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Structures and stabilities of hydrofullerenes. Completion of a tetrahedral fused quadruple crown structure and a double crown structure at C60H36
Journal of Molecular Structure (Theochem), 304 (1994) 18 l-189 0166-1280/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved
181
Structures and stabilities of hydrofullerenes. Completion of a tetrahedral fused quadruple crown structure and a double crown structure at Ce0Hj6 Brian W. Clare, David L. Kepert* Research Centre for Advanced W. A. 6009, Australia
(Received
7 September
Mineral and Materials
1993; accepted
28 September
Processing,
University of Western Australia, Nedlands,
1993)
Abstract The calculation of the stabilities of different isomers of C6sHn has been extended from n = 24 to n = 36. The stepwise addition of pairs of hydrogen atoms to C60H24, which contains a C,sH,s crown as the dominant structural feature, forms a fused double crown structure at C6,,Hs, and a fused quadruple crown structure of tetrahedral T symmetry at C&H,,. Another isomer of C60Hs6, consisting of two independent crowns, is slightly more stable than the fused quadruple crown structure. Further hydrogenation is relatively unfavourable. The centres of the crowns contain C6 rings in which the bonding is unusually delocalised.
Introduction Compounds formed by successive additions of Hz onto C6s to form C6sHn have been described previously, where n = 2-12 [l] and n = 14-24 [2]. Two structures were found which may be of particular importance. The first of these is the highly symmetrical Th structure of C6sHt2 in which the centres of six CH-CH groups, each forming an edge between two hexagonal faces, form an octahedron. The compound C6sHt2 has not been characterised but a clue to its structure is C&Pt(PEts)& in which the metal atoms are octahedrally arranged about C6e, each outside a hexhex edge [3]. A second important structure resulting from the further successive addition of H2 is C6,,Hz4, in which three of the CH-CH groups in C&HI2 are linked together to form a CtsHts crown, with three * Corresponding
author.
SSDI 0166-1280(93) 03487-R
remaining isolated CH-CH groups. A feature of this structure is the isolated C6 ring at the centre of the CtsHts crown. A second series of molecules, which is more stable than the first series for n = 18-26, is based on the crown structure of C60H18 with no isolated CH-CH groups. C6aH1s has been obtained by the transfer hydrogenation of CGO with dihydroanthracene at 350°C [4] but its structure is not known. The structure of C6eHs6 is of particular interest as it appears to be the product of exhaustive reduction using lithium in liquid ammonia [5], or a large excess of dihydroanthracene at 350°C [4], or hydrogen at 7 MPa and 400°C in the presence of methyl iodide [6]. The stereochemistry most commonly considered [4,5,7,8] for C6sHs6 is a high symmetry Tt, structure which has one set of 12CH groups as in CSOHt2 above, another set of 24CH groups attached to the first set, and a third set of 24 hydrogen-free carbon atoms which form C=C bonds along pent-hex
B. W. Glare and D.L. KepertjJ. Mol. Strut.
calculations;
any symmetry
mised structure Diagrams
(Theochem)
is noted
304 (1994) 181-189
observed
in the opti-
in the appropriate
were drawn with Schakal
table.
[l I].
Results Only a limited number of isomers was able to be considered for each stoichiometry C60Hn. The first
Fig. 1. Numbering system used in this work. Atoms are consecutively numbered around planes perpendicular to a five-fold axis, commencing at a mirror plane.
edges, one such bond for each of the 12 pentagonal faces. A second structure proposed [6] is of Djd symmetry and contains C6 rings on opposite sides of the molecule with each C6 attached to three C6H2 rings in which there is a double bond along a pent-hex edge. A third structure of lower symmetry has also been considered [6], with isolated C6 rings on opposite sides of the molecule and six isolated C6H, rings, again with double bonds localised on pent-hex edges. The number of isomers possible for (&Hs6 is 6.01 x lOI4 [9], but if the restrictions are made that each pair of hydrogen atoms adds onto a hex-hex edge and that pairs of optical isomers are counted only once, 7.23 x 10’ isomers are possible [lo]. Calculations The numbering system for CeO is shown in Fig. 1 and is the same as used previously [I]. Calculations were carried out using the AM1 hamiltonian and the program MOPAC 6.0 as before [l]. Geometry optimisation was done with the eigenvector following (EF) option and the option PRECISE was used throughout. No symmetry was enforced in the
series of structures was based on the addition of a pair of hydrogen atoms onto a hex-hex edge of the most stable structure obtained previously for C60Hnp2, the series commencing from C60. The isomer C60Hn formed by the addition onto the A,B sites of isomer x of C6,,Hnp2 is written [C60Hn_2: x]A,B. The starting point for this paper is C60H24 with hydrogen atoms on the 1,6; 2,7; 3,8; 15,16; 19,20; 23,33; 26,36; 27,37; 28,38; 29,39; 41,50; 53,58 sites. A second series of structures was based on the addition of pairs of hydrogen atoms onto the hexhex edges of the crown structure of C6aH1s with hydrogen atoms on the 1.6; 2,7; 3,8; 15,16; 19,20; 26,36; 27,37; 28,38; 29,39 sites. A third series of structures was based on a double crown structure of C60H36 discovered during the course of these calculations. Many other calculations were carried out which followed various hints of perceived structural trends but the resulting structures were less stable than those reported here. First series
There are only six possible isomers of C6eHz6 formed by the addition of H2 onto isomer 1 of C60H24, which contains the C,sH,s crown plus three CH-CH groups (Table 1). The most stable structure, isomer 6, has the pair of additional hydrogen atoms on the C6 ring on the three-fold axis of C60H24, opposite the C,sH,s crown (Fig. 2). The least stable structure, isomer 1, has the additional pair of hydrogen atoms on the C6 ring in the centre of the C,sH,s crown. Indeed, this [C60H24: 1]4,9 isomer, in which both hydrogen atoms are exo to the CGO skeleton, is only
B. W. C/are and D.L. Kepert/J.
Mol. Strut.
(Theochem)
304 (1994) 181-189
183
Table 1 (&Hz6 isomers, first series: heats of formation
Table 2 C,,H,, isomers, first series:heatsof formation
Isomer
Isomer
AEPf (kcal mol-‘) [CM,HS,:ll4,9 [C6,,Hz4: 1111, 12 [CG0Hz4:1]13,14 [C60H24:1]21,31 [Ce0H14: 1]22,32 [Ce0Hz4: 1]42,43
1
2 3 4 5 6
3.59 kcal mol-’
more
498.04 482.79 473.05 470.63 469.67 462.98 stable
than
I
[Ce0Hz4: 1]4-
endo, 9-exo, as the C6 ring in the centre of the
C,sH,s
crown is in an almost planar environment
and either side can be added on to. Isomer 14 of (&,H2s is substantially
more stable
2 3 4 5 6 7 8 9 10 11 12
:i
than the other 1.5possible isomers formed by add-
15
ing a pair of hydrogen atoms to isomer 6 of Ce0Hz6
16
AWf (kcal mol-‘) [CK,HX:614,9 LIHx,: 615,lO [C60H*6:6]llr12 [C6,,H26:6]13r 14 [C60H26:6]17r 18 [C,,H,,: 6]21,31 [&Hz6: 6]22,32 [Ce0Hz6:6]24,34 [Cb0Hz6: 6]25,35 [Ce0Hz6: 6]30,40 [C,,H,,: 6]44,45 [C&Hz6:6]46,47 [C,oH,,: 6]48,49 [C6,,H2,.,: 6]51,56 [Ce0Hz6:6]54,59 [&HS6: 6]55,60
462.59 462.45 448.68 435.27 462.60 438.92 447.93 439.27 448.24 436.18 437.75 436.92 447.22 424.55 435.96 435.03
(Table
2). The hydrogen atoms are again located on the C6 ring opposite the C,sH,s crown. The three least stable isomers 1, 2 and 5 again have the hydrogen atoms on the C6 ring in the centre of the C1sH1s crown. The two structures of lowest energy for C&HjO are isomer 4, [C6,,H2s: 14]13,14 and isomer 10, [&,H2s: 14]30,40 (Table 3). Hydrogen atoms at
all four sites, 13,14,30,40, complete a second crown and this is the most stable structure for CG0Hj2 (isomer 2, Table 4, Fig. 3). This second crown is centred about the hexagonal face 11,12,22,32,31,21. The string of CH groups 7,2,1,6,20,19 are common to both crowns. In addition to this fused double crown, hydrogen atoms Table 3 C6,,HX0isomers, first series: heats of formation AWf (kcal mol-‘) 1 2 3 4 5 6 7 8 9 IO 11 12 13 14 15 16
Fig. 2. Structure of CG0Hz6,isomer 6, first series. For clarity, the HC-CH carbon atoms and bonds are blackened.
[CmHz8:141439 [C,,,Hzs:1415,10 [C,,,H,,: 14111,12 [&,Hz8: 14113,14 [C6,,Hz8:14117,18 [C60H28: 14]21,31 [C60H28: 14]22,32 [C,,Hzs: 14]24,34 [ChoH,,:14]25,35 [C60H28: 14]30,40 [C60H28: 14]44,45 [&,Hz8: 14]46,47 [C6,,Hz8:14]48,49 [&,HZB: 14152, 57a [C6,,HZB: 14]54,59 [C&HS8:14]55,60
a C3 axis through 42,43,52,57,56,51.
midpoints
424.37 424.07 411.55 395.91 424.19 405.73 415.13 400.53 409.60 395.39 399.88 397.61 409.87 397.78 400.29 408.86 of
4,5,10,18,17,9
and
184
B. W. C/are and D.L. KepertlJ.
Mol. Struct.
(Themhem)
Table 4 C60H32 isomers, first series: heats of formation
Table 5 C6,,Hs6 isomers: heats of formation
Isomer
AH? (kcal mol-‘)
Isomer
381.31 365.58 393.66 387.96 371.55 380.66 371.05 361.43 387.41 367.80 372.13 385.27
First series
1 2 3 4 5 6 I 8 9 10 11 12
[C6,,Hj0: 10111, 12 [C6,,H3s: 10]13,14” [C6,,H3,,: 10]21,31 [C6,,H3s: 10]22,32 [&,Hss: 10]24,34 [C6aH3,,: 10]25,35 [C6aH3,,: 10]44,45 [C6aH3,,: 10]46,47 [C6sHs0: 10]48,49 [C6,,H3a: 10]52,57 [C60H30:10]54,59 [C,,H,,: 10]55,60
a C2 axis through midpoints
of 1,6 and 53,58.
are on the 53,58 edge as part of the octahedral C60H12 structure. The two next most stable isomers of (&Hs2, isomers 8 and 10, have hydrogen atoms on the 46,47 and 52,57 edges, and continuation of this same building principle will form C60H34([C60H32: 2]30,40 AWr = 336.70 kcal mall’) and C&H36 based on the fusion of four crowns
1
AWr (kcal mol-‘)
[C6eHs2: 2]46,47; 52, 57a
Literature
304 (1994) 181-189
304.50
structures
2
[C6sHz4:2]l,6; 15,16; 23,33; 28,38; 4150; 53,58a.b
320.14
3
1,2,3,7,11,12,13,14,16,19,21, 24,25,26,27,28,29,30,31,32, 33,34,35,36,39,40,41,44,46, 47,48,49,54,58,59,60’
341.57
4
1,2,3,6,7,11,13,15,16,19,21, 23,24,25,26,27,28,29,3 1,32, 33,34,36,38,39,40,41,44,46, 48,50,53,54,58,59,60d
305.28
a C3 axes through midpoints of 4,5,10,18,17,9 and 42,43,52,57,56,51; 2,3,8,14,13,7 and 48,49,55,60,59,54; 19, 20,30,40,39,29 and 24,25,35,45,44,34; 26,21,31,47,46,36 and 11,12,22,32,31,21. Cz axes through midpoints of 1,6 and 53,58; 15,16 and 41,50; 23,33 and 28,38. b Mirror planes through 1,6,58,53; 15,16,50,41; 23,33,38,28. ’ C3 axis through midpoints of 4,5,10,18,17,9 and 42,43,52,57,56,51. C2 axes through midpoints of 1I,12 and 46,47; 13,14 and 48,49; 25,35 and 30,40. Mirror planes through 2,7,59,54; 21,31,36,26; 24,34,39,29. dS, axis through midpoints of 4,5,10,18,17,9 and 42,43,52,57,56,51.
(isomer
1, Table 5, Fig. 4). The structure
has high
symmetry, with four three-fold axes, each passing through the centre of a crown, and three two-fold axes, each passing through CH-CH edges common to two crowns. Views down a three-fold axis and a two-fold axis are shown in Fig. 4. Table 5 also lists the calculated heats of forma-
Fig. 3. Structure of CsOHs2, isomer 2, first series, viewed down the two-fold axis. For clarity, the HC-CH carbon atoms and bonds are blackened.
tion of the three structures of Ch0Hj6 which have been proposed in the literature. Isomer 2 is of Th symmetry which has been proposed by a number of workers [4,5,7,8]. Isomer 3 of Djd symmetry and isomer 4 of S, symmetry have been proposed by Attalla et al. [6]. These three isomers are less stable than isomer 1 found in this work. Only one isomer of C6sHss can be formed by hydrogenation of tetrahedral CMHs6: 1. Hydrogenation must occur at one of the very unfavourable hex-hex edges at the centre of one of the crowns,
B. W. Glare and D.L. KepertlJ. Mol. Siruct. (Theochem)
304 (1994)
185
181-189
Table 6 C6aHz6 isomers, second series: heats of formation AK+ (kcal mol-‘)
Isomer 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
(a) n
Q
A
LoHx:
614,9 [G&~:615,10
495.61 495.54
[C6,,H24:6111,12 [C60H24:6]13r 14 [C6sHz4: 6]17,18 [&Hz+,: 6]21,31 [C6,,Hz4:6]22,32 [C6,,Hz4:6]24,34 [C6,,Hz4:6]25,35 [C6aHz4: 6]30,40 (C6,,Hz4:6]41,50 [C6,,Hz4:6]44,45 [C6aHz4: 6]46,47 [CmHz4: 6]48,49 [C6,,Hz4:6]52,57 [CbOHz4:6]53,58 [C6,,Hz4:6]54,59 [C6,,H2z,:6155, 60a
416.47 468.83 496.10 413.20 477.05 461.25 473.86 471.66 461.03 466.95 469.72 472.44 469.92 459.89 466.31 458.24
a Mirror plane through 21,31,36,26.
(b)
CGOHls, isomer 27, addition of pairs of hydrogen atoms occurs at the C6 ring opposite the crown to form isomer 20 of C6sHz0 [2] and isomer 18 of C&H22 [2]. These are the same sites as for the addition of the first two pairs of hydrogen atoms to C60H24:1 in the first series, see above. The next pair of hydrogen atoms adds onto one of the “octahedral” sites of C6,,Ht2 [2] to form C6aHz4: 6 and these structures are all fragments of the first series, compound [C6sH2s: 141.
v
“6
Fig. 4. Structure of C6aHs6, isomer 1, first series, viewed (a) down a three-fold axis and (b) down a two-fold axis. For clarity, the HC-CH carbon atoms and bonds are blackened.
and the heat of formation is less than 1 kcal mol-’ less than C6eHs6, in contrast to the typical decrease in progressing of approximately 36 kcal mol-’ from C60Hn_2 to C6aHn: Ce0H3s, isomer
1: [C6sHs6: 1]4,9
AWr = 303.65 kcal mol-’ Second series Commencing
with
the
crown
structure
of
Table 7 CaH2s isomers, second series: heats of formation A H”f (kcal mol-‘)
Isomer 17 18
19 20 21 22 23 24 25 26
[CWHX: 2414,9 [C6sHz6: 2415,1Oa [C6aHz6: 24111, 12 [C&,H16:24]13,14 [C6aHz6: 24121, 31a [C6,,Hz6:24]22,32 [C60H26:24]24,34 [C,,H,,: 24]25,35 [C6,,Hz6:24]44,45 [C6sHz6: 24152, 57a
458.83 451.77
438.24 431.42 436.57 438.74 429.59 436.04 428.81 435.19
a Mirror plane through 21,31,36,26.
B. W. Ciare and D.L. KepertlJ. Mol. Struct. iThrochem)
186
304
(1994J
Table 8 C6eH3a isomers, second series: heats of formation
Table 9 C6eH, isomers, third series: heats of formation
Isomer
Isomer
17 18 19 20
AH; (kcal mol-‘) [C,,H2s:25]21,31 [C,,H,s: 25]24,34 [C6,,H2s: 25153, 5Ea [C+,,,H>s:25]54,59
406.93 402.72 403.60 404.37
181-189
AH”r(kcal mot-‘)
C60H36
5
[C6aHz4: 1]21,31; 22,32; 24,34; 44,45; 54,59; 55,60a
303.72
[C6,,H36:5]p54,59b [C6,,H36:5]-55,60
339.77 341.96
[C,,H,,: 5]4,9c (C60H36:5]11,12d
300.89 297.97
C60H34
’ Mirror plane through 21,3 1,36,26.
Hydrogenation of C6sH14: 6 (Table 6), however, leads to CG0Hz6: 24 which has a mirror plane and is not a fragment of the first series of compounds which is based on a three-fold symmetry motif, and these two series of compounds do not converge to a common structure. Further hydrogenation of the second series of compounds forms C6aH2s:25 (Table 7) and C60H30:18 (Table 8) which are less stable than the corresponding first series isomers and studies of this second series were not continued to higher members. The same general features are observed for this second series as were observed for the first series, including the formation of (CH), strings and the particular instability of compounds formed by the hydrogenation of the C6 ring at the centre of the crown. Third series
14
I5 c60H38
2 3
a C3 axis through midpoints of 4,5,10,18,17,9 and 42, 43,52,57,56,51. C2 axes through midpoints of 1I,12 and 46,47; 13,14 and 48,49; 25,35 and 30,40. Mirror planes through 2,7,59,54; 21,31,36,26; 24,34,39.29. b Mirror plane through 2,7,59,54. ’ Mirror plane through 24,34.39,29. d C2 axis through midpoint of 1I,12 and 46,47.
Discussion The step-wise addition of pairs of hydrogen atoms to C&HnP2 to form CeOHn culminates in the aesthetically beautiful structure of C6sHs6 shown in Fig. 4. This structure does not appear to have been considered by previous workers. The
n
n
The particular stability of the CtsHts crown structure in C&HI8 (second series) prompted examination of C6sHs6 based on two separate crowns (Table 9, Fig. 5). This is the most stable structure for C&H36 found to date, but disruption of this structure to form &OH34 by the removal of two hydrogen atoms leads to structures (Table 9) which are less stable than that found in the first series. Further hydrogenation occurs at a hexhex edge between the two crowns to form C6sHss: 2 (Table 9) which is more stable than CboHss: 3 formed by hydrogenation of the C6 ring at the centre of a crown, or C6,,H3s: 1 in the first series in which hydrogenation can only occur at the centre of a crown.
Fig. 5. Structure of C60H36, isomer 5, third series. For clarity the HC-CH carbon atoms and bonds are blackened.
B. W. Glare and D.L. KepertlJ.
structure together, carbon
Mol. Struct. (Theochem)
is composed of four CisHts crowns fused each with a C6 ring at its centre, all other atoms
being attached
to hydrogen
atoms.
There are four three-fold symmetry axes, each running through the centre of a C6 ring on one side of the molecule and the centre of a C6H6 ring on the other. There are also three two-fold axes, each bisecting
a pair of three-fold
187
304 (1994) 181-I89
axes. Figure 4 clearly
shows the general tetrahedral shape of the molecule, the tetrahedral edges being formed by a chain of CH groups with a C6H6 ring at each tetrahedral vertex and a C6 ring at the centre of each tetrahedral face. The edges of the C6H6 and C6 rings are twisted by about 10” relative to the tetrahedral edges so that there are no mirror planes in the molecule, the overall symmetry being T. It is important to appreciate that the step-wise addition of H2 onto the most stable isomer of C&Hn-2 is not the only route to the tetrahedral Cb0Hs6. For example, addition of H2 to C6aHZ forms eight possible isomers of Ce0H4, all of which are fragments of the C6eHj6 structure [l]. As the degree of hydrogenation increases, it is observed that those isomers that are en route to C6aHj6 are more stable than those that are incapable of reaching this structure. For example, hydrogenation of r&Hi4 forms 15 isomers of C60H16, the seven most stable all being capable of leading to C6sHj6. It can be seen that early on in the hydrogenation sequence the carbon atoms have become sufficiently distinguishable that those which will be eventually hydrogenated in tetrahedral Ce0Hj6 have already been identified and are queuing up ready for reaction, and the exact order in which this occurs is of lesser importance. A chemically important feature of CeO and partially hydrogenated fullerenes is the localisation of the double bonds along the hex-hex edges. In C6a itself, each C6 ring has alternating strong and weak bonds around the ring, with bond orders of 1.49 for the hex-hex edges and 1.lO for the penthex edges [l]. Hydrogenation of some of the hexhex edges leads to increasing localisation of double bonds along the remaining hex-hex edges [ 11, with the bond order increasing to 1.8-l .9 for those C=C
attached
to four CH groups,
where the CH groups
are lifted out of the spherical C60 surface, creating a relatively planar double bonded structure. The exception to this general structural trend is the isolated
C6 rings
at the
centre
of the crowns
observed, for example, in C6,,Hz4, C&Hj2 and C60H36 (first series) and CGOH1s (second series) in which the bond orders are much more equal, with alternating values of 1.48 and 1.28 for the hex-hex edges and pent-hex edges respectively. It is therefore predicted that these rings may behave more like arenes than is usually observed for fullerenes. During the extensive searches for stable structures made in the course of this work, it was found that the structure of Ce0Hj6 in which there are two independent C,sH,s crowns (Fig. 5) is 0.8 kcal mol-’ more stable than C&Hs6 found in the first series (Fig. 4). Each crown has a delocalised C6 ring at its centre, with alternating bonds of order 1.48 and 1.28 similar to the crowns described above; the six hex-hex edges between the two crowns have highly localised double bonds of order 1.87. In addition to the three-fold axis through the centres of the crowns there are twofold axes through the centres of each of the highly localised double bonds and three vertical mirror planes between the two-fold axes, the overall symmetry being Djd. This structure is more stable than the D,, structure described by Attalla et al. [6], but both isomers would show the 13C NMR pattern of 2 : 2(sp2 carbon atoms):2 : 2 : 1 : l(sp3 carbon atoms) observed for (an impure sample of) C60H36 161. The tetrahedral Th structure for C6,,Hs6 proposed by a number of workers [4,5,7,8], isomer 2, is of lower stability than others described here. In the T,, structure, all 12 localised double bonds lie along pent-hex edges with bond orders of 1.86 and it has been shown previously [ 1,121that this is a less stable arrangement than double bonds along hexhex edges. In the D3d and s6 structures, isomers 3 and 4, there are six localised double bonds along pent-hex edges with bond order of 1.85. In the more stable tetrahedral T and double crown D3, structures described in this paper, isomers 1 and
188
B. W. Ciarr
and D.L.
Kepert/J.
Mol.
Struct.
(Themhem)
304 (1994)
181-189
990.0 First series
985.0
I
980.0
975.0
970.0
965.0
s +
First
960.0
series
g
Third series
955.0
950.0
945.0
940.0
935.0
930.0
925.0
920.0
Fig. 6. Heats
1
2
4
of formation
6
6
10 12 14 16 16 20 22 24 26 20 30 32 34 36 36
of C6,,Hn, plotted
5, all double bonds are in the favourable locations along hex-hex edges. Successive addition of Hz onto C6a leads to a decrease in the heat of formation or approximately 18 kcal mall’ for each hydrogen atom. Variations in this value for any C&H, can be conveniently visualised by plotting (AWr + 18~) against n, Fig. 6. It is clear that many of the C6aH, molecules described here are unstable with respect to disproportionation into lower and higher hydrides: (Xf L')C6OH, 'yC60Hn-,
as (AHi
Regions
of particular
against
stability
C60H22_28 and particularly
n.
may be C6eH2_i2,
C6,,Hj6.
Acknowledgement This work is funded by the Australian Research Council through its Special Research Centres programme. References 1
fXC60Hn+,
+ 1%~) (kcal mol-‘)
B.W. Clare and D.L. them), 281 (1993) 45.
Kepert,
J. Mol.
Struct.
(Theo-
B. W. Glare and D.L. Kepert/J.
2
Mol. Struct. (Theochem)
304 11994) 181-189
B.W. Clare and D.L. Kepert, J. Mol. Struct. (Theothem), 303 (1994) I. 3 P.J. Fagan, J.C. Calabrese and B. Malone, J. Am. Chem. Sot., 113 (1991) 9408. 4 C. Rtichardt, M. Gerst, J. Ebenhoch, H.-D. Beckhaus, E.E.B. Campbell, R.Tellgmann, H. Schwarz, T. Weiske and S. Pitter, Angew. Chem., Int. Ed. Engl., 32 (1993) 584. 5 R.E. Haufler, J. Conceicao, L.P.F. Chibante, Y. Chai, N.E. Byrne, S. Flanagan, M.M. Haley, S.C. O’Brien, C. Pan, Z. Xiao, W.E. Billups, M.A. Ciufolini, R.M. Hauge, J.L. Margrave, L.J. Wilson, R.F. Curl and R.E. Smalley, J. Phys. Chem., 94 (1990) 8634.
189
6 M.I. Attalla, A.M. Vassallo, B.N. Tattam and J.V. Hanna, J. Phys. Chem., 97 (1993) 6329. 7 B.I. Dunlap, D.W. Brenner, J.W. Mintmire, R.C. Mowrey and T.C. White, J. Phys. Chem., 95 (1991) 5763. 8 T. Guo and G.E. Scuseria, Chem. Phys. Lett., 191 (1992) 527. 9 K. Balasubramanian, Chem. Phys. Lett., 182 (1991) 257. 10 A.G. Kepert, personal communication, 1993. 11 E. Keller, J. Appl. Cryst., 22 (1989) 19. 12 N. Matsuzawa, D.A. Dixon and T. Fukunaga, J. Phys. Chem., 96 (1992) 7594.
181
Structures and stabilities of hydrofullerenes. Completion of a tetrahedral fused quadruple crown structure and a double crown structure at Ce0Hj6 Brian W. Clare, David L. Kepert* Research Centre for Advanced W. A. 6009, Australia
(Received
7 September
Mineral and Materials
1993; accepted
28 September
Processing,
University of Western Australia, Nedlands,
1993)
Abstract The calculation of the stabilities of different isomers of C6sHn has been extended from n = 24 to n = 36. The stepwise addition of pairs of hydrogen atoms to C60H24, which contains a C,sH,s crown as the dominant structural feature, forms a fused double crown structure at C6,,Hs, and a fused quadruple crown structure of tetrahedral T symmetry at C&H,,. Another isomer of C60Hs6, consisting of two independent crowns, is slightly more stable than the fused quadruple crown structure. Further hydrogenation is relatively unfavourable. The centres of the crowns contain C6 rings in which the bonding is unusually delocalised.
Introduction Compounds formed by successive additions of Hz onto C6s to form C6sHn have been described previously, where n = 2-12 [l] and n = 14-24 [2]. Two structures were found which may be of particular importance. The first of these is the highly symmetrical Th structure of C6sHt2 in which the centres of six CH-CH groups, each forming an edge between two hexagonal faces, form an octahedron. The compound C6sHt2 has not been characterised but a clue to its structure is C&Pt(PEts)& in which the metal atoms are octahedrally arranged about C6e, each outside a hexhex edge [3]. A second important structure resulting from the further successive addition of H2 is C6,,Hz4, in which three of the CH-CH groups in C&HI2 are linked together to form a CtsHts crown, with three * Corresponding
author.
SSDI 0166-1280(93) 03487-R
remaining isolated CH-CH groups. A feature of this structure is the isolated C6 ring at the centre of the CtsHts crown. A second series of molecules, which is more stable than the first series for n = 18-26, is based on the crown structure of C60H18 with no isolated CH-CH groups. C6aH1s has been obtained by the transfer hydrogenation of CGO with dihydroanthracene at 350°C [4] but its structure is not known. The structure of C6eHs6 is of particular interest as it appears to be the product of exhaustive reduction using lithium in liquid ammonia [5], or a large excess of dihydroanthracene at 350°C [4], or hydrogen at 7 MPa and 400°C in the presence of methyl iodide [6]. The stereochemistry most commonly considered [4,5,7,8] for C6sHs6 is a high symmetry Tt, structure which has one set of 12CH groups as in CSOHt2 above, another set of 24CH groups attached to the first set, and a third set of 24 hydrogen-free carbon atoms which form C=C bonds along pent-hex
B. W. Glare and D.L. KepertjJ. Mol. Strut.
calculations;
any symmetry
mised structure Diagrams
(Theochem)
is noted
304 (1994) 181-189
observed
in the opti-
in the appropriate
were drawn with Schakal
table.
[l I].
Results Only a limited number of isomers was able to be considered for each stoichiometry C60Hn. The first
Fig. 1. Numbering system used in this work. Atoms are consecutively numbered around planes perpendicular to a five-fold axis, commencing at a mirror plane.
edges, one such bond for each of the 12 pentagonal faces. A second structure proposed [6] is of Djd symmetry and contains C6 rings on opposite sides of the molecule with each C6 attached to three C6H2 rings in which there is a double bond along a pent-hex edge. A third structure of lower symmetry has also been considered [6], with isolated C6 rings on opposite sides of the molecule and six isolated C6H, rings, again with double bonds localised on pent-hex edges. The number of isomers possible for (&Hs6 is 6.01 x lOI4 [9], but if the restrictions are made that each pair of hydrogen atoms adds onto a hex-hex edge and that pairs of optical isomers are counted only once, 7.23 x 10’ isomers are possible [lo]. Calculations The numbering system for CeO is shown in Fig. 1 and is the same as used previously [I]. Calculations were carried out using the AM1 hamiltonian and the program MOPAC 6.0 as before [l]. Geometry optimisation was done with the eigenvector following (EF) option and the option PRECISE was used throughout. No symmetry was enforced in the
series of structures was based on the addition of a pair of hydrogen atoms onto a hex-hex edge of the most stable structure obtained previously for C60Hnp2, the series commencing from C60. The isomer C60Hn formed by the addition onto the A,B sites of isomer x of C6,,Hnp2 is written [C60Hn_2: x]A,B. The starting point for this paper is C60H24 with hydrogen atoms on the 1,6; 2,7; 3,8; 15,16; 19,20; 23,33; 26,36; 27,37; 28,38; 29,39; 41,50; 53,58 sites. A second series of structures was based on the addition of pairs of hydrogen atoms onto the hexhex edges of the crown structure of C6aH1s with hydrogen atoms on the 1.6; 2,7; 3,8; 15,16; 19,20; 26,36; 27,37; 28,38; 29,39 sites. A third series of structures was based on a double crown structure of C60H36 discovered during the course of these calculations. Many other calculations were carried out which followed various hints of perceived structural trends but the resulting structures were less stable than those reported here. First series
There are only six possible isomers of C6eHz6 formed by the addition of H2 onto isomer 1 of C60H24, which contains the C,sH,s crown plus three CH-CH groups (Table 1). The most stable structure, isomer 6, has the pair of additional hydrogen atoms on the C6 ring on the three-fold axis of C60H24, opposite the C,sH,s crown (Fig. 2). The least stable structure, isomer 1, has the additional pair of hydrogen atoms on the C6 ring in the centre of the C,sH,s crown. Indeed, this [C60H24: 1]4,9 isomer, in which both hydrogen atoms are exo to the CGO skeleton, is only
B. W. C/are and D.L. Kepert/J.
Mol. Strut.
(Theochem)
304 (1994) 181-189
183
Table 1 (&Hz6 isomers, first series: heats of formation
Table 2 C,,H,, isomers, first series:heatsof formation
Isomer
Isomer
AEPf (kcal mol-‘) [CM,HS,:ll4,9 [C6,,Hz4: 1111, 12 [CG0Hz4:1]13,14 [C60H24:1]21,31 [Ce0H14: 1]22,32 [Ce0Hz4: 1]42,43
1
2 3 4 5 6
3.59 kcal mol-’
more
498.04 482.79 473.05 470.63 469.67 462.98 stable
than
I
[Ce0Hz4: 1]4-
endo, 9-exo, as the C6 ring in the centre of the
C,sH,s
crown is in an almost planar environment
and either side can be added on to. Isomer 14 of (&,H2s is substantially
more stable
2 3 4 5 6 7 8 9 10 11 12
:i
than the other 1.5possible isomers formed by add-
15
ing a pair of hydrogen atoms to isomer 6 of Ce0Hz6
16
AWf (kcal mol-‘) [CK,HX:614,9 LIHx,: 615,lO [C60H*6:6]llr12 [C6,,H26:6]13r 14 [C60H26:6]17r 18 [C,,H,,: 6]21,31 [&Hz6: 6]22,32 [Ce0Hz6:6]24,34 [Cb0Hz6: 6]25,35 [Ce0Hz6: 6]30,40 [C,,H,,: 6]44,45 [C&Hz6:6]46,47 [C,oH,,: 6]48,49 [C6,,H2,.,: 6]51,56 [Ce0Hz6:6]54,59 [&HS6: 6]55,60
462.59 462.45 448.68 435.27 462.60 438.92 447.93 439.27 448.24 436.18 437.75 436.92 447.22 424.55 435.96 435.03
(Table
2). The hydrogen atoms are again located on the C6 ring opposite the C,sH,s crown. The three least stable isomers 1, 2 and 5 again have the hydrogen atoms on the C6 ring in the centre of the C1sH1s crown. The two structures of lowest energy for C&HjO are isomer 4, [C6,,H2s: 14]13,14 and isomer 10, [&,H2s: 14]30,40 (Table 3). Hydrogen atoms at
all four sites, 13,14,30,40, complete a second crown and this is the most stable structure for CG0Hj2 (isomer 2, Table 4, Fig. 3). This second crown is centred about the hexagonal face 11,12,22,32,31,21. The string of CH groups 7,2,1,6,20,19 are common to both crowns. In addition to this fused double crown, hydrogen atoms Table 3 C6,,HX0isomers, first series: heats of formation AWf (kcal mol-‘) 1 2 3 4 5 6 7 8 9 IO 11 12 13 14 15 16
Fig. 2. Structure of CG0Hz6,isomer 6, first series. For clarity, the HC-CH carbon atoms and bonds are blackened.
[CmHz8:141439 [C,,,Hzs:1415,10 [C,,,H,,: 14111,12 [&,Hz8: 14113,14 [C6,,Hz8:14117,18 [C60H28: 14]21,31 [C60H28: 14]22,32 [C,,Hzs: 14]24,34 [ChoH,,:14]25,35 [C60H28: 14]30,40 [C60H28: 14]44,45 [&,Hz8: 14]46,47 [C6,,Hz8:14]48,49 [&,HZB: 14152, 57a [C6,,HZB: 14]54,59 [C&HS8:14]55,60
a C3 axis through 42,43,52,57,56,51.
midpoints
424.37 424.07 411.55 395.91 424.19 405.73 415.13 400.53 409.60 395.39 399.88 397.61 409.87 397.78 400.29 408.86 of
4,5,10,18,17,9
and
184
B. W. C/are and D.L. KepertlJ.
Mol. Struct.
(Themhem)
Table 4 C60H32 isomers, first series: heats of formation
Table 5 C6,,Hs6 isomers: heats of formation
Isomer
AH? (kcal mol-‘)
Isomer
381.31 365.58 393.66 387.96 371.55 380.66 371.05 361.43 387.41 367.80 372.13 385.27
First series
1 2 3 4 5 6 I 8 9 10 11 12
[C6,,Hj0: 10111, 12 [C6,,H3s: 10]13,14” [C6,,H3,,: 10]21,31 [C6,,H3s: 10]22,32 [&,Hss: 10]24,34 [C6aH3,,: 10]25,35 [C6aH3,,: 10]44,45 [C6aH3,,: 10]46,47 [C6sHs0: 10]48,49 [C6,,H3a: 10]52,57 [C60H30:10]54,59 [C,,H,,: 10]55,60
a C2 axis through midpoints
of 1,6 and 53,58.
are on the 53,58 edge as part of the octahedral C60H12 structure. The two next most stable isomers of (&Hs2, isomers 8 and 10, have hydrogen atoms on the 46,47 and 52,57 edges, and continuation of this same building principle will form C60H34([C60H32: 2]30,40 AWr = 336.70 kcal mall’) and C&H36 based on the fusion of four crowns
1
AWr (kcal mol-‘)
[C6eHs2: 2]46,47; 52, 57a
Literature
304 (1994) 181-189
304.50
structures
2
[C6sHz4:2]l,6; 15,16; 23,33; 28,38; 4150; 53,58a.b
320.14
3
1,2,3,7,11,12,13,14,16,19,21, 24,25,26,27,28,29,30,31,32, 33,34,35,36,39,40,41,44,46, 47,48,49,54,58,59,60’
341.57
4
1,2,3,6,7,11,13,15,16,19,21, 23,24,25,26,27,28,29,3 1,32, 33,34,36,38,39,40,41,44,46, 48,50,53,54,58,59,60d
305.28
a C3 axes through midpoints of 4,5,10,18,17,9 and 42,43,52,57,56,51; 2,3,8,14,13,7 and 48,49,55,60,59,54; 19, 20,30,40,39,29 and 24,25,35,45,44,34; 26,21,31,47,46,36 and 11,12,22,32,31,21. Cz axes through midpoints of 1,6 and 53,58; 15,16 and 41,50; 23,33 and 28,38. b Mirror planes through 1,6,58,53; 15,16,50,41; 23,33,38,28. ’ C3 axis through midpoints of 4,5,10,18,17,9 and 42,43,52,57,56,51. C2 axes through midpoints of 1I,12 and 46,47; 13,14 and 48,49; 25,35 and 30,40. Mirror planes through 2,7,59,54; 21,31,36,26; 24,34,39,29. dS, axis through midpoints of 4,5,10,18,17,9 and 42,43,52,57,56,51.
(isomer
1, Table 5, Fig. 4). The structure
has high
symmetry, with four three-fold axes, each passing through the centre of a crown, and three two-fold axes, each passing through CH-CH edges common to two crowns. Views down a three-fold axis and a two-fold axis are shown in Fig. 4. Table 5 also lists the calculated heats of forma-
Fig. 3. Structure of CsOHs2, isomer 2, first series, viewed down the two-fold axis. For clarity, the HC-CH carbon atoms and bonds are blackened.
tion of the three structures of Ch0Hj6 which have been proposed in the literature. Isomer 2 is of Th symmetry which has been proposed by a number of workers [4,5,7,8]. Isomer 3 of Djd symmetry and isomer 4 of S, symmetry have been proposed by Attalla et al. [6]. These three isomers are less stable than isomer 1 found in this work. Only one isomer of C6sHss can be formed by hydrogenation of tetrahedral CMHs6: 1. Hydrogenation must occur at one of the very unfavourable hex-hex edges at the centre of one of the crowns,
B. W. Glare and D.L. KepertlJ. Mol. Siruct. (Theochem)
304 (1994)
185
181-189
Table 6 C6aHz6 isomers, second series: heats of formation AK+ (kcal mol-‘)
Isomer 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
(a) n
Q
A
LoHx:
614,9 [G&~:615,10
495.61 495.54
[C6,,H24:6111,12 [C60H24:6]13r 14 [C6sHz4: 6]17,18 [&Hz+,: 6]21,31 [C6,,Hz4:6]22,32 [C6,,Hz4:6]24,34 [C6,,Hz4:6]25,35 [C6aHz4: 6]30,40 (C6,,Hz4:6]41,50 [C6,,Hz4:6]44,45 [C6aHz4: 6]46,47 [CmHz4: 6]48,49 [C6,,Hz4:6]52,57 [CbOHz4:6]53,58 [C6,,Hz4:6]54,59 [C6,,H2z,:6155, 60a
416.47 468.83 496.10 413.20 477.05 461.25 473.86 471.66 461.03 466.95 469.72 472.44 469.92 459.89 466.31 458.24
a Mirror plane through 21,31,36,26.
(b)
CGOHls, isomer 27, addition of pairs of hydrogen atoms occurs at the C6 ring opposite the crown to form isomer 20 of C6sHz0 [2] and isomer 18 of C&H22 [2]. These are the same sites as for the addition of the first two pairs of hydrogen atoms to C60H24:1 in the first series, see above. The next pair of hydrogen atoms adds onto one of the “octahedral” sites of C6,,Ht2 [2] to form C6aHz4: 6 and these structures are all fragments of the first series, compound [C6sH2s: 141.
v
“6
Fig. 4. Structure of C6aHs6, isomer 1, first series, viewed (a) down a three-fold axis and (b) down a two-fold axis. For clarity, the HC-CH carbon atoms and bonds are blackened.
and the heat of formation is less than 1 kcal mol-’ less than C6eHs6, in contrast to the typical decrease in progressing of approximately 36 kcal mol-’ from C60Hn_2 to C6aHn: Ce0H3s, isomer
1: [C6sHs6: 1]4,9
AWr = 303.65 kcal mol-’ Second series Commencing
with
the
crown
structure
of
Table 7 CaH2s isomers, second series: heats of formation A H”f (kcal mol-‘)
Isomer 17 18
19 20 21 22 23 24 25 26
[CWHX: 2414,9 [C6sHz6: 2415,1Oa [C6aHz6: 24111, 12 [C&,H16:24]13,14 [C6aHz6: 24121, 31a [C6,,Hz6:24]22,32 [C60H26:24]24,34 [C,,H,,: 24]25,35 [C6,,Hz6:24]44,45 [C6sHz6: 24152, 57a
458.83 451.77
438.24 431.42 436.57 438.74 429.59 436.04 428.81 435.19
a Mirror plane through 21,31,36,26.
B. W. Ciare and D.L. KepertlJ. Mol. Struct. iThrochem)
186
304
(1994J
Table 8 C6eH3a isomers, second series: heats of formation
Table 9 C6eH, isomers, third series: heats of formation
Isomer
Isomer
17 18 19 20
AH; (kcal mol-‘) [C,,H2s:25]21,31 [C,,H,s: 25]24,34 [C6,,H2s: 25153, 5Ea [C+,,,H>s:25]54,59
406.93 402.72 403.60 404.37
181-189
AH”r(kcal mot-‘)
C60H36
5
[C6aHz4: 1]21,31; 22,32; 24,34; 44,45; 54,59; 55,60a
303.72
[C6,,H36:5]p54,59b [C6,,H36:5]-55,60
339.77 341.96
[C,,H,,: 5]4,9c (C60H36:5]11,12d
300.89 297.97
C60H34
’ Mirror plane through 21,3 1,36,26.
Hydrogenation of C6sH14: 6 (Table 6), however, leads to CG0Hz6: 24 which has a mirror plane and is not a fragment of the first series of compounds which is based on a three-fold symmetry motif, and these two series of compounds do not converge to a common structure. Further hydrogenation of the second series of compounds forms C6aH2s:25 (Table 7) and C60H30:18 (Table 8) which are less stable than the corresponding first series isomers and studies of this second series were not continued to higher members. The same general features are observed for this second series as were observed for the first series, including the formation of (CH), strings and the particular instability of compounds formed by the hydrogenation of the C6 ring at the centre of the crown. Third series
14
I5 c60H38
2 3
a C3 axis through midpoints of 4,5,10,18,17,9 and 42, 43,52,57,56,51. C2 axes through midpoints of 1I,12 and 46,47; 13,14 and 48,49; 25,35 and 30,40. Mirror planes through 2,7,59,54; 21,31,36,26; 24,34,39.29. b Mirror plane through 2,7,59,54. ’ Mirror plane through 24,34.39,29. d C2 axis through midpoint of 1I,12 and 46,47.
Discussion The step-wise addition of pairs of hydrogen atoms to C&HnP2 to form CeOHn culminates in the aesthetically beautiful structure of C6sHs6 shown in Fig. 4. This structure does not appear to have been considered by previous workers. The
n
n
The particular stability of the CtsHts crown structure in C&HI8 (second series) prompted examination of C6sHs6 based on two separate crowns (Table 9, Fig. 5). This is the most stable structure for C&H36 found to date, but disruption of this structure to form &OH34 by the removal of two hydrogen atoms leads to structures (Table 9) which are less stable than that found in the first series. Further hydrogenation occurs at a hexhex edge between the two crowns to form C6sHss: 2 (Table 9) which is more stable than CboHss: 3 formed by hydrogenation of the C6 ring at the centre of a crown, or C6,,H3s: 1 in the first series in which hydrogenation can only occur at the centre of a crown.
Fig. 5. Structure of C60H36, isomer 5, third series. For clarity the HC-CH carbon atoms and bonds are blackened.
B. W. Glare and D.L. KepertlJ.
structure together, carbon
Mol. Struct. (Theochem)
is composed of four CisHts crowns fused each with a C6 ring at its centre, all other atoms
being attached
to hydrogen
atoms.
There are four three-fold symmetry axes, each running through the centre of a C6 ring on one side of the molecule and the centre of a C6H6 ring on the other. There are also three two-fold axes, each bisecting
a pair of three-fold
187
304 (1994) 181-I89
axes. Figure 4 clearly
shows the general tetrahedral shape of the molecule, the tetrahedral edges being formed by a chain of CH groups with a C6H6 ring at each tetrahedral vertex and a C6 ring at the centre of each tetrahedral face. The edges of the C6H6 and C6 rings are twisted by about 10” relative to the tetrahedral edges so that there are no mirror planes in the molecule, the overall symmetry being T. It is important to appreciate that the step-wise addition of H2 onto the most stable isomer of C&Hn-2 is not the only route to the tetrahedral Cb0Hs6. For example, addition of H2 to C6aHZ forms eight possible isomers of Ce0H4, all of which are fragments of the C6eHj6 structure [l]. As the degree of hydrogenation increases, it is observed that those isomers that are en route to C6aHj6 are more stable than those that are incapable of reaching this structure. For example, hydrogenation of r&Hi4 forms 15 isomers of C60H16, the seven most stable all being capable of leading to C6sHj6. It can be seen that early on in the hydrogenation sequence the carbon atoms have become sufficiently distinguishable that those which will be eventually hydrogenated in tetrahedral Ce0Hj6 have already been identified and are queuing up ready for reaction, and the exact order in which this occurs is of lesser importance. A chemically important feature of CeO and partially hydrogenated fullerenes is the localisation of the double bonds along the hex-hex edges. In C6a itself, each C6 ring has alternating strong and weak bonds around the ring, with bond orders of 1.49 for the hex-hex edges and 1.lO for the penthex edges [l]. Hydrogenation of some of the hexhex edges leads to increasing localisation of double bonds along the remaining hex-hex edges [ 11, with the bond order increasing to 1.8-l .9 for those C=C
attached
to four CH groups,
where the CH groups
are lifted out of the spherical C60 surface, creating a relatively planar double bonded structure. The exception to this general structural trend is the isolated
C6 rings
at the
centre
of the crowns
observed, for example, in C6,,Hz4, C&Hj2 and C60H36 (first series) and CGOH1s (second series) in which the bond orders are much more equal, with alternating values of 1.48 and 1.28 for the hex-hex edges and pent-hex edges respectively. It is therefore predicted that these rings may behave more like arenes than is usually observed for fullerenes. During the extensive searches for stable structures made in the course of this work, it was found that the structure of Ce0Hj6 in which there are two independent C,sH,s crowns (Fig. 5) is 0.8 kcal mol-’ more stable than C&Hs6 found in the first series (Fig. 4). Each crown has a delocalised C6 ring at its centre, with alternating bonds of order 1.48 and 1.28 similar to the crowns described above; the six hex-hex edges between the two crowns have highly localised double bonds of order 1.87. In addition to the three-fold axis through the centres of the crowns there are twofold axes through the centres of each of the highly localised double bonds and three vertical mirror planes between the two-fold axes, the overall symmetry being Djd. This structure is more stable than the D,, structure described by Attalla et al. [6], but both isomers would show the 13C NMR pattern of 2 : 2(sp2 carbon atoms):2 : 2 : 1 : l(sp3 carbon atoms) observed for (an impure sample of) C60H36 161. The tetrahedral Th structure for C6,,Hs6 proposed by a number of workers [4,5,7,8], isomer 2, is of lower stability than others described here. In the T,, structure, all 12 localised double bonds lie along pent-hex edges with bond orders of 1.86 and it has been shown previously [ 1,121that this is a less stable arrangement than double bonds along hexhex edges. In the D3d and s6 structures, isomers 3 and 4, there are six localised double bonds along pent-hex edges with bond order of 1.85. In the more stable tetrahedral T and double crown D3, structures described in this paper, isomers 1 and
188
B. W. Ciarr
and D.L.
Kepert/J.
Mol.
Struct.
(Themhem)
304 (1994)
181-189
990.0 First series
985.0
I
980.0
975.0
970.0
965.0
s +
First
960.0
series
g
Third series
955.0
950.0
945.0
940.0
935.0
930.0
925.0
920.0
Fig. 6. Heats
1
2
4
of formation
6
6
10 12 14 16 16 20 22 24 26 20 30 32 34 36 36
of C6,,Hn, plotted
5, all double bonds are in the favourable locations along hex-hex edges. Successive addition of Hz onto C6a leads to a decrease in the heat of formation or approximately 18 kcal mall’ for each hydrogen atom. Variations in this value for any C&H, can be conveniently visualised by plotting (AWr + 18~) against n, Fig. 6. It is clear that many of the C6aH, molecules described here are unstable with respect to disproportionation into lower and higher hydrides: (Xf L')C6OH, 'yC60Hn-,
as (AHi
Regions
of particular
against
stability
C60H22_28 and particularly
n.
may be C6eH2_i2,
C6,,Hj6.
Acknowledgement This work is funded by the Australian Research Council through its Special Research Centres programme. References 1
fXC60Hn+,
+ 1%~) (kcal mol-‘)
B.W. Clare and D.L. them), 281 (1993) 45.
Kepert,
J. Mol.
Struct.
(Theo-
B. W. Glare and D.L. Kepert/J.
2
Mol. Struct. (Theochem)
304 11994) 181-189
B.W. Clare and D.L. Kepert, J. Mol. Struct. (Theothem), 303 (1994) I. 3 P.J. Fagan, J.C. Calabrese and B. Malone, J. Am. Chem. Sot., 113 (1991) 9408. 4 C. Rtichardt, M. Gerst, J. Ebenhoch, H.-D. Beckhaus, E.E.B. Campbell, R.Tellgmann, H. Schwarz, T. Weiske and S. Pitter, Angew. Chem., Int. Ed. Engl., 32 (1993) 584. 5 R.E. Haufler, J. Conceicao, L.P.F. Chibante, Y. Chai, N.E. Byrne, S. Flanagan, M.M. Haley, S.C. O’Brien, C. Pan, Z. Xiao, W.E. Billups, M.A. Ciufolini, R.M. Hauge, J.L. Margrave, L.J. Wilson, R.F. Curl and R.E. Smalley, J. Phys. Chem., 94 (1990) 8634.
189
6 M.I. Attalla, A.M. Vassallo, B.N. Tattam and J.V. Hanna, J. Phys. Chem., 97 (1993) 6329. 7 B.I. Dunlap, D.W. Brenner, J.W. Mintmire, R.C. Mowrey and T.C. White, J. Phys. Chem., 95 (1991) 5763. 8 T. Guo and G.E. Scuseria, Chem. Phys. Lett., 191 (1992) 527. 9 K. Balasubramanian, Chem. Phys. Lett., 182 (1991) 257. 10 A.G. Kepert, personal communication, 1993. 11 E. Keller, J. Appl. Cryst., 22 (1989) 19. 12 N. Matsuzawa, D.A. Dixon and T. Fukunaga, J. Phys. Chem., 96 (1992) 7594.