4731
J . Phys. Chem. 1992,96,4731-4733
Standard Enthalpy of Formation of Buckminsterfullerene W. V. Steele,*Vt R. D. Chirico,f N. K. Smitht W. E. Billup,$ P. R. Elmore,$and A. E. Wheeler* IIT Research Institute, National Institute for Petroleum and Energy Research, P.O. Box 2128, Bartlesville, Oklahoma 74005. and Department of Chemistry, Rice University. P.O. Box 1892, Houston, Texas 77251 (Received: March 10, 1992; In Final Form: April 22, 1992)
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The recent discovery of an efficient synthesis of buckminsterfullerene, Cso,has opened the way to the eventual determination of its thermodynamic properties. This paper reports results of energy of combustion measurements made on 125-mg samples of Cw using a specially developed technique to ensure combustion of the small samples in a conventional bomb calorimeter. Energy of combustion measurements on a sample of graphite gave &UO,/M = -(32774.6 f 3.7) J g-l, in excellent agreement with earlier results at NIST. For pure Csoa value &UO,/M = -(36 123.7 f 9.7) J g-' is obtained. The corresponding standard enthalpy of formation of crystalline Cw is AfHo, (C6,,, cr, 298.15 K) = (2422 f 14) kJ mol.-' Using literature values for the enthalpy of sublimation and ideal-gas-phase heat capacity for Cw,in conjunction with unpublished solid-phase heat capacity measurements made at NIPER, an ideal gas standard enthalpy of formation A&P, (Ca,g, 298.15 K) = (2656 f 25) kJ mol-' is derived. That value is listed along with values in the literature estimated using various semiempirical molecular-orbital-type computational methods.
Introduction Cso (buckminsterfullerene; abstracted in Chemical Abstracts under the name footballene, registry number 99685-96-8) has caught the imagination of chemists worldwide. The list of articles written in the popular press continues to increase at an impressive rate.' A search of Chemical Abstracts from Jan 1,1991 through Dec 27, 1991, revealed no less than 228 articles on buckminsterfullerene. This burst of activity has followed the publication of an efficient synthesis of this allotrope of carbon.24 Full exploitation of the possibilities of this new material requires accurate and precise thermodynamic property measurements. With the preparation of macroscopic samples2-" and the development of methods of purification of the individual f~llerenes,~ it has become possible to make such measurements. To date, in the literature the only values for the standard enthalpy of formation of Cso are gas-phase values estimated using various semiempirical molecular-orbital-type computational methods.613 Measurements of other thermodynamic properties such as heat capacity have not yet been reported. This paper reports experimental measurements of the energy of combustion of buckminsterfullerene leading to the determination of the standard enthalpy of formation of Cw in crystalline form at 298.15 K. Measurements of the energy of combustion of pure graphite (99.9995 mol % carbon) were also made to verify the methodology used (combustion calorimetry on milligram samples is not yet a well-defined procedure) and for direct comparison with the values obtained for the Cw samples. Experimental Section Materials. C:, The calorimetric samples of Cw [two of four separate samples (approximately 150 mg per sample) transmitted to NIPER from Rice University over the time interval Apr to Dec 19911 were prepared in the following manner. Crude soot was prepared as described previously and extracted with toluene using established prooedures.4 The extract was then washed with diethyl ether until a fine, black powder was obtained. Purification was achieved by column chromatography using a 30.5-cm-diameter column packed with 10 cm of neutral alumina. The fullerenes were introduced as a dry plug prepared by evap orating the solvent in vacuo from a slurry prepared by the addition of 1.5 L of a saturated solution of the fullerenes in toluene to 200 g of alumina.5 Elution of the column using hexane required about 8 h to obtain a 2.5-cm separation of Cso (purple band) and C70 To whom correspondence should be addressed at NIPER. Contribution no. 333 from the Thermodynamics Rescarch Laboratory at the National Institute for Petroleum and Energy Research. National Institute for Petroleum and Energy Research. *Rice University. 0022-3654/92/2096-4731$03.00/0
TABLE I: Summary of Experimental Energy of Combustion Results"
Graphite -32 764.9 -32 771.7 -32 779.6 -32 764.9 -32 718.9 -32 787.2 (((k,V",/M),l/(J g-I)) -32 774.6 f 3.7 C60(with Solvent Present)"
( ( k v m / M ) / ( J g-')l -35 926.2 -36 281.5
(((&Vm/M)]/(Jg-I)) -36 123.7 f 9.7 a T = 298.15 K, andpo = 101.325 P a . The uncertainties shown are 1 standard deviation of the mean for graphite and 2 standard devia-
tions for Cm bCombustionsmade before the affinity of CWfor solvent
was fully realized. (burgundy band). Analysis of the Ca fraction was achieved using a Hewlett-Packard Series 1050 HPLC with a variablewavelength UV detector fitted with a Altech Econsphere 5-pm silica column (250 X 4.6 mm) and a Hewlett-Packard Series 3396 integrator. Samples with a purity of 99.99 mol % Cw (UV detector) were combined, and the remaining solvent was removed in vacuo. The sample was then washed exhaustively with diethyl ether and then with methylene chloride until the 'HNMR spectrum showed no resonances. Solvent removal was crucial to the determination of a consistent energy of combustion for Cso. Complete removal of solvent was achieved by heating the sample to 450 K under a pressure of
