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J. Phys. Chem. B 2007, 111, 10352-10356
ARTICLES Gas-Phase Methylation of [60]Fullerene S. M. C. Vieira,*,† T. Drewello,‡ S. G. Kotsiris,‡ C. A. Rego,† and P. R. Birkett† DiVision of Chemistry and Materials, School of Biology, Chemistry & Health Science, Manchester Metropolitan UniVersity, Chester Street, Manchester, M1 5GD, United Kingdom, and Department of Chemistry, UniVersity of Warwick, CoVentry, CV4 7AL, United Kingdom ReceiVed: May 2, 2007; In Final Form: June 16, 2007
Direct methylation of [60]fullerene via a gas-phase reaction in a CH4/H2 atmosphere was performed using a modified hot filament chemical vapor deposition method. Pressures were varied from 10 to 60 mbar and the substrate was maintained at 690 °C. High-resolution matrix-assisted laser desorption ionization (MALDI) mass spectrometry analysis showed signals corresponding to C60H18-2n(H,CH3)n. Collision-induced dissociation experiments confirmed a maximum of 18 ligands possible to the [60]fullerene cage.
Introduction The development of a method to produce macroscopic quantities of fullerenes by Kra¨tschmer and co-workers1 made possible studies into their properties and more importantly their reactivity. In particular, [60]fullerene is a good electron pair acceptor that can form charge-transfer complexes with electron pair donors and has a small band gap of ∼1.8 eV.2 This gives [60]fullerene the capability to accommodate up to 12 electrons in subsequent reversible reduction steps. This redox property and energy storage capacity makes it an interesting candidate in electron- and energy-transfer reactions. Reactions involving alkylation resulted in more than 20 methyl groups being attached to the cage,3 and benzyl and methyl radicals react readily with [60]fullerene to give stable derivatized products.4 This high reactivity of [60]fullerenes with reactive free radicals makes it a potential molecule for biomedical applications.5 For instance, the remarkable capability of derivatized [60]fullerene to prevent the HIV virus from attacking healthy cells has been reported.6 In addition, derivatized [60]fullerene also has the capability of acting as an antibacterial agent7 and as an antiaging8 ingredient that can be used in cosmetic industry. From the various derivatized [60]fullerenes studied, hydrofullerenes have attracted considerable attention, in part, due to their important acidic properties and the prospect of using these compounds in synthetic strategies. However, characterization of hydrogenated fullerene adducts can be somewhat difficult. Hence characterization techniques have been optimized/ developed to facilitate identification of the hydrogenation reaction products obtained and their true synthetic routes, in addition to those of other [60]fullerene derivatives.9 We have previously reported studies on the stability of [60]fullerene using the modified hot filament chemical vapor deposition (HFCVD) method in a hydrogen only atmosphere (30 mbar, 30 min, 690 °C). The result was the hydrogenation * Address correspondence to this author. E-mail:
[email protected]. † Manchester Metropolitan University. ‡ University of Warwick.
of crude [60]fullerene with C3V C60H18 as the major reaction product in good yields.10 Since a gas-phase chemical reaction was “modifying” a solid the technique was named chemical vapor modification (CVM). However, hot filament CVD has been used to synthesize other forms of carbon-based materials besides the common deposition of thin diamond films, namely, carbon nanostructures11 and carbon nanotubes (CNTs).12 For instance, the CVD system used to deposit larger areas of thick diamond films resulted also in the growth of carbon nanostructures.13 Hence modified HFCVD is an appropriate description of this solvent-free derivatization method of [60]fullerene. In this paper, the potential of the modified HFCVD process for the synthesis of new derivatives based on [60]fullerene, in particular the addition of methyl groups to the [60]fullerene cage, is reported. Experimental Section The starting material, [60]fullerene, was purchased from the Kurchatov Institute, Moscow and was used without further purification. The reaction was performed with a HFCVD reactor, which consists of a stainless steel chamber evacuated by a rotary pump and is further described in ref 10. Finely ground [60]fullerene (∼3 g) was placed on the molybdenum substrate holder (5 × 5 cm2) and positioned 2 mm directly below a coiled tungsten filament (Figure 1a). The gas flow rate of methane was kept constant (14.4 sccm) and the flow rate of hydrogen was varied to set the hydrogen concentration between 85 and 96.5 vol %. The tungsten filament was resistively heated with a DC power supply to set the desired filament temperature at ∼1620 °C. This corresponded to a substrate holder temperature, measured by a thermocouple in direct contact with the central region, of 690 °C. The filament temperature was found to be the minimum temperature required for the efficient formation of radicals and atoms at the W surface and the corresponding substrate temperature was the maximum temperature with which the [60]fullerene does not decompose. Other process parameters varied were the reaction time (2-15 min) and total gas pressure (15-60 mbar). Beneath the filament colored material was
10.1021/jp0733623 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/17/2007
Gas-Phase Methylation of [60]Fullerene
Figure 1. Schematic diagram of the modified substrate-filament assembly (a) before and (b) after the reaction between [60]fullerene and the gas precursors. Note the distance between the filament and [60]fullerene was taken from the top layer of the solid material and the bottom of the filament coil. Concentric rings of differently colored materials which vary from unreacted black [60]fullerene directly beneath the filament to pale yellow (outer circle) indicate that a mixture of reaction products is obtained. Colored material was also observed in the filament supporting rods.
observed almost as soon as the gas precursors were introduced into the system. This indicated the high reactivity of [60]fullerene under the HFCVD conditions. Characterization of the reaction products by X-ray diffraction (Philips PW 1729 X-ray diffractometer), Kompact matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry, and proton (1H) nuclear magnetic resonance (NMR) (Jeol Delta GSX, 270 MHz, CDCl3) spectroscopy were performed. The computer simulation program SENKIN estimated the gas-phase composition close to the filament surface. Results and Discussion Figure 1b shows the reaction products obtained after the gasphase CH4/H2 reactions. Concentric rings of differently colored materials, which vary from dark brown directly beneath the filament to light yellow, indicate that a mixture of reaction products is obtained. The changes in colors are expected since the conjugated electronic system of the [60]fullerene is reduced as the cage becomes more functionalized. The red-brown reaction product has been reported to be characteristic of C60H18,14,15 whereas lighter colored material corresponds to a higher number of functional groups attached to the cage, e.g., C60H.3616 The more highly hydrogenated [60]fullerene molecules migrate further from the center of the substrate holder, which enables a crude separation of the product mixture to be achieved. This may be explained since the higher the degree of derivatization of the [60]fullerene the lower its enthalpy of sublimation relative to the parent [60]fullerene.17,18 From the initial starting material an approximate yield of 65% of colored material was obtained. This was determined by weighting the starting material and colored products after HFCVD. X-ray Diffraction. The black and gray residues were collected from the central region directly below the filament and characterized by X-ray diffraction (XRD). These residues were formed regardless of the particular experimental conditions used. Thus 100% yield of derivatized [60]fullerene was never attained. Figure 2 shows the XRD patterns of the starting material, dark gray and black residues collected subsequent to HFCVD experiments. The XRD pattern for the dark gray residue
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Figure 2. X-ray diffraction patterns obtained for starting material and gray and black residues collected in the central region after HFCVD. The crystal structure for both [60]fullerene and dark-gray residue is FCC with a determined cell parameter a ≈ 14.1 Å. The black residue does not have a crystalline structure.
and [60]fullerene is similar to that with a calculated unit cell of a ≈ 14.1 Å. This value is in agreement with the face-centered cubic (FCC) crystal structure previously calculated for [60]fullerene.19 The X-ray diffraction patterns for the black residue indicate the absence of a crystalline structure due to possible decomposition of [60]fullerene into amorphous carbon. MALDI-ToF Mass Spectrometry Analysis. The ionization method applied in the mass spectrometry characterization of the reaction products was MALDI, which has been successfully used in analyzing hydrofullerenes.9 MALDI involves laser activation of a composite target material in which the analyte is embedded into a matrix material. The matrix is commonly present in excess and acts as the laser light absorbing material, which upon activation desorbs into the gas phase, thus carrying the analyte with it. The desorption is accompanied by the formation of analyte ions, which result from the interaction of the analyte with primary ions of the initially activated matrix material. The most common ion formation mechanism in MALDI of fullerene derivatives involves electron-transfer reactions between the neutral analyte and matrix-derived ions. The matrix material used in the present investigation was 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB), which is currently the most universally applicable electron-transfer matrix for the analysis of derivatized fullerenes, featuring high analyte ion production with often only minor fragmentation.20 Low-Resolution MALDI MS. Figure 3 shows a series of lowresolution MALDI mass spectra of the reaction products collected at different distances from the hot filament (HFCVD conditions: 10 vol % CH4, ∼50 mbar, 5 min). In all spectra the C60 ion (m/z 720) is accompanied by a signal at m/z 738, with up to six additional peaks at higher masses featuring a spacing of about 14-16 mass units. As [60]fullerene treated in a H2 only atmosphere produced C60H18 as the major product, it most likely that the peak at m/z 738 is from this species. In Figure 3 the labels I-V indicate the areas of different colors of the samples collected from the substrate holder, e.g., dark brown is represented by label I and light orange by label V. There is a clear increase in the relative concentration of more highly derivatized [60]fullerene in the lighter colored materials. Consequently, MALDI MS confirms the color pattern is mainly caused by fractional sublimation, where derivatized [60]fullerene separates from unreacted material by travelling further away from the filament. However, the mass resolution and accuracy in these experiments is insufficient to establish unequivocally
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Figure 3. Low-resolution MALDI-ToF mass spectra of reaction products collected at different distances from the hot filament (HFCVD conditions: 10 vol % CH4, ∼50 mbar, 5 min). The sample labeled I corresponds to the dark brown solid collected near the gray residue. Sample II is the brown material, III is the light brown, IV is the dark orange, and V is the light orange.
the elemental composition of the reaction products observed. The composition of the higher mass species needs further analysis. Under the HFCVD conditions employed the sample can only be composed of carbon, hydrogen, and oxygen, the latter being a potential impurity due to handling the product in air. Negative-ion mass spectrometry MALDI of the samples showed that oxygenated compounds, which have a particularly high response in the negative-ion mode, are of such low abundance that they can be discounted as a major sample component. High-Resolution MALDI Analysis. Figure 4a shows the highresolution MALDI mass spectrum obtained for the lighter colored sample (labeled V in Figure 3). Figure 4b compares the measured mass with the masses that were calculated for different possible derivatized fullerenes. It can be seen that the smallest difference for calculated and measured mass is obtained for the C60H18-2n(H,CH3)n composition with n ) 1-3. The C60H18-2n(H,CH3)n composition of the major products, established here by accurate mass measurements, is further corroborated by collision-induced dissociation (CID) experiments. MALDI mass spectrometry also confirmed that increasing the proportion of methane in the precursor gas mixture up to 15 vol % CH4 results in an increase in the number of methyl groups addended to the fullerene cage. For methane concentrations above 15 vol % the efficiency of the derivatization process rapidly diminishes as amorphous carbon deposits on the surface of the W filament causing a decrease in the filament temperature and, therefore, a reduction in the efficiency of radical formation. Collision-Induced Dissociation Experiments. Parent ions were selected by an ion gate and underwent CID with air at a kinetic energy of 1 kV. Figure 5 shows the resulting CID spectra for the radical cations of C60H18-2n(H, CH3)n with n ) 0 (Figure 5a: m/z 738, C60H18+•), n ) 1 (Figure 5b: m/z 752, C60H17CH3+•, n ) 2 (Figure 5c: m/z 766, C60H16(CH3)2+•), and n ) 3 (Figure 5d: m/z 780, C60H15(CH3)3+•), respectively. The selectivity of the ion gate was approximately 6 Da, so that adjacent ions within this mass range were co-selected with the chosen parent ion. Consequently, the CID spectra are a reflection of the dissociations of all ions selected in this mass window, rather than representing the decomposition of only one ion. Nevertheless, the dissociation pattern of the selected collective reveals structural insight into the individual. The CID mass spectrum of C60H18 (Figure 5a) confirms findings obtained in
Figure 4. (a) High-resolution MALDI performed on the sample labeled V (HFCVD conditions: 10 vol % CH4, ∼50 mbar, 5 min). (b) Comparison of the measured mass with the masses that were calculated for different possible addended fullerene compositions. The smallest deviations were for the elemental compositions labeled 1, 5, 12, and 22. This corresponds to the compounds C60H18, C60H16(H,CH3)1, C60H14(H,CH3)2, and C60H12(H,CH3)3, respectively.
Figure 5. Schematic diagram of collision-induced dissociation (CID) experiments performed on the high-resolution MALDI.
earlier investigations into the unimolecular dissociation behavior in which it was found that hydrogen loss from the fullerene is accompanied by hydrocarbon loss involving the expulsion of carbon atoms from the cage.21 The two enhanced signal clusters in the CID spectrum of C60H18+ are caused by the loss of CmHn, with m ) 1 and 2, from the fullerene cage. The loss of these alkyl entities is accompanied by the loss of exohedral hydrogen. The signal at m/z 720 is enhanced and accompanied by a signal
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Figure 6. 1H NMR (270 MHz, CDCl3) spectrum of the light orange material (HFCVD conditions: 10 vol % CH4, ∼50 mbar, 5 min).
Figure 7. Mole fractions of the major gas-phase species during CVD as a function of filament temperature, using an input gas mixture of 1 vol % CH4 in H2 at a pressure of 30 mbar. At filament temperatures used in HFCVD the major species present are •H and C2H2. The major reactive carbon specie is •CH3.
at m/z 696, which indicates that the pure [60]fullerene is formed by complete hydrogen loss. The dissociation pattern of C60H18+• is repeated below m/z 738 in the CID spectra of the other precursor ions. In addition to hydrogen losses, the high mass end of the CID spectra of the methylated precursors features the loss of exohedral CHn units. Figure 5b shows the loss of one CHn moiety from C60H17CH3+•, Figure 5c shows the loss of two units from C60H16(CH3)2+•, and Figure 5d shows the loss of three from C60H16(CH3)3+•. The loss of these exohedral CHn groups is more facile (and thus more abundantly observed) than alkyl loss from [60]fullerene cage rupture seen in the low mass region and the loss of hydrogen from the cage. The resolution of the CID spectra is not sufficient to establish the daughter ion composition by accurate mass measurement. However, the assignment of the high mass fragments as losses of exohedral CHn units is beyond doubt as the presence of oxygen has been discounted because of the negative-ion behavior. Therefore, the only alternative composition for the parent ions above m/z 738 would be C60Hn with n ) 32, 46, and 60. None of these hydrogen compositions is particularly stable, and in the latter case may not be attainable. The dissociations of the corresponding radical cations would undergo alkyl loss involving cage-carbon atoms; while each of these precursors would lead to the same preferred daughter ion in mass (just below m/z 738), the carbon core for these daughter ions would be different in each case, consisting of C59Hn in Figure 5b, C58Hm in Figure 5c, and C58Hx in Figure 5d. The observed fragmentation behavior is at odds with these alternative structures and in complete agreement with a precursor structure of C60H18-2n(H,CH3)n. Proton NMR Analysis. A crude separation of the colored samples was performed and the light-colored samples obtained with different process conditions were analyzed by NMR. All the 1H NMR spectra had a similar broad resonance signal between δ 4 and 3 due to the presence of several isomers or due to the decomposition of the sample during data acquisition. Figure 6 shows the preliminary 1H NMR spectrum from the crude separated light orange sample (HFCVD conditions: 10 vol % CH4, ∼50 mbar and 5 min reaction time). Four signals are present in a 1:2:1:2 ratio at 4.4, 4.1, 3.9, and 3.4, and are similar to the values reported for the HPLC isolated C3V symmetric isomer of C60H18. The broad baseline signal from 4.6 to 3.1, however, could be indicative of the presence of a mixture of polyhydrogenated/methylated [60]fullerene regioisomers within the sample. In addition, resonance signals at 2.61.6 are also present. The more functionalized the [60]fullerene cage the less electron withdrawing it becomes; therefore, the signals in the 1H NMR spectrum will be expected more upfield,22 i.e. in the δ 3-1 region relative to tetramethylsilane (TMS, δ 0). However, a blank 1H NMR (270 MHz) performed
with only CDCl3 (not shown) indicated the presence of resonance signals in the same δ 2.5-2.0 region. These signals derived from impurities in the CDCl3 solvent, thus a conclusive analysis of the samples with signals more upfield is not possible under these NMR acquisition conditions. It only indicates with certainty the presence of C3V C60H18 possibly as a result of either decomposition of the mixed reaction products during NMR acquisition or decomposition into C60H18 subsequent to HFCVD experiments. Modeling of the Gas-Phase CVD Environment. For the modified HFCVD to work, the precursor gas mixture must be activated to form atoms/radicals otherwise no reaction with [60]fullerene occurs. To understand the gas-phase chemistry near the filament, the composition of the gas mixture was calculated by using the Fortran computer program SENKIN that computes the time evolution of a homogeneous mixture in a closed system.23 The model does not include electron impact dissociation, ionic reactions, or surface chemistry and does not account for the mass transport mechanism in the CVD reactor. The results can only be taken as being characteristic of the gaseous environment close to the filament. For modeling the CVD gasphase reaction mechanism the Arrhenius equation is considered where the reaction rate constant, k, is given by the following: k ) ATβ exp(-Ea/(RT)), where A is the pre-exponential factor, Ea is the activation energy, and β is the temperature exponent. Modeling studies in the case of a hydrogen only atmosphere indicated that H atoms formed in the filament vicinity react with solid [60]fullerene in a multiple radical addition mechanism to produce C60Hx (x ) 1-18).10 To model a CH4/H2 atmosphere a total of 34 C/H gas-phase elementary reactions were used and the corresponding kinetic parameters (A, Ea, and β) for each reaction were obtained from ref 24. Figure 7 shows the calculated mole fractions of the major gas-phase species as a function of temperature for a 1 vol % methane in hydrogen input gas mixture at ∼50 mbar. An increase in the filament temperature results in an increase in the concentration of atomic hydrogen via the dissociation of H2. Consequently, this results in the conversion of CH4 into primarily C2H2 and •CH3 species with C2H2 being the most stable hydrocarbon at elevated gas temperatures.25 The concentration of •CH3 reaches a maximum at approximately 1620 °C, which is the optimized filament temperature used in the modified HFCVD experiments. The concentration of •CH3 increases almost linearly with the concentration of methane in the input gas mixture. Since the concentration of •H present is much higher compared to that of •CH it is understandable that the stable C H adduct is formed 3 60 18 readily and subsequently -H adducts are replaced by -CH3 groups so that 18 ligands are always addened to the fullerene cage.
10356 J. Phys. Chem. B, Vol. 111, No. 35, 2007 From the modeling results, a possible mechanism for the production of methylated fullerenes can be proposed. When a methane/hydrogen precursor gas mixture is introduced into the CVD system first H atoms are formed at the filament surface, then H atom abstraction from the CH4 results in the formation of •CH3. This is the most abundant carbon containing radical species and this is confirmed by experimental measurements in diamond CVD systems.25 From MALDI spectral data it seems that C60H18 is formed rapidly under the high-temperature CVD conditions. Subsequently, the C60H18 is able to react further with •CH to form the methylated product. The resulting derivatized 3 reaction products migrate away from the heat source due to their higher volatility. Conclusions Analysis of HFCVD treated [60]fullerene samples with use of CH4/H2 precursor gas mixtures indicates a H/CH3 addition pattern to the [60]fullerene with a maximum of 18 ligands possible to the [60]fullerene cage. MALDI mass spectrometry was a vital characterization technique which confirmed that the lowest-mass component is C60H18 and is accompanied by up to seven replacements of a hydrogen atom by a CH3 moiety leading to a series of C60H18-2n(H,CH3)n adducts. There is an increase in the number of methyl groups addended to the fullerene cage as the concentration of methane in the precursor gas mixture increases (up to 15 vol %). For methane concentrations above 15 vol % the efficiency of the derivatization process rapidly diminishes as amorphous carbon deposits on the surface of the W filament causing a decrease in the filament temperature and, therefore, a reduction in the efficiency of radical formation. Collision-induced dissociation experiments confirmed a maximum of 18 ligands addended to the [60]fullerene cage. In addition, the loss of these exohedral CHn groups from the derivatized fullerenes is more facile than alkyl loss from [60]fullerene cage rupture or the loss of exohedral hydrogen. 1H NMR indicated the presence of C C H together with 3V 60 18 other byproducts. X-ray diffraction confirmed the presence of unreacted [60]fullerene along with amorphous carbon of the material collected at the center of the substrate holder subsequent to HFCVD. Acknowledgment. We would also like to thank Paul Warren, Norman Jenkinson, and William Ellison for technical support. The authors are extremely grateful to Dr. Matthias Glu¨ckmann and Dr. Dietmar Waidelich from Applied Biosystems Darmstadt for the performance of the high-resolution MALDI experiments on their 4700 Proteomics Analyzer.
Vieira et al. References and Notes (1) Kra¨tschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (2) Haddon, R. C.; Brus, L. E.; Raghavachari, K. Chem. Phys. Lett. 1986, 131, 165. (3) Bausch, J. W.; Prakash, G. K. S.; Olah, G. A.; Tse, D. S.; Lorents, D. C.; Bae, Y. K.; Malhotra, R. J. Am. Chem. Soc. 1991, 113, 3205. (4) Tebbe, F. N.; Harlow, R. L.; Chase, D. B.; Firment, L. E.; Holler, E. R.; Malone, B. S.; Krusic, P. J.; Wasserman, E. J. Am. Chem. Soc. 1991, 113, 9900. (5) Krusic, P. J.; Wasserman, E.; Keizer, P. N.; Morton, J. M.; Preston, K. F. Science 1991, 254, 1183. (6) Friedman, S. H.; Ganapati, P. S.; Rubin, Y.; Kenyon, G. L. J. Med. Chem. 1998, 41, 2424. (7) Da Ros, T.; Prato, M.; Novello, F.; Maggini, M.; Banfi, E. J. Org. Chem. 1996, 61, 9070. (8) Bisaglia, M.; Natalini, B.; Pellicciari, R.; Straface, E.; Malorni, W.; Monti, D.; Franceschi, C.; Schettini, G. J. Neurochem. 2000, 74, 1197. (9) Rogner, I.; Birkett, P. R.; Campbell, E. B. Int. J. Mass Ion Proc. 1996, 156, 103. (10) Vieira, S. M. C.; Birkett, P. R.; Rego, C. A. Chem. Phys. Lett. 2001, 347, 355. (11) Bonnot, A. M.; Deldem, M.; Beaugnon, E.; Fournier, T.; Schouler, M. C.; Mermoux, M. Diamond Relat. Mater. 1999, 8, 631. (12) Spatenka, P.; Pacal, F.; Taschner, Ch.; Leonhardt, A.; Blazek, J. J. Phys. D: Appl. Phys. 2004, 37, 2709. (13) Gan, B.; Ahn, J.; Zhang, Q.; Yoon, S. F.; Rusli Huang, Q. F.; et al. Diamond Relat. Mater. 2000, 9, 897. (14) Darwish, A. D.; Avent, A. G.; Taylor, R.; Walton, D. R. M. J. Chem. Soc., Perkin Trans. 2 1996, 2051. (15) Loufty, R. O.; Wexler, E. M. Feasibility of Fullerene Hydride as a High Capacity Hydrogen Storage Material; NREL/CP-570-30535; 2001. (16) Darwish, A. D.; Sada, A. K. A.; Langley, G. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Synth. Met. 1996, 77, 303. (17) Jin, C.; Hettich, R.; Compton, R.; Joyce, D.; Blencoe, J.; Burch, T. J. Phys. Chem. 1994, 98, 4215. (18) Harries, S. J.; Weiner, A. M.; Perry, T. A. Appl. Phys. Lett. 1998, 53, 1605. (19) Flemming, R. M.; Siegrist, T.; March, P. M.; Hessen, B.; Kortan, A. R.; Murphy, D. W.; Haddon, R. C.; Tycko, R.; Dabbagh, G.; Mujsce, A. M.; Kaplan, M. L.; Zahurak, S. M. Mater. Res. Soc. Sym. Proc. 1991, 206, 691. (20) Brown, T.; Clipston, N. L.; Simjee, N.; Luftmann, H.; Hungerbu¨hler, H.; Drewello, T. Int. J. Mass Spectrom. 2001, 210, 249. (21) Mo¨der, M.; Nu¨chter, M.; Ondruschko, B.; Czira, G.; Ve´key, K.; Barrow, M. P.; Drewello, T. Int. J. Mass Spectrom. 2000, 195/196, 599. (22) Al-Matar, H.; Sada, A. K. A.; Avent, A. G.; Fowler, P. W.; Hitchcock, P. B.; Rogers, K. M.; Taylor, R. J. Chem. Soc., Perkin Trans. 2002, 2, 53. (23) Lutz, A. E.; Kee, R. J.; Miller, J. A. SENKIN, A Fortran Program for Predicting Homogeneous Gas Phase Chemical Kinetics with Sensitivity Analysis; Sandia National Laboratories, SAND87-8248; 1987. (24) Miller, J. A.; Smooke, M. D.; Green, R. M.; Kee, R. J. Combust. Sci. Technol. 1983, 34, 149. (25) Morell, G.; Canales, E.; Weiner, B. R. Diamond Relat. Mater. 1999, 8, 166.
