J. Phys. Chem. 1995,99, 7707-7714
7707
Structures of Carbon Clusters from Polychlorinated Graphitic Precursors: Investigations of C&l,+ (x = 0-10) Using the Ion Chromatography Method Gert von Helden, Edwart Porter,? Nigel G. Gotts,* and Michael T. Bowers* Department of Chemistry, University of Califomia, Santa Barbara, Califomia 93106 Received: September 2, 1994; In Final Form: January 5, 1995@
Various C12ClX+ions (x = 0-10) are formed from electron impact on decachloroacenaphthene, C12C110. The structures of these ions are determined using ion chromatography. It is found that the graphitic carbon backbone persists down to C12C14+. Small amounts of monocyclic structures appear for CI2Cl6+and C12CL++and are exclusive for C12+. The linear structure first appears at C12C14+ and is exclusive for C12C12+ and C12C1+. For the most part, these results are consistent with predictions from semiempirical PM3 calculations, indicating thermochemistry plays a major role in determining isomer distributions. In several cases, other factors have to be considered, the most important being isomerization barriers and the effect of entropy. Doubly charged ions are observed for a significant number of cases. Graphitic structures disappear below C12C162+and are replaced by monocyclic and linear isomers, again consistent with PM3 calculations.
Introduction Carbon clusters have been of interest for many years' because carbon is such a versatile, ubiquitous element and plays a central role in chemistry. The interest in these species has accelerated enormously in the past decade, however, primarily due to the discovery of the unique hollow cage compounds (the fullerenes2), especially CW,and the ability to produce bulk quantities of these specie^.^ Consequently, the properties of c60, c70, and several other fullerenes larger than C70 have been probed using traditional structural and reactive chemical methods. For carbon clusters smaller than (260, more exotic methods must be applied to determine their structural and reactive properties. In these instances, the clusters must be generated, isolated (if possible), and investigated in the rarefied atmosphere of the gas phase. This makes traditional structure analysis methods essentially impossible, except for the smallest sizes, where spectroscopic methods have been e m p l ~ y e d . ' ~It. ~also virtually requires that any systematic studies be done on ionic clusters, allowing the powerful separation and analysis methods of mass spectrometry to be utilized. Of particular interest here is the new method of ion chromatography5 that has allowed structural determination of carbon clusters over the range of C5-Cso for both positive6 and negative ions.' These studies indicate that carbon first forms linear species, around Clo planar monocyclic rings become dominant, and near C20 planar bicyclic rings appear with tricyclic and tetracyclic rings appearing near C30 and C40. The first hollow cage fullerene is observed at c30, and for positive ions, these species become dominant by For negative ions, the planar ring systems are more robust and fullerenes do not become important until c60. Curved graphitic networks of five- and six-membered rings were not observed at any cluster size. This absence is surprising since it was initially felt that these species were logical precursors for fullerene formation?%*with growth occumng by addition of small carbon fragments to the outer rim and closure to form fullerenes occumng at thermodynamically favored sizes.
* Author to whom correspondence
should be addressed. Present address: Department of Chemistry, University College Dublin, Dublin, Ireland. Resent address: Battelle Pacific Northwestem Laboratories, Richland, WA 99352. Abstract published in Advance ACS Abstracts, May 1, 1995. +
*
@
For very small clusters, theory indicates that these species are high in energy and not observing them is reasonable. However, for C20 and larger, theory predicts these cuplike structures to be quite table,^ certainly competitive with planar mono- and bicyclic rings. Thus, failure to observe them for n L 20 is a bit of a puzzle. One possible answer to this question is that the distribution of structures emitted from the laser desorption source is kinetically controlled. In this scenario, the graphitic cuplike structures do become stable near C20 but barriers to rearrangement from the monocyclic rings are higher than dissociation thresholds of the clusters. If this scenario is correct, then there may be alternative ways to make these species that bypass the kinetic bottlenecks. Such a process was found to explain the appearance of fullerenes at n 2 30 where no apparent precursors are present below C30'. In this instance, the large planar ring systems with n 35 were annealed by collisional heating with the result that massive rearrangement occurred and fullerenes were formed with high efficiency.1° Such a process does not yield graphitic networks of five- and six-membered rings at any size, however, so an alternate method to address this question is required. In this paper, we describe a novel approach for searching for the missing graphitic isomers. It is well-known that polycyclic aromatic hydrocarbons (PAHs) containing five- and sixmembered rings are stable at very small sizes. This occurs because hydrogen atoms tie up the dangling bonds of the pure carbon centers and stabilize these species. It would be interesting to sequentially remove the hydrogen atoms one by one and follow the structure of the molecule and see how long the PAH backbone remained. Unfortunately, the C-H bonds are so strong that the carbon skeleton fragments under electron impact before a significant number of H atoms are lost. The problem can be circumvented if perchlorinated polycyclic aromatic compounds (PPCs) are utilized, as recently demonstrated by Griitzmacher, Lifshitz, and co-workers.ll These authors were able to form relatively intense C,+ ion beams by electron impact on C,C& PPCs. In this paper, we will report our first results on decachloroacenaphthene, C12C110. Our strategy will be to follow structural changes in the carbon backbone as C1 atoms are sequentially lost under electron impact. Semiempirical and ab initio quantum chemical calculations will be used to provide
0022-365419512099-7707$09.00/0 0 1995 American Chemical Society
von Helden et al.
7708 J. Phys. Chem., Vol. 99, No. 19, 1995 structures for mobility modeling for comparison with experiment and to determine the relative energies of the various isomeric structures of each C,Cl, species (x = 0- 10) for neutral, singly, and doubly charged ions.
C12C18+
Experiment
The experimental setup is similar to that published prev i o ~ s l y . ~Briefly, -~ cluster ions are generated by electron impact (100 eV) on decachloroacenaphthene" vapor. After mass selection by a reverse-geometry, double-focusing mass spectrometer, a short pulse (2-5 p s ) of mass-selected ions is decelerated to a few electronvolts (2-10 eV) and injected into a high-pressure drift cell (the "chromatography cell") containing He gas at 2-5 Torr. The ion packet is quickly thermalized by collisions and drifts through the cell under the influence of a weak electric field. Field strengths are typically between 2 and 20 Vlcm, depending on the helium pressure. Ions exiting the cell are mass selected by a quadrupole mass filter, and their arrival time distribution (ATD) is collected on a multichannel scaler with 2-ps time resolution. The drift time of an ion depends on its geometric shape. In cases where different isomers with different mobilities are present, these isomers can be separated as different peaks in the ATD. When the mobilities of two or more isomers are very close to each other, the experimental resolution might not be sufficient to resolve different peaks. In these cases, the ATD can be fit with a theoretical model6b in order to deconvolute the ATD and to obtain accurate mobilities and relative isomeric abundances. It has been shown previously, for pure carbon clusters, that mobilities for theoretical structures can be calculated accurately and corresponding Monte Carlo generated mobilities can be compared to experiment.6b The algorithm for calculating mobilities has been described previously.6b This procedure has two adjustable parameters, the sum of the carbon-helium van der Waals (vdW) radii and the sum of the chlorine-helium vdW radii. The sum of the carbon-helium vdW radii is taken as 2.70 8, (1.09 8, for helium and 1.61 8, for carbon). This value has been used in previous publications on pure carbon clusters6b and reproduced experimental mobilities better than 2% for carbon clusters containing from 5 to 80 atoms. The chlorinehelium vdW radius is taken as 2.79 8, (1.09 8, for helium and 1.70 8, for chlorine12).
A
I \"\
c12c16+
f
c12c14+
!1 100
200
150
Drifl Time [PSI
Figure 1. Ion arrival time distribution (chromatogram) for three ionic systems. The points (+) represent the data. The dotted lines correspond to contributions of individual components. The solid lines are the convolution of the dotted lines. For C&ls+, there is only one component; for C12CI6+,there are two components; and for C12Ch+, there are three components.
Results (A) Nominal Assignment of Ion Chromatograms. Arrival time distributions were taken for all species with the nominal formula C12C1,+ for x = 0-10 except for x = 9 and x = 3 where intensities were very low. A sampling of these distributions for C12C18+, C12C16+, and C&14+ is given in Figure 1. These three chromatograms are typical and were chosen to exemplify the analysis done on the peak shapes to obtain isomer distributions. The data are given by the points. The dotted lines represent individual isomeric contributions, and the solid lines are the convolution of the dotted lines. The relative intensities and peak positions of the isomers are computer generated using a least-squares fitting procedure, and the line widths are obtained by solving the diffusion equations for the systems of interest. Details are given elsewhereFb What is clear from Figure 1 is that the ATD of C12Clg+ can be fit by a single structure, C12C16+ by two structures, and C12C4+ by three structures. This procedure cannot distinguish between isomers with very similar rotationally averaged shapes, and components with differences in mobilities of a few percent are difficult to quantify even if they are structurally dissimilar.
0
100
200
300
Drift Time [PSI Figure 2. Ion chromatograms for two values of mlz. For mlz = 214, there are two possible ions, 12C1235C12+ and '2c1235c1637c1For 22+ mlz . = 213, there is only one possibility: 12C1235C1737C112f. The vertical dotted lines are drawn through the peak centers to guide the eye.
A different kind of data analysis complication is exemplified in Figures 2 and 3. In the bottom panel of Figure 2, the chromatogram for mlz = 214 is given. This mass-to-charge ratio can be satisfied by either 12C1235C12+ singly charged ions or by 12c1235c1637c122+ doubly charged ions. However, mlz = 213 is uniquely satisfied only by the doubly charged ion
J. Phys. Chem., Vol. 99, No. 19, 1995 7709
C Clusters from Polychlorinated Graphitic Precursors
Cl2C110 Exp.: 5.04 Calc.: 4.95
0
50
100
150
200
Drift Time [ps]
Figure 3. Chromatograms for mlz = 144, 143, and 142. The ions
c12c18 Exp.: 5.36 Calc.: 5.39
CUC17 Exp.: 5.48 Calc.: 5.62
'BC16 Exp.: 5.82 Calc.: 5.83
c12c16 Exp.: 5.40 Calc.: 5.35
c12c16 Exp.: 5.40 Calc.: 5.52
cl2c15 Exp.: 6.09 Calc.: 6.07
Cd-34 Exp.: 6.52 Calc.: 6.60
cr2c14 Exp.: 6.03 Calc.: 6.07
Figure 4. Structures of various C12Cl,+ isomers at the semiempirical
that can contribute to these values are shown in the various panels. The vertical dotted lines are to guide the eye. See text for assignments.
(PM3) level. The carbon atoms are given by the darker crosshatched circles and the chlorine atoms by the larger lighter circles. The experimental mobilities (in cm2 V-' s-' ) are given below each figure along with a theoretical mobility calculated for the structures shown.
12C1235C1737C112+. The chromatogram in the upper panel shows only a single peak with exactly the same arrival time as the faster species at mlz = 214. Hence, the unambiguous identification shown in the figure is obtained, and there is only one isomer of each species present. The most complex set of chromatograms obtained in this study occur near the nominal mass of Cl2+. The chromatogram for mlz = 144 is given in the bottom panel of Figure 3 and contains at least four separate peaks. Species consistent with m/z = 144 are 12C12+,12c635c137c1+, and 12C1235C1237C122+. The largest peak in this panel comes at longest times and is quite broad, suggesting it can have several components in it. When mlz = 143 is selected, the chromatogram in the middle panel is obtained. This mass-to-charge ratio can contain 12C935C1+and 12C1235C1337C12+ species. The largest peak in the mlz = 144 chromatogram is now gone, and a small peak at slightly longer times has appeared. This new peak is almost certainly due to 12C935Cl+ since it only appears at mlz = 143. The three fastest peaks in the m/z = 143 panel are the same in both their arrival times and relative intensities as those in the mlz = 144 panel and thus must be due to three separate isomers of the doubly charged ion c12c142+.Finally, the top panel is for mlz = 142 and can contain only 12C635C12+ and 12c1235c1 species. 42+ The three fast peaks are still present, confirming the C12C14~+ion, and a new peak appears at longer times that must be due to 12C635C12+.The dotted lines are drawn through peak centers of all the resolved assigned ATDs. When this is done, it appears that a small amount of C6Cl2+ is present in the mlz = 144 chromatogram, causing broadening in the peak at the longest times, which can be assigned as primarily due to C12+. The individual contributions are easily deconvoluted using methods exemplified in Figure 1.
(B) Theoretical Structure Calculations. Structure of C12Clx (0 5 x 5 10) as well as some CgCl, and CsCl, clusters have been calculated using a semiemprical PM3 l3 Hamiltonian with the program GAMESS.I4 Restricted open-shell wave functions have been used in all open-shell calculations. All optimizations were done in Cartesian coordinates assuming no symmetry. All clusters with an even number of chlorine atoms were calculated as singlets and those with an odd number of chlorine atoms as doublets. In addition, all clusters were reoptimized as singly charged cations (doublets or singlets for an even or odd number of chlorine atoms, respectively) and for doubly charged ions (with the same multiplicities as the neutrals). Some structures and energies were also calculated using AM1 l5 with results very similar to the PM3 calculations. The lowest energy structures for the C12Clx species are given in Figure 4 and their energies in Table 1. Ab initio calculations have also been done for graphitic, monocyclic, and linear structures of (212. Geometries have been optimized in the Hartree-Fock limit (RHF for closed shells and ROHF for open shells) using a standard 6-31G(d) basis set. Monocyclic C12 has also been optimized at the MP2 level using the same basis set. Graphitic and linear structures could not be optimized at the correlated level since gradient optimization at the restricted open-shell MP2 (ROMP2) level was not included in the programs available to us. Heavy spin contamination made the use of an unrestricted reference wave function impractical. Single-point calculations were done at the CCSD(t) level with a RHF or ROHF reference function using the Dunning cc-PVDZ basis set.I6 The ab initio structures are shown in Figure 5 and their energies in Table 2. At all levels of theory, a monocyclic ring was found to be the lowest energy structure. At the HF level,
7710 J. Phys. Chem., Vol. 99, No; 19, 1995
von Helden et al. TABLE 2: Ab Initio Energies for Neutral C12O symHF/ MP2/ MP2/ CCSD(t)/ structure metry 6-31G(d) 6-31G(d) cc-PVDZ cc-PVDZ cyclic C6h -453.9021 +26.9 +33.7 +10.1 &hh +30.1 -455.4173 -455.4276 -455.5103 +49.2 D,h +58.1 linear +88.6 +85.7 graphitic C2
Graphitic
5
4
“Energies for the lowest energy structure in hartrees; all other energies relative to that one in kcaymol. hoptimized at the MP2/631G(d) level. All other structures are optimized at the Hartree-Fock level; see text.
3
Cyclic I
3
4
4
c6h
D6h Linear
Figure 5. Structures of C12 isomers calculated using ab initio methods.
The number is used to identify bond length and angles in Table 4. TABLE 1: Energies of C12Clx,C12CIx+, and C12CIx2+ Clusters at the PM3 Level PM3 heat of formation, kcaYmol ion structure neutral cation dications graphitic -2.2 196 514 graphitic 30.9 213 48 1 monocyclic 127 320 585 graphitic 88.9 564 302 bicyclic 127 584 322 monocyclic 186 612 354 graphitic 145 329 600 bicyclic 149 609 336 monocyclic 168 620 356 graphitic 205 684 42 1 bicyclic 415 687 210 monocyclic 435 249 674 linear 463 286 691 graphitic 259 446 723 bicyclic 468 267 755 monocyclic 236 426 688 linear 465 299 662 graphitic 432 599 903 monocyclic 511 348 767 linear 298 495 750 graphitic 475 686 985 monocyclic 576 385 853 linear 396 591 819 graphitic 534 750 1065 c 1 2 monocyclic 619 415 924 linear 666 497 901
the lowest energy ring structure was of C6h symmetry; while optimizing at the MP2 level, the structure changed point groups to a &h structure. The lowest energy linear structure has been found to have an 32gelectronic state. At the ROHF level, the bond length varied between 1.266 and 1.281 A. The graphitic structure was found to have C2 symmetry and a 3B ground electronic state. This structure was optimized at the ROHF level. The C2 unit attached to the Clo naphthalenic
framework to form a five-membered ring has a bond length of 1.26 8, that is close to a typical triple bond. It is attached to the naphthalenic framework by two single bonds (1.46 A). The molecule thus does not conjugate the C2 n electrons with the naphthalenic n system. Doing so would probably destroy the aromaticity of the naphthalenic unit and turn the molecule into a formally “antiaromatic” compound. The two unpaired electrons reside dominantly on the naphthalenic carbon atoms opposite the C2 unit. This result is consistent with the fact that six carbon atoms in the naphthalene unit (three on each side) have electrons in “in-plane n orbitals’’ but only four (two on each side) can couple to form a bond, leaving two unpaired electrons. (C) Structural Assignments of Peaks in the Chromatograms. It is straightforward to calculate an ion mobility given a calculated structure assuming a hard-sphere interaction potential between the ion and the He buffer gas.6b Theoretical mobilities of the structures shown in Figure 4 are given in units of cm2 V-’ s-’ in the figure. Also shown in Figure 4 are experimental mobilities determined from ATDs for those structures, where a close match between experiment and calculated mobilities occurred. Previous studies on pure carbon clusters6bindicate that experiment and theory should match to at least 2% in order to be confident in the assignment. Virtually all of the species in Figure 4 easily meet this criteria in spite of the fact that inclusion of C1 atoms in the structures requires us to include a value for the C1 van der Waals radius, a quantity that has not yet been accurately determined. We have found that structural families exhibit regular changes in mobility as their size increases or decreases. The most useful way to show these changes is in a plot of the reduced inverse mobility, KO-’, vs cluster size. In Figure 6, we show a plot of KO-‘ vs the number of chlorine atoms for C12ClX+,x = 0-10. The data points clearly separate into three distinct groups or families. The calculated KO-’ values from theoretical structures for each family as a function of the number of chlorine atoms are shown as the solid lines for graphitic, monocyclic ring, and linear structures. Clearly the calculated graphitic KO-’ values nicely fit the family plotted as filled squares, and the monocyclic rings fit that with filled circles. The family of triangles, which occurs at the longest times in an ATD and thus with the largest values of KO-’, is assigned as linear even though experimental values of KO-’ are systematically 7% lower than theoretical estimates. This kind of discrepancy has been noted earlier in pure carbon cluster anions for linear and is attributed to the fact that calculated mobilities correspond to rigid linear structures, while in reality these species are quite floppy and are bent in a time average, therefore leading to larger mobilities than expected for rigid linear species. Also shown in Figure 6 is a dotted line for calculated values of KO-’ for bicyclic species. The mobility of these species falls between those of the graphitic structures and the monocyclic rings. Further, at the PM3 level of theory, these structures are relatively low in energy for x = 4-6 C1 atoms attached to C12.
J. Phys. Chem., Vol. 99, No. 19, 1995 7711
C Clusters from Polychlorinated Graphitic Precursors
o'22
c Linear
0.20
100
I 80
0.18 Graphitic
F
.e
0.16
E 8
z
9)
Bicyclic
a
C 1
+ isomers vs number of C1 atoms. The assigned isomers are graphitic (a),monocyclic (0),and linear (A). A line is drawn through the graphitic isomers and through the remaining isomers to guide the eye.
observed, and it corresponds in each case to the lowest energy isomer predicted by calculations. These observations indicate that thermodynamic stability is an important driving force in determining which isomers are observed and that kinetic considerations might play a lesser role. As mentioned earlier, however, it is really free energy that determines the thermodynamic contribution to the isomer abundance. In all cases, entropy favors the less compact structures, and in these systems, there is only marginal evidence that entropy plays a significant role in determining the isomer distribution. Additionally, there certainly are major isomerization barriers in going from the graphitic to the cyclic and linear structures. The role these play is not clear. However, in pure carbon clusters, large barriers between planar ring systems and either graphitic or fullerene structures kinetically enhance the abundance of ring systems, especially above C ~ O In . C12Clx, the situation is quite different. The PM3 calculations in Table 1 are in reasonable agreement with the observed isomer abundances except for C12C14+ and perhaps C12C15+. For C&L+, theory predicts essentially 100% monocyclic ring should be observed in contrast to experimental results which yield 69% graphitic, 23% monocyclic, and 8% linear. The appearance of a small percentage of the linear isomer can be justified on entropy grounds, but only a substantial energy barrier can rationalize the large graphitic fraction if the PM3 calculations are to be believed (comparing the relative energies calculated at the PM3 level to the ab initio results in Table 2 indicates a relative accuracy of about 20-30 kclaymol for the semiempirical calculations). Since the experiment begins with a C12C110 molecule with a graphitic backbone, formation of any other structure involves crossing one or more isomerization barriers. Also, electron impact deposits a broad energy distribution into a moleculle, and by the time ionization occurs and six C1 atoms are lost, this distribution could be strongly skewed to low energies, favoring graphitic structures because insufficient energy is available to isomerize. This question can be addressed by annealing studies1* where collisional heating energizes a cluster allowing isomerization to occur. We have successfully accomplished such experiments on small carbon clusters and have been able to obtain important information on potential energy surfaces and on the relative stabilities of various
J. Phys. Chem., Vol. 99, No. 19, 1995 7713
C Clusters from Polychlorinated Graphitic Precursors i ~ 0 m e r s . lAnnealing ~ studies on carbon-chlorine systems are underway and will be reported elsewhere. The C12C11+ system is interesting. This ion is wedged between C12C12+, which PM3 theory predicts to be linear, and, C12+, which theory predicts to be monocyclic (by 47 kcaymol). Theory predicts C12C11+ should be monocyclic (by 15 kcaU mol), yet experiment indicates only the linear isomer is observed. This could, of course, be due to entropy effects. However, in a separate experiment?O where C12H1+ was made by adding H2 to the He expansion gas in a graphite laser desorption source, the isomer distribution is 65% monocyclic and 35% linear, consistent with the PM3 result for C12H1 where the cyclic isomer has been calculated to be more stable than the linear isomer by about 5 kcal/mol.20 The answer to why these systems are so different may be in the way the C12Cl1+ and C12H1+ ions are made. In the chlorine case, it is probable that the process is
while for the hydrogen system it is most likely C1,:
(cyclic)
+ H - Cl,,H+
(cyclic and linear)
(2)
In reaction 1, the large majority of the C12C11+ product ions may simply not have enough energy to isomerize to the more stable cyclic form. In reaction 2, the energy of the reactants is far above any isomerization barriers in C12H1+ (a C-H bond is worth more than 4 eV). Hence, both isomers have an opportunity to be sampled, although the cyclic isomer may be favored due to the cyclic structure of C12+ (again an annealing experiment could shed light on the system energetics and dynamics). Of course, the cyclic and linear isomers can have different dissociation kinetics as well which might affect the observed isomer distribution. To a first-order approximation, the doubly charged ions mimic the singly charged ions, with the graphitic species dominating if six or more C1 atoms remain adached to the carbon framework. For fewer C1 atoms, the graphitic structure rapidly diminishes and first monocyclic structure and then the linear structures dominate. These trends are in agreement with predictions of PM3 calculations (Table l), again suggesting thermochemistry plays a major role in determining isomer distributions. The linear species become the most stable isomers by C 12Cb2+, almost certainly due to Coulombic repulsion between the two charges. Interestingly, the most stable form of the C1z2+ ion is predicted by PM3 theory to be linear, and it is unfortunate that we had insufficient intensity to determine its structure. It is interesting to look at the structural evolution with size in some more detail. C12C110 has a naphthalenic aromatic backbone with a C2C4 unit attached, forming an additional fivemembered ring. The semiempirical calculations indicate that the first Clz pair is lost from this group so that formally an additional double bond is formed in the five-membered ring. All other structures are calculated to be much higher in energy. According to the calculations, the next chlorine atom is most likely lost from the naphthalenic unit, with loss from any of the three possible locations being nearly isoenergetic. Loss of a chlorine atom from the five-membered ring is calculated to be about 10 kcaUmol higher in energy. Other possible monoor bicyclic structures for C12C17 are calculated to be at least 40 kcaumol higher in energy than graphitic structures. When C12C15+ is formed, the most stable graphitic isomer involves loss of chlorine atoms from adjacent sites on one of the sixmembered rings. This can be rationalized by the fact that an
additional carbon-carbon bond can be formed, introducing a formal triple bond in the system. This bond results from the overlap of two “in-plane n” orbitals on the carbon atoms. This bond is expected to be somewhat weaker than a real triple bond due to the poor overlap of these orbitals, which results in strain in the molecule. The calculations indicate that for c12c16+,biand monocyclic structures become competitive in energy to the graphitic structures. Independent of the charge state and whether PM3 or AM1 is used, these structures (Figure 4) are always within 10 kcal/mol of the energy of the lowest energy graphitic structure. Interestingly, C12c16’ is the first ion where a monoor bicyclic structure is observed as discussed above. The detailed mechanism involved in this rather massive rearrangement is unknown; however, it should be noted that carbon atoms in naphthalene, for example, are observed to be rather mobile, and switching places of two atoms is experimentally observed at elevated temperatures? We also expect the chlorine atoms to be rather mobile on the carbon backbone so that migration of a chlorine atom from one site to another probably does not require much activation energy. Only one low-energy bicyclic ring and one low-energy monocyclic structure have been found for C&16+, with other possible structures at least 30 kcaymol higher in energy. The general feature of these two structures is that one can easily assign bond orders to the bonds and all carbon valencies are satisfied. For C12C4+, many low-energy isomers have been calculated, some of them shown in Figure 4. It also represents the first ion where a low-energy linear form is theoretically predicted. In this structure, two chlorine atoms are attached to each end and force the carbon chain into a cummulenic form. The lowest energy graphitic isomer has two C1 atoms lost from the fivemembered ring and a total of four chlorine atoms lost from the six-membered rings, again maximizing the number of C-C bonds that can be formed. Summary and Conclusions
The structural evolution of C12Cl,+ (x = 0-10) and of C12ClX2+(x = 8, 6, 5, 4) was studied using gas-phase ion chromatography. It was found that the ion retains the graphitic backbone down to about C12C14+ for the singly charged ions and down to about C12C152+ for the doubly charged ions. No graphitic structure was observed for C&l,+ with x = 2, 1, 0. Ab initio and semiempirical calculations show that the range where these structural transitions occur coincides with the size range where graphitic structures become higher in energy compared to linear and monocyclic structures. It is, therefore, likely that thermodynamics, rather than kinetics, is the key factor in determining which structures are experimentally observed. The technique described here can and will be used to test the stability of larger graphitic fragments, such as C20+, where theory indicates graphitic structures to be very stable, while laser vaporization sources only produce monocyclic ring cluster cations. Acknowledgment. The support of the Air Force Office of Scientific Research under Grant FA9620-93-1-0134 and, in part, the National Science Foundation under Grant CHE91-19752 is gratefully acknowledged. We also thank Prof. Hans Griitzmacher for providing the decachloroacenaphthene sample. Finally, we are most pleased to be included in this special issue honoring the life work of Mostafa El-Sayed. Mostafa is one of the true gentlemen of science, and it is a pleasure to count him as a friend and a valued colleague.
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7714 J. Phys. Chem., Vol. 99, No. 19, 1995
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(10) (a) von Helden, G.; Gotts, N. G.; Bowers, M. T. Nature 1993,363, 60. (b) von Helden, G.; Gotts, N. G.; Bowers, M. T. J . Am. Chem. SOC. 1993, 115, 4363. (c) Hunter, J.; Fye, 3.; Jarrold, M. F. Science 1993, 260, 784. (11) (a) Lifshitz, C.; Peres, T.; Agranat, I. Int. J . Muss Spectrom. Ion Process. 1989, 93, 149. (b) Sun, J.; Griitzmacher, H.-F.; Lifshitz, C. J. Am. Chem. SOC.1993,115,8382. (c) Sun,J.; Griitzmacher, H.-F.; Lifshitz, C. Int. J. Mass Spectrom. Ion Process. 1994, 138, 49. (12) The C1 vdW radius has been obtianed by calculating the potential
energy curve of H3CCl+ He at the Hartree-Fock limit. The distance where the potential energy is equal to the thermal energy is taken as the sum of the He and C1 vdW radii. The C1 radius obtained in this way is 1.64 8, and has been adjusted to 1.70 8,in order to give the best fit with the experiment. (13) Steward, J. J. P. J. Comput. Chem. 1989, 101, 209. (14) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S . T.; Gordon, M. S . ; Jensen, J. H.; Koseki, S . ; Matsunaga, N.; Nguyen, K. A.; Su, S . J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J . Comput. Chem. 1993, 14, 1347. (15) Dewar, M. J. S . ; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J . Am. Chem. SOC. 1985, 107, 3902. (16) Dunning, T. H. J . Chem. Phys. 1989, 90, 1007. (17) Kemper, P. R.; Creasey, C.; Bowers, M. T. To be published. (18) Jarrold, M. F.; Honea, E. C. J. Am. Chem. SOC.1992, 114, 459. (19) von Helden, G.; Gotts, N. G.; Palke, W. E.; Bowers, M. T. Int. J . Mass Spectrom. Ion Phys. 1994, 138, 33. von Helden, G.; Gotts, N. G.; Bowers, M. T. Chem. Phys. Lett. 1993, 212, 241. (20) Lee, S.; Gotts, N. G.; von Helden, G.; Bowers, M. T. To be published. (21) Scott, L. T.; Hashemi, M. M.; Schultz, T. H.; Wallace, M. B. J. Am. Chem. SOC.1991, 113, 9692.
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