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Smaller carbon species in the laboratory and space
Mass
Spectrometry and Ion Processes
ELSEVIER
International Journal of Mass Spectrometry and Ion Processes 138 (1994) 1-15
Smaller carbon species in the laboratory and space H.W. Kroto School of Chemistry and Molecular Sciences, University of Sussex, Brighton BNI 9QJ, UK
Received 15 January 1994; accepted 6 March 1994
Abstract
The recent intense activity in carbon research which has resulted from the discovery and extraction of C6e has uncovered a wealth of fascinating evidence for species of intermediate size between C, C2 and C3 that have been known for decades, and the stable fullerenes. Carbon species with 4-30 atoms appear to form linear chains and monocyclic rings as well as some, as yet, less well characterised structures. They appear to be key species in the creation of the fullerenes. Some of the key, mainly mass spectrometric, studies which appear to shed light on the structures of these intermediates and their role in fullerene formation are briefly surveyed and the likelihood that these species and/or their derivatives are present and/or detectable in space is briefly assessed. Keywords:
Fullerenes; Interstellar medium; Red giant; Smaller carbon species
1. Introduction
After hydrogen and helium, oxygen and carbon are the next most abundant elements in the Universe and it is becoming very clear that some fascinating extended carbon species are extremely abundant in the interstellar medium. Indeed it now seems apparent that some very large species reside in the massive dark clouds of dust and gas that are strewn across the Galaxy. Early studies in 1930s on CH, CH+ and CN in space and later work on C3 in comets were a prelude to the revelations made by radioastronomy in the 1970s that long carbon chains HC,N (n = 3, 5, 7, 9, 11 etc.) were also abundant (1). Furthermore it gradually became clear that the carbon chain species which had been discovered might in some way be related to the carbon dust blown out
from the shells of carbon-rich red giants [l, 21. The discovery of long carbon chain molecules and radicals in carbon stars has recently culiminated in exciting new discoveries made by joint laboratory/radioastronomy studies [3-61 which have uncovered all sorts of fascinating carbon chain radicals such as CCCCCCH, CCCCH2 and HCCCO etc. Elegant work by Bernath et al. [7] has resulted in the detection of the bare carbon species such as C3 and C5 in a red giant star by means of the molecules’ IR active antisymmetric stretching vibrations. Equally fascinating are related discoveries such as that of CCCCSi by Ohishi et al. [8]. As far as recent terrestrial studies of vapour phase carbon are concerned, mass spectrometry has been a most important tool in the initial development of our understanding of gaseous carbon, especially as far as species larger than C, C2 and Cs are concerned. In
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the first phase there were early experiments in 1943 by Mattauch et al. who identified carbon species up to Cis in an arc [9] and this culminated in 1961 in the work of Hintenberger and coworkers who showed that carbon forms aggregates with as many as 33 atoms [lo-131. Although laser ablation of graphite was first carried out by Berkowitz and Chupka [14], the next major advance was made by Rohlfing et al. [15] in 1984 who showed that carbon aggregates with many more than 33 atoms could be produced using the laser cluster beam technique developed by Dietz et al. [16]. Rohlfing et al. [15] found a new family of clusters consisting, apparently, only of even numbers of carbon atoms from C30 to C+,,,. This work was closely followed by a photofragmentation study of this new family by Bloomfield et al. [17], which showed that the new family photofragmented by ejecting Cz. In 1985 a third study of graphite clusters aimed at connecting these types of laboratory experiments with the discoveries of carbon chains in space (above) serendipitously revealed that, under certain physicochemical conditions, one member of the new family consisting of 60 carbon atoms appeared to be exceptionally stable [18]. The discovery was made because this series of experiments probed a wide range of clustering [ 181 and cluster reaction [19,20] conditions and uncovered the fact that only under certain conditions did the intrinsic stability of CbObecome clearly apparent. It was proposed that this stability could be rationalised on the basis of a closed truncated icosahedral cage structure-fullerene-60 [ 181. The early studies of carbon species are well surveyed in the classic 1967 review by Palmer and Shelef [21] which was been nicely supplemented by 1989 by Weltner and Van Zee (22) who also include a summary of some of the early work on fullerene-60. The discovery of CbO however has added a new dimension to our understanding of the gas phase constituents of carbon vapour and the early work
between 1985 and 1990 on this species has been comprehensively reviewed [23]. The phenomenal explosion in fullerene research which followed the Kratschmer-Huffman breakthrough in CbOproduction [24] has necessarily led to more specialised reviews. Cambell and Hertel [25] and McElvany and coworkers [26, 271 have focused attention on a range of fascinating mass spectrometric observations. As well as the primary aim of the experiments which uncovered the existence of C6,,, i.e. to simulate circumstellar gas phase processes, a second incentive was to probe the possible role that carbon chain molecules might have as carriers of the Diffuse Interstellar Bands (DIBs) [28]. The identity of the carriers of these interstellar absorption bands has puzzled astronomer-spectroscopists ever since their identification as being due to interstellar matter some sixty years ago. In 1977 Douglas had proposed that carbon chain species might be responsible for the DIBs [29]. Thus a fascination with carbon molecules in space has provided a most rewarding impetus for the development of our fundamental understanding of carbon. Indeed the origins of the Kratschmer-Huffman breakthrough [24] is to be found in a fascination with the possible role carbonaceous dust might play in the UV interstellar extinction [30]. There has been a consequent rapid recent advance in our understanding of the richness of the molecular architecture that can be constructed from carbon, and here we survey mass spectrometric studies on clusters, focusing mainly on species with less than 60 carbon atoms, their possible relationships to fullerene formation and their likely existence and detectability in space.
2. The structures of extended carbon species, C,(n < 30)
The original work by Heath and coworkers [19,20] showed that species such as HC,N
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(n = 5, 7, 9 etc.), which had been detected in space, could form in a laser initiated carbon plasma which nucleated in the presence of hydrogen and nitrogen. Indeed evidence for long chain species such as HCzOH and NCZ4N was obtained in these studies. The experiments complemented previous observations of species such as CnNu2 (n = 20.. . etc.) in the Rohlfing et al. study [15]. Thus it was clear that species in which the main chemical units are cumulene or polyyne carbon chains must form in carbon vapour. The results have been elegantly confirmed in IR studies by Bernath and co-workers [7] as well as by Heath and Saykally [31] and Heath [32]. They also were broadly consistent with the suggestion that monocyclic carbon rings formed [33]. It is worthwhile briefly summarising, here, some of the work which pertains to our ideas about the nature of carbon clusters. In hypotheses about the structures of other carbon clusters, it is particularly important not to lose sight of the fact that the originally proposed structure of ChO itself [18] was hypothetical, based on circumstantial evidence, and not finally confirmed until after the material has been extracted [24] and the X-ray [24] and NMR measurements [34] completed, which put any lingering doubts finally to rest. The proposal was based on the discovery of a highly dominant mass spectrometric line at 72Ou, coupled with an instinctive act of faith that so elegant a truncated icosahedral Buckminsterfullerene structure and one which was so satisfyingly consistent with the known bonding behaviour of carbon, just had to be right [l&35]. Priro to the breakthrough in extraction of C6s in 1990 [24], a significant amount of support for the structure had been accumulated, albeit circumstantial. A careful series of cluster beam experiments was carried out [36] which supported the closed cage proposal as did a wide range of theoretical studies
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3
Initially the circumstantial evidence for the unique stability of Cm was to be found in the conflation of three key points [35,37]. (i) A generalisation of Euler’s Law, as it pertains to a mixed hexagonal/pentagonal network, indicates that closure requires the presence of exactly 12 pentagons [38,39]. (Note that the number of hexagons is unspecified, it may be any value other than one [40]). (ii) Molecules
with abutting pentagons, i.e. pentalene and its derivatives, are very unstable as is well known empirically and theoretically [41]. (iii) The final step in Barth and Lawton’s corannulene synthesis was relatively facile suggesting that a curved (saucer shaped) structure in which a pentagon is surrounded by hexagons is perfectly (and apparently unexpectedly!) stable [42]. Thus on the basis of (i) As 12 x 5 = 60 the structural uniqueness of C6, is a trivially straightforward consequence of Euler’s law in that it must be the smallest cage that can close for which all the 12 necessary and suficient pentagons are isolated. Furthermore the stability of an isolated pentagon isomer of C6, is entirely consistent with (ii) and (iii). Indeed
the existence of corannulene was important in the original theoretical conception of fullerene-60 by Osawa [43,44] and Yoshida and Osawa [45]. Schmalz et al. discussed theoretical aspects of pentagon isolation [46]. Prior to 1990 the most clear-cut support for the fullerene closed cage structure in general and CGOin particular was to be found in a simple, but not obvious, topological extension of points (i)-(iii). An investigation of the implications of pentagon isolation, as it applies to fullerene cages in general, indicates that the second (i.e. smallest after C,& cage which can possess isolated pentagons is fullerene- 70 [47,48]. This initially unexpected result, i.e.
that between 60 and 70 no isolated pentagon closed cage can exist, immediately explained the original observation that CT0 was the second favoured structure [18]. This result is the basis of the Isolated Pentagon Rule: the
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l-Photon Fluorine
Ionisation Laser (7.9eV)
:60
=50 c28
I
80 Carbon Atoms per Cluster Fig. 2. Mass spectrum from Cox et al. [51] showing a very strong even numbered dominant pattern of clusters down to C&. The spectrum shows a particularly strong signal for Css, a very abrupt cut-off at Cz4 and absolutely no evidence for C22.
the fullerene network decreased [47]. A particularly important study by O’Brien et al. [50], which followed on from the study of Bloomfield et al. [17] indicated that it was possible to photofragment C6s according to a scheme Fig. 1. (Top) (T,,) Fullerene-28 [47,37]; (bottom) (Z’J tetrahydrofullerene-28 [47,37].
hv
C60 z
hv
Css -$ Cj6 --) . . *C32 Jf+ C, 42
possession of isolated pentagon isomers is the primary criterion of fullerene stability. So far no pure fullerene that contravenes this rule has been isolated. During the early fullerene experiments [18, 361 various carbon cluster distributions exhibiting magic numbers (< 60) were observed. For instance under some circumstances C2s and other clusters in the range n < 60 appeared to be prominent [49]. These results could be explained on the basis of a generalisation of the Isolated Pentagon Rule which postulated that stability should increase as the multiplicity of the abutting pentagons in
. .2. (II
=
1220)
which appeared to have a cut-off at C32. further irradiation of Cs2 resulted in a wide range of carbon fragments rather than CsOthe next possible smaller fullerene. Ejection of C4, C6 etc. also occurred. Consequently it was suggested that Cs2 might be the smallest fullerene that could form [50]. A simple analysis of the structures of small fullerenes between C20 and C50 together with the isolated pentagon multiplet concept however suggested that smaller fullerenes might be feasible and a simple hypothetical general picture of small fullerene stability evolved [47]. CzOwould not only
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0%
H
,--k---t-----I-+ -t-+
16
18
20
+-!-
-++
22
24 Number
+-I--
26
++-
28
++
++
30
32
of Carbon Atoms
Fig. 3. Reaction experiments by Hallet et al. [52] followed by mass spectrometry. The position of the bare carbon cluster peak for a given number of carbon atoms is identified by the left hand fiducial mark and the right hand mark indicates the position of the product with six H atoms i.e. C,Hs. Only the even clusters are shown. At very low added hydrogen (1%) the product patterns of C,(m = 16, 18 and 22) are almost identical and different from those for Cn(n = 24,28 and 30) which also are similar. It is possible that CzOshows very weak composite (blended) character.
be the smallest fullerene but also almost certainly the least stable; C22 could not exist [40] and C24 would be the first partly stabilised cage. It was furthermore suggested that C2s might have the elegant Td structure shown in Fig. l(a) and possess a degree of extra stability over other small fullerenes. It was noted [47] that this molecule bore a close chemical and with Gomberg’s relationship structural famous triphenylmethyl free radical. It was also noted [37,47] that these observations were consistent with the strong C2s signal in a mass spectrum published by Cox et al. [51] (Fig. 2) which most interestingly also shows a
very clear cut-off in the even carbon cluster distribution at C24. (Note that there is a second family (both even and odd), weakly apparent, underlying the much stronger alleven set.) What is very clear in this spectrum is the fact that there is no 22 atom member of the all-even family-a result in perfect accord with a fullerene explanation because as noted above no fullerene-22 cage is possible. This spectrum (Fig. 2) is the strongest evidence, albeit circumstantial, that fullerenes down to Cz4 and even perhaps Czo might form-at least in a clustering source under certain carefully controlled conditions. This spectrum was
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obtained using very gentle F2 excimer laser radiation to ionise the clusters with a single photon. The generalised isolated pentagon rule study [47] empirically predicted that the set of fullerenes 24, 28, 32, 36, 50, 60 and 70 should exhibit varying degrees of stability, relative to neighbouring members of the family, a result neatly consistent with the overall pattern depicted in Fig. 2. Further circumstantial support for this result was obtained by Hallett et al. [52] (Fig. 3) using reactivity techniques similar to those employed by Heath et al. [19], Rohlfing [53] and Doverstal et al. [54]. Hallet et al. [52] showed that the relative reactivity in the range C16-C30 was consistent with at least two types of cluster. In particular it was found that Ci6, Cis and Cz2 behaved in a similar manner on hydrogenation which was different from the behaviour of Cz4, Cz6 and C2s etc. In addition CzO appeared to show incipient composite character. The observations are thus broadly consistent with a family relationship with the number pattern expected for the fullerenes down to Cz4 and perhaps also for 0. The interesting aspect of fullerene-28 is that it has four lone sp3 type orbitals pointing along the tetrahedral axes which should be readily able to bond to four atom such as H or groups such as Me as indicated in Fig. l(b). Thus Czs may be considered to behave like a giant supercluster which can form bonds [47] similar to the way sp3 hybrids do in methane. The possibility that small fullerenes might be stabilised by reaction to form derivatives has been generalised [55] to fullerenes other than Czs. Indeed attention may be drawn to an intuitive chemical perspective which would classify dodecahedrane, first synthesised by Paquette et al. [56], as a fullerene stabilised by perhydrogenation [35,55]. The tetravalent (tetrahedral) C2s structure proposal [47] has recently gained support from the study by Guo et al. [57] who have obtained evidence for a stable,
extractable endohedral C2s complex of uranium which is thought to be U@C2s. Not only is this result interesting because U is tetravalent but also because it highlights the fascinating possibility that the bonding in a fullerene or its “valency” may be satisfied internally. In the studies of the reaction products formed during clustering in the presence of reactive gases [52-541, empirical evidence was obtained for more than one type of cluster in the range Cz0-C44. Indeed it appeared that linear chains which as many as 40 or more atoms formed [53]. This work cannot be regarded as absolutely definitive structurally, as far as pure carbon is concerned, as reactions with added gas can obviously interfere with the clustering process and lead to species different from those found in pure carbon. They are however quite convincing with regard to the viability of very long (C20_40 etc.) chains either linear or monocyclic. In the presence of hydrogen the reactive ends may add H atoms and seal whereas in the absence of hydrogen it is possible that the ends of the chains might link to produce monocyclic cumulene or polyyne rings or perhaps some as yet structurally unspecified more complex intermediate in the carbon clustering scenario. There has been much discussion of the structure of carbon clusters with about 10 or more atoms and early studies suggested that the well-known magic number sequence y1= 11, 15, 19, 23 [lo-13,211 might be explained on the basis of monocyclic rings by Pitzer and Clementi [34]. Thus the cyclic Cii,Ci5, Ci9 and Cz3 cations would have 10, 14, 18 and 22 pi electrons-numbers consistent with the Hickel 4n + 2 rule. Convincing evidence for a change in structure in the region n = 7-10 comes from the work of McElvany and coworkers [58,59] and also from the negative ion photoelectron study of Yang et al. [60]. After the pioneering matrix studies of Weltner and Van Zee [22] the recent gas-phase
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Fig. 4. Schematic diagrams of a hypothetical spirodimerization process [64] proposed to explain the coalescence of two CjO cumulene or polyacetylene rings to form Cso as observed by Diederich and coworkers [62,63,65], cf. Fig. 5. (a) The associated Schlegel diagram; (b) a perspective view highlighting one of the C30 units.
spectroscopic analyses by Bernath et al. [7] and Heath and Saykally [31] and Heath [32] provide the first completely definitive evidence that linear chains occur with up to nine carbon atoms. However the relative amounts of
7
non-linear isomers for C,(n = 4-9) and what structures they may be, still need to be determined. It is very much easier to detect and assign the spectra (rotational, vibrational or electronic) of linear species that non-linear ones due mainly to density of state and related spectrometric pattern and intensity factors. The evidence that some fraction of the carbon clusters in this range possess chain structures is now clear, however the assignment of other isomers as monocyclic rings can, as yet, only be considered to be based on circumstantial evidence and is still in need of unequivocal confirmation. Recently technical advances by von Helden et al. [61] have yielded some very important new information on the range of possible structural isomers that can exist in carbon cluster distributions (see below). Prior to the breakthrough in production of C6O by Kratschmer et al. [24] a fascinating result was obtained by Rubin et al. [62]. In a series of experiments aimed at a study of pure carbon monocyclic rings, various ingenious carbon oxide precursors were generated which on decarbonylation were expected to form such rings with 18, 24 and 30 carbon atoms. Rubin et al. discovered, however, that in the gas phase the larger rings can zip up to form fullerene cages fairly readily [62]. The experiments were carried out by laser desorption mass spectrometry. The key result was that two CsO rings can dimerise readily to form C6s. A hypothetical concerted cyclization process, depicted schematically in Fig. 4, was suggested [64] to explain this observation. During the intermolecular/intramolecular crosslinking process the resulting hot cage of carbon atoms cools either radiatively or by collisions and also fragmentation. The resulting cages isomerise to form the stable isolated pentagon isomers. In the case of CsOring polymerisation, the dominant product is fullerene60. the predominant processes for C24 involve the coalescence of three rings to form a nascent Cy2 cage which ejects 2 carbon atom fragments
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to form CTO. A similar scenario appears to operate in the case of Cis in which four rings coalesce to form CT2which again appears to be the precursor of CTO. This study has been extended and an equally fascinating and important result obtained. McElvany et al. [65] have shown that, as the rings coalesce to form fullerenes, CT0 appears able to grow by sequential ingestion of carbon fragments. Cm, on the other hand, appears to be more of a dead end and once formed is less susceptible to further growth. The key results are shown in Fig. 5 where four Cis molecules have aggregated to form a CT2precursor which has ejected C2 to form the favoured CT0 species, which has then undergone further accretion to form C70+n1s fullerenes by further ingestion of Cis precursors. Cs,, on the other hand shows a much lower tendency for further growth (Fig. 5(b)). This result fits in neatly with earlier results of Ulmer et al. [66] which showed that laser irradiation of C& leads to fragmentation/deflation to smaller fullerenes with concomitant accretion/inflation, as ejected fragments are captured and ingested by other &a molecules which grew into larger fullerenes (Fig. 6). This series of studies was the first to shed light on the fullerene formation process. These results offer empirical evidence to support a mechanism proposed to explain the growth of carbon nanotubes [67,68] (and their helical wall structures) as well as a mechanism proposed by Heath to explain how C6a itself might grow from smaller fullerenes [69]. A technique has recently been developed by Bowers and coworkers which has added greatly to our understanding of the range of species formed by carbon. Using a gas phase
ion drift technique akin to chromatography or electrophoresis, van Helden et al. [61] have shown that several carbon isomers may occur for a given mass number, each with its own characteristic drift velocity. The main results [61,70] are summarised in Fig. 7 where the drift velocities are plotted against the number of carbon atoms. Complex energy dependent annealing, fragmentation, processes occur which lead to the formation of fullerenes [71], and the results have been rationalised on the basis of a series of hypothetical monocyclic, dicyclic and tricyclic structures [61,72], the proposed structures being supported on the basis of theoretically estimated drift velocities [61]. Schwarz has recently given a succinct analysis of various aspects of the pertinent work on the fullerene formation process [73]. In Fig. 7 we see that in the van Helden and coworkers study [61, 741 the family which extrapolates to the fullerenes stops at Cs2-a result consistent with the early result of O’Brien et al. [50] who were (as mentioned above) unable to produce fullerenes with less than 32 atoms by fragmentation of a larger one. On the other hand, as also discussed above, the Exxon mass spectrum [Sl] shown in Fig. 2 was interpreted as compelling evidence that fullerenes down to at least CZ4, and even possibly C2s, form in a cluster beam source [37,47]. van Helden et al. have also addressed the fact that their study does not support the existence of fullerenes below Cs2 [74]. It should be noted that the Exxon data [51] and those of Hallet et al. [52] were carried out using relatively gentle one-photon ionisation of neutrals whereas the O’Brien experiments [50] produced clusters in a relatively high energy process. If these small
Fig. 5. Positive ion laser desorption mass spectra of McElvany et al. [65] showing: (a) the coalescence of Crs to produce Cr2 followed by fragmentation to CT0with subsequent sequential coalescence of further CL8species to produced C70+n1s;(b) the coalescence of C24 to produce CT2followed by fragmentation to CT0with subsequent sequential coalescence of further (224species to produce Crs+,,a4; (c) the coalsecence of Css to form mainly Cm. Further coalescence occurs but is less important. Note that that Cw is weaker than Css suggesting that Css is the precursor of the latter, whereas in (a) for Crs and (b) for C24 the strong coalescence products are C+s+ rs and CT,,+24 respectively.
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C,,(W,
70
66 106 90(5n)
I
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c=60(2n)
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fullerenes are produced in the clustering source used by van Helden et al. [61] they do not appear to have survived the collisions in the drift region. Also this study pertains to ionic properties which may differ significantly from neutral behaviour. In the drift method the ions are produced directly in the nozzle by the laser ablation process. It is not obvious that the dramatic and abrupt change in intensity observed in the cluster distribution between CZ2 and CZ4 (shown in Fig. 2) has a counterpart in the drift experiments. Nor is it clear how the very prominent odd/even pattern displayed in Fig. 2, and which has generally been taken to be the tell-tale signature of fullerene structure, has an analogue in Fig. 7. The possibility that fullerenes with less than 32 atoms might form, albeit stabilised, may also explain the reported stability of U@C2s [57].
3. Carbon clusters in space As far as carbon space is concerned it is clear that in red giant stars such as IRC + 10216 the conditions are such that chains with up to 11 carbon atoms form at the same time as carbon dust. It is thus reasonable to consider which of the many newly discovered carbon species may also be present in such stars and ejected into the general interstellar medium. The likelihood that C6s is present has been discussed [75,76]. On the basis of our present knowledge it seems unlikely that more than 1% of the interstellar or circumstellar carbon budget is tied up in C6s. However there are certain carbon rich red giant stars, in particular the “so-called” RCorBor stars, which eject voluminous clouds of carbonaceous dust. The physicochemical conditions in such stars appear to be rather similar to those in which C6s is found to form in laboratory experiments. Consequently clouds in the neighbourhood of such stars would seem to be ideal regions for detecting ChO. However even if some 1.0% of the
carbon budget is in the form of C&-or was during the earlier stellar ejection phase-it would be very difficult to detect the molecule by standard spectroscopic techniques. There is, for instance, a strong possibility that any C6s formed will have reacted with other species-in particular H atoms and other abundant species. Only if the hydrogenation level is very low and species such as C6,,H or C6sH2 only are formed is there a chance of detection, otherwise “dilution” into a very large number of possible derivatives would make detection essentially impossible. The possibility still exists that charge transfer spectra of species such as exohedral Na+C& and other related species might be detectable. Transitions of this kind are very strong; however the studies of Maier [77] indicate that the spectra are so broadened that they are unlikely to be definitively identifiable. They may contribute to some of the broad banded emission features seen from some carbon rich red objects. Gasyna et al. [78] and Fulara et al. [79] have observed the spectra of the positive (and negative) ions of ChOin a matrix, which might be possible candidates for detection in space [75,76]. The spectra do not appear to correlate with any known astrophysical feature. Thus as far as astrophysical measurements are concerned it is profitable to focus on the smaller carbon species for which there is good evidence that detectable amounts are present. The early work on the cyanopolyynes showed that carbon chains with as many as 9 or 11 atoms exist [ 1,2] and more recently the detections of numerous chain radicals has added to the collection [3-81. Earlier studies [58-601 and more recent ones by van Helden et al. [80] have shed light on the existence of isomers for the small carbon species with 7-10 atoms. However the most recent and exciting breakthrough in our understanding of the possible types of carbon chain species present in space has been achieved by Fulara et al. [81]. They have developed an ion beam technique to
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Direct Ions 248nm 30 mJ cm
Direct Ions 248nm 20 mJ cmq2
11
-2
‘60
x 20
-
140 160
180
200
220
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Time of Flight / micrwec.
Time of Flight I microsee. Direct Ions 248nm 600 mJcmS2
1500 -
_ .Z I d ti ; P 8 9
‘60
1000 -
500 -
140
160 Time of Flight I micrasee.
Fig. 6. Mass spectra by Uhner et al. [66] of ions produced directly by laser desorption of Cm and C,,, at various fluences. Photofragmentation and concomitant coalescence has occurred to produce a mixture of deflated and inflated fullerenes.
deposit wuzss selected ions in a neon matrix and neutral&e them by photodetachment [811. The optical quality of the neon matrix allows multiple reflection “waveguide” transmission
techniques to be applied in order to obtain the spectra of these species in the visible region. In this way the species C,H, (m = 4, 6, 8, 10, 12 and n = 1 or 2) have been selectively deposited
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I
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Fig. 7. Plots obtained by van Helden and coworkers [61,70] of ion drift mobilities vs. cluster size for positively charged cluster ions. Note that there is no evidence for fullerenes below n = 30
spectra and their electronic absorption measured. In this study Fulara et al. [81] have obtained a very significant number of coincidences between their laboratory measurements and the Diffuse Interstellar Bands [28]. Parts of the observed spectra are shown in Fig. 8. This appears to be the first time that any significant coincidences have been obtained and furthermore the carriers proposed are species which make sense on the basis of our present understanding of the chemical constitution of the interstellar medium.
4. Conclusions There is now definitive evidence that linear
carbon chains and closed cage fullerene structures form spontaneously in a nucleaton the ing carbon plasma. Information smaller clusters containing 4-30 carbon atoms is starting to become available and it is clear that as well as linear species several other types of structure occur of which the evidence for monocyclic rings, though not yet definitive, appears to be quite good. The results suggest that chains of more than 7 atoms appear able to cyclise readily but in the presence of other (reactive) species-such as hydrogen-the dangling bonds at the ends of the carbon chains may be sealed. Very long chains, with more than 20 atoms, can form in these circumstances. This may be the case in space
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Wavelength (nm) Fig. 8. Part of the absorption spectra of species in a 5 K neon matrix recorded by Fulara et al. [81] after co-deposition of mass-selected C,H; with m = 6,8, 10 and 12 (spectra A-D respectively). The value of n = 1 or 2. The matrix was simultaneously irradiated with UV to photodetach the electrons to produce neutrals. The bands marked by an asterisk lie within 36 cm-’ of a Diffuse Band. The number of coincidences and the proximity (which is within the expected matrix shifts) are statistically significant.
where chains with 9 and 11 carbon atoms have been detected. The studies of Rubin and coworkers [62,63] and McElvany et al. [65] have made most important contributions to our understanding of the part played by smaller clusters in fullerene formation. The polymerisation of the cumulene or polyyne monocyclic rings and/or chains into fullerenes is consistent with a high temperature concerted ring cyclisation/ restructuring process for the spontaneous creation of C6o [64]. The recent ion-drift studies of van Helden et al. [61] have added significantly to this perspective in that they show that cyclic chains can anneal into
fullerenes. Evidence has been uncovered for some other additional types of structure and the associated drift behaviour has been rationalised theoretically on the basis of a range of hypothetical polycyclic as well as monocyclic ring structures [72] which do not relate intuitively with more familiar carbon bonding concepts. It is worth noting that the carbon potential is very poorly understood and until the subtly complex linearity, quasilinearity and non-linearity of species such as C,(n = 3-9) can be reproduced satisfactorily it would be unwise to place to much reliance on theory for even more extended systems. The analysis of the drift behaviour appears to
14
H. W. Krotollnternational Journal of Mass Spectrometry and Ion Processes 138 (1994) l-15
indicate that open, curved graphene sheets do not occur. Whether the smallest and most unstable fullerenes (C, with y1= 20-30) can form is not yet clear. However it is clear that to observe them the most gentle cage formation/ionisation conditions are required as they are likely to be the most strained and least electronically stabilised of the fullerene family. At present, astrophysical studies of carbon species are limited to clusters with less than a dozen carbon atoms and it is not obvious how to enhance the spectroscopic sensitivity so that much larger species are likely to be detectable. Furthermore it will also be particularly difficult to detect the non-linear species that mass spectrometric measurements have uncovered. This is due to intrinsic spectroscopic weighting factors which spread the overall transition strength into a large number of lines for nonlinear species. Thus the mass spectrometric techniques have been outstandingly successful as the primary agent in unravelling the hidden treasures in the Carbon Pandora’s Box.
Acknowledgements Thanks are due to the Royal Society for support and to J.P. Hare M. Jura, A.J. State and D.R.M. Walton for helpful discussions. References [1] H.W. Kroto, Int. Rev. Phys. Chem., 1 (1981) 309-376. [2] H.W. Kroto, Chem. Sot. Rev., 11 (1982) 435-491. [3] M. Gublin, J. Cemicharo, C. Kahane, J. GomezGonzalez and C.M. Walmsley, Astron. Astrophys., 175 (1987) L5. [4] J. Cemicharo, C.A. Gottlieb, M. Guilin, T.C. Killian, P. Thaddeus and J.M. Vrtilek, Astrophys. J. Lett., 368 (1991) L43-45. [5] A.L. Cooksy, J.K.G. Watson, C.A. Gottleib and P. Thaddeus, Astrophys. J. Letts., 386 (1992) L27-30. [6] M. G&in and J. Cemicharo, in Winnewisser G. and Armstrong J.T. (Eds.), The Physics and Chemistry of Molecular Clouds, Vol. 33 1, Springer Lecture Notes on Physics, 1989.
[7] P.F. Bemath, K.H. Hinkle and J.J. Ready, Science, (1991) 244 3129. [8] M. Oshishi, N. Kaifu, K. Kawaguchi, A. Murakami, S. Saito, S. Yamamoto, S. Ishikawa, Y. Fujita, Y. Shiratori and W.M. Irvine, Astrophys. J. Lett., 345 (1989) L83-86. [9] J. Mattauch, H. Ewald, 0. Hahn and F. Strassmann, Z. Phys. 120 (1943) 598-617. [lo] E. Doernenburg and H. Hintenberger, Z. Naturforsch. Teil A, 14 (1959) 765-767. [1 1] E. Doemenburg, H. Hintenberger and J. Franzen, Z. Naturforsch. Teil A, 16 (1961) 532-534. [12] J. Franzen and H. Hintenberger, Z. Naturforsch. Teil A, 16 (1961) 535-539. [13] H. Hintenberger, J. Franzen and K.D. Schuy, Z. Naturforsh. Teil A, 18 (1963) 1236-1237. [14] J. Berkowitz and W.A. Chupka, J. Chem. Phys., 40 (1964) 2735. [15] E.A. Rohlfing, D.M. Cox and A. Kaldor, J. Chem. Phys., 81 (1984) 3322-3330. [16] T.G. Dietz, M.A. Duncan, D.E. Powers and R.E. Smalley, J. Chem. Phys., 74 (1981) 6511-6512. [17] L.A. Bloomfield, M.E. Geusic, R.R. Freeman and W.L. Brown, Chem. Phys. Lett., 121 (1985) 33-37. [18] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl and R.E. Smalley, Nature, 318 (1985) 162-163. [19] J.R. Heath, Q. Zhang, S.C. O’Brien, R.F. Curl, H.W. Kroto and R.E. Smalley, J. Am. Chem. Sot., 109 (1987) 359-363. [20] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl and R.E. Smalley, Astrophys. J., 314 (1987) 352-355. [21] H. Palmer and M. Shelef, in J.R. Walker, Jr., (Ed.) Chemistry and Physics of Carbon, Vol. 4, Marcel Dekker, New York, 1967, pp. 85-135. [22] W. Weltner, Jr., and R.J. Van Zee, Chem. Rev., 89 (1989) 1713-1747. [23] H.W. Kroto, W. Allaf and S.P. Balm, Chem. Rev., 91 (1992) 1213-1235. [24] W. Krltschmer, L.D. Lamb, K. Fostiropoulos and D.R. Huffman, Nature, 347 (1990) 354-358. [25] E.E.B. Cambell and I.V. Hertel, Carbon 30 (1992) 11571165. [26] SW. McElvany and M.M. Ross, J. Am. Sot. Mass. Spectrom., 3 (1992) 268-280. [27] S.W. McElvany, M.M. Ross and J.H. Callghan, Act. Chem. Res., 25 (1992) 162-168. [28] G. Herbig, Astrophys. J., 196 (1975) 129. [29] A.E. Douglas, Nature, 269 (1977) 130-132. [30] W. Kratschmer and D.R. Huffman, in H.W. Kroto, J.E. Fisher and D.E. Cox (Eds.), The Fullerenes, Pergamon Press, Oxford, 1993, pp. 5-9. [31] J.R. Heath and R.J. Saykally, J. Chem. Phys., 94 (1991) 1724. [32] J.R. Heath, Spectroscopy, 5 (1990) 36-43. [33] K.S. Pitzer and E. Clementi, J. Am. Chem. Sot., 81 (1959) 4477. [34] T. Taylor, J.P. Hare, A. Abdul-Sada and H.W. Kroto, J. Chem. Sot. Chem. Commun., (1990) 1423.
H. W. Krotollnternational
Journal of Mass Spectrometry and Ion Processes 138 (1994) 1-15
[35] H.W. Kroto, Angew. Chem., 31 (1992) 111-246.
[36] J.R. Heath, SC. O’Brien, R.F. Curl, H.W. Kroto and R.E. Smalley, Comments Condens. Matter Phys., 13 (1987) 119-141. [37] H.W. Kroto, Science 242 (1988) 1139-1145. [38] D.W. Thompson, On Growth and Form, Cambridge University Press, Cambridge, 1942. [39] D.E.H. Jones, The Inventions of Daedalus, Freeman, Oxford, 1982, pp. 118-l 19. [40] P.W. Fowler and J.I. Steer, J. Chem. Sot. Chem. Commun., (1987) 1403-1405. [41] E. Heilbronner and H. Bock, The HMO Model and its Application, Vols. 1-3, Wiley, London, 1976. [42] W.E. Barth and R.G. Lawton, J. Am. Chem. Sot., 93 (1971) 1730. [43] E. Osawa, Kagaku (Kyoto), 25 (1970) 854. [44] E. Osawa, in H.W. Kroto and D.R.M. Walton, (Eds.), Fullerenes, Cambridge University Press, Cambridge, 1993. [45] Z. Yoshida and E. Osawa, Aromaticity, Kagakudojin, Kyoto, 1971. [46] T.G. Schmaltz, W.A. Seitz, D.J. Klein and G.E. Hite, Chem. Phys. Lett., 130 (1986) 203-207. [47] H.W. Kroto, Nature, 329 (1987) 529-531. [48] T.G. Schmaltz, W.A. Seitz, D.J. Klein and G.E. Hite, J. Am. Chem. Sot., llO(1988) 1113-1127. [49] J.R. Heath, S.C. O’Brien, R.F. Curl, H.W. Kroto and R.E. Smalley, unpublished observations. [50] S.C. O’Brien, J.R. Heath, R.F. Curl and R.E. Smalley, J. Chem. Phys., 88 (1988) 220-230. [5l] D.M. Cox, D.J. Trevor, K.C. Reichmann and A. Kaldor, J. Am. Chem. Sot., 108 (1986) 2457-2458. [52] R.A. Hallett, K.G. McKay, S.P. Balm, A.W. Allaf, H.W. Kroto and A.J. State, in press. [53] E.A. Rohlfing, J. Chem. Phys., 93 (1990) 7851-7862. [54] M. Doverstal, B. Lindgren, U. Sassenberg and H. Yu, Phys. Scripta, 43 (1991) 542. [55] H. Kroto and D.R.M. Walton, Chem. Phys. Lett., 214 (1993) 353-356. [56] L.A. Paquette, R.J. Ternansky, D.W. Balogh and G. Kentgen, J. Am. Chem. Sot., 105 (1983) 5446. [57] T. Guo, M.D. Diener, Y. Chai, M.J. Alford, R.E. Haufler, S.M. McClure, T. Ohno, J.H. Weaver, G.E. Scuseria and R.E. Smalley, Science, 257 (1992) 1661-1664. [58] SW. McElvany, B.I. Dunlap and J. O’Keefe, J. Chem. Phys., 86 (1987) 715-725. [59] S.W. McElvany, Int. J. Mass Spectrom. Ion Processes, 102 (1990) 81-98.
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[60] S.H. Yang, C.L. Pettiette, J. Conceicao, 0. Cheshnovsky and R.E. Smalley, Chem. Phys. Lett., 139 (1987) 233-238. [61] G. von Helden, M-T. Hsu, N. Gotts and M.T. Bowers, J. Phys. Chem., 97 (1993) 8182-8192. [62] Y. Rubin, M. Kahr, C.B. Knobler, F. Diederich and C.L. Wilkins, J. Am. Chem. Sot., 113 (1991) 495-600. [63] F. Diederich, Y. Rubin, C.B. Knobler, R.L. Whetten, K.E. Schriver, K.N. Houk and Y. Li, Science, 245 (1989) 10881090. [64] H.W. Kroto and D.R.M. Walton, in E. Osawa and 0. Yonemitsu, (Eds.), Chemistry of Three Dimensional Polycyclic Molecules, Verlag Chemie International, 1992. [65] SW. McElvany, M.M. Ross, N.S. Goroff and F. Diederich, Science, 259 (1993) 1594-1596. [66] G. Uhner, E.E.B. Campbell, R. Kuhnle, H.-G. Busmann and I.V. Hertel, Chem. Phys. Lett., 182 (1991) 114-119. [67] M. Endo and H. Kroto, J. Phys. Chem., 96 (1992) 69416944. [68] M. Endo, K. Takeuchi, S. Igarishi, K. Kobori, M. Shiraishi and H. Kroto, J. Phys. Chem. Solids, 54 (1993) 1841. [69] J.R. Heath, in G.S. Hammond and V.J. Kuck, (Eds.), Fullerene-Synthesis, Properties and Chemistry of Large Carbon Clusters, ACS Symp. Ser. 481, American Chemical Society, Washington, DC, 1992, Chapter 1, pp. 1-21. [70] G. von Helden, M.-T. Hsu, P.R. Kemper and M.T. Bowers, J. Chem. Phys., 95 (1991) 3835-3837. [71] G. von Helden, N.G. Gotts and M.T. Bowers, J. Am. Chem. Sot., 115 (1993) 4363-4364. [72] G. von Helden, N.G. Gotts and M.T. Bowers, Nature, 363 (1993) 60-63. [73] G. von Helden, M.-T. Hsu, N.G. Gotts, P.R. Kemper and M.T. Bowers, Chem. Phys. Lett., 204 (1993) 15-22. [74] H. Schwarz, Angew. Chem. Int. Ed. Engl., 32 (1993) 14121415. [75] H.W. Kroto and M. Jura, Astron. Astrophys., 263 (1992) 275-280. [76] J.P. Hare and H.W. Kroto, Act. Chem. Res., 25 (1992) 106-112. [77] J.P. Maier, personal communication. [78] Z. Gasyna, L. Andrews and P.M. Schatz, J. Phys. Chem., 96 (1992) 1525. [79] J. Fulara, M. Jakobi and J.P. Maier, Chem. Phys. Lett., 211 (1993) 227-234. [80] G. von Helden, N.G. Gotts and M.T. Bowers, Chem. Phys. Lett., 212 (1993) 241-246. [81] J. Fulara, D. Lessen, P. Freivogel and J.P. Maier, Nature, 366 (1993) 439.
Spectrometry and Ion Processes
ELSEVIER
International Journal of Mass Spectrometry and Ion Processes 138 (1994) 1-15
Smaller carbon species in the laboratory and space H.W. Kroto School of Chemistry and Molecular Sciences, University of Sussex, Brighton BNI 9QJ, UK
Received 15 January 1994; accepted 6 March 1994
Abstract
The recent intense activity in carbon research which has resulted from the discovery and extraction of C6e has uncovered a wealth of fascinating evidence for species of intermediate size between C, C2 and C3 that have been known for decades, and the stable fullerenes. Carbon species with 4-30 atoms appear to form linear chains and monocyclic rings as well as some, as yet, less well characterised structures. They appear to be key species in the creation of the fullerenes. Some of the key, mainly mass spectrometric, studies which appear to shed light on the structures of these intermediates and their role in fullerene formation are briefly surveyed and the likelihood that these species and/or their derivatives are present and/or detectable in space is briefly assessed. Keywords:
Fullerenes; Interstellar medium; Red giant; Smaller carbon species
1. Introduction
After hydrogen and helium, oxygen and carbon are the next most abundant elements in the Universe and it is becoming very clear that some fascinating extended carbon species are extremely abundant in the interstellar medium. Indeed it now seems apparent that some very large species reside in the massive dark clouds of dust and gas that are strewn across the Galaxy. Early studies in 1930s on CH, CH+ and CN in space and later work on C3 in comets were a prelude to the revelations made by radioastronomy in the 1970s that long carbon chains HC,N (n = 3, 5, 7, 9, 11 etc.) were also abundant (1). Furthermore it gradually became clear that the carbon chain species which had been discovered might in some way be related to the carbon dust blown out
from the shells of carbon-rich red giants [l, 21. The discovery of long carbon chain molecules and radicals in carbon stars has recently culiminated in exciting new discoveries made by joint laboratory/radioastronomy studies [3-61 which have uncovered all sorts of fascinating carbon chain radicals such as CCCCCCH, CCCCH2 and HCCCO etc. Elegant work by Bernath et al. [7] has resulted in the detection of the bare carbon species such as C3 and C5 in a red giant star by means of the molecules’ IR active antisymmetric stretching vibrations. Equally fascinating are related discoveries such as that of CCCCSi by Ohishi et al. [8]. As far as recent terrestrial studies of vapour phase carbon are concerned, mass spectrometry has been a most important tool in the initial development of our understanding of gaseous carbon, especially as far as species larger than C, C2 and Cs are concerned. In
0168-l 176/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0168-1176(94)04062-l
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the first phase there were early experiments in 1943 by Mattauch et al. who identified carbon species up to Cis in an arc [9] and this culminated in 1961 in the work of Hintenberger and coworkers who showed that carbon forms aggregates with as many as 33 atoms [lo-131. Although laser ablation of graphite was first carried out by Berkowitz and Chupka [14], the next major advance was made by Rohlfing et al. [15] in 1984 who showed that carbon aggregates with many more than 33 atoms could be produced using the laser cluster beam technique developed by Dietz et al. [16]. Rohlfing et al. [15] found a new family of clusters consisting, apparently, only of even numbers of carbon atoms from C30 to C+,,,. This work was closely followed by a photofragmentation study of this new family by Bloomfield et al. [17], which showed that the new family photofragmented by ejecting Cz. In 1985 a third study of graphite clusters aimed at connecting these types of laboratory experiments with the discoveries of carbon chains in space (above) serendipitously revealed that, under certain physicochemical conditions, one member of the new family consisting of 60 carbon atoms appeared to be exceptionally stable [18]. The discovery was made because this series of experiments probed a wide range of clustering [ 181 and cluster reaction [19,20] conditions and uncovered the fact that only under certain conditions did the intrinsic stability of CbObecome clearly apparent. It was proposed that this stability could be rationalised on the basis of a closed truncated icosahedral cage structure-fullerene-60 [ 181. The early studies of carbon species are well surveyed in the classic 1967 review by Palmer and Shelef [21] which was been nicely supplemented by 1989 by Weltner and Van Zee (22) who also include a summary of some of the early work on fullerene-60. The discovery of CbO however has added a new dimension to our understanding of the gas phase constituents of carbon vapour and the early work
between 1985 and 1990 on this species has been comprehensively reviewed [23]. The phenomenal explosion in fullerene research which followed the Kratschmer-Huffman breakthrough in CbOproduction [24] has necessarily led to more specialised reviews. Cambell and Hertel [25] and McElvany and coworkers [26, 271 have focused attention on a range of fascinating mass spectrometric observations. As well as the primary aim of the experiments which uncovered the existence of C6,,, i.e. to simulate circumstellar gas phase processes, a second incentive was to probe the possible role that carbon chain molecules might have as carriers of the Diffuse Interstellar Bands (DIBs) [28]. The identity of the carriers of these interstellar absorption bands has puzzled astronomer-spectroscopists ever since their identification as being due to interstellar matter some sixty years ago. In 1977 Douglas had proposed that carbon chain species might be responsible for the DIBs [29]. Thus a fascination with carbon molecules in space has provided a most rewarding impetus for the development of our fundamental understanding of carbon. Indeed the origins of the Kratschmer-Huffman breakthrough [24] is to be found in a fascination with the possible role carbonaceous dust might play in the UV interstellar extinction [30]. There has been a consequent rapid recent advance in our understanding of the richness of the molecular architecture that can be constructed from carbon, and here we survey mass spectrometric studies on clusters, focusing mainly on species with less than 60 carbon atoms, their possible relationships to fullerene formation and their likely existence and detectability in space.
2. The structures of extended carbon species, C,(n < 30)
The original work by Heath and coworkers [19,20] showed that species such as HC,N
H. W. Krotollnternational Journal of Mass Spectrometry and Ion Processes 138 (1994) I-15
(n = 5, 7, 9 etc.), which had been detected in space, could form in a laser initiated carbon plasma which nucleated in the presence of hydrogen and nitrogen. Indeed evidence for long chain species such as HCzOH and NCZ4N was obtained in these studies. The experiments complemented previous observations of species such as CnNu2 (n = 20.. . etc.) in the Rohlfing et al. study [15]. Thus it was clear that species in which the main chemical units are cumulene or polyyne carbon chains must form in carbon vapour. The results have been elegantly confirmed in IR studies by Bernath and co-workers [7] as well as by Heath and Saykally [31] and Heath [32]. They also were broadly consistent with the suggestion that monocyclic carbon rings formed [33]. It is worthwhile briefly summarising, here, some of the work which pertains to our ideas about the nature of carbon clusters. In hypotheses about the structures of other carbon clusters, it is particularly important not to lose sight of the fact that the originally proposed structure of ChO itself [18] was hypothetical, based on circumstantial evidence, and not finally confirmed until after the material has been extracted [24] and the X-ray [24] and NMR measurements [34] completed, which put any lingering doubts finally to rest. The proposal was based on the discovery of a highly dominant mass spectrometric line at 72Ou, coupled with an instinctive act of faith that so elegant a truncated icosahedral Buckminsterfullerene structure and one which was so satisfyingly consistent with the known bonding behaviour of carbon, just had to be right [l&35]. Priro to the breakthrough in extraction of C6s in 1990 [24], a significant amount of support for the structure had been accumulated, albeit circumstantial. A careful series of cluster beam experiments was carried out [36] which supported the closed cage proposal as did a wide range of theoretical studies
WI.
3
Initially the circumstantial evidence for the unique stability of Cm was to be found in the conflation of three key points [35,37]. (i) A generalisation of Euler’s Law, as it pertains to a mixed hexagonal/pentagonal network, indicates that closure requires the presence of exactly 12 pentagons [38,39]. (Note that the number of hexagons is unspecified, it may be any value other than one [40]). (ii) Molecules
with abutting pentagons, i.e. pentalene and its derivatives, are very unstable as is well known empirically and theoretically [41]. (iii) The final step in Barth and Lawton’s corannulene synthesis was relatively facile suggesting that a curved (saucer shaped) structure in which a pentagon is surrounded by hexagons is perfectly (and apparently unexpectedly!) stable [42]. Thus on the basis of (i) As 12 x 5 = 60 the structural uniqueness of C6, is a trivially straightforward consequence of Euler’s law in that it must be the smallest cage that can close for which all the 12 necessary and suficient pentagons are isolated. Furthermore the stability of an isolated pentagon isomer of C6, is entirely consistent with (ii) and (iii). Indeed
the existence of corannulene was important in the original theoretical conception of fullerene-60 by Osawa [43,44] and Yoshida and Osawa [45]. Schmalz et al. discussed theoretical aspects of pentagon isolation [46]. Prior to 1990 the most clear-cut support for the fullerene closed cage structure in general and CGOin particular was to be found in a simple, but not obvious, topological extension of points (i)-(iii). An investigation of the implications of pentagon isolation, as it applies to fullerene cages in general, indicates that the second (i.e. smallest after C,& cage which can possess isolated pentagons is fullerene- 70 [47,48]. This initially unexpected result, i.e.
that between 60 and 70 no isolated pentagon closed cage can exist, immediately explained the original observation that CT0 was the second favoured structure [18]. This result is the basis of the Isolated Pentagon Rule: the
H. W. Krotollnternational
Journal of Mass Spectrometry
and Ion Processes 138 (1994) l-15
l-Photon Fluorine
Ionisation Laser (7.9eV)
:60
=50 c28
I
80 Carbon Atoms per Cluster Fig. 2. Mass spectrum from Cox et al. [51] showing a very strong even numbered dominant pattern of clusters down to C&. The spectrum shows a particularly strong signal for Css, a very abrupt cut-off at Cz4 and absolutely no evidence for C22.
the fullerene network decreased [47]. A particularly important study by O’Brien et al. [50], which followed on from the study of Bloomfield et al. [17] indicated that it was possible to photofragment C6s according to a scheme Fig. 1. (Top) (T,,) Fullerene-28 [47,37]; (bottom) (Z’J tetrahydrofullerene-28 [47,37].
hv
C60 z
hv
Css -$ Cj6 --) . . *C32 Jf+ C, 42
possession of isolated pentagon isomers is the primary criterion of fullerene stability. So far no pure fullerene that contravenes this rule has been isolated. During the early fullerene experiments [18, 361 various carbon cluster distributions exhibiting magic numbers (< 60) were observed. For instance under some circumstances C2s and other clusters in the range n < 60 appeared to be prominent [49]. These results could be explained on the basis of a generalisation of the Isolated Pentagon Rule which postulated that stability should increase as the multiplicity of the abutting pentagons in
. .2. (II
=
1220)
which appeared to have a cut-off at C32. further irradiation of Cs2 resulted in a wide range of carbon fragments rather than CsOthe next possible smaller fullerene. Ejection of C4, C6 etc. also occurred. Consequently it was suggested that Cs2 might be the smallest fullerene that could form [50]. A simple analysis of the structures of small fullerenes between C20 and C50 together with the isolated pentagon multiplet concept however suggested that smaller fullerenes might be feasible and a simple hypothetical general picture of small fullerene stability evolved [47]. CzOwould not only
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H. W. Krotojlnternational Journal of Mass Spectrometry and Ion Processes 138 (1994) l-15
0%
H
,--k---t-----I-+ -t-+
16
18
20
+-!-
-++
22
24 Number
+-I--
26
++-
28
++
++
30
32
of Carbon Atoms
Fig. 3. Reaction experiments by Hallet et al. [52] followed by mass spectrometry. The position of the bare carbon cluster peak for a given number of carbon atoms is identified by the left hand fiducial mark and the right hand mark indicates the position of the product with six H atoms i.e. C,Hs. Only the even clusters are shown. At very low added hydrogen (1%) the product patterns of C,(m = 16, 18 and 22) are almost identical and different from those for Cn(n = 24,28 and 30) which also are similar. It is possible that CzOshows very weak composite (blended) character.
be the smallest fullerene but also almost certainly the least stable; C22 could not exist [40] and C24 would be the first partly stabilised cage. It was furthermore suggested that C2s might have the elegant Td structure shown in Fig. l(a) and possess a degree of extra stability over other small fullerenes. It was noted [47] that this molecule bore a close chemical and with Gomberg’s relationship structural famous triphenylmethyl free radical. It was also noted [37,47] that these observations were consistent with the strong C2s signal in a mass spectrum published by Cox et al. [51] (Fig. 2) which most interestingly also shows a
very clear cut-off in the even carbon cluster distribution at C24. (Note that there is a second family (both even and odd), weakly apparent, underlying the much stronger alleven set.) What is very clear in this spectrum is the fact that there is no 22 atom member of the all-even family-a result in perfect accord with a fullerene explanation because as noted above no fullerene-22 cage is possible. This spectrum (Fig. 2) is the strongest evidence, albeit circumstantial, that fullerenes down to Cz4 and even perhaps Czo might form-at least in a clustering source under certain carefully controlled conditions. This spectrum was
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H. W. Krotollnternational Journal of Mass Spectrometry and Ion Processes 138 (1994) I-15
obtained using very gentle F2 excimer laser radiation to ionise the clusters with a single photon. The generalised isolated pentagon rule study [47] empirically predicted that the set of fullerenes 24, 28, 32, 36, 50, 60 and 70 should exhibit varying degrees of stability, relative to neighbouring members of the family, a result neatly consistent with the overall pattern depicted in Fig. 2. Further circumstantial support for this result was obtained by Hallett et al. [52] (Fig. 3) using reactivity techniques similar to those employed by Heath et al. [19], Rohlfing [53] and Doverstal et al. [54]. Hallet et al. [52] showed that the relative reactivity in the range C16-C30 was consistent with at least two types of cluster. In particular it was found that Ci6, Cis and Cz2 behaved in a similar manner on hydrogenation which was different from the behaviour of Cz4, Cz6 and C2s etc. In addition CzO appeared to show incipient composite character. The observations are thus broadly consistent with a family relationship with the number pattern expected for the fullerenes down to Cz4 and perhaps also for 0. The interesting aspect of fullerene-28 is that it has four lone sp3 type orbitals pointing along the tetrahedral axes which should be readily able to bond to four atom such as H or groups such as Me as indicated in Fig. l(b). Thus Czs may be considered to behave like a giant supercluster which can form bonds [47] similar to the way sp3 hybrids do in methane. The possibility that small fullerenes might be stabilised by reaction to form derivatives has been generalised [55] to fullerenes other than Czs. Indeed attention may be drawn to an intuitive chemical perspective which would classify dodecahedrane, first synthesised by Paquette et al. [56], as a fullerene stabilised by perhydrogenation [35,55]. The tetravalent (tetrahedral) C2s structure proposal [47] has recently gained support from the study by Guo et al. [57] who have obtained evidence for a stable,
extractable endohedral C2s complex of uranium which is thought to be U@C2s. Not only is this result interesting because U is tetravalent but also because it highlights the fascinating possibility that the bonding in a fullerene or its “valency” may be satisfied internally. In the studies of the reaction products formed during clustering in the presence of reactive gases [52-541, empirical evidence was obtained for more than one type of cluster in the range Cz0-C44. Indeed it appeared that linear chains which as many as 40 or more atoms formed [53]. This work cannot be regarded as absolutely definitive structurally, as far as pure carbon is concerned, as reactions with added gas can obviously interfere with the clustering process and lead to species different from those found in pure carbon. They are however quite convincing with regard to the viability of very long (C20_40 etc.) chains either linear or monocyclic. In the presence of hydrogen the reactive ends may add H atoms and seal whereas in the absence of hydrogen it is possible that the ends of the chains might link to produce monocyclic cumulene or polyyne rings or perhaps some as yet structurally unspecified more complex intermediate in the carbon clustering scenario. There has been much discussion of the structure of carbon clusters with about 10 or more atoms and early studies suggested that the well-known magic number sequence y1= 11, 15, 19, 23 [lo-13,211 might be explained on the basis of monocyclic rings by Pitzer and Clementi [34]. Thus the cyclic Cii,Ci5, Ci9 and Cz3 cations would have 10, 14, 18 and 22 pi electrons-numbers consistent with the Hickel 4n + 2 rule. Convincing evidence for a change in structure in the region n = 7-10 comes from the work of McElvany and coworkers [58,59] and also from the negative ion photoelectron study of Yang et al. [60]. After the pioneering matrix studies of Weltner and Van Zee [22] the recent gas-phase
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Fig. 4. Schematic diagrams of a hypothetical spirodimerization process [64] proposed to explain the coalescence of two CjO cumulene or polyacetylene rings to form Cso as observed by Diederich and coworkers [62,63,65], cf. Fig. 5. (a) The associated Schlegel diagram; (b) a perspective view highlighting one of the C30 units.
spectroscopic analyses by Bernath et al. [7] and Heath and Saykally [31] and Heath [32] provide the first completely definitive evidence that linear chains occur with up to nine carbon atoms. However the relative amounts of
7
non-linear isomers for C,(n = 4-9) and what structures they may be, still need to be determined. It is very much easier to detect and assign the spectra (rotational, vibrational or electronic) of linear species that non-linear ones due mainly to density of state and related spectrometric pattern and intensity factors. The evidence that some fraction of the carbon clusters in this range possess chain structures is now clear, however the assignment of other isomers as monocyclic rings can, as yet, only be considered to be based on circumstantial evidence and is still in need of unequivocal confirmation. Recently technical advances by von Helden et al. [61] have yielded some very important new information on the range of possible structural isomers that can exist in carbon cluster distributions (see below). Prior to the breakthrough in production of C6O by Kratschmer et al. [24] a fascinating result was obtained by Rubin et al. [62]. In a series of experiments aimed at a study of pure carbon monocyclic rings, various ingenious carbon oxide precursors were generated which on decarbonylation were expected to form such rings with 18, 24 and 30 carbon atoms. Rubin et al. discovered, however, that in the gas phase the larger rings can zip up to form fullerene cages fairly readily [62]. The experiments were carried out by laser desorption mass spectrometry. The key result was that two CsO rings can dimerise readily to form C6s. A hypothetical concerted cyclization process, depicted schematically in Fig. 4, was suggested [64] to explain this observation. During the intermolecular/intramolecular crosslinking process the resulting hot cage of carbon atoms cools either radiatively or by collisions and also fragmentation. The resulting cages isomerise to form the stable isolated pentagon isomers. In the case of CsOring polymerisation, the dominant product is fullerene60. the predominant processes for C24 involve the coalescence of three rings to form a nascent Cy2 cage which ejects 2 carbon atom fragments
8
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to form CTO. A similar scenario appears to operate in the case of Cis in which four rings coalesce to form CT2which again appears to be the precursor of CTO. This study has been extended and an equally fascinating and important result obtained. McElvany et al. [65] have shown that, as the rings coalesce to form fullerenes, CT0 appears able to grow by sequential ingestion of carbon fragments. Cm, on the other hand, appears to be more of a dead end and once formed is less susceptible to further growth. The key results are shown in Fig. 5 where four Cis molecules have aggregated to form a CT2precursor which has ejected C2 to form the favoured CT0 species, which has then undergone further accretion to form C70+n1s fullerenes by further ingestion of Cis precursors. Cs,, on the other hand shows a much lower tendency for further growth (Fig. 5(b)). This result fits in neatly with earlier results of Ulmer et al. [66] which showed that laser irradiation of C& leads to fragmentation/deflation to smaller fullerenes with concomitant accretion/inflation, as ejected fragments are captured and ingested by other &a molecules which grew into larger fullerenes (Fig. 6). This series of studies was the first to shed light on the fullerene formation process. These results offer empirical evidence to support a mechanism proposed to explain the growth of carbon nanotubes [67,68] (and their helical wall structures) as well as a mechanism proposed by Heath to explain how C6a itself might grow from smaller fullerenes [69]. A technique has recently been developed by Bowers and coworkers which has added greatly to our understanding of the range of species formed by carbon. Using a gas phase
ion drift technique akin to chromatography or electrophoresis, van Helden et al. [61] have shown that several carbon isomers may occur for a given mass number, each with its own characteristic drift velocity. The main results [61,70] are summarised in Fig. 7 where the drift velocities are plotted against the number of carbon atoms. Complex energy dependent annealing, fragmentation, processes occur which lead to the formation of fullerenes [71], and the results have been rationalised on the basis of a series of hypothetical monocyclic, dicyclic and tricyclic structures [61,72], the proposed structures being supported on the basis of theoretically estimated drift velocities [61]. Schwarz has recently given a succinct analysis of various aspects of the pertinent work on the fullerene formation process [73]. In Fig. 7 we see that in the van Helden and coworkers study [61, 741 the family which extrapolates to the fullerenes stops at Cs2-a result consistent with the early result of O’Brien et al. [50] who were (as mentioned above) unable to produce fullerenes with less than 32 atoms by fragmentation of a larger one. On the other hand, as also discussed above, the Exxon mass spectrum [Sl] shown in Fig. 2 was interpreted as compelling evidence that fullerenes down to at least CZ4, and even possibly C2s, form in a cluster beam source [37,47]. van Helden et al. have also addressed the fact that their study does not support the existence of fullerenes below Cs2 [74]. It should be noted that the Exxon data [51] and those of Hallet et al. [52] were carried out using relatively gentle one-photon ionisation of neutrals whereas the O’Brien experiments [50] produced clusters in a relatively high energy process. If these small
Fig. 5. Positive ion laser desorption mass spectra of McElvany et al. [65] showing: (a) the coalescence of Crs to produce Cr2 followed by fragmentation to CT0with subsequent sequential coalescence of further CL8species to produced C70+n1s;(b) the coalescence of C24 to produce CT2followed by fragmentation to CT0with subsequent sequential coalescence of further (224species to produce Crs+,,a4; (c) the coalsecence of Css to form mainly Cm. Further coalescence occurs but is less important. Note that that Cw is weaker than Css suggesting that Css is the precursor of the latter, whereas in (a) for Crs and (b) for C24 the strong coalescence products are C+s+ rs and CT,,+24 respectively.
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fullerenes are produced in the clustering source used by van Helden et al. [61] they do not appear to have survived the collisions in the drift region. Also this study pertains to ionic properties which may differ significantly from neutral behaviour. In the drift method the ions are produced directly in the nozzle by the laser ablation process. It is not obvious that the dramatic and abrupt change in intensity observed in the cluster distribution between CZ2 and CZ4 (shown in Fig. 2) has a counterpart in the drift experiments. Nor is it clear how the very prominent odd/even pattern displayed in Fig. 2, and which has generally been taken to be the tell-tale signature of fullerene structure, has an analogue in Fig. 7. The possibility that fullerenes with less than 32 atoms might form, albeit stabilised, may also explain the reported stability of U@C2s [57].
3. Carbon clusters in space As far as carbon space is concerned it is clear that in red giant stars such as IRC + 10216 the conditions are such that chains with up to 11 carbon atoms form at the same time as carbon dust. It is thus reasonable to consider which of the many newly discovered carbon species may also be present in such stars and ejected into the general interstellar medium. The likelihood that C6s is present has been discussed [75,76]. On the basis of our present knowledge it seems unlikely that more than 1% of the interstellar or circumstellar carbon budget is tied up in C6s. However there are certain carbon rich red giant stars, in particular the “so-called” RCorBor stars, which eject voluminous clouds of carbonaceous dust. The physicochemical conditions in such stars appear to be rather similar to those in which C6s is found to form in laboratory experiments. Consequently clouds in the neighbourhood of such stars would seem to be ideal regions for detecting ChO. However even if some 1.0% of the
carbon budget is in the form of C&-or was during the earlier stellar ejection phase-it would be very difficult to detect the molecule by standard spectroscopic techniques. There is, for instance, a strong possibility that any C6s formed will have reacted with other species-in particular H atoms and other abundant species. Only if the hydrogenation level is very low and species such as C6,,H or C6sH2 only are formed is there a chance of detection, otherwise “dilution” into a very large number of possible derivatives would make detection essentially impossible. The possibility still exists that charge transfer spectra of species such as exohedral Na+C& and other related species might be detectable. Transitions of this kind are very strong; however the studies of Maier [77] indicate that the spectra are so broadened that they are unlikely to be definitively identifiable. They may contribute to some of the broad banded emission features seen from some carbon rich red objects. Gasyna et al. [78] and Fulara et al. [79] have observed the spectra of the positive (and negative) ions of ChOin a matrix, which might be possible candidates for detection in space [75,76]. The spectra do not appear to correlate with any known astrophysical feature. Thus as far as astrophysical measurements are concerned it is profitable to focus on the smaller carbon species for which there is good evidence that detectable amounts are present. The early work on the cyanopolyynes showed that carbon chains with as many as 9 or 11 atoms exist [ 1,2] and more recently the detections of numerous chain radicals has added to the collection [3-81. Earlier studies [58-601 and more recent ones by van Helden et al. [80] have shed light on the existence of isomers for the small carbon species with 7-10 atoms. However the most recent and exciting breakthrough in our understanding of the possible types of carbon chain species present in space has been achieved by Fulara et al. [81]. They have developed an ion beam technique to
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Direct Ions 248nm 30 mJ cm
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Fig. 6. Mass spectra by Uhner et al. [66] of ions produced directly by laser desorption of Cm and C,,, at various fluences. Photofragmentation and concomitant coalescence has occurred to produce a mixture of deflated and inflated fullerenes.
deposit wuzss selected ions in a neon matrix and neutral&e them by photodetachment [811. The optical quality of the neon matrix allows multiple reflection “waveguide” transmission
techniques to be applied in order to obtain the spectra of these species in the visible region. In this way the species C,H, (m = 4, 6, 8, 10, 12 and n = 1 or 2) have been selectively deposited
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Fig. 7. Plots obtained by van Helden and coworkers [61,70] of ion drift mobilities vs. cluster size for positively charged cluster ions. Note that there is no evidence for fullerenes below n = 30
spectra and their electronic absorption measured. In this study Fulara et al. [81] have obtained a very significant number of coincidences between their laboratory measurements and the Diffuse Interstellar Bands [28]. Parts of the observed spectra are shown in Fig. 8. This appears to be the first time that any significant coincidences have been obtained and furthermore the carriers proposed are species which make sense on the basis of our present understanding of the chemical constitution of the interstellar medium.
4. Conclusions There is now definitive evidence that linear
carbon chains and closed cage fullerene structures form spontaneously in a nucleaton the ing carbon plasma. Information smaller clusters containing 4-30 carbon atoms is starting to become available and it is clear that as well as linear species several other types of structure occur of which the evidence for monocyclic rings, though not yet definitive, appears to be quite good. The results suggest that chains of more than 7 atoms appear able to cyclise readily but in the presence of other (reactive) species-such as hydrogen-the dangling bonds at the ends of the carbon chains may be sealed. Very long chains, with more than 20 atoms, can form in these circumstances. This may be the case in space
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Wavelength (nm) Fig. 8. Part of the absorption spectra of species in a 5 K neon matrix recorded by Fulara et al. [81] after co-deposition of mass-selected C,H; with m = 6,8, 10 and 12 (spectra A-D respectively). The value of n = 1 or 2. The matrix was simultaneously irradiated with UV to photodetach the electrons to produce neutrals. The bands marked by an asterisk lie within 36 cm-’ of a Diffuse Band. The number of coincidences and the proximity (which is within the expected matrix shifts) are statistically significant.
where chains with 9 and 11 carbon atoms have been detected. The studies of Rubin and coworkers [62,63] and McElvany et al. [65] have made most important contributions to our understanding of the part played by smaller clusters in fullerene formation. The polymerisation of the cumulene or polyyne monocyclic rings and/or chains into fullerenes is consistent with a high temperature concerted ring cyclisation/ restructuring process for the spontaneous creation of C6o [64]. The recent ion-drift studies of van Helden et al. [61] have added significantly to this perspective in that they show that cyclic chains can anneal into
fullerenes. Evidence has been uncovered for some other additional types of structure and the associated drift behaviour has been rationalised theoretically on the basis of a range of hypothetical polycyclic as well as monocyclic ring structures [72] which do not relate intuitively with more familiar carbon bonding concepts. It is worth noting that the carbon potential is very poorly understood and until the subtly complex linearity, quasilinearity and non-linearity of species such as C,(n = 3-9) can be reproduced satisfactorily it would be unwise to place to much reliance on theory for even more extended systems. The analysis of the drift behaviour appears to
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indicate that open, curved graphene sheets do not occur. Whether the smallest and most unstable fullerenes (C, with y1= 20-30) can form is not yet clear. However it is clear that to observe them the most gentle cage formation/ionisation conditions are required as they are likely to be the most strained and least electronically stabilised of the fullerene family. At present, astrophysical studies of carbon species are limited to clusters with less than a dozen carbon atoms and it is not obvious how to enhance the spectroscopic sensitivity so that much larger species are likely to be detectable. Furthermore it will also be particularly difficult to detect the non-linear species that mass spectrometric measurements have uncovered. This is due to intrinsic spectroscopic weighting factors which spread the overall transition strength into a large number of lines for nonlinear species. Thus the mass spectrometric techniques have been outstandingly successful as the primary agent in unravelling the hidden treasures in the Carbon Pandora’s Box.
Acknowledgements Thanks are due to the Royal Society for support and to J.P. Hare M. Jura, A.J. State and D.R.M. Walton for helpful discussions. References [1] H.W. Kroto, Int. Rev. Phys. Chem., 1 (1981) 309-376. [2] H.W. Kroto, Chem. Sot. Rev., 11 (1982) 435-491. [3] M. Gublin, J. Cemicharo, C. Kahane, J. GomezGonzalez and C.M. Walmsley, Astron. Astrophys., 175 (1987) L5. [4] J. Cemicharo, C.A. Gottlieb, M. Guilin, T.C. Killian, P. Thaddeus and J.M. Vrtilek, Astrophys. J. Lett., 368 (1991) L43-45. [5] A.L. Cooksy, J.K.G. Watson, C.A. Gottleib and P. Thaddeus, Astrophys. J. Letts., 386 (1992) L27-30. [6] M. G&in and J. Cemicharo, in Winnewisser G. and Armstrong J.T. (Eds.), The Physics and Chemistry of Molecular Clouds, Vol. 33 1, Springer Lecture Notes on Physics, 1989.
[7] P.F. Bemath, K.H. Hinkle and J.J. Ready, Science, (1991) 244 3129. [8] M. Oshishi, N. Kaifu, K. Kawaguchi, A. Murakami, S. Saito, S. Yamamoto, S. Ishikawa, Y. Fujita, Y. Shiratori and W.M. Irvine, Astrophys. J. Lett., 345 (1989) L83-86. [9] J. Mattauch, H. Ewald, 0. Hahn and F. Strassmann, Z. Phys. 120 (1943) 598-617. [lo] E. Doernenburg and H. Hintenberger, Z. Naturforsch. Teil A, 14 (1959) 765-767. [1 1] E. Doemenburg, H. Hintenberger and J. Franzen, Z. Naturforsch. Teil A, 16 (1961) 532-534. [12] J. Franzen and H. Hintenberger, Z. Naturforsch. Teil A, 16 (1961) 535-539. [13] H. Hintenberger, J. Franzen and K.D. Schuy, Z. Naturforsh. Teil A, 18 (1963) 1236-1237. [14] J. Berkowitz and W.A. Chupka, J. Chem. Phys., 40 (1964) 2735. [15] E.A. Rohlfing, D.M. Cox and A. Kaldor, J. Chem. Phys., 81 (1984) 3322-3330. [16] T.G. Dietz, M.A. Duncan, D.E. Powers and R.E. Smalley, J. Chem. Phys., 74 (1981) 6511-6512. [17] L.A. Bloomfield, M.E. Geusic, R.R. Freeman and W.L. Brown, Chem. Phys. Lett., 121 (1985) 33-37. [18] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl and R.E. Smalley, Nature, 318 (1985) 162-163. [19] J.R. Heath, Q. Zhang, S.C. O’Brien, R.F. Curl, H.W. Kroto and R.E. Smalley, J. Am. Chem. Sot., 109 (1987) 359-363. [20] H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl and R.E. Smalley, Astrophys. J., 314 (1987) 352-355. [21] H. Palmer and M. Shelef, in J.R. Walker, Jr., (Ed.) Chemistry and Physics of Carbon, Vol. 4, Marcel Dekker, New York, 1967, pp. 85-135. [22] W. Weltner, Jr., and R.J. Van Zee, Chem. Rev., 89 (1989) 1713-1747. [23] H.W. Kroto, W. Allaf and S.P. Balm, Chem. Rev., 91 (1992) 1213-1235. [24] W. Krltschmer, L.D. Lamb, K. Fostiropoulos and D.R. Huffman, Nature, 347 (1990) 354-358. [25] E.E.B. Cambell and I.V. Hertel, Carbon 30 (1992) 11571165. [26] SW. McElvany and M.M. Ross, J. Am. Sot. Mass. Spectrom., 3 (1992) 268-280. [27] S.W. McElvany, M.M. Ross and J.H. Callghan, Act. Chem. Res., 25 (1992) 162-168. [28] G. Herbig, Astrophys. J., 196 (1975) 129. [29] A.E. Douglas, Nature, 269 (1977) 130-132. [30] W. Kratschmer and D.R. Huffman, in H.W. Kroto, J.E. Fisher and D.E. Cox (Eds.), The Fullerenes, Pergamon Press, Oxford, 1993, pp. 5-9. [31] J.R. Heath and R.J. Saykally, J. Chem. Phys., 94 (1991) 1724. [32] J.R. Heath, Spectroscopy, 5 (1990) 36-43. [33] K.S. Pitzer and E. Clementi, J. Am. Chem. Sot., 81 (1959) 4477. [34] T. Taylor, J.P. Hare, A. Abdul-Sada and H.W. Kroto, J. Chem. Sot. Chem. Commun., (1990) 1423.
H. W. Krotollnternational
Journal of Mass Spectrometry and Ion Processes 138 (1994) 1-15
[35] H.W. Kroto, Angew. Chem., 31 (1992) 111-246.
[36] J.R. Heath, SC. O’Brien, R.F. Curl, H.W. Kroto and R.E. Smalley, Comments Condens. Matter Phys., 13 (1987) 119-141. [37] H.W. Kroto, Science 242 (1988) 1139-1145. [38] D.W. Thompson, On Growth and Form, Cambridge University Press, Cambridge, 1942. [39] D.E.H. Jones, The Inventions of Daedalus, Freeman, Oxford, 1982, pp. 118-l 19. [40] P.W. Fowler and J.I. Steer, J. Chem. Sot. Chem. Commun., (1987) 1403-1405. [41] E. Heilbronner and H. Bock, The HMO Model and its Application, Vols. 1-3, Wiley, London, 1976. [42] W.E. Barth and R.G. Lawton, J. Am. Chem. Sot., 93 (1971) 1730. [43] E. Osawa, Kagaku (Kyoto), 25 (1970) 854. [44] E. Osawa, in H.W. Kroto and D.R.M. Walton, (Eds.), Fullerenes, Cambridge University Press, Cambridge, 1993. [45] Z. Yoshida and E. Osawa, Aromaticity, Kagakudojin, Kyoto, 1971. [46] T.G. Schmaltz, W.A. Seitz, D.J. Klein and G.E. Hite, Chem. Phys. Lett., 130 (1986) 203-207. [47] H.W. Kroto, Nature, 329 (1987) 529-531. [48] T.G. Schmaltz, W.A. Seitz, D.J. Klein and G.E. Hite, J. Am. Chem. Sot., llO(1988) 1113-1127. [49] J.R. Heath, S.C. O’Brien, R.F. Curl, H.W. Kroto and R.E. Smalley, unpublished observations. [50] S.C. O’Brien, J.R. Heath, R.F. Curl and R.E. Smalley, J. Chem. Phys., 88 (1988) 220-230. [5l] D.M. Cox, D.J. Trevor, K.C. Reichmann and A. Kaldor, J. Am. Chem. Sot., 108 (1986) 2457-2458. [52] R.A. Hallett, K.G. McKay, S.P. Balm, A.W. Allaf, H.W. Kroto and A.J. State, in press. [53] E.A. Rohlfing, J. Chem. Phys., 93 (1990) 7851-7862. [54] M. Doverstal, B. Lindgren, U. Sassenberg and H. Yu, Phys. Scripta, 43 (1991) 542. [55] H. Kroto and D.R.M. Walton, Chem. Phys. Lett., 214 (1993) 353-356. [56] L.A. Paquette, R.J. Ternansky, D.W. Balogh and G. Kentgen, J. Am. Chem. Sot., 105 (1983) 5446. [57] T. Guo, M.D. Diener, Y. Chai, M.J. Alford, R.E. Haufler, S.M. McClure, T. Ohno, J.H. Weaver, G.E. Scuseria and R.E. Smalley, Science, 257 (1992) 1661-1664. [58] SW. McElvany, B.I. Dunlap and J. O’Keefe, J. Chem. Phys., 86 (1987) 715-725. [59] S.W. McElvany, Int. J. Mass Spectrom. Ion Processes, 102 (1990) 81-98.
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[60] S.H. Yang, C.L. Pettiette, J. Conceicao, 0. Cheshnovsky and R.E. Smalley, Chem. Phys. Lett., 139 (1987) 233-238. [61] G. von Helden, M-T. Hsu, N. Gotts and M.T. Bowers, J. Phys. Chem., 97 (1993) 8182-8192. [62] Y. Rubin, M. Kahr, C.B. Knobler, F. Diederich and C.L. Wilkins, J. Am. Chem. Sot., 113 (1991) 495-600. [63] F. Diederich, Y. Rubin, C.B. Knobler, R.L. Whetten, K.E. Schriver, K.N. Houk and Y. Li, Science, 245 (1989) 10881090. [64] H.W. Kroto and D.R.M. Walton, in E. Osawa and 0. Yonemitsu, (Eds.), Chemistry of Three Dimensional Polycyclic Molecules, Verlag Chemie International, 1992. [65] SW. McElvany, M.M. Ross, N.S. Goroff and F. Diederich, Science, 259 (1993) 1594-1596. [66] G. Uhner, E.E.B. Campbell, R. Kuhnle, H.-G. Busmann and I.V. Hertel, Chem. Phys. Lett., 182 (1991) 114-119. [67] M. Endo and H. Kroto, J. Phys. Chem., 96 (1992) 69416944. [68] M. Endo, K. Takeuchi, S. Igarishi, K. Kobori, M. Shiraishi and H. Kroto, J. Phys. Chem. Solids, 54 (1993) 1841. [69] J.R. Heath, in G.S. Hammond and V.J. Kuck, (Eds.), Fullerene-Synthesis, Properties and Chemistry of Large Carbon Clusters, ACS Symp. Ser. 481, American Chemical Society, Washington, DC, 1992, Chapter 1, pp. 1-21. [70] G. von Helden, M.-T. Hsu, P.R. Kemper and M.T. Bowers, J. Chem. Phys., 95 (1991) 3835-3837. [71] G. von Helden, N.G. Gotts and M.T. Bowers, J. Am. Chem. Sot., 115 (1993) 4363-4364. [72] G. von Helden, N.G. Gotts and M.T. Bowers, Nature, 363 (1993) 60-63. [73] G. von Helden, M.-T. Hsu, N.G. Gotts, P.R. Kemper and M.T. Bowers, Chem. Phys. Lett., 204 (1993) 15-22. [74] H. Schwarz, Angew. Chem. Int. Ed. Engl., 32 (1993) 14121415. [75] H.W. Kroto and M. Jura, Astron. Astrophys., 263 (1992) 275-280. [76] J.P. Hare and H.W. Kroto, Act. Chem. Res., 25 (1992) 106-112. [77] J.P. Maier, personal communication. [78] Z. Gasyna, L. Andrews and P.M. Schatz, J. Phys. Chem., 96 (1992) 1525. [79] J. Fulara, M. Jakobi and J.P. Maier, Chem. Phys. Lett., 211 (1993) 227-234. [80] G. von Helden, N.G. Gotts and M.T. Bowers, Chem. Phys. Lett., 212 (1993) 241-246. [81] J. Fulara, D. Lessen, P. Freivogel and J.P. Maier, Nature, 366 (1993) 439.