische Chemie. ETH-Zentruni
Unibersititstrasse 16. CH-8092 Zurich (Switrerland)
Telefax: Int. code + (3)?61-3524
Dr. H - U . ter Meer, Dr. H. Mohn. Dr. W. Miiller
Hoechst AG. D-65916 Frankfurt am Main ( F R G )
This uork was supported by the Schweizerischen Nationalfonds 7ur FBrderung
der wiasenschaftlichen Forschung. hy thc Deutsche Forschui~gsgemeinschaft.
and by the Fonds der Chemischen Industrie.
pure samples were provided by Hoechst AG. In order to exclude
any contamination by solvent molecules,"gold grade C,," and
"gold grade C,," were both subjected to fractional sublimation
using helium as the transport gas at 600 and 650 "C/atmospheric
pressure, respectively, and stored under argon prior to use. Taking into account the differences in molar extinction coefficients
of the two fullerenes, the composition of the C,, sample (99.82%
C,, and 0.18% C,,O) and the C,, sample (0.44% C,, and
99.56% C,J was determined by analytical HPLC (Fig. 1).
t [mml
Fig. 1. HPL chromatograms of the sublimed C,, (A) and C-,) (B) samples. Conditiona. Vydac 201TP54 C18 reversed-phase column. 250 x 4.6 mm, acetonitrile,
toluene 1:l. 1 m L m i n - ' . i' = 310 nm. S = solvent peak.
Chromatographic analysis with pure hexane as the eluent and
detection at 254 nm showed that the sublimed samples did not
contain any toluene from previous operations.
The heat of combustion measurements were performed in an
isoperibolic aneroid ~nicrocalorimeter[~~
and in a water-stirred
isoperibolic macrocalorimeter['O1 (Table 1 ) . Adsorbed argon
was removed from the pelletized samples by heating at 350' C i
bar for 2 h. The pellets were cooled under vacuum and
exposed to N, just prior to weighing. Preliminary experiments
were used to test the sample stability against oxygen. because
under the experimental conditions preceding ignition (30 bar
0 2 H,O
saturation), a reversible weight increase was observed.
In fact, 1-2 moles of 0, per mole of fullerene were absorbed in
0.5 h, and after 1 d of exposure. even more than 5 moles of 0,
had been absorbed per mole of C70. According to analytical
HPLC, however. this did not lead to a chemical transformation
of the fullerenes; in particular, the amount of fullerene oxide did
not increase. A control experiment using a C,, sample sealed in
polyethylene under N, gave a AH,+ value within the experimental error of the measurements performed on nonsealed samples
(Table 1, Experiment 2). Whereas the combustion of C,, was
complete in each experiment, that of C,,, in some cases, led to
the formation of traces of soot which were weighed and taken
into account in the calculation." The reduction to the isothermal bomb process with c p = 0.71 (C,,) and 0.79 J K - l g - ' (C7")
Table I . Rewits from representative combustion experiments for C,, and C,, [a].
micro Ibl
macro [c]
- 12.67
micro [b]
macro [c]
( f 0.020)
( 2 0.0042)
- 3.34
- 1593.68
- 5.07
- 1409.91
- 25896
micro [b]
- 30086
- 35780
- 30083
- 30096
[a] Measured and calculated quantities see ref. [12]. buoyancy correctlon of the weighing carried out with p =1.76 (C6J and 1.69 gem-' (C,"). T, = 25 C , p' (gas) =
30.00 atm (30.45 bar). T' = 25.00k0.01 "C. Au,(fuse) = -16945k4 J g - ' . [b] Aneroid microcalorimeter ref. [9],V,,,, = 0.046 I, A€,,,,, = 1.51 J, m'(H,O) = 0.23 g. mPlallnum
= 3.461 g. [c] Water-stirred macrocalorimeter ref. [lo]. Ybollih
= 0.266 1. AE,,,,,, = 1.46 J, m'(H,O) = 0.78 g, mp,.,,,num
= 13.1 3 g. [d] From calibration measurements with benzoic
acid, Nutional Bureau of Standards. ref. sample 3%; s : standard deviation from the mean value. [el Polyethylene ampoule. Au,(PE) = - 46372 9 2.9 J g - l . [f] ATc = T'
- r'
A7, ,,,,. [g] cLUn, ( - AT') = t&,(T' - T,) + E' Lnn, (Th- T ' + AT,,,,). [h] Sum of the items 81-85, 87-90, 93 and 94 in ref. [12]. [i] Heat correction for incomplete
combustion (33 J per mp soot), ref. [l I ] . [k] Heat correction on account of the formation of nitrous and nitric acid (titrated).
and to standard conditions was carried out according to the
usual procedure.['21 Table 1 shows representative results obtained with both kinds of calorimeters. Table 2 contains the
different AHc+ values, their averages, and the resulting standard
heats of formation AH? of C,, and C,,.
Table 3. C,tloi iinetricall~determined heats of combustion AH,'(c)
heats of formation AH;(cj of C,, and C,, [kJmol-'1.
Table 3. Heat of formation A H 7 (8, C) per C atom of C,, and C,, [kcalmol-'1 [a].
and standard
- 25966
- 30083
_ f s [a]
+ s [b]
Afi,-(c.C) per C atoni
- 301 24
- 30096
- 30078
- 10
(= 0033%)
f 12
36.50 [d]
[a] Standard deviation of the mean value. [b] Standard deviations of all measurements considered. [c] 9.269i0.0621 kcalmol-'. [d] 8.72420.041 kcalmol-'.
Since the deviations in the enthalpy of combustion caused by
the impurities of 0.44% C,, in the C,, sample and 0.18 YOC,,O
[13] in the C,,
sample are no larger than 0.7 kJmol-' and
-0.5 kJ mol-', respectively, no corrections were made to the
measured values.
The experimental results were compared to calculated standard gas phase heats of formation AH? (g, C) obtained with the
use of the enthalpy of sublimation AH,:,, determined for C,,
(Table 3). Iterative MM3['61 force-field-MO calculations on
both fullerenes afforded the A H p ( g , C) values given in Table 3.
The redetermination of the heat of formation AH," (g, C) per
C atom of C,, using sublimed samples led to a value
(1 0.1 6 kcalmol-') that is slightly higher than the one determined previously by us (I 0.01 kcal mol- '). Both numbers are in
A n ~ r u .~. ' l w m
Inr. Ed. Engl. 1994. 33, No. Y
[I 81
a h initio
- 16
( = 0.061°/b)
38.78 [c]
[I 71
~ 9 1
C) per C atom
+s [b]
(A -0.91, C,,= 0.0) [c]
[a] With AH.Yb(c) (298 K. per C atom) = 0.93 kcalmol-' [14]. AH,(298 K. per C
atom, graphite) = 0. AH,(298 K, per C atom, diamond) = 0.4 kcal mol- ' (1 kcal =
4.184 kJ). [b] From the spread of the heat ofcombustion of the crystalline samples.
[c] Hartree-Fockidouble zeta plus polarization (HFIDZP) energy of M N D O structures relative to C,, .
relatively good agreement with the results of Kiyobayashi et
a1.[6]and Diogo et al.[71.The present experimental data are best
reproduced by M M 3 and the incremental method, both affording values which are only slightly lower and higher, respectively.
In contrast, the numbers derived from M N D O (MND O = Modified Neglect of Differential Overlap) calculations
are much higher. The difference in energy between the two fullerenes is reasonably well reflected by relative a b initio energies
(LYP/DZP)(LYP = Lee-Yang-Parr/double zeta plus polarization)["I obtained from calculations based on MNDO-approximated structures.
The AH? (g, C) value determined for C,, (9.65 kcalmol-I) is
lower by 0.51 kcalmol-' compared to that of buckminsterfullerene. This can be interpreted as the first confirmation of the
prediction stating that fullerenes show a decrease in AH,"(g, C)
with increasing number of C atoms. It should not be forgotten,
however, that higher fullerenes C, with n 2 7 8 can exist in a
VCH Vrrlug~gL'rPIl.~I.liujff
m h H , 0-69451 Wi,inherm, 1YY4
0570-0833~94/0YUY-0997S 10.00
+ .2VO
number of isomeric forms which may differ considerably in
strain and hence in energy content.[20]
Among the theoretical methods used for the calculation of the
heat of formation of C,,, MM3 and the incremental method
give the best results (both providing values which are slightly
lower than the experimental ones), whereas the number resulting from MNDO calculations is much higher. From these comparisorls it can he concluded that the first two methods mentioned are best suited for the prediction of the heat of forrnatioii
of higher fullerenes. Due to the lack of availability of these
carbon spheres in quantities large enough for experimental measurements, knowledge about their stability will have to rely on
theoretical calculations for some time.
Synthesis of Unsaturated Amino Acids by
[3,3]-Sigmatropic Rearrangement of
Chelate-Bridged Glycine Ester Enolates **
Uli K a z m a i e r *
Dedicated to PrqfiJssor Ulrick Schmidt
the occasion of'liis 70th hirtlidcij,
The synthesis of ?,&-unsaturated amino acids has received a
great deal of attention for a long time. Some of these amino
acids. which also occur in nature,"' exhibit pronounced antibiotic activity, and can be iised as enzyme inhibitors.[21 These
unsaturated amino acids are also very interesting from a synthetic point of view, since more complex compounds can be
Recekcd: December 11, 1993 [Z65571E]
readily constructed by functionalization of the double bond l3I
(krm;in version Angeii . Choii. 1994. 106, 1033
In 1975. Steglich et al. described the thermal Claisen rearrangement of N-benzoyl-a-amino acid allyl esters on treatment with
a dehydrating agent. in which an 5-allyloxazole intermediate
a ) R Taylor. D. R . M . Walton. N r i t i i r c {Loiiihn/ 1993. 363. h85-6')3: h) A.
Hirsch, 4iigcii.. ( % C I I I . 1993. (0.1189-1 192: A i i , q r , ~C / i ~ 7 i/./ i t . Ed. EtiRI.
was formed.["] Since the [3,3]-sigmatropic rearrangement takes
1993. 32. ll3X 1141.
place highly diastereoselectively, this elegant method found nuW.Kritschmcr, L. D. Lamb, K. Fostiropoulos. 0 .Huffman. .Viiiiir<,(Loiiiiii~i)
merous applications. in particular, for the synthesis of n-alkylat1990.347. 354 358.
ed amino acids.[5]In 1982. Bartlett et al. investigated the Ire/ i i / / r v e i i c . \ (Ed\ : W. E. Billups. M A. Ciufdini). VCH. Nci\
York. 1993: b) T/ic Fii//mw,.s(Eds: H. W. Kroto. J. E Fischer. D E. Cox).
land -Chisen rearrangementc6'of N-acylated glycine allyl esters
Pergainon Pi-ess. Oxford, 1993: c ) .4<i. C ' / i c i i i , R r % 1992. 7.5. 98 175.
in detail."' In the rearrangement of crotyl esters they found in
H.-D. Beckhaus. C Ruchardt. M. Kao. F. Diederich. C. S Foote. Ai7gm.
some cases very high diastereoselectivities. The observed . Y J ~
C ' h ~ ~ i 1992.
104. 69-70. .$iigc,ii.C'/ir,iii, l i 7 i . El/. Gig/. 1992. 3/. 63 64.
selectivity can be explained by a preferred formation of the
W. V. Steek. R. D Chiiico. N. K . Smith, ti'. E Billups. P R. Elniore. A. E.
Wheeler. .I / ' / i n . C%eiii. 1992. 96. 4731 4733
(E)-lithium enolate and its chelation by the anionic acylaniide
T, Kiyohayashi. M . Sakiqamii, A i ~ ~ i i i uRr,porr
of M i r , r o r , i i / ( j r i i i i c t r i , R i v i i r d i
group in the r-position. Similar chelate-bridged enolates have
('ruitcr (Faculty of Science. Osaka University) 1992. 13, 5X: Fiiiliwiic Sr i.
also been postulated in sigmatropic rearrangements of x-alkoxyTwhiid. 19Y3. 1. 269-273
substituted ally1 esters.[*' An interesting asymmetric boron enoH P. Diogo. M . E. Minas dii Pielade. T. J. S Dennis. J. P. Hara. H . W Kroto.
R . Taylor. D R . M . Walton. J. ( ' h r i i i . Sor. Fiiriidfij. T,mr,. 1993. N9. 3541
late variant of the Claisen rearrangement was described by
Corey and Lee in 1991."I1
a ) B. L. Zhang. C. H. Xu. C. Z. Wang, C. T. C h i . K. M. Ho. P/ii.\. Rei.. B.
Whereas the hitherto described procedures for the Claisen
1992. 46. 7331 7336, h) J. Tcrsoff, Ihili. 1992. 46. 15546 15546.
rearrangement of ester enolates of r-amino acid allyl esters
H.-D. Beckhnus. C. Ruchardt. M. Sniisek. T / i i w i i i d i i i i i . I c r o 1984, 79. 149
make use of the Ireland silylketene acetal variant,[''] we now
S. Suniier i n ( ' o i i i h i i \ i i ~ i C " i / o r i i i i r , i r i . Lbl. 1 (Eds.: S. Suiinei-. M. MBnison).
describe a rearrangement of chelate-bridged metal enolates. AtPergamon Press, Oxford, 1979, ('11. 2. The cnlorimeter from 'he Chemical
tempts to rearrange lithiated glycine allyl esters directly did not
Center. Uniwrsitit Lund, Sweden. w a s equippcd with ii tempering unit foi- the
succeed. since the desired rearrangement only occurs at temperexteriial bath by BASk AG, Ludw~igshafen(temperature stability iO.001 K ) .
We thank BASF for the gcncrous gift of the ciilorimeter.
atures at which the lithium enolates decompose. However, by
A . .I. Head. W D. Good. C. Mosselman in C ' ( i i i i I x i . ~ l i ( i Cii/i)riiit<
adding salts of chelatable metals to the lithium enolate solution.
(Ed S. Suniier. M. Mbnsson), Pergiimon Press. Oxford. 1979. Cli. 8.
stable glvcine enolate chelates are obtained which do not deW N . Hubbard. D. W. Scott. G. Waddingon iii E\pi~?iinrnio/ ~ / ~ ~ , r i i i ~ ) [ / i i , i ~ i i compose when warmed to room temperature.'"' In fact. cop\ i r \ ' , C i i / . / (Ed.: F.L). Rossini). Interscience, New York, 1956. Ch. 6.
For Cb(,O a \ d u e for A H ; ( y ) o f 2 1 1 0 k.lmo1-I m a y he calculated from the
per(1r) and nickel(r1) salts form very stable c h e h e s with amino
isodesniic reaction ( A H ; ( g i [ k J m o l ~ ' ] )C,.,,H:
(2078.5 from M M 3 calculaacids and peptides;" '1 however, the corresponding enolates are
tion [ l i a j ) + oumiie ( - 52.6 [13h])+etlianc ( ' X3.X [13h]) + C,,,O. t k ) For
not reactive enough to undergo rearrangement.
t h e preparation of and force-field calculation o i i hydrogenated fullerrncs see
We have now investigated the influence of a number of metal
M. Gerst. H.-D. Beckhauh. C Ruchnrdt. E . E . B. Canipbcll. R Tellginann,
/ e t r i i / i r ~ / r ~Lctr.
1993. 34, 7729 7732. h) J. B. Pedlc). R. D. Ka)lor, S . B.
salts on the yield and diastereoselectivity of the rearrangement
Kirby. ~ / i ~ ~ ~ i i i / J ~ / i [I/~ , Doiro
i 7 i i r of 0 , x o i i i c C ( l i i i / J i ~ i l ~ l l /C
\ . h a p m m a n d HnII. Lonof the N-benzyloxycarbonylglycinecrotyl ester I to 2, and have
don. 1986.
obtained interesting results (Table 1 ) . The best results with reAH.,, (29X K , per C atom) = 0 93 kcalmol-I ezrnipolated [IS] ftom 1liemc;rsspect to both yield and selectivity bere obtained on addition of
ui-ed enthalpy of sublimation AH.,,, of C,,, ( 7 0 7 K . per C a t o m ) =
0.67 kcalmol-' [ S ] \\ith C.,, ( c , measurcd) [5] and C,, (g. estimated) [ S ]
zinc(]]) chloride. If a solution of the zinc enolate (No. 1 ) is
C Pan. M. P.Sanipson. Y, Chdl. R. H. Haugr. J. L . Mnrgravc. J P/i>,.s C7/ii,iii.
heated from - 78 ' C to room temperature, the rearrangement
1991. 95. 2044-2946.
at - 20 ' C and is complete after 3 h at room temperature.
N.L. Allingcr. Y. H. Yuli. J.-k1. LII. J h i . C/iwi. Srir.. 1989, / / I . 8551. 8566.
In the reaction with tetraisopropylorthotitanate (No. 5 ) . the
X i 7 6 Tec1iiiic;il Utiliciition Coi-p. Inc.. 235 Glen Village Court. Powell. O H
43065. USA
crotyl ester is partially split or even transesterified by titaniD. A . Annitage. C. W. Bird. fi,1riilier/roi~Lett. 1993. 34. 5811 5812.
um(rv) catalysis[' 'I resulting in lower yields. Otherwise. the
D. Bakowies. W. Thiel. J. ,4111. C / i ~ i i iSoc.
1991. 113. 3704&3714.
metal enolates are clearly superior to silylketene acetals (Nos. 6
I. C i ~ s l o w s k i .C/irwii.Phi.\. Lpii 1993. 216. 389-393.
and 7) both in terms of their reactivity and selectivity.
; i j F. Dicdcrich. R L. Whctten. C. Thilgen. R. Ettl, I Cliao. M. M. Alvarer.
Sciwic<, f H i i d i i i i g m i i j 1992. 354. 1768-1770:
Kolh. D Bakohies. W. Tliiel. F i r l l i ~ r r ~Sci.
h ) 2. S h h i n a . J:P.
Franpis, M .
1993. I . 221 2317.
I*]Dr. U. Karmaier
0rf"nisch-cheniisches Iiistitut der Universitgt
Im Neiicnheimei- Fcld 270. D-69120 Heidelberg ( F R G )
Telefax: Int. code + (62213564205
Reactions olChelated Eno1;ire.i. Part 1. This work was aupported hy the Fonds
der Chemicchen Indurtrie. I thank Pro].. G Helmchen for his i~ivalunhleaiipport.
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c60, experimentov, c70, heat, formation, determination, morel, stable
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