J. Phys. Chem. 1992,96,1584-I589
7584
the cluster has been emphasized. In the present study, all different on-top binding sites were investigated for the different clusters. From these calculations, possible relations between metal coordination and chemisorption energy at that site could be investigated. For the two clusters where shell closing effects are observed, highly coordinated metal atoms are preferred as adsorption sites. This can be interpreted as due to an electron deficiency at these atoms coming from electron delocalization out into the bonding region, combined with the fact that the carbon lone pair should fit into the electronic structure of the cluster. For similar reasons, high coordination is preferred for most cationic clusters, whereas the situation for the neutral clusters is less clear. A similar study is under way for ammonia adsorption on nickel clusters where direct comparisons to recent experimentsI9 are possible.
References and Notes (I) Hermann, K.; Bagus, P. S.;Nelin, C. J. Phys. Reu. 1987,835,9467. (2) Tracy, J. C. J. Chem. Phys. 1972, 56, 2748. McConville, C. F.; Woodruff. D. P.; Prince, N. C.; Paolucci, G.;Chab, V.; Surman, M.; Bradshaw, A. M. Surf. Sci. 1986, 166, 221. (3) Leuchtner, R. E.; Harms, A. C.; Castleman, A. W., Jr. J . Chem. Phys. 1990,92,6527. (4) Siegbahn, P. E. M.; Wahlgren, U. In Reaction Energetics on Metal Surfaces: Theory and Applications; Shustorovich, E., Ed.; VCH: New York, 1991. (5) Wahlgren, U.; Siegbahn, P. In A New Tool for the Study of Chemisorption on Transition Metals: The Quantum Chemistry Approach to Sur-
face Reactions; Ruette, F., Ed.; Kluwer Academic: Dordrecht, 1991. (6) Panas, I.; Schale, J.; Siegbahn, P.; Wahlgren, U. Chem. Phys. Lerr. 1988, 149, 265. (7) Siegbahn, P. E. M.; Pettrrsson, L. G.M.; Wahlgren, U. J . Chem. Phys. 1991, 94, 4024. (8) Siegbahn, P. E. M.; Wahlgren, U. Int. J . Quantum Chem. 1992.42, 1149. (9) Post, D.; Baerends, E. J. J . Chem. Phys. 1983, 78, 5663. (10) Akeby, H.; Panas, I.; Pettersson, L. G.M.; Siegbahn, P.; Wahlgren, U. J. Phys. Chem. 1990, 94, 5471. (1 1 \ Wachters. A. J. H. J . Chem. Phvs. 1970.52. 1033. (12j Mattsson,’A.; Panas, I.; Siegbahn,‘P.; Wahlgren, U.;Akeby, H.Phys. Rev. 1987, B36, 7389. (13) Dunning, T. H. J . Chem. Phys. 1970, 53, 2823. (14) Ahlrichs, R.; Scharf. P.; Erhardt, C. J . Chem. Phvs. 1985.82. 890. (15) Maller, W.; Flesch, J.; Meyer, W. J. Chem. Phy;. 1984, 80, 3297. (16) Pettenson, L. G. M.; Akeby, H. J . Chem. Phys. 1991, 94, 2968. (17) Pettersson, L. G.M; Akeby, H.; Siegbahn, P. E. M.; Wahlgren, U. J . Chem. Phys. 1990, 93, 4954. (18) Nygren, M. A.; Siegbahn, P. E. M.; Jin, C.; Guo,T.; Smalley, R. E. J. Chem. Phys. 1991, 95, 6181. (19) Klots, T. D.; Winter, B. J.; Parks, E. K.; Riley, S.J. J . Chem. Phys. 1990,92,2110. Parks, E . K.; Winter, B. J.; Mots, T. D.; Riley, S. J. J. Chem. Phys. 1991, 94, 1882. (20) Fantucci, P.; Koutecky, J. In Elemental and Molecular Clusrers; Benedek, G.,Martin, T. P., Pacchioni, G.,Eds.; Springer-Verlag: Berlin, 1988. (21) Leopold, D. G.;Ho, J.; Lineberger, W. C. J . Chem. Phys. 1987,86, 1715. (22) Pettiette, C. L.; Yang, S.H.; Craycraft, M. J.; Conceicao, J.; Laaksonen, R. T.; Chesnovsky, 0.;Smalley, R. E. J . Chem. Phys. 1988,88, 5377. (23) Knickelbein, M. B. Chem. Phys. Lett. 1992, 192, 129. (24) Perdew, J. P. Phys. Rev. 1986, 33, 8822.
Characterization and Stability of Highly Fluorinated Fullerenes Albert A. Tuinman, Pumendu Mukberjee, James L. Adcock, Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996- 1600
Robert L. Hettich, and Robert N. Compton*it Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6125 (Received: April 9, 1992;
In Final Form: June 5, 1992)
Several procedures were used to fluorinate neat samples of CWand mixtures of Cso/C70. Both positive and negative ion mass spectrometry showed C60FX and C70Fy product mixtures, with X I48 and Y 5 56, representing the highest degree of fullerene substitution recorded by mass spectrometry to date. Small but significant differences are observed in the mass spectra obtained using field desorption, thermal deposition, and laser desorption ionization techniques, indicating facile loss of fluorine from the highly substituted fullerenes. slow decomposition in C6F6 solution was also observed. Infrared and UV-vis absorption spectra of c60Fx are also reported.
1. Introduction Since the discovery’ and production of large quantitiesZof the fullerenes Cso and C7* there has been considerable interest in the chemical alteration of these new allotropes of carbon. The Birch reduction was first used to hydrogenate Csoto form c.&36,which was observed to be a “light cream to off-white subtance”.3 Mass analysis of molecules emitted from a heated probe gave C H *+ and C&36’+ at low temperatures as well as c ~ ~ H ?nd ~ > C&I1;+ at higher temperatures. It was not clear whether C&II8 is a product of the Birch reduction or a pyrolysis product of C a w These authors favored one particular isomer of the more than 6 X lOI4 possibilities. Assuming that two hydrogens attack a single conjugated bond, Haufler et al.’ suggested that a lone double bond is left on each of the 12 pentagons of C60H36.Recent theoretical Address correspondence to this author. ‘Also Department of Chemistry, The University of Tennessee, Knoxville, TN 37996-1600.
calculations indicate that hydrogen atoms can energetically reside inside the c 6 0 cage as ell.^,^ These studies have ushered in the fullerene chemistry era. Of great interest is the construction of organic superconductors from fullerenes. For example, codeposition of Ca with alkali atoms has led to high-T, superconducting solidse6It is also possible to substitute atoms inside, outside, and into the closed fullerenes cages. Smalley and co-workers have trapped atoms inside’ the carbon cage as well as substituted other atoms such as boron into the carbon framework.* The Smalley group has suggested new nomenclature to accommodate this new chemistry: A @ c 6 0 means that atom A is contained inside Ca. Three recent theoretical studies have considered the possible perfluorination of c60; however, the results are inconclusive as to whether c60F60 is stable. Scuseria9 has performed ab initio calculations at the Hartree-Fock level employing a Hartree double-zeta plus polarization (DZP)method which predicts that C6OFm should be observed in the laboratory. However, Cioslow-
0022-3654/92/2096-7 584%03.00/0 0 1992 American Chemical Society
Highly Fluorinated Fullerenes
skilo has performed similar calculations and concludes that "the C,Fm molecule may not be easily isolable". Both calculations show large steric repulsions between the F atoms lengthening the C-C bonds to a record 1.627 A. A further calculation" suggests that "twisting" the carbon skeleton may relieve this steric strain in part, while retaining I symmetry. In a separate study, Dunlap et aL5have used local density functional methods to predict that C,Fm is stable, and that the C-F bond strength is 15%smaller than that for CF4. Recently, two experimental p a p e r ~ l have ~ . ~ ~described the fluorination of Cw and C7@Selig et al.I2have fluorinated powder samples of fullerenes containing C, and C70 in various ratios. Gravimetrically measured mass changes of the C, samples upon fluorination corresponded to F/Cm = 38 f 0.1, The positive ion mass spectrum of CmFx gave a distribution of mass peaks extending from C,F30*+ to CmFU*+with the maximum intensity occurring at CmF38*+.Only ions containing an even number of fluorine atoms were observed. The corresponding C70Fx spectra showed even-numbered fluorine compounds from C70F36'+ to C70F46*.+with the maximum intensity at C7oF40'+. The infrared absorpuon spectra of this brown to tan colored material (-CaFs8) showed a strong, very broad absorption at 1165 cm-l, charactembc of a C-F stretch vibration. In a separate experimental study of fullerene fluorination, Holloway et al." have reported a stepwise fluorination leading to an "almost white" material. The IR spectrum of the crystalline material gave two main peaks at 1067 and 1027 cm-',considerably different than that reported elsewhere.I2 The I9F NMR spectrum of the products resulting from long fluorination times show either of two single peaks at -150.5, -152.7 ppm, or both. These authors attribute one or both of these peaks to C6oF6o since the singlets require that all of the fluorines be equivalent. Unfortunately, the mass spectra of these samples were less conclusive. Electron ionization was unsuccessful, and fast atom bombardment produced interpretablepeaks only for partially fluorinated samples, indicating CmF6and CmF42 as major constituents. Olah et al.I4have partially chlorinated and brominated Cm and C70. Heating C, in C12 gas at 250 OC for 5 h produced a maximum uptake of 24 C1 atoms onto the fulerene skeleton. Mass analysis of orange C,Clx and CmBrx samples was unsuccessful. Thermal dehalogenation of these compounds was observed. In this study, we report the fluorination of pure C, and of the C60/C70(-91 1) mixture in F2/He atmosphere using different temperature regimes. The resulting C,F, and C7& samples were extensively studied by a variety of mass spectrometry techniques using both positive and negative ion mass analysis. Infrared and UV-vis absorption spectra were recorded for samples obtained from pure Cm. N
2. Experimental Section Fluorination. Pure C, and a 9 / 1 Cm/C70 mixture were obtained from Texas Fullerene (Houston, TX). A 6 mm diameter, 250 mm long quartz tube and a 2 mm diameter, 50 mm long quartz capillary were cleaned with water and acetone and dried in an oven overnight. Four milligrams of the fullerene was deposited in the capillary, which was placed in the quartz tube. The latter was fitted with an external heating coil and an internal thermocouple and was connected via Swagelok fittings to sources of dry fluorine and helium. The tubes were purged with 50 mL/min He for 2 h at 160 OC to remove t r a m of moisture, then 1 mL/min F2 was added to the flow. At intervals of 30 min the temperature was raised to 185 and 210 OC without visible change to the material. At this temperature, the He flow was interrupted (thus increasing the F2concentration) causing a notable swelling of the material and a color transition from black to yellow. Because no further color change was observed, the temperature was raised to 275 OC (experience had shown that above this temperature F2 started to aggressively attack the quartz tubes). Some of the material sublimed out of the inner tube to a cooler area of the outer tube downstream. After 4 h, F2flow and heating were discontinued, and the samples (residue and sublimate) were c o l l d separately as white powders. Mass spectrometric analyw
The Journal of Physical Chemistry, Vol. 96, No. 19. 1992 1585 of these samples showed them to be essentially identical. Fourier T d o m (ET)Mass Spectrometry. FT mass spectra were obtained at the Oak Ridge National Laboratory using an Extrel FTMS-2000 equipped with a Nd:YAG laser (266 nm) as well as thermal desorption/electron impact ionization. A few micrograms of solid sample were loaded onto a stainless steel probe tip for examination by either laser desorption (LD) or thermal desorption/electron capture. For LD experiments the fourth harmonic of the YAG (266 nm) was used at 106-107 W/cm2 to desorb and simultaneously ionize the fluorinated fullerenes. The laser power density was calculated from estimates of the laser spot size (300 pm) and measurement of the output power of the YAG laser incorporating losses at mirrors and the vacuum window. Alternately, the samples were thermally desorbed (TD) from a stainless steel solids probe (-250 "C) into the vacuum chamber where they were ionized by low-energy electron capture (EC). For this ionization technique, an electron filament (-8 V, 3 PA) was used to inject low-energy electrons into the FTMS cell. The electrons can be collisionally cooled and trapped in the cell for a relatively long period of time (100 ms). Thus, the electron energies are influenced by the trapping plate voltages and the pressure in the FTMS ion cell and are difficult to precisely determine. Depending on experimental conditions, a distribution of electrons can be trapped, with energies ranging from thermal to the trapping plate voltages (3 V). Ions formed by either LD or TD/EC were trapped, manipulated, and ultimately detected in the FTMS ion cell. The data were acquired under frequency-sweep excitation (0-2 MHz) at 2 kHz/ps and broad band detection (64K data points, apodized with no zero-fill). Magnetic Sector Mass Spectrometry. Field desorption (FD), 70-eV electron ionization (EI), electron capture (EC), and collision induced dissociation (CID) mass spectra were obtained at the University of Tennessee using a double focusing magnetic sector/quadrupole hybrid instrument, the ZAB-EQ from VG-Analytical, at resolution 2000 (8000 for the exact mass measurement giving elementary compositions). The FD spectrum was obtained. on a home-grown emitter, using emitter heating current at 14-22 mA and 12 kV total extraction potential (8 kV accelerating voltage and -4 kV extraction voltage). The E1 and EC mass spectra were obtained by loading the sample dissolved in C6F6 onto a direct exposure probe (0.13 mm X 12 mm Pt wire coiled to a five-turn helix) which could be heated resistively in the ion source. Thermal desorption from the wire occurred within the source at 0.7-0.9 A coil heating current. For EC, "thermal" electrons were generated by moderating 70-eV electrons with N2 ( N 1 mTorr within the source). Calibration of the 1200-2000 amu mass range was accomplished with fast atom bombardment of CsI. Accurate masses for elementary composition determination were assigned using the C,F, and C70Fx peaks as internal reference. Masses so determined deviated from theoretical values by 5% of the base peak, and by
