© Copyright 2000 by the American Chemical Society
VOLUME 104, NUMBER 23, JUNE 15, 2000
LETTERS Enthalpy of Formation and C-F Bond Enthalpy of Fluorofullerene C60F36 T. S. Papina, V. P. Kolesov,* V. A. Lukyanova, O. V. Boltalina, A. Yu. Lukonin, and L. N. Sidorov Department of Chemistry, Moscow State UniVersity, 119899 Moscow, Russia ReceiVed: February 2, 2000; In Final Form: April 20, 2000
The standard molar enthalpy of combustion of fluorofullerene C60F36 was determined by rotating-bomb calorimetry, ∆cH°m ) -24692 ( 199 kJ‚mol-1. Using this result and the standard molar enthalpy of sublimation, ∆subH°m ) 139 ( 8 kJ‚mol-1, the values of the standard molar enthalpies of formation of C60F36 in the crystalline and gaseous states were found to be -5362 ( 201 kJ‚mol-1and -5223 ( 201 kJ‚mol-1, respectively. The enthalpy of reaction C60F36(g) ) C60(g) + 36F(g) was calculated, ∆rH°m(298.15 K) ) (10617 ( 202) kJ‚mol-1, or 294.9 ( 5.6 kJ‚mol-1 per one C-F bond. The comparison with the enthalpy of reaction C60F48 ) C60(g) + 48F(g), ∆rH°m(298.15 K) ) 287.5 ( 3.5 kJ‚mol-1 per C-F bond leads to a preliminary conclusion that C-F bond enthalpy decreases with amount of F atoms attached to the fullerene cage.
Introduction For the decade past since the discovery of fullerenes1 the chemistry of fullerenes has developed into an exciting and dynamic field, providing an unprecedented wealth of the chemical derivatives.2 Fullerene molecule can be functionalized inside carbon cage (endo derivatives) or outside carbon cage (exo derivatives), or form products of the replacement of the cage carbon atoms with heteroatom (heterofullerenes). One of the key characteristics of any compound is its thermodynamic stability, which can be measured, at the first approach, as enthalpy of formation from elements. Up to the present, only for two most abundant fullerenes, C60 and C70, the ∆fH°m values were published (for the most recent review, see ref 3) In our previous paper,4 we reported on the first determination of ∆fH°m of the fluorofullerene C60F48. The C-F bond in this compound appeared to be stronger than it was predicted theoretically.5 C60F36 is another recently obtained individual fluorofullerene.6 Its molecular structure and crystal structure as well as some physicochemical properties have been reported.7-9 This * To whom correspondence should be addressed. E-mail: kolesov@ capital.ru. Fax: 7-(095)-932-8846).
fluorofullerene can be prepared in the reaction of C60 with MnF3 in the form of two major isomers having structures T and C3 in the approximate ratio 1:3. The molecular structures were determined by 19F NMR and 13C NMR,10 and the experiments involving preparation of (3He@C60)F36 followed by the 3He NMR and theoretical calculations of the respective chemical shifts were performed thus confirming the assignments of the structures.11 The aim of this work was to measure the enthalpy of formation of the fluorofullerene C60F36 and to compare the C-F bonds in this compound with those determined previously in C60F48. Experimental Section The sample of C60F36 was prepared using MnF3 as a fluorinating agent. The mixture of thoroughly ground powders C60 and MnF3 taken in the stoichiometric ratio was heated at T ) 653 K under reduced pressure (∼2 Pa) during 4 h. The sample of fluorofullerene appeared as a yellow-brownish powder and was subsequently purified by sublimation at T ) 573 K under the same reduced pressure. Chemical analysis of the purified sample showed the ratio n(F)/n(C60) ) 36 ( 1.
10.1021/jp000409a CCC: $19.00 © 2000 American Chemical Society Published on Web 05/18/2000
5404 J. Phys. Chem. B, Vol. 104, No. 23, 2000
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TABLE 1: Combustion Experiments on Fluorofullerene C60F35.800O.099 at T ) 298.15 K -∆cu° (J‚g-1)
q(CF4) (J) a
N
(m/g)
′‚∆Rc (J)
q(aux) (J)
C
1 2 3 4 5
0.070 019 0.070 338 0.057 636 0.068 986 0.072 002
12 687.7 12 974.8 12 916.1 12 975.3 12 994.4
11 432.7 11 695.3 11 855.3 11 719.2 11 682.1
0.7 0 0.3 4.0
F
b
0 0 2.0 3.9 2.2
q(res) (J)
q(HNO3) (J)
q(s) (J)
2.3 0.6 0 2.5 2.6
3.7 1.1 6.7 5.9 2.7
37.6 38.4 38.0 38.3 38.4 mean
weighted mean 〈∆cu°〉 ) -17603 ( 142 J‚g-1 a
C
a
17 377 17 630 17 608 17 747 17 590 ( 246
Fb 17 367 17 638 17 664 17 660 17 722 17 610 ( 173
The values are based on deficiency of carbon in combustion products. b The values are based on deficiency of fluorine in combustion products.
The molecular composition of the sample was characterized by Knudsen cell mass spectrometry (KCMS). As the volatility of fluorofullerenes with close fluorine content is similar, it allowed us to apply KCMS for estimation of solid phase composition from the molecular composition of the vapor phase. According to this method, the following values of mole fraction x were obtained: x(C60F36) ) 0.831; x(C60F34) ) 0.054; x(C60F34O) ) 0.061; x(C60F36O) ) 0.038; x(C60F38) ) 0.015. On the basis of these data, the Brutto formula of the sample was found to be C60F35.800O0.099. Its molecular mass, equal to 1402.3868, was calculated using relative atomic masses recommended by IUPAC.12 The energy of combustion of the sample was measured in a rotating-bomb isoperibolic calorimeter. The inner surface of the bomb was lined with platinum. The apparatus, the details of calorimetric procedure, and the analysis of combustion products were described in our previous paper4 related to C60F48. The sample was pressed into pellets and then crushed to small pieces, prior to the combustion. Then it was burnt in the sealed Terylene-film bags at the initial temperature T ) 298.15 ( 0.03 K under initial pressure of oxygen p ) 3.5 MPa together with a pellet of benzoic acid. Before all experiments, 10.04 cm3 of distilled water was introduced into the bomb to dissolve hydrogen fluoride. The sum of combustion energies of benzoic acid and Terylene film was about 90-92% of the total energy evolved in calorimetric experiments. After each run, the products of combustion were analyzed for CO2(g), HF(soln), and HNO3(soln) using the same procedure as described in reference.4 Small amounts of CF4 and unburnt residue in the crucible (about 0.4 × 10-5 g) were detected after most of the runs and could be considered as side products of combustion. The amount of CF4 was calculated in each run by two ways: from the deficiency in combustion products of carbon or fluorine. The corresponding energy correction q(CF4) was calculated basing on the molar energy of reaction of the CF4 hydrolysis, ∆rU°m ) -173.1 ( 1.3 kJ‚mol-1.13 It was shown by laser mass spectrometry that the unburnt residue contained neither fluorine nor C60; for that reason it was considered to be soot. Its massic combustion energy, -32 762 J‚g-1, was calculated from the value ∆fH°m(CO2,g) taken from ref 14. Because of the small quantity of the residue, the corresponding energy correction did not depend seriously on its real composition. Results and Discussion The results of the calorimetric experiments are presented in Table 1, where m denotes the sample mass, ′ the energy equivalent of the calorimeter corrected for heat capacity of the final bomb contents, ∆Rc the increase of the thermometer resistance corrected for heat exchange, q(aux) the sum of
combustion energies of auxiliary substances (benzoic acid and Terylene film), q(CF4) the correction for CF4 formation, q(res), the correction for the unburnt residue, q(HNO3) the correction for the energy of formation of aqueous nitric acid from nitrogen, oxygen, and water, q(s) the correction to standard states, and ∆cu° the standard massic energy of combustion of C60F36. Two columns of ∆cu° values correspond to two series of q(CF4) calculated from the deficiency of carbon or fluorine. The uncertainty of the mean value of ∆cu° in each column is given as the standard deviation of the mean multiplied by the Student’s coefficient t for the 0.05 significance level. The final value of ∆cu° of the sample, ∆cu° ) -17 603 ( 142 J‚g-1, is taken as the weighted mean of these two values. The molar energy of combustion of the sample is equal to -24 686 ( 199 kJ‚mol-1. This value corresponds to the sample having the above composition. Its large uncertainty is a consequence of the large fraction of auxiliary energy, which was necessary for the successful calorimetric experiments. To derive the molar combustion energy of C60F36, the corrections for impurities were introduced. The energies of combustion of impurities were estimated as follows. For the impurities, containing no oxygen, the molar combustion energy was assumed to change proportionally to the number of fluorine atoms in the series C60 to C60F36 and C60F36 to C60F48. The value of ∆cU°m(C60,cr) ) -25 965 ( 12 kJ‚mol-1 was taken from ref 15, and ∆cU°m(C60F48,cr) ) -24 668 ( 163 kJ‚mol-1 from ref 4. At the estimation of ∆cU°m of the impurities containing oxygen it was assumed that addition of oxygen atom to fluorofullerene molecule leads to the same change of ∆fH°m value as in the case of pairs of compounds: ethene-ethene oxide, or propene-propene oxide. The necessary ∆fH°m values for the last four compounds were taken from the book.16 As a result of this estimation, following ∆cU°m values were obtained (in kJ‚mol-1): C60F34 -24 783; C60F38 -24 706; C60F34O -24 673; C60F36O -24 604. Using the ∆cU°m values cited above, corrections for impurities were introduced and the energy of combustion of C60F36 was calculated: ∆cU°m(C60F36,cr) ) -24 714 ( 199 kJ‚mol-1. This value corresponds to the reaction:
C60F36(cr) + 51O2(g) + 738H2O(l) ) 60CO2(g) + 36(HF‚20H2O)(aq) (1) The overall correction for impurities turned out to be 28 kJ‚mol-1. It is much higher than in the case of C60F48 (3 kJ‚mol-1),4 owing to the higher contents of impurities. The standard molar energy and enthalpy of combustion of C60F36, as well as the standard molar enthalpy and Gibbs energy of formation values, are listed in Table 2. The values of the enthalpies of formation of CO2(g), H2O(l), and F-(aq) and the
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J. Phys. Chem. B, Vol. 104, No. 23, 2000 5405
TABLE 2: Results and Derived Quantities for C60F36 at 298.15 K
a
-∆cU°m(cr) (kJ‚mol-1)
-∆cH°m(cr) (kJ‚mol-1)
-∆fH°m(cr) (kJ‚mol-1)
∆subH°m (kJ‚mol-1)
-∆fH°m(g) (kJ‚mol-1)
S°ma (J‚K-1‚mol-1)
-∆fG°m(cr) (kJ‚mol-1)
24714 ( 199
24692 ( 199
5362 ( 201
139 ( 8 135 ( 8b
5223 ( 201
1014.5 ( 5.1
4339 ( 201
The entropy was determined in ref 17. b Experimental value at T ) 466 K.18
standard entropies S°m of C(cr,graphite) and F2(g) used in the calculations were taken from ref 14. The enthalpy of sublimation of C60F36(cr) was measured experimentally at T ) 466 K and was equal to 135 ( 8 kJ‚mol-1.18 This value was corrected to T ) 298.15 K (see Table 2) using the difference {Cp,m(g) Cp,m(cr)} ∼-26 J‚K-1‚mol-1 taken as approximate to analogous difference of the Cp values of gaseous and crystal C60 (a similar difference of Cp values was also obtained using Cp,m(C60F36,cr) from ref 17 and Cp,m(C60F36,g) estimated from the geometry parameters and vibrational frequencies calculated by the semiempirical PM3 method). The above-mentioned estimation is rather rough, but it could not distort seriously the ∆fH°m(g) value, taking into account that the temperature correction is small as compared with large uncertainty of the ∆cU°m. The enthalpy of the reaction
C60F36(g) ) C60(g) + 36F(g)
(2)
was calculated using the enthalpy of formation of C60F36(g) listed in Table 2 and the following values: ∆fH°m(C60,cr) ) 2355 ( 15 kJ‚mol-1,15 ∆subH°m(C60,cr) ) 181 ( 2 kJ‚mol-1,19 and ∆fH°m(F,g) ) 79.38 ( 0.30 kJ‚mol-1.14 The resulting value is ∆rH2 ) 10 617 ( 202 kJ‚mol-1, or 294.9 ( 5.6 kJ‚mol-1 per C-F bond. We compared the latter value with the enthalpy of analogous reaction of C60F48 decomposition
C60F48(g) ) C60(g) + 48F(g)
(3)
which was obtained in our previous work:4 ∆rH3 ) 13 800 ( 167 kJ‚mol-1, or 287.5 ( 3.5 kJ‚ mol-1 per C-F bond. It is also possible, using ∆rH2 and ∆rH3, to calculate the enthalpy of reaction
C60F48(g) ) C60F36(g) + 12F(g)
(4)
∆rH4 ) 3184 ( 261 kJ‚mol-1. This result leads to the value 265 ( 22 kJ‚mol-1 per C-F bond, which is needed for breaking C-F bonds in going from C60F48 to C60F36. Though these data need further refinement when C60F36 samples in larger quantities and higher purity are available, one can draw the conclusion from the obtained results that there is a tendency of C-F bond enthalpy decrease with the amount of F atoms attached to the fullerene cage.
Acknowledgment. This research was supported by the Russian Foundation for Basic Research (Grant 00-03-32623), by MNTP “Fullerenes and Atomic Clusters” (Grant “Sfera” 98064), and INTAS97-30027. We thank Mr. A. Popov for estimation of the heat capacity of gaseous C60F36. References and Notes (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) The Chemistry of Fullerenes; Taylor, R., Ed.; World Scientific: Singapore 1995. (3) Diogo, H. P.; da Piedade, M. In Recent adVances in the chemistry and physics of fullerenes and related materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society, Inc.: Pennington, NJ 08534-2896, 1998; Vol. 8, p 627. (4) Papina, T. S.; Kolesov, V. P.; Lukyanova, V. A.; Boltalina, O. V.; Galeva, N. A.; Sidorov, L. N. J. Chem. Thermodyn. 1999, 31, 1321. (5) Matsuzava, N.; Fukunaga, T.; Dixon, D. A. J. Phys. Chem. 1992, 96, 10747. (6) Boltalina, O. V.; Borschevskii, A. Ya.; Sidorov, L. N.; Street, J. M.; Taylor, R. Chem. Soc., Chem. Commun. 1996, 529. (7) Kawasaki, S.; Aketa, T. Touhara, H.; Okino, F.; Boltalina, O. V.; Gol’dt, I.; Troyanov, S. I.; Taylor, R. J. Phys. Chem. B. 1999, 103, 1223. (8) Liu,.N.; Morio, Y.; Okino, F.; Touhara, H.; Boltalina, O. V.; Pavlovich, V. K. Synth. Met. 1997, 86, 2289. (9) Mitsumoto, R.; Araki, T.; Ito, E.; Ouchi, Y.; Seki, K.; Kikuchi, K.; Achiba, A.; Kurosaki, H.; Sonoda, T.; Kobayashi, H.; Boltalina, O. V.; Pavlovich, V. K.; Sidorov, L. N.; Hattori, Y.; Liu, N.; Yajima, S.; Kawasaki, S.; Okino, F.; Touhara, H. J. Phys. Chem. 1997, 101, 516. (10) Boltalina, O. V. Street, J.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1998, 649. (11) Boltalina, O. V.; Bu¨hl, M.; Khong, A.; Saunders: M.; Street, J. M.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1999, 1475. (12) Atomic Weights of the Elements 1993, IUPAC Comission on Atomic Weights and Isotopic Abundances. J. Phys. Chem. Ref. Data 1995, 24, 1561. (13) Cox, J. D.; Gundry, H. A.; Head, A. J. Trans. Faraday Soc. 1965, 61, 1594. (14) Cox, J. D.; Wagman, D. D.; Medvedev, V. A. CODATA Key Values for Thermodynamics; Hemisphere: New York, 1989. (15) Kolesov, V. P.; Pimenova, S. M.; Pavlovich, V. K.; Tamm, N. B.; Kurskaya, A. A. J. Chem. Thermodyn. 1996, 28, 1121. (16) Pedley, J. B.; Naylor, R. D.; Kirbi, S. P. Thermochemical Data of Organic Compounds; Chapman and Hall: London, 1986. (17) Druzhinina, A. J.; Galeva, N. A.; Varushchenko, R. M.; Boltalina, O. V.; Sidorov, L. N. J. Chem. Thermodyn. 1999, 31, 1469. (18) Boltalina, O. V.; Markov, V. Yu.; Borshchevskii, A. Ya.; Galeva, N. A.; Sidorov, L. N.; Gigli, G.; Balducci, G. J. Phys. Chem. B 1999, 103, 3828. (19) Piacente, V.; Gigli, G.; Scordala, P.; Giustini, A.; Ferro, D. J. Phys. Chem. 1995, 99, 14052.
