J. Phys. Chem. 1990, 94. 2057-2062 by the adsorbed C2H2. The Y(=) modes of two *-bonded C2H2 species and of silver acetylide are observed at 1950, 1912, and 18 16 cm-I, respectively, and agree well with those published previously.2 The v(C-H) stretching vibrations of these species are observed at 3206 and 3 17 1 cm-I. These were previously not resolved. The perturbation of the modes due to adsorbed water on C2H2 adsorption results from the release of H + on silver acetylide formation. In addition to the formation of new hydroxyl groups at 3655 and 3584 cm-', the loss of the features due to adsorbed water (I648 and 3590-3000 cm-]) indicates a strong possibility that H30+is formed. A weak band observed ca. 2500 cm-I, which was also seen in the previous study,2 is assigned to the oxonium ion (H30+.H20). Corma et a1.18 have reported a band at 2510 cm-I for this ion in type Y zeolites. After C2H2 was in contact with Ag-A zeolite for 16 h, the v(OH), v(CH), and v(C=C) bands disappeared and two new bands were observed at 1719 and 1684 cm-I. These are assigned to v(C0) of ethanal hydrogen bonding to the framework O H groups and adsorbed on the Ag+ cations, re~pectively.~'The deformation and bending modes of the CH, and C-H groups are observed in the 1500-1 300-cm-' region. These assignments were confirmed by the adsorption on ethanal on Ag-A, which is described in detail elsewhere." After 16 h contact, difference spectra clearly indicate the reaction of both the *-bonded C2H2 and acetylide complexes. However, the present data do not provide sufficient evidence that both species are active in the formation of ethanal. It should be note that in our previous study a band observed at 1380 cm-l remained unassigned. In view of our assignment of a band in the same region for ZnNa-A C2H2to a mode of the zinc acetylide complex, a similar assignment to silver acetylide seems reasonable.
+
Conclusions The formation of both reacting and stable ethyne species has (18) Corma, A.; Agudo, A. L.; Fornes, V. J . Chem. Soc., Chem. Commun. 1983, 42.
2057
been observed when C2H2 (C2D2) is adsorbed onto ZnNa-A zeolite. A stable species, characterized by bands at 3238 (2403), r-bonding 1955 (1731), and 763 cm-', has been identified as C2H2 to the Na+ cations through a side-on interaction. A second adsorbed species, characterized by bands at 3203 (2378), 1943 (1731), 1366, and 750 cm-l, was found to react when left in contact with the zeolite. This complex has been identified as zinc acetylide (Zn-C=C-H). The zinc acetylide was observed to react with zeolitic water to form ethanal, characterized by its adsorption bands at 1685, 1414, and 1355 cm-l. In the case of C2D2,the release of D+ into the zeolite on acetylide formation resulted in the very rapid exchange of zeolitic O H and HzO, leading to a randomly deuterated product. The deuteration of the zeolitic water and OH groups was observed to be very much more efficient than that obtained by exchange with DzO. Ethyne adsorbed on low-exchanged ZnNa-A was found to interact with only the Na+ cations. It was concluded that the Zn2+ cations were located in the sodalite cages where they are unable to form adsorption complexes with ethyne. No hydration of C2H2 to ethanal was observed. The adsorption of ethyne on Ag-A zeolite has been reinvestigated. The reaction of the adsorbed ethyne with zeolitic water to form ethanal was observed. Although both the acetylide and *-bonded C2H2were found to react, we envisage that the r-bonded species convert to the silver acetylide before hydration. The hydration of ethyne to ethanal in the presence of zeolitic water has been found to occur via acetylide formation. N o evidence for acetylide or ethanal formation was observed on lowexchanged ZnNa-A or Na-A zeolites, where only Na+ ions interacted with the adsorbed ethyne. The formation of acetylide complexes thus seems to occur only in the presence of transition-metal cations and is the important step in the hydration of ethyne to ethanal. Registry No. HC=CH, 74-86-2; CH,CHO, 75-07-0.
Production and Fragmentation of Tantalum Carbide Cluster Ions Stephen W. McElvany* and Carolyn J. Cassadyt Code 61 1 IIChemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000 (Received: July 20, 1989)
The production and fragmentation of tantalum carbide cluster ions, Ta,Cy+ (x = 1-1 1, y = 1-26), have been studied by Fourier transform mass spectrometry. The cluster ions were generated by direct laser vaporization of a variety of metaland carbon-containingsamples, and the results were compared with previous studies that used Knudsen effusion mass spectrometry for the production of metal carbides. A study using isotopically labeled precursors was performed to determine the origin of the carbon species present in the Ta,CY+ions. All results are consistent with a mechanism involving gas-phase recombination reactions in the laser-generated plasma for the production of the Ta,Cy+ ions. The low-energy collision-induced dissociation reactions are dominated by loss of neutral C, and Cl0. The TaCy+ions can be considered to be an ionic metal center attached to a carbon cluster having either a linear or cyclic structure. Bridging metal-carbido ligands are suggested to be important for the larger Ta,Cy+ ions.
I. Introduction Transition-metalcarbides are characterized by their high melting pints, great hardness, and metallic conductivity. These unique properties have led to numerous studies on the electronic structure and type of bonding present in the bulk carbides.'-, The majority of gas-phase metal carbide (M,C,) studies have used ' N R C / N R L Postdoctoral Research Associate. Current address: Department of Chemistry, Miami University, Oxford, OH 45056.
Knudsen effusion mass spectrometry to monitor the species in equilibrium in high-temperature ovens containing the metal of interest and This technique has been used to study (1) Storms, E. K. In Refractory Materials; Margrave, J. L., Ed.; Academic Press: New York, 1967; Vol. 2. (2) Silberbach, H.; Merz, H. Z . Phys. B: Condens. Matter 1985,59, 143. (3) Gesheva, K.; Vlakhov, E. Mater. Lett. 1987, 5 , 276. (4) Gingerich, K. A. In Current Topics in Materials Science; Kaldis, E., Ed.; North-Holland: Amsterdam, 1980; Vol. 6, p 347.
This article not subject to U S . Copyright. Published 1990 by the American Chemical Society
2058
T h e Journal of Physical Chemistry, Vol. 94, No. 5, I990
more than 15 different metal and mixed-meta19J0carbide systems and has provided both trends for carbide production and limited information on the structures of these species. Typically, the gas-phase metal carbides, MC,,, produced by this technique contain 1-4 carbon atoms. The MC2 and MC, species exhibit enhanced stability which has been attributed to the pseudo-oxygen character of the C2 ligand.6 The early transition metals tend to form larger carbide species, i.e., MC with y 2 4. The structures of these molecules have been infYerred from assumed models and the consistency of the thermodynamic results obtained from equilibrium measurements as a function of temperature. The most recent studies of larger atomic metal carbides (YC,' and CeC,,;" y I8 and 6, respectively) suggest that these species consist of a linear (or possibly bent) carbon chain with the metal attached at one end. The one exception is MC, which is believed to have the dicarbide structure, C2-M-C2. Cyclic carbon structures, however, have not been considered for the metal carbide species. Secondary ion mass spectrometry (SIMS) has also been used to study the distribution of ionic carbides ( M,C,,+) produced directly from bulk metal carbidesi2which are similar to the distributions observed in the oven experiments. I n this study, metal carbide cluster ions (M,C,+) produced by direct laser vaporization (DLV) from solid samples were investigated by Fourier, transform ion cyclotron resonance mass spectrometry (FTMS). This is an extension of our previous work on carbon cluster ions produced by DLV of graphite.I3-l6 Preliminary studies" on the DLV of samples containing tungsten and carbon showed that W,Cy+ species are formed and were attributed to reactions occurring in the laser-generated plasma. Similar results have been obtained by DLV of W03/carbon samples.'* Tantalum was chosen for this detailed investigation because it is both monoisotopic, which facilitates M S / M S studies, and relatively high in mass ( m / z 181). which makes the TaC,+ species higher in mass than the abundant low-mass carbon cluster ions (C,"; n < 1513) that are also produced by DLV and may interfere in the study of the metal carbides. The goal of this initial study is to examine the formation mechanisms and fragmentation patterns of tantalum carbide cluster ions. Ta,C,+. Various tantalum, carbon, and tantalum carbide samples, including isotopically labeled compounds, were studied to obtain information on the formation mechanisms of Ta,Cr+ species in the laser-generated plasma. The MS/MS capabilities of FTMS19 were used to perform low-energy collision-induced dissociation (CID) experiments in an attempt to obtain thermodynamic and structural information on the metal carbide ions. Future studies will include ion/molecule reactions of the Ta,$,+ species. ~~~
~
(5) Pelino, M.; Haque, R.; Bencivenni, L.; Gingerich, K. A. J . Chem. Phys. 1988, 88, 6534. (6) Chupka, W . A.; Berkowitz, J.; Giese, C. F.; Inghram, M. G. J . Phys. Chem. 1958, 62, 61 I . (7) Kohl, F. J.; Stearns, C. A. High Temp. Sci. 1974, 6, 284. (8) DeMaria, G.; Guido, M.; Malaspina, L.; Pesce, B. J . Chem. Phys. 1965, 43, 4449. (9) Pelino, M.; Gingerich, K. A,; Haque, R.; Bencivenni, L. J . Phys. Chem. 1986, 90, 4358. (IO) Pelino, M.; Gingerich, K. A,; Haque, R.; Kingcade, J. E. J . Phys. Chem. 1985, 89, 4257. (11) Kingcade, J. E.; Cocke, D. L.; Gingerich, K. A. High Temp. Sei. 1983, 16, 89. (12) Leleyter, M.; Joyes, P. Surf. Sei. 1985, 156, 800. ( I 3) McElvany, S. W.: Creasy, W. R.; O'Keefe, A . J . Chem. Phys. 1986, 85, 632. (14) McElvany, S. W.; Dunlap, B. I.; OKeefe, A. J . Chem. Phys. 1987, 86, 715. (15) McElvany, S. W. J . Chem. Phys. 1988, 89, 2063. (1 6) Parent, D. C.; McElvany, S. W. J . A m . Chem. SOC.1989, 111 , 2393. (17) McElvany, S. W.: Ross, M. M.; Baronavski, A. P. Anal. Insrrum. ( N . Y . ) 1988, 17, 23. (18) Dietze, H. J.; Becker, S. Int. J . Mass Spectrom. Ion Processes 1988, 82, 47. (19) A recent review of the FTMS technique may be found in: Fourier Transform Mass Spectrometry; Evolution, Innovation and Applications; Buchanan, M. V., Ed.; ACS Symposium Series 359; American Chemical Society: Washington, DC, 1987.
McElvany and Cassady
TaC,,+ D T
I
I
r=2
i
I: 4
3
I
6
.+
1
1
-L 153
260
250 MASS
IN
4 M U
34c
4d3
Figure 1. Direct laser vaporization mass spectrum of a tantalum powder/graphite pellet (Ta/C(g)) producing TaC,+ ions o/ values listed above the peaks).
11. Experimental Section
Experiments were performed with a Fourier transform ion cyclotron resonance mass spectrometer (FTMS) which has been described in detail previously.16 The details of FTMS and ion cyclotron resonance have also been described e l ~ e w h e r e . ~ ~ , ~ ~ ~ * Briefly, the system uses both a Nicolet 3-T superconducting magnet and FTMS/1000 data system. The cubic trapped ion cell has been replaced with a 1-in. X 2-in. (z-axis) rectangular cell to provide a larger ion trapping volume for the study of ions in low abundance. A pressed pellet sample (0.5-in. diameter) was placed on a solid probe and positioned flush with one of the trapping plates (90% transparent nickel mesh) of the cell located Torr. in the vacuum chamber which is maintained at 1 5 X The frequency-doubled output of a Quanta-Ray DCR-2 Nd:YAG laser (532 nm, 1-5 mJ/pulse, operated at 10 Hz) was focused by a 1-m lens so that it traversed the cell and vaporized the solid sample forming both ionic and neutral species. The ionic species formed in the laser-generated plasma were trapped in the ICR cell (typically 2.0 V trapping potential) for further study. Collision-induced dissociation (CID) experiments were performed by isolation of a particular ion ( m / z ) using swept resonant frequency ejection techniques to remove all other ions from the cell. The parent ion of interest was then accelerated with a fixed-frequency pulse (at its cyclotron frequency) of known amplitude and duration, from which a maximum translational energy of the ion may be calculated. The maximum collision energies that may be attained in the present system are limited by the size of the cell and the magnetic field strength. Typical collision energies employed were 0-100 eV (lab). The ions were then allowed to undergo collisions with the neutral target gas (Ar or Xe) at static pressures of 1.0 X lo-' to 1.0 X 10" Torr. All ions in the cell, including daughter ions which were formed by fragmentation, were then detected by using conventional techniques. The tantalum/carbon samples were prepared from tantalum powder (325 mesh, Johnson Matthey) with either graphite powder (325 mesh, Alfa Inorganics) or amorphous I3C (99.0%, Isotec) in approximately a 1:4 (Ta/C) molar ratio. The tantalum carbide, TaC (99%, Johnson Matthey), was used as supplied. Pressed pellets of these samples were used with the exception of Ta/ amorphous carbon due to the inability to press amorphous carbon into a rigid pellet. The Ta/amorphous carbon mixture was placed in a stainless steel tube which had nickel mesh (0.001 in. wide, 90 lines/in.) spot-welded over the end. The powder was supported by the mesh and an aluminum rod inserted in the tube. This allowed the sample to be positioned vertically next to the FTMS (20) Comisarow, M. B.; Marshall, A. G. J . Chem. Phys. 1975, 62, 293. (21) Ledford, Jr., E. B.; Ghaderi, S.; Wilkins, C. L.; Gross, M. L. Ado. Mass Specrrom. 1980, B8, 1707. (22) Beauchamp. J. Annu. Reu. Phys. Chem. 1971, 22, 527.
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 2059
Production and Fragmentation of Ta,C,+
1-
0
Ta/Clal
1
Ta/Clg) A TaC
W V C
: 10-1c 3
n
Q: W
> 3 4-
10
10-2-
d
U W
1 10-3
2 3CO
41
500 600 MASS I N A M U
700
800
Figure 2. DLV mass spectrum of tantalum/amorphous carbon sample (Ta/C(a)) showing the larger Ta,Cy+ cluster ions.
4
6 B 10 y , No. o f Carbons
12
14
Figure 3. Semilog plot of relative abundance vs number of carbon atoms (JJ) on the monometal carbide ions, TaC,+, produced from various samples.
cell for laser vaporization. Vaporization of the nickel mesh did not affect the distribution of the Ta,Cy+ species. This problem has also been addressed recently by Morse and co-~orkers,2~ who used halogenated wax as a binder for the amorphous carbon and resonance-enhanced multiphoton ionization to selectively ionize only the carbon species. This technique, however, would probably not be amenable to laser ablation because of the nonselective ionization which occurs in the laser-generated plasma. 111. Results and Discussion A . Formation of Metal Carbides by DLV. A typical mass spectrum obtained by direct laser vaporization (DLV) of a tantalum powder/graphite (Ta/C(g)) sample is shown in Figure 1. The Ta+ ion (m/z 181) was ejected from the cell during ion formation by using resonant frequency ejection techniques to enhance the signal of the less abundant metal carbide cluster ions. The base peak in Figure 1 (TaC2+)is typically 5% of the Ta+ ion signal prior to ejection. Figure 1 shows that DLV produces monometal carbide ions (TaC +) with y = 1-14 in addition to ditantalum carbide ions (Ta2CA. Figure 2 shows the larger metal carbide cluster ion distribution (Ta,C,+ with x = 1-4, y = 1-16) produced from a tantalum/amorphous carbon (Ta/C(a)) sample. Larger metal carbide ions were observed (not shown) up to T a l l C +. The TaC,' distributions (normalized to TaC2+) from both t i e graphite and amorphous carbon containing samples are shown in Figure 3. The distributions are an average of several spectra (each consisting of several hundred laser pulses) obtained under various laser conditions. The only difference in the distributions produced by the graphite and amorphous carbon containing samples is that greater abundances (a factor of 10) of the larger carbides (Le., y > 5) are observed with amorphous carbon. This effect is not fully understood at this time but may be due to the greater vaporization efficiency of amorphous carbon compared to that of graphite. The enhanced abundances of TaC2+and TaC4+ in the observed distributions (Figure 3) are consistent with the many Knudsen effusion mass spectrometric studies in which early transition metals and graphite are present in the high-temperature oven.6,7,24*25 The enhanced stability of MC2 and MC4 species has been attributed to strong metal-dicarbide (M-C2 and C2-M-C2) bonds which result from the pseudo-oxygen character of the C2 radicaL6 In general, the M - 0 bond is stronger than the M-C, bond by approximately 1 e V (note that D(Ta+-0) = 8.20 eV26). For y > 4, there is a gradual decrease in the TaC,+ abundances with no anomalies in the distribution. There is a dramatic decrease in (23) Lemire, G. W.; Fu, Z.; Hamrick, Y. M.; Taylor, S.; Morse, M. D. J . Phys. Chem. 1989, 93, 231 3. (24) Kohl, F. J.; Stearns, C. A. J . Phys. Chem. 1970, 74, 2714. (25) Gupta, S. K.; Kingcade, Jr., J . E.; Gingerich, K. A. Adu. Mass Spectrom. 1980, 8, 445. (26) Thermodynamic values taken and derived from: Lias, S.G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J . L.; Levin, R.D.; Mallard, W. G. J . Phys. Chem. Re$ Data 1988, 17 (Suppl. 1 ) .
the abundance between y = 13 and 15 which limits the ions for further study toy I14. It should be noted that the low abundance of TaC15+(m/z 361) and Ta2+ (m/z 362) is due in part to ion excitation which occurs at one-half the frequency (Le., at m/z 362) of the ejection pulse to remove Ta+ from the ion trap during ion formation. Spectra obtained without this ejection pulse show that the abundance of TaCI5+is approximately 50% of TaCI4+. Also, spectra obtained at higher resolution than in Figure 2 show that TaC,+ ions are produced up t o y = 26. In general, the species produced by DLV of Ta/carbon samples are quite similar to those observed previously by using high-temperature ovens containing metal and graphite powders. The DLV technique has several apparent advantages including the formation of species with more carbon atoms (MCy+ with up to y = 14 compared t o y = 8 for M = Y5and La27and y = 6 for S C ,U,29 ~~ Th,30and Cell) and also larger metal-containing carbides (up to TallC,+ by DLV compared to dimetal carbides by high-temperature oven p r o d ~ c t i o n ~ I 3 ~DLV ~ ) . studies of other metals will show whether these differences occur only for the tantalum/carbon system. Also, DLV easily produces metal carbide species of much more refractory metals (boiling point of Ta is approximately 5400 OC)l than can be studied by high-temperature ovens which typically use tungsten or tantalum Knudsen cells with graphite liners, although TaC+ and TaC2+were observed from the tantalum oven used in the study of hafnium carbide^.^ The similar relative abundances of the smaller metal carbide species (MC,+; y < 6) produced by direct laser vaporization and in Knudsen cells may suggest that these species are formed by analogous mechanisms. Thus, DLV production of these species may be due to gas-phase reactions that occur in the laser-generated plasma since the bulk samples consist initially of pure tantalum and carbon. This type of mechanism has been postulated previously for the formation of larger carbon cluster ions (n > 50) by DLV.33-35 Since the sample is not rotated and many (10-100) laser pulses are required to generate the mass spectra, it is possible that the laser vaporization process chemically alters the outer layers of the bulk, e.g., to form bulk metal carbide species, which are subsequently desorbed and ionized by the next laser pulse. Under DLV conditions similar to those used to generate the distributions ~~~
(27) Gingerich, K. A.; Pelino, M.; Haque, R.High Temp. Sci. 1981, 14, 137. (28) Haque, R.;Gingerich, K. A. J . Chem. Phys. 1981, 74, 6407. (29) Gupta, S.; Gingerich, K. A. J . Chem. Phys. 1979, 71, 3072. (30) Gupta, S.; Gingerich, K. A. J . Chem. Phys. 1980, 72, 2795. (31) Pelino, M.; Gingerich, K. A.; Nappi, B.; Haque, R.J . Chem. Phys. 1984, 80,4478. (32) Kingcade, Jr., J. E.; Cocke, D. L.; Gingerich, K. A. Inorg. Chem. 1984, 23, 1334. (33) OKeefe, A.; Ross,M. M.; Baronavski, A. P. Chem. Phys. Len. 1986, 130, 17. (34) McElvany, S. W.; Nelson, H. H.; Baronavski, A. P.; Watson, C. H.; Eyler, J. R. Chem. Phys. Lett. 1987, 134, 214. (35) Creasy, W. R.;Brenna, J. T. Chem. Phys 1988, 126, 453.
2060 The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
McElvany and Cassady
y.2
I
TaO-
1:l Molar 12C:13C .Tal3Cy
+
~
1
OTa12C'3Cy-1t
j 1
T%C,'
i I 1
_-
-_
i-
Figure 4. DLV mass spectrum of bulk tantalum carbide (TaC) under laser conditions similar to those used to produce the spectra in Figures 1 and 2.
220
L.
243
262 285 MASS I N i M
IC:
22;
1 : l Ta'2C/'3C(amorphous) Pellet P)
0
5
0.6
2 .-
U 5 c m
-
1
I
I
I
0
4
8
12
y,
4
E?Z Statistical
I Experimental
I -
J
1
0.4
-1 I
-
Ij 5,O
N o . o f Carbons
Figure 5. Ta3C,+ distribution produced from Ta/carbon and bulk TaC samples.
in Figures 1 and 2, a bulk tantalum carbide (TaC) sample yields the mass spectrum shown in Figure 4. Note that the base peak in the spectrum is not TaC2+,as in Figures 1 and 2, but is TaO+ ( m / z 197). The TaC,+ distribution from TaC is also plotted in Figure 3 for comparison. The TaC spectrum contains only small 0, < 5), low-abundance TaC,+ ions with many 0 and H impurities incorporated in the observed ions. Larger metal carbide ions, Ta,C,+ with x L 2, are produced from bulk TaC but contain fewer carbon atoms than those produced by D L V of Ta/carbon (graphite or amorphous carbon) samples. This is illustrated in the Ta3Cy+ distributions from TaC and Ta/carbon targets (Figure 5). The above results show that TaC (bulk) is not the primary source of the metal carbide ions from the Ta/carbon samples of Figures 1 and 2. The Ta,C,+ ions are significantly enhanced when carbon species, either C, or C,,+ from graphite or amorphous carbon, are also present in the laser-generated plasma. Two possible mechanisms for Ta,C,+ production are the following:
+ Tax Tax+ + C, C,+
-
-
I
Figure 6. Mass spectrum produced by DLV of a sample containing equimolar TaI2C and amorphous I3C. It is evident that the carbon on TaCy+originates primarily from the amorphous carbon in the sample and not from the bulk TaC. Ta'2C,'3C,+ ( x t y = 5 ) Distribution
1 n-2 _.
L'
Ta,C,+
+ C,
(1)
Ta,Cy+
+ C,,
(2)
Subsequent reactions of Ta,Cy+ with neutral carbon species would result in the formation of larger carbide ions. If reaction 1 occurred exclusively, the TaC,+ distribution might be expected to show some resemblance to the nascent C,' distribution generated by D L V of carbon which exhibits an alternation of An = 4 with maxima at n = 7, 11, 15, .... A pattern of this type (Le., An = 4) is not observed in the TaC,+ distribution (Figures 1-3) and suggests that reaction 1 is not a major reaction channel in the plasma reactions although it cannot be ruled out entirely. A i2C/f3C-labeledsample was examined to gain further insight on the origin of the carbon species necessary for the formation
4,1
32
23
1.4
0,5
isotopic Composition (x,y)
Figure 7. Comparison of the statistical isotopic distribution of TaC5+ (expected assuming equal probability of incorporation of I2C and I3C) and the experimentally observed distribution illustrating the nonstatistical 12C/13Cratio of the TaCy+cluster ions. See text for complete explana-
tion. of Ta,C,+ ions. Similar experiments using the laser microprobe mass analysis (LAMMA) technique have shown that recombination reactions during laser ablation are significant for mixed A ~ / C UNi,S,,,37 , ~ ~ and small carbon cluster ions.38 The sample for the present experiment contained both TaC (with natural abundance 98.9% I2C) and amorphous I3C (99.0%) in a 1:l I2C/I3Cmolar ratio. A typical D L V mass spectrum showing the TaC,' ions produced from the Tai2C/I3C(a)sample is shown in Figure 6. The most abundant monometal carbide ions observed in Figure 6 correspond to TaI3C,+ with no incorporation of the I2C produced from TaC. The ions to the left of Tai3Cy+are due to substitution of I2C for 13C,Le., Ta'2Ci3C,1+, Tai2C2i3C,2+, etc. (The ions 1 amu higher than Ta13C,+ are due to the carbon cluster ion i3C,+!4+.) The nonstatistical isotopic ratio is illustrated in Figure 7, which shows both the experimental l2C/I3C ratios for TaC5+and the statistical distribution assuming equal probability for both production and incorporation of '*C (from TaC) and I3C (from C(a)). The extent of carbon scrambling is relatively constant as the number of carbon atoms increases; e.g., for TaC4+ to T a C l i +the 13Cy:i2Ci3C,l:1zC213C,2ratio is 0.78:0.17:0.05 ( u = 0.03). The same behavior is observed in the TazC,+ and Ta3C,,+ (36) Wurster, R.; Haas, U.; Wieser, P. Fresenius' Z . Anal. Chem. 1981, 308,206. (37) Musselman, I . H.; Linton, R. W.; Simons, D. S. Anal. Chem. 1988, 60,1 IO. ( 3 8 ) Bruynseels, F.; Van Grieken, R. In?. J . Mass Specirom. Ion Processes 1986, 74, 16 1.
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 2061
Production and Fragmentation of Ta,CyS
-
TABLE I: Low-Energy Collision-Induced Dissociation of TaC,': TaC,,+ + C, TaC,' neutral C, loss for z = parent ion 2 3 4 5 7 1 0 1 4 TaC2* TaC,' TaC,' TaCSt TaC6' TaC7' TaC8+ TaC9' TaClot TaCIIt TaCI2' TaCI,;
100
IO
IO0 15
75
15 100
25
100 100 15
85 30
80
70 20
55
45 50
50
65
TaC14
35
TABLE 11: Low-Energy Collision-Induced Dissociation of Ta2C,' fragmentation channels Parent ion Ta2Ct Ta2C2+ Ta2C,' Ta2C4+ Ta2CSt Ta2C6+ Ta2C7'
Tat
+ TaC,
Ta,C,?'
+ Cq
100 100 100 100 100 100 100
ion distributions in which the Ta,13Cy+:Tax12C13C~I+ ratio is 0.75:0.25. The extent of carbon scrambling for the larger Ta,C,+ (x > 3) ions could not be quantitatively determined due to insufficient mass resolution but does not appear to increase significantly. The isotopic ratios of the Ta,Cy+ species did not change considerably at long irradiation times (several thousand laser pulses) indicating that 12C/13Cscrambling does not occur to a significant extent in the bulk prior to vaporization. This is also consistent with the pure TaC results in that the Ta,Cy+ ions (y >> x) do not arise from bulk (or condensed-phase) tantalum carbide species. The small extent of I2C incorporation in the metal carbide ions is consistent with the mechanism of recombination reactions that occur in the laser-generated plasma. Incorporation of one 12C suggests that TaC species vaporized from the bulk metal carbide may be involved in the recombination reactions. DLV of TaC probably results predominantly in the production of carbon atoms which do not efficiently form Ta,Cy+ ions. A recent matrix isolation study has shown that predominantly carbon atoms are formed by laser vaporization (CW C 0 2 laser) of TaC.39 In contrast, carbon vapor over heated graphite consists primarily of C340941but extends beyond C1042over graphite irradiated by a pulsed ruby laser. Therefore, larger carbon species (C, or C,+) that are produced from either amorphous carbon or graphite are necessary for larger Ta,Cy+ formation, possibly through reactions 1 and 2. B. Fragmentation of Ta,C +. The fragmentation channels observed in the low-energy codsion-induced dissociation (CID) = 2-14) and Ta2Cy+(y = 1-7) are listed in Tables of TaC,' (j I and 11, respectively. The fragmentations observed are identical with either Ar or Xe as the collision gas and are the same for Ta,Cy+ formed from graphite or amorphous carbon containing samples. The branching ratios in Tables I and I1 correspond to near-threshold fragmentations (Le., at collision energies sufficient to fragment 5-10% of the parent ions and in an energy range where the branching ratios do not vary due to higher energy processes). At greater ion translational energies, higher energy (39) Ortman, B. J.; Hauge, R. H.; Margrave, J. L. J . Quant. Spectrosc. Radiar. Transfer 1988, 40, 439. (40) Honig, R. E. J . Chem. Phys. 1954, 22, 126. (41) Drowart, J.; Burns, R. P.; DeMaria, G.; Inghram, M. G. J . Chem. Phys. 1959, 31, 1131. (42) Berkowitz, J.; Chupka, W. A. J . Chem. Phys. 1964, 40, 2735.
fragmentations and/or sequential multiple-collision dissociations are observed but will not be discussed. No fragment ions were observed for Ta,Cy+ (x L 3) with Xe at energies up to approximately 80 eV (lab). The TaC,' and Ta2Cy+ions also required relatively high translational energies (>60-70 eV (lab)) before fragment ions were observed, suggesting that these Ta,C,+ species are very strongly bound. The fragmentations of TaCy+ (Table I) all result in the loss of a neutral C, species. This is consistent with the large difference in ionization potentials of Ta (7.40 eV26)and small carbon clusters, e.g., C2-C9 (12.1 to 9.4 eV4,), indicating that the charge is localized on the metal of the TaC,+ species. One might expect the CID fragmentations to provide information on the number and types of C, ligands on the metal ion; e.g., M+-C, would lose C, whereas C,-M+-Cb would lose either C, or Cb. This does not appear to be the case, however, for the TaC,+ ions in Table I. Only small TaC,+ 0,= 2-5) lose the entire carbon ligand to form Ta+, and this channel decreases with increasing number of carbons. The predominant fragmentation in low-energy CID of TaC is loss of neutral C3. This C, loss has also been p r e d i ~ t e d l ~ l ~ ~ ~ a n d observed experimentally in CIDI4 and p h o t o d i s s o c i a t i ~ nof~ ~both ~~ positive and negative bare carbon cluster ions, C,+ and C; ( n < 20). The TaC,+ fragmentations should exhibit characteristics of neutral Cy, however, since the charge is localized on the metal. Ab initio calculations also show C3 loss to be the favored loss for small neutral C, species.43 The propensity for larger TaCy+ to lose C3 may suggest that C, species are ligated to the ionic metal center, although this seems unlikely. It is more probable that a linear M+-C structure loses C3 (the stable loss for neutral Cy) rather than greaking the M+-C, bond. The latter process is supported in the comparison of the calculated fragmentation energies of neutral c6-ClO to lose C3 (3.5-6.0 eV4,) to the M-C2 bond which is believed to be approximately 1 eV less than the M-0 bond (cf. D(Ta-0) = 8.7 eV;26D(Ta+-0) = 8.2 eV26). Although no values are available for neutral or ionic Ta/carbon species, ionization of the metal atom seems to strengthen the M+-C, bond for the other group V metals, e.g., D(Vt-C2) = 6.07 eV46 > D(V-C,) = 5.93 eV4' and D(Nb+-C) > 5.93 eV4* > D(Nb-C) = 5.85 eV.47 Other stable neutral losses are observed for the larger TaC,+ ions in Table I. For example, neutral C5 loss occurs for TaC,+ and TaClo+. This has been observed previously in the photod i s s ~ c i a t i o nand ~ ~ unimolecular decompo~ition~~ of Cn+,although this channel does not become significant until n I 15. This transition in fragmentation of neutral C, for smaller n has been predicted for Clo in which C3 and Cs loss are energetically eq~ivalent.~~ The most interesting transition in the fragmentations of TaC,+ occurs at y = 10 in which neutral Cloloss becomes a major channel for dissociation. Loss of Cs is also observed for TaClo+, but variation of both collision energies and pressure indicates that C,, loss does not result from sequential loss of two Cs units. This Cl0 loss occurs for all TaCyt withy I 10 with the exception of TaCII+. This may be due to the formation of the relatively unstable TaC+ ion (see Figure 3) following C l oloss. T a C l l + is also unusual in that it loses neutral C7, but this results in the formation of the stable TaC,+ ion. Neutral C,, loss is particularly interesting because there is a great deal of experimental evidence that the transition between linear and cyclic structures of both C, and C,+ occurs between n = 9 and 10.14-16344.50 Thus, the transition to favor loss of Clo for TaC,+ with y I 10 may be due to the loss of neutral cyclic +
(43) Raghavachari, K.; Binkley, J. S. J . Chem. Phys. 1987, 87, 2191.
(44) Geusic, M. E.; Jarrold, M. F.; Mcllrath, T. J.; Freeman, R. R.; Brown, W. L. J . Chem. Phys. 1987,86, 3862. (45) Deluca, M. .I.Johnson, ; M. A. Chem. Phys. L e f t . 1988, 152, 67. (46) Aristov, N.; Armentrout, P. B. J . A m . Chem. SOC.1984, 106,4065. (47) Gupta, S. K.; Gingerich, K. A. J . Chem. Phys. 1981, 74, 3584. (48) Hettich, R. L.; Freiser, B. S. J . A m . Chem. SOC.1987, 109, 3543. (49) Radi, P. P.; Bunn, T. L.; Kemper, P. R.; Molchan, M. E.; Bowers, M. T. J . Chem. Phys. 1988, 88, 2809. (50) Yang, S.; Taylor, K. J.; Craycraft, M. J.; Conceicao, J.; Pettiette, C. L.; Chesnovsky, 0.;Smalley, R. E. Chem. Phys. Leff. 1988, 144, 431.
2062 The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 C l ospecies. Once again, the loss of Clo is not observed for C,+ until n = 24?9 The loss of Clodoes not necessarily imply, however, that the TaC,+ ions have metallocene structures containing an intact cyclic C l oligand. Also, one might expect TaCIl+to TaC,,+ to lose C I 1to C,,, respectively, since these are also stable cyclic species, but only C l oloss is observed. It is interesting that TaC,,+ fragments by loss of C,, in addition to CI0. The C I 4neutral loss has been observed previously for larger C,+ (n 2 25) and is consistent with the predicted enhanced stability of neutral cyclic carbon species containing 4n 2 carbon atoms.51 Another intriguing possibility for these larger TaC,+ ions may involve metallacyclic structures
+ Tc
,/'
\,
I
I / '
