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Sublimation of hydrofullerenes C60H36 and C60H18
9 March 2001
Chemical Physics Letters 336 (2001) 39±46
www.elsevier.nl/locate/cplett
Sublimation of hydrofullerenes C60H36 and C60H18 P.A. Dorozhko a, A.S. Lobach b, A.A. Popov a,*, V.M. Senyavin a, M.V. Korobov a b
a Department of Chemistry, Moscow State University, Moscow, 119899, Russia Institute of Problems of Chemical Physics RAS at Chernogolovka, Russian Academy of Science, Chernogolovka, Russia
Received 18 October 2000; in ®nal form 11 January 2001
Abstract Thermal behavior of two hydrofullerenes, C60 H36 and C60 H18 , was studied by means of Knudsen cell mass-spectrometry and infrared spectroscopy. Vapor pressures and enthalpies of sublimation at T 550±685 K were measured. Sublimation of the hydrofullerenes was accompanied by partial loss of hydrogen. Decomposition of C60 H36 was con®rmed to be a stepwise process with formation of C60 H18 as an intermediate product. The material of the Knudsen cell strongly aected the partial pressures and mass-spectra of the hydrofullerene vapor species. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction It has been claimed that potential application of hydrogenated fullerenes [1] and nanotubes [2] is in hydrogen gas storage devices. The concentration of hydrogen in solid hydrofullerene (HyF) could be as high as 6±8 wt%, much more than in metal hydrides. Two solid highly hydrogenated HyFs with distinct compositions have been reported in the literature, namely C60 H36 and C60 H18 . A variety of synthetic methods have been proposed in order to produce C60 H36 [3±8]. The samples of C60 H36 , prepared by means of dierent hydrogenation techniques, have been extensively characterized by IR spectroscopy [4,6,9], mass-spectrometry [3± 8,10], 1 H NMR [3,5±7,10], 13 C NMR [4,10] and electron diraction [11] and no samples have been *
Corresponding author. E-mail address: [email protected] (A.A. Popov).
found to be completely identical. This has provoked speculations about the possible formation of dierent isomers of C60 H36 or of their mixtures. Indeed, 3 He NMR spectroscopy proved that C60 H36 synthesized by hydrogen transfer and in the Birch reaction is a mixture of two isomers [12]. The structures of the C60 H36 unit, usually discussed, were T, Th ; D3d , S6 and C3 [3,4,9±16]. Recently C60 F36 was proved to be a mixture of T and C3 isomers [17] and 3 He NMR data strongly suggest that C60 H36 and C60 F36 are isostructural [18]. C60 H18 was often detected as a pyrolysis product of C60 H36 [3,5,6,10]. It was synthesized individually also by the transfer hydrogenation technique [5,10] or by direct reaction of C60 with hydrogen at high temperature and pressure [19]. Molecules of C60 H18 have C3v symmetry as evidenced from the 1 H NMR spectrum [19]. The thermodynamic data on C60 H36 and C60 H18 are limited. Only heat capacity measurements have been carried out for solid C60 H36 [20].
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 1 0 3 - 8
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P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
In the present study, thermal behavior of two typical HyFs, C60 H36 and C60 H18 , produced by transfer hydrogenation, has been examined using Knudsen cell mass-spectrometry and IR spectroscopy. We have attempted both to determine the vapor pressures of HyF species and to follow the removal of hydrogen from the solid samples. 2. Experimental C60 H36 and C60 H18 were prepared via transfer hydrogenation with 9,10-dihydroanthracene as described earlier [10]. C60 H18 was additionally heated at 600 K for 5 h under high vacuum to remove anthracene impurity. IR spectra of C60 H36 and C60 H18 are presented in Fig. 1a,b. All the samples were contaminated with 0.5±1% anthracene. The electron impact (EI) mass-spectra (U 65±70 eV) of the vapor phase over HyFs were recorded using a MI 1201 static mass-spectrometer (resolving power 700). HyF samples were heated in vacuum inside the quartz and metal eusion cells (Pt, Au, Cu, Ni and Fe) up to a temperature of 770 K. The evolution of massspectra over HyFs sublimed from quartz eusion cell with temperature and with time at constant temperature was followed.
Fig. 1. IR spectra of C60 H36 (a) and C60 H18 (b). Intensities of C±H stretches are reduced by factors of 8 and 4, respectively.
Vapor pressures P of HyF species were calculated from the corresponding ion currents I using the conventional equation P k=rIT ;
1
where k, r, T are calibration constant, total ionization cross-section, calculated by the additivity rule and absolute temperature, respectively. Ion current/vapor pressure calibration constant k has been measured in separate runs with PbCl2 and C60 . The vapor pressures of both these substances closely approximate those of HyFs. IR spectra of solid samples pelletted in KBr were recorded on a Specord M-80 (Carl±Zeiss) spectrophotometer at room temperature. 3. Results and discussion 3.1. Isothermal treatment The initial mass-spectra of C60 H36 obtained from quartz, Pt, and Ni eusion cells and massspectrum of C60 H18 sublimed from the quartz cell are presented in Fig. 2a±d. As it is seen from Fig. 2 using of metal cells caused a rapid change in the mass-spectrum of C60 H36 . The dehydrogenation rate was signi®cantly higher in these cases and C60 H 36 peak disappeared already at the very beginning of the experiment (see below). Only quartz cells were used in further investigation of the HyFs thermal behavior. Fig. 3a±d presents the evolution of the mass-spectrum of the vapor phase over a C60 H36 solid sample with time in the course of an isothermal run at T 660 K. The initial massspectrum (Figs. 2a and 3a) included the strong peaks of C60 H 36 with typical fullerene isotopic distribution and weaker lines with m=e 755±720. This mass-spectrum was observed for 5±7 h (and longer at lower temperatures) and was attributed to the electron impact dissociation/fragmentation of the C60 H36 species. The parent ion dominates in the spectrum, with the ratio of I(C60 H 36 , m=e 756, 757, 758) to the total ion current produced from the C60 H36 species being 0.37. Further thermal treatment led to a relative decrease of the C60 H 36 peak and to a signi®cant increase of C60 H18 and C60 peaks (Fig. 3b). A small increase of peaks
P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
41
a
b
c
d
Fig. 2. EI mass-spectra from quartz eusion cell of C60 H36 at 660 K (a) and C60 H18 at 665 K (b); EI mass-spectra of C60 H36 from metal eusion cells at 600 K from Pt (c) and at 650 K from Ni (d).
with m=e 746±750 was also observed. Later (Fig. 3c) all lines, but C60 H 18 ; C60 and peaks with m=e 762 and 764 actually disappeared from the massspectrum. Prolonged treatment did not lead to signi®cant changes in the mass-spectra, peaks of C60 H 18 and other minor components of the massspectra decreased simultaneously (Fig. 3d) except for the C 60 signal remaining constant. The mass-spectrum of vapor over the C60 H18 samples, presented in Fig. 2b, is similar to the EI mass-spectrum of C60 H18 synthesized at high temperature and pressure [19]. The parent ion C60 H 18 (m=e 738, 739, 740) contributed 28% of the total ion intensity from these species. Thermal
treatment of C60 H18 at 620±660 K led to a decrease of the parent and fragment ions, which originated from the C60 H18 vapor species. Group of peaks with m=e 760±764 was observed again. Based on the data described above it was assumed that during thermal treatment of solid C60 H36 at 660 K sublimation of the HyF was accompanied by its slow dehydrogenation with the formation of C60 H18 . Our results fall in line with previously reported rapid formation of C60 H18 when C60 H36 was heated at higher temperatures [3,6]. A group of peaks with m=e 760±764 are still unassigned. Possible candidates for these peaks are C60 fCH2 g [6] or C60 H 3 42±44 [21].
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P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
a
b
c
d
Fig. 3. Time evolution of the mass-spectrum of the vapor over a C60 H36 sample at 660 K (a)±(c); time dependence of C60 H 36 and C60 H 18 signals in a typical experiment at the constant temperature (d). Vertical lines denote the stages at which IR spectra of the samples were measured.
IR-spectra, as described below, have not shown the presence of any other species except for C60 H36 and C60 H18 . To clarify whether observed evolution of the HyF mass-spectra corresponds to the process in the bulk of the samples, in several experiments isothermal holding of C60 H36 was interrupted at dierent stages to measure the IR-spectra of a residual at ambient conditions. Fig. 4a±d presents these spectra in the most informative region of 400±900 cm 1 . The spectrum (a) is that of the initial C60 H36 sample, spectra (b), (c) and (d) correspond to the samples with the mass-spectra marked by the vertical dashed lines in Fig. 3d. The
IR-spectrum of a C60 H18 sample is shown for comparison in Fig. 4e. It was found that even after the peak of C60 H 18 became dominant in the mass-spectrum of the thermally treated C60 H36 , surprisingly the content of C60 H18 in the bulk was still negligible (Fig. 4b). The IR spectrum shown in Fig. 4b is similar to that of the initial sample with the exception of bands at 484, 600, 729, 745 cm 1 and some weaker features. All of them originated from an anthracene impurity in the initial C60 H36 sample. This impurity was sublimed during the heating. It is worth noting that the IR-spectrum of C60 H36 synthesized by radical hydrogenation [4] is quite similar to the one
P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
43
ferent stages of the C60 H36 thermal treatment the IR spectra of the solid can be regarded as a superposition of those of C60 H18 and C60 H36 species without any intermediate hydrofullerenes. Decomposition of C60 H36 under our conditions can be considered a stepwise process with selective formation of C60 H18 as intermediate product. With the increase of the HyF sample temperature, C 60 began to dominate the mass-spectrum over 700 K. However IR spectra of the samples heated up to 700 K could not be attributed to the C60 itself. Characteristic fullerene lines at 528, 576, 1182 and 1429 were absent except for the weak feature at 528 cm 1 . Intense absorptions in the C±H stretch region were preserved and broad bands at 750, 850 and 1450 cm 1 have also appeared. Treatment at temperature up to 790 K led to a relative increase of the fullerene line at 528 cm 1 in the IR spectra of the residuals, however in other regions the spectra still diered from C60 spectrum signi®cantly. Thus, the pristine C60 was not recovered under the conditions of our experiment.
Fig. 4. IR spectra of: the initial C60 H36 (a); the solids obtained by interrupting the thermal treatment of C60 H36 at the stages denoted by the vertical lines at Fig. 3(d) (b)±(d); C60 H18 (e).
shown in Fig. 4b. In the spectrum (c) the C60 H18 bands at 544 and 602 cm 1 are observed but the content of solid C60 H36 is obviously much higher. Note that the mass-spectrum of this sample consists solely of C60 H18 peaks. C60 H18 could be considered as the main component of the solid only after prolonged thermal treatment of the sample, when bands at 544, 576, 602, 659, 689 and, probably, 744 cm 1 increased suciently. However, even after this a signi®cant amount of C60 H36 was still present in the sample. Thus, it was concluded that during the thermal treatment of C60 H36 the mass-spectra of the vapor phase do not correspond with the bulk composition of the sample. Possibly, rapid evolution of mass-spectra (compared to the decomposition of the solid) resulted from the process on the sample surface. However, it is remarkable that at the dif-
4. Thermodynamic data Using Eq. 1 vapor pressures of C60 H36 were calculated from the mass-spectra, measured over the corresponding solid samples at the initial period of heating (see Fig. 2a). The temperature dependence of the vapor pressure is given by the equation: ln P atm 13:98 0:54
19:47 0:33
1000=T ; obtained in the interval 580±680 K using 152 experimental points. The pressure values are accurate within a factor of 2.5 and hopefully give reasonable estimation of the saturated vapor pressures of C60 H36 . Above 680 K the peak of C60 H 36 dropped o rapidly. The second law enthalpy of sublimation of C60 H36 was derived from the temperature dependence of lnfI C60 H36 T g at T 560±680 K to be 162 5 kJ/mol. The third law enthalpy of sublimation was estimated employing experimental data of the vapor
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P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
pressures and thermodynamic functions calculated for the solid and gaseous C60 H36 . Gas-phase thermodynamic functions were calculated with ab initio harmonic vibrational frequencies of the C3 isomer [22] (Table 1), scaled by a factor of 0.908. The real sample is believed to be a mixture of two isomers [12] (possibly, C3 and T but other versions still cannot be excluded). However, gas phase thermodynamic functions of the T isomer calculated with ab initio vibrational frequencies from [23] were found to be very close to those of the C3 isomer. C60 H36 was treated as the C3 isomer only. Heat capacity, Cp , of solid C60 H36 from zero to 340 K was taken from reference [20]. Above 340 K Cp was calculated as a sum of vibrational contributions (considered to be the same as in the gas state), lattice contribution (equal to 6R at the temperatures studied) and the dierence between Cp and Cv . The latter was calculated in the temperature region where experimental data on Cp were available and then extrapolated to higher temperatures. The third law sublimation enthalpy of C60 H36 was obtained to be 175 kJ/mol at 298 K and 152 kJ/mol at 630 K in reasonable agreement with the second law value. Vapor pressures of C60 H18 varied from sample to sample. The highest pressure of C60 H18 at T const was observed over the samples of C60 H36 subjected to thermal treatment within the Knudsen cell (Fig. 2c), namely 2:7 10 8 atm. at 660 K. The pressure value measured over synthetic C60 H18 at 660 K was 0:45 10 8 atm. These two values can be considered as a conservative estimation of the saturated pressure of C60 H18 species over solid C60 H18 at T 660 K. The second law sublimation enthalpy of C60 H18 was estimated to be P186 kJ=mol: Table 1 Thermodynamic functions of C60 H36 in the gaseous state, C3 isomer T (K)
Cp (J/mol K)
S0 (J/mol K)
H 0 H00 (kJ/mol)
298 400 500 630
621 913 1157 1405
660 884 1114 1411
73 151 255 423
4.1. Electron impact mass-spectra and in¯uence of the Knudsen cell material The initial EI Knudsen cell mass-spectrum of C60 H36 recorded in this study (Figs. 2a and 3a) can be compared with the EI mass-spectra reported for the substance produced by dierent synthetic methods [4±6,8,10]. Our spectrum with no pronounced C60 H 18 peak is similar to the one presented by Osaki et al. [8] for the sample obtained by catalytic hydrogenation in toluene, and by Darwish et al. [6] for C60 H36 synthesized via reduction of C60 by Zn/HCl in the toluene solution. In other cases, C60 H 18 and C60 peaks have been found, in addition to C60 H36 [5,10]. Attalla et al. [4] had not detected C60 H 36 peaks over the HyF sample prepared by radical hydrogenation. Only C60 H 18 and lesser hydrogenated ions have been found. The data on thermal decomposition of C60 H36 are also somewhat controversial. All of the authors agreed to the fact that C60 H36 can be converted to C60 when heated in vacuum or inert atmosphere, even though the decomposition temperature and rate reported by dierent groups varied noticeably. Thus, in [3] for the C60 H36 obtained in the Birch reduction conditions 633 K was mentioned as the onset temperature. For C60 H36 synthesized via hydrogen transfer in [5] total decomposition was reported at 820 K for 15 min in the inert atmosphere and in [10] it was shown that after 3 h at 573 K C 60 became the major component in the mass-spectrum. Darwish et al. [6] reported that after heating of C60 H36 (prepared by the Zn/HCl reduction in toluene solution) at 920 K for 10 min, only small peaks of C60 H 18 and C60 have ap peared and C60 began to dominate in the massspectrum after only 1 h. The above-mentioned dierences in the massspectra and temperatures of decomposition can be explained by the possible contact of the samples with dierent metal parts of the experimental equipment and by the sample prehistory. In the present study, it was found that in Au, Ni and Fe Knudsen cells at 600±650 K the evolution of mass-spectra over C60 H36 followed the same
P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
scheme as in the quartz cell, though the C60 H 36 peak decreased 4±5 times more rapidly. In the case of Pt and Cu cells at the beginning of the experiment the intensity of C 60 was already ten times higher than that of the HyF peaks and the latter disappeared completely after 20±30 min of heating at 600±650 K. The samples treated in metallic cells were similar to those treated in quartz cells. Both displayed certain disagreement between IR spectra representing bulk of the samples and MS data representing surface of the samples. While the peaks of C60 H 18 (from Ni) or C60 (from Pt, Cu) dominated in the mass-spectra at 630 K, IR spectra of the residual solids were nearly the same and surely belonged to C60 H36 . If C60 H36 was exposed to air for a signi®cant amount of time, decrease of the C60 H 36 signal occurred very rapidly. A similar observation was made by Billups [12] who has shown that freshly synthesized C60 H36 made by the Birch reduction gave only a C60 H 36 peak while if the sample was exposed to air, a mixture of lesser hydrogenated C60 H x peaks was observed as in the study of Banks et al. [24]. Selectivity of the Birch reduction which yields C60 H36 only was also con®rmed by Vasil'ev et al. [25] using matrix-assisted laser desorption ionization mass-spectrometry. In conclusion, the saturated vapor pressure in the temperature region of 580±680 K and the sublimation enthalpy of C60 H36 were measured in this study. Conservative estimations of C60 H18 vapor pressure and sublimation enthalpy were also performed. The composition of starting materials and the process of their thermal decomposition were monitored by IR-spectroscopy. The Knudsen cell material together with the samples prehistory was shown to dramatically aect the mass-spectra over solid hydrofullerenes. This can provide an explanation for disagreement in the experimental data obtained by dierent groups. Acknowledgements This study was supported by the Russian State Program `Fullerenes and Atomic Clusters'.
45
References [1] J. Withers, R. Loufty, T. Lowe, Full. Sci. Tech. 5 (1) (1997) 1. [2] A. Dillon, K. Jones, T. Bekkedahl, C. Kiang, D. Bethune, M. Heben, Nature 386 (1997) 377. [3] R. Hau¯er, J. Conceicao, L. Chibante, Y. Chai, N. Byrne, S. Flanagan, M. Haley, S. O'Brien, C. Pan, Z. Xiao, W. Billups, M. Ciofolini, R. Hauge, J. Margrave, L. Wilson, R. Curl, R. Smalley, J. Phys. Chem. 94 (1990) 8634. [4] M. Attala, A. Vassalo, B. Tattam, J. Hanna, J. Phys. Chem. 97 (1993) 6329. [5] C. R uchardt, M. Gerst, J. Ebenhoch, H. Beckhaus, E. Campbell, R. Tellgmann, H. Schwarz, T. Weiske, S. Pitter, Angew. Chem. Int. Ed. Engl. 32 (1993) 584. [6] A.D. Darwish, A.K. Abdul-Sada, G.J. Langley, H.W. Kroto, R. Taylor, D.R.M. Walton, J. Chem. Soc. Perkin Trans. 2 (1995) 2359. [7] Y. Sui, J. Qian, J. Zhang, X. Zhou, Z. Gu, Y. Wu, H. Fu, J. Wang, Full. Sci. Tech. 4 (5) (1996) 813. [8] T. Osaki, T. Tanaka, Y. Tai, Phys. Chem. Chem. Phys. 1 (1999) 2361. [9] R. Bini, J. Ebenhoch, M. Fanti, P.W. Fowler, S. Leach, G. Orlandi, Ch. R uchardt, J.P.B. Sandall, F. Zerbetto, Chem. Phys. 232 (1998) 75. [10] A.S. Lobach, A.I. Rebrov, A.A. Perov, O.S. Roshchupkina, V.A. Tkacheva, A.N. Stepanov, Russ. Chem. Bull. 46 (1997) 641 English Translation. [11] L.E. Hall, D.R. McKenzie, M.I. Attala, A.M. Vassalo, R.L. Davis, J.B. Dunlop, D.J.H. Cockayne, J. Phys. Chem. 97 (1993) 5741. [12] W. Billups, A. Gonzalez, C. Gesenberg, W. Luo, T. Marriott, B. Alemany, M. Saunders, H. Jimenez-Vazquez, A. Khong, Tetrahedron Letters 38 (1997) 175. [13] R. Taylor, J. Chem. Soc., Perkin Trans. 2 (1992) 1667. [14] B.W. Clare, D.L. Kepert, J. Mol. Struct. (Theochem) 315 (1994) 71. [15] M. B uhl, W. Thiel, U. Schneider, J. Am. Chem. Soc. 117 (1995) 4623. [16] B.W. Clare, D.L. Kepert, J. Mol. Struct. (Theochem) 466 (1999) 177. [17] O.V. Boltalina, J.M. Street, R. Taylor, J. Chem. Soc., Perkin Trans. 2 (1998) 649. [18] O.V. Boltalina, M. B uhl, A. Khong, M. Saunders, J.M. Street, R. Taylor, J. Chem. Soc., Perkin Trans. 2 (1999) 1475. [19] A.D. Darwish, A.G. Avent, R. Taylor, D.R.M. Walton, J. Chem. Soc., Perkin Trans. 2 (1996) 2051. [20] B.V. Lebedev, T.A. Bykova, A.S. Lobach, Doklady Chemistry 368 (1999) 237. [21] M. Gerst, H.-D. Beckhaus, C. R uchardt, E.E.B. Campbell, R. Tellgmann, Tetrahedron Letters 34 (48) (1993) 7729. [22] V.M. Senyavin, A.A. Popov, A.A. Granovsky, Moscow State University. Quantum-chemical calculations were perfomed at the Hartree-Fock level using 6-31G basis set, unpublished results.
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[23] L.G. Bulusheva, A.V. Okotrub, A.V. Antich, A.S. Lobach, Vibrational frequencies were calculated at the HartreeFock level using 6-31G basis set, THEOCHEM, 2001, accepted for publication. [24] M. Banks, M.J. Dale, I. Gosney, P.K.G. Hodgson, R.C.K. Jennings, A.C. Jones, J. Lecoultre, P.R.R. Langrodge-
Smith, J.P. Maier, J.H. Scrivens, M.J.C. Smith, C.J. Smyth, A.T. Taylor, P. Thorburn, A.S. Webster, J. Chem. Soc., Chem. Commun. (1993) 1149. [25] Y. Vasil'ev, D. Wallis, M. N uchter, B. Ondruschka, A. Lobach, T. Drewello, Chem. Commun. (2000) 1233.
Chemical Physics Letters 336 (2001) 39±46
www.elsevier.nl/locate/cplett
Sublimation of hydrofullerenes C60H36 and C60H18 P.A. Dorozhko a, A.S. Lobach b, A.A. Popov a,*, V.M. Senyavin a, M.V. Korobov a b
a Department of Chemistry, Moscow State University, Moscow, 119899, Russia Institute of Problems of Chemical Physics RAS at Chernogolovka, Russian Academy of Science, Chernogolovka, Russia
Received 18 October 2000; in ®nal form 11 January 2001
Abstract Thermal behavior of two hydrofullerenes, C60 H36 and C60 H18 , was studied by means of Knudsen cell mass-spectrometry and infrared spectroscopy. Vapor pressures and enthalpies of sublimation at T 550±685 K were measured. Sublimation of the hydrofullerenes was accompanied by partial loss of hydrogen. Decomposition of C60 H36 was con®rmed to be a stepwise process with formation of C60 H18 as an intermediate product. The material of the Knudsen cell strongly aected the partial pressures and mass-spectra of the hydrofullerene vapor species. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction It has been claimed that potential application of hydrogenated fullerenes [1] and nanotubes [2] is in hydrogen gas storage devices. The concentration of hydrogen in solid hydrofullerene (HyF) could be as high as 6±8 wt%, much more than in metal hydrides. Two solid highly hydrogenated HyFs with distinct compositions have been reported in the literature, namely C60 H36 and C60 H18 . A variety of synthetic methods have been proposed in order to produce C60 H36 [3±8]. The samples of C60 H36 , prepared by means of dierent hydrogenation techniques, have been extensively characterized by IR spectroscopy [4,6,9], mass-spectrometry [3± 8,10], 1 H NMR [3,5±7,10], 13 C NMR [4,10] and electron diraction [11] and no samples have been *
Corresponding author. E-mail address: [email protected] (A.A. Popov).
found to be completely identical. This has provoked speculations about the possible formation of dierent isomers of C60 H36 or of their mixtures. Indeed, 3 He NMR spectroscopy proved that C60 H36 synthesized by hydrogen transfer and in the Birch reaction is a mixture of two isomers [12]. The structures of the C60 H36 unit, usually discussed, were T, Th ; D3d , S6 and C3 [3,4,9±16]. Recently C60 F36 was proved to be a mixture of T and C3 isomers [17] and 3 He NMR data strongly suggest that C60 H36 and C60 F36 are isostructural [18]. C60 H18 was often detected as a pyrolysis product of C60 H36 [3,5,6,10]. It was synthesized individually also by the transfer hydrogenation technique [5,10] or by direct reaction of C60 with hydrogen at high temperature and pressure [19]. Molecules of C60 H18 have C3v symmetry as evidenced from the 1 H NMR spectrum [19]. The thermodynamic data on C60 H36 and C60 H18 are limited. Only heat capacity measurements have been carried out for solid C60 H36 [20].
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 1 0 3 - 8
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P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
In the present study, thermal behavior of two typical HyFs, C60 H36 and C60 H18 , produced by transfer hydrogenation, has been examined using Knudsen cell mass-spectrometry and IR spectroscopy. We have attempted both to determine the vapor pressures of HyF species and to follow the removal of hydrogen from the solid samples. 2. Experimental C60 H36 and C60 H18 were prepared via transfer hydrogenation with 9,10-dihydroanthracene as described earlier [10]. C60 H18 was additionally heated at 600 K for 5 h under high vacuum to remove anthracene impurity. IR spectra of C60 H36 and C60 H18 are presented in Fig. 1a,b. All the samples were contaminated with 0.5±1% anthracene. The electron impact (EI) mass-spectra (U 65±70 eV) of the vapor phase over HyFs were recorded using a MI 1201 static mass-spectrometer (resolving power 700). HyF samples were heated in vacuum inside the quartz and metal eusion cells (Pt, Au, Cu, Ni and Fe) up to a temperature of 770 K. The evolution of massspectra over HyFs sublimed from quartz eusion cell with temperature and with time at constant temperature was followed.
Fig. 1. IR spectra of C60 H36 (a) and C60 H18 (b). Intensities of C±H stretches are reduced by factors of 8 and 4, respectively.
Vapor pressures P of HyF species were calculated from the corresponding ion currents I using the conventional equation P k=rIT ;
1
where k, r, T are calibration constant, total ionization cross-section, calculated by the additivity rule and absolute temperature, respectively. Ion current/vapor pressure calibration constant k has been measured in separate runs with PbCl2 and C60 . The vapor pressures of both these substances closely approximate those of HyFs. IR spectra of solid samples pelletted in KBr were recorded on a Specord M-80 (Carl±Zeiss) spectrophotometer at room temperature. 3. Results and discussion 3.1. Isothermal treatment The initial mass-spectra of C60 H36 obtained from quartz, Pt, and Ni eusion cells and massspectrum of C60 H18 sublimed from the quartz cell are presented in Fig. 2a±d. As it is seen from Fig. 2 using of metal cells caused a rapid change in the mass-spectrum of C60 H36 . The dehydrogenation rate was signi®cantly higher in these cases and C60 H 36 peak disappeared already at the very beginning of the experiment (see below). Only quartz cells were used in further investigation of the HyFs thermal behavior. Fig. 3a±d presents the evolution of the mass-spectrum of the vapor phase over a C60 H36 solid sample with time in the course of an isothermal run at T 660 K. The initial massspectrum (Figs. 2a and 3a) included the strong peaks of C60 H 36 with typical fullerene isotopic distribution and weaker lines with m=e 755±720. This mass-spectrum was observed for 5±7 h (and longer at lower temperatures) and was attributed to the electron impact dissociation/fragmentation of the C60 H36 species. The parent ion dominates in the spectrum, with the ratio of I(C60 H 36 , m=e 756, 757, 758) to the total ion current produced from the C60 H36 species being 0.37. Further thermal treatment led to a relative decrease of the C60 H 36 peak and to a signi®cant increase of C60 H18 and C60 peaks (Fig. 3b). A small increase of peaks
P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
41
a
b
c
d
Fig. 2. EI mass-spectra from quartz eusion cell of C60 H36 at 660 K (a) and C60 H18 at 665 K (b); EI mass-spectra of C60 H36 from metal eusion cells at 600 K from Pt (c) and at 650 K from Ni (d).
with m=e 746±750 was also observed. Later (Fig. 3c) all lines, but C60 H 18 ; C60 and peaks with m=e 762 and 764 actually disappeared from the massspectrum. Prolonged treatment did not lead to signi®cant changes in the mass-spectra, peaks of C60 H 18 and other minor components of the massspectra decreased simultaneously (Fig. 3d) except for the C 60 signal remaining constant. The mass-spectrum of vapor over the C60 H18 samples, presented in Fig. 2b, is similar to the EI mass-spectrum of C60 H18 synthesized at high temperature and pressure [19]. The parent ion C60 H 18 (m=e 738, 739, 740) contributed 28% of the total ion intensity from these species. Thermal
treatment of C60 H18 at 620±660 K led to a decrease of the parent and fragment ions, which originated from the C60 H18 vapor species. Group of peaks with m=e 760±764 was observed again. Based on the data described above it was assumed that during thermal treatment of solid C60 H36 at 660 K sublimation of the HyF was accompanied by its slow dehydrogenation with the formation of C60 H18 . Our results fall in line with previously reported rapid formation of C60 H18 when C60 H36 was heated at higher temperatures [3,6]. A group of peaks with m=e 760±764 are still unassigned. Possible candidates for these peaks are C60 fCH2 g [6] or C60 H 3 42±44 [21].
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P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
a
b
c
d
Fig. 3. Time evolution of the mass-spectrum of the vapor over a C60 H36 sample at 660 K (a)±(c); time dependence of C60 H 36 and C60 H 18 signals in a typical experiment at the constant temperature (d). Vertical lines denote the stages at which IR spectra of the samples were measured.
IR-spectra, as described below, have not shown the presence of any other species except for C60 H36 and C60 H18 . To clarify whether observed evolution of the HyF mass-spectra corresponds to the process in the bulk of the samples, in several experiments isothermal holding of C60 H36 was interrupted at dierent stages to measure the IR-spectra of a residual at ambient conditions. Fig. 4a±d presents these spectra in the most informative region of 400±900 cm 1 . The spectrum (a) is that of the initial C60 H36 sample, spectra (b), (c) and (d) correspond to the samples with the mass-spectra marked by the vertical dashed lines in Fig. 3d. The
IR-spectrum of a C60 H18 sample is shown for comparison in Fig. 4e. It was found that even after the peak of C60 H 18 became dominant in the mass-spectrum of the thermally treated C60 H36 , surprisingly the content of C60 H18 in the bulk was still negligible (Fig. 4b). The IR spectrum shown in Fig. 4b is similar to that of the initial sample with the exception of bands at 484, 600, 729, 745 cm 1 and some weaker features. All of them originated from an anthracene impurity in the initial C60 H36 sample. This impurity was sublimed during the heating. It is worth noting that the IR-spectrum of C60 H36 synthesized by radical hydrogenation [4] is quite similar to the one
P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
43
ferent stages of the C60 H36 thermal treatment the IR spectra of the solid can be regarded as a superposition of those of C60 H18 and C60 H36 species without any intermediate hydrofullerenes. Decomposition of C60 H36 under our conditions can be considered a stepwise process with selective formation of C60 H18 as intermediate product. With the increase of the HyF sample temperature, C 60 began to dominate the mass-spectrum over 700 K. However IR spectra of the samples heated up to 700 K could not be attributed to the C60 itself. Characteristic fullerene lines at 528, 576, 1182 and 1429 were absent except for the weak feature at 528 cm 1 . Intense absorptions in the C±H stretch region were preserved and broad bands at 750, 850 and 1450 cm 1 have also appeared. Treatment at temperature up to 790 K led to a relative increase of the fullerene line at 528 cm 1 in the IR spectra of the residuals, however in other regions the spectra still diered from C60 spectrum signi®cantly. Thus, the pristine C60 was not recovered under the conditions of our experiment.
Fig. 4. IR spectra of: the initial C60 H36 (a); the solids obtained by interrupting the thermal treatment of C60 H36 at the stages denoted by the vertical lines at Fig. 3(d) (b)±(d); C60 H18 (e).
shown in Fig. 4b. In the spectrum (c) the C60 H18 bands at 544 and 602 cm 1 are observed but the content of solid C60 H36 is obviously much higher. Note that the mass-spectrum of this sample consists solely of C60 H18 peaks. C60 H18 could be considered as the main component of the solid only after prolonged thermal treatment of the sample, when bands at 544, 576, 602, 659, 689 and, probably, 744 cm 1 increased suciently. However, even after this a signi®cant amount of C60 H36 was still present in the sample. Thus, it was concluded that during the thermal treatment of C60 H36 the mass-spectra of the vapor phase do not correspond with the bulk composition of the sample. Possibly, rapid evolution of mass-spectra (compared to the decomposition of the solid) resulted from the process on the sample surface. However, it is remarkable that at the dif-
4. Thermodynamic data Using Eq. 1 vapor pressures of C60 H36 were calculated from the mass-spectra, measured over the corresponding solid samples at the initial period of heating (see Fig. 2a). The temperature dependence of the vapor pressure is given by the equation: ln P atm 13:98 0:54
19:47 0:33
1000=T ; obtained in the interval 580±680 K using 152 experimental points. The pressure values are accurate within a factor of 2.5 and hopefully give reasonable estimation of the saturated vapor pressures of C60 H36 . Above 680 K the peak of C60 H 36 dropped o rapidly. The second law enthalpy of sublimation of C60 H36 was derived from the temperature dependence of lnfI C60 H36 T g at T 560±680 K to be 162 5 kJ/mol. The third law enthalpy of sublimation was estimated employing experimental data of the vapor
44
P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
pressures and thermodynamic functions calculated for the solid and gaseous C60 H36 . Gas-phase thermodynamic functions were calculated with ab initio harmonic vibrational frequencies of the C3 isomer [22] (Table 1), scaled by a factor of 0.908. The real sample is believed to be a mixture of two isomers [12] (possibly, C3 and T but other versions still cannot be excluded). However, gas phase thermodynamic functions of the T isomer calculated with ab initio vibrational frequencies from [23] were found to be very close to those of the C3 isomer. C60 H36 was treated as the C3 isomer only. Heat capacity, Cp , of solid C60 H36 from zero to 340 K was taken from reference [20]. Above 340 K Cp was calculated as a sum of vibrational contributions (considered to be the same as in the gas state), lattice contribution (equal to 6R at the temperatures studied) and the dierence between Cp and Cv . The latter was calculated in the temperature region where experimental data on Cp were available and then extrapolated to higher temperatures. The third law sublimation enthalpy of C60 H36 was obtained to be 175 kJ/mol at 298 K and 152 kJ/mol at 630 K in reasonable agreement with the second law value. Vapor pressures of C60 H18 varied from sample to sample. The highest pressure of C60 H18 at T const was observed over the samples of C60 H36 subjected to thermal treatment within the Knudsen cell (Fig. 2c), namely 2:7 10 8 atm. at 660 K. The pressure value measured over synthetic C60 H18 at 660 K was 0:45 10 8 atm. These two values can be considered as a conservative estimation of the saturated pressure of C60 H18 species over solid C60 H18 at T 660 K. The second law sublimation enthalpy of C60 H18 was estimated to be P186 kJ=mol: Table 1 Thermodynamic functions of C60 H36 in the gaseous state, C3 isomer T (K)
Cp (J/mol K)
S0 (J/mol K)
H 0 H00 (kJ/mol)
298 400 500 630
621 913 1157 1405
660 884 1114 1411
73 151 255 423
4.1. Electron impact mass-spectra and in¯uence of the Knudsen cell material The initial EI Knudsen cell mass-spectrum of C60 H36 recorded in this study (Figs. 2a and 3a) can be compared with the EI mass-spectra reported for the substance produced by dierent synthetic methods [4±6,8,10]. Our spectrum with no pronounced C60 H 18 peak is similar to the one presented by Osaki et al. [8] for the sample obtained by catalytic hydrogenation in toluene, and by Darwish et al. [6] for C60 H36 synthesized via reduction of C60 by Zn/HCl in the toluene solution. In other cases, C60 H 18 and C60 peaks have been found, in addition to C60 H36 [5,10]. Attalla et al. [4] had not detected C60 H 36 peaks over the HyF sample prepared by radical hydrogenation. Only C60 H 18 and lesser hydrogenated ions have been found. The data on thermal decomposition of C60 H36 are also somewhat controversial. All of the authors agreed to the fact that C60 H36 can be converted to C60 when heated in vacuum or inert atmosphere, even though the decomposition temperature and rate reported by dierent groups varied noticeably. Thus, in [3] for the C60 H36 obtained in the Birch reduction conditions 633 K was mentioned as the onset temperature. For C60 H36 synthesized via hydrogen transfer in [5] total decomposition was reported at 820 K for 15 min in the inert atmosphere and in [10] it was shown that after 3 h at 573 K C 60 became the major component in the mass-spectrum. Darwish et al. [6] reported that after heating of C60 H36 (prepared by the Zn/HCl reduction in toluene solution) at 920 K for 10 min, only small peaks of C60 H 18 and C60 have ap peared and C60 began to dominate in the massspectrum after only 1 h. The above-mentioned dierences in the massspectra and temperatures of decomposition can be explained by the possible contact of the samples with dierent metal parts of the experimental equipment and by the sample prehistory. In the present study, it was found that in Au, Ni and Fe Knudsen cells at 600±650 K the evolution of mass-spectra over C60 H36 followed the same
P.A. Dorozhko et al. / Chemical Physics Letters 336 (2001) 39±46
scheme as in the quartz cell, though the C60 H 36 peak decreased 4±5 times more rapidly. In the case of Pt and Cu cells at the beginning of the experiment the intensity of C 60 was already ten times higher than that of the HyF peaks and the latter disappeared completely after 20±30 min of heating at 600±650 K. The samples treated in metallic cells were similar to those treated in quartz cells. Both displayed certain disagreement between IR spectra representing bulk of the samples and MS data representing surface of the samples. While the peaks of C60 H 18 (from Ni) or C60 (from Pt, Cu) dominated in the mass-spectra at 630 K, IR spectra of the residual solids were nearly the same and surely belonged to C60 H36 . If C60 H36 was exposed to air for a signi®cant amount of time, decrease of the C60 H 36 signal occurred very rapidly. A similar observation was made by Billups [12] who has shown that freshly synthesized C60 H36 made by the Birch reduction gave only a C60 H 36 peak while if the sample was exposed to air, a mixture of lesser hydrogenated C60 H x peaks was observed as in the study of Banks et al. [24]. Selectivity of the Birch reduction which yields C60 H36 only was also con®rmed by Vasil'ev et al. [25] using matrix-assisted laser desorption ionization mass-spectrometry. In conclusion, the saturated vapor pressure in the temperature region of 580±680 K and the sublimation enthalpy of C60 H36 were measured in this study. Conservative estimations of C60 H18 vapor pressure and sublimation enthalpy were also performed. The composition of starting materials and the process of their thermal decomposition were monitored by IR-spectroscopy. The Knudsen cell material together with the samples prehistory was shown to dramatically aect the mass-spectra over solid hydrofullerenes. This can provide an explanation for disagreement in the experimental data obtained by dierent groups. Acknowledgements This study was supported by the Russian State Program `Fullerenes and Atomic Clusters'.
45
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