8556
J. Phys. Chem. 1995,99, 8556-8561
Addition Chemistry of c60: A Theoretical Analysis of Substitutional Preferences for Halogenation and Hydrogenation of c 6 0 Shu-Hsien Wang and Susan A. Jansen” Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122 Received: November 8, 1994; In Final Form: February 17, 1995@
Reactions involving Cm with a variety of species have produced a series of new fullerene derivatives. Such reactions include hydrogenation, methylation, halogenation, and epoxidation to name a few. The observed reaction chemistry traces the chemistry of aromatics in some cases; however, in others it is considerably less well understood. In this work, our efforts are directed at understanding mechanistic concems in halogenation. To date several studies have analyzed the energetics of fluorination, bromination, and hydrogenation without serious consideration of orbital mechanism or isomer preferences. In this work, the orbital mechanism for reactivity is assessed for addition of halogens. The site preference for addition is analyzed with respect to symmetry distinct sites in c60. These include the well-described 6-6‘ and 5-6’ sites in Cm. The site directed preference for subsequent additions is discussed as well. Different site preferences are observed for F and Br; the effects are due to orbital driven reaction chemistry and thermodynamic stabilities of products. “Ortho” substitution appears favored for F substitution while 1,3 or 1,4 product predominants for Br addition.
Introduction The availability of relatively large quantities of fullerene’ has aroused increasing research interest in fullerene chemistry. Multiple endo- and exohedrally metal-substituted fullerenes have been prepared. The reactivity of the 6-6’ linkage of the fullerene toward metal chelation and substitution is now well ~haracterized~-~ and understood in terms of simple frontier orbital effects5 The aromatic nature of c 6 0 has been exploited in a plethora of new synthetic objectives in which the regiochemistry is of great interest. Hydrogenation and halogenation of Cm have been studied both theoretically2%6-8and experimen tall^;^.^-'^ however, the former work has not attracted the same amount of attention as the latter. It has been reported that c 6 0 has the tendency to possess an even number of addenda and that the site preference for addition depends only on the size of the incoming addenda.2.3,7-9*11-13 In these studies, it was concluded that either 1,2 addition across the 6-6’ bond or 1,4 addition was the common addition motif for halogens. The maximum number of addenda c 6 0 can possess varies depending on the particular substituent, with the maximum being 36 for hydrogenation, 60 for fluorination, and 24 for chlorination and bromination. To date, theoretical studies on Cm and its derivatives have included ab initioI6 and semiempirical MND0,23’7-’9IND0,2°*21 AM1, PM3, and MOPAC molecular orbital methods, and local density functional method^.^,^ Many of these methods, MNDO and INDO, do not predict frontier orbital energies well. Others, AM1 and PM3, require inclusion of multiple CI states for an analysis of the electronic structure, and thus most current studies describe the energetics or stabilities of fullerene derivatives but offer little conclusive evidence as to the orbital mechanism of the fullerene reactivity. In some cases, the model used was based on the “cyclohexatriene” structure, and thus olefinic bond character was emphasized or possibly exaggerated. In this work, an orbital description of reactivity and site preference is provided. The stability of the new adducts is addressed by comparison of relative energies obtained from both extended Hiickel and molecular mechanics methods. The site preference ‘Abstract published in Advance ACS Absrructs, May 1, 1995.
Figure 1. Two types of bonds in C ~ O .
for 1,2 substitution is readdressed considering both 6-6’ and 5-6’ bond types. Models for Cm were selected to include both localized and delocalized structures. A comparison of molecular and free radical addition chemistry is provided through consideration of bonding between halogens along the reaction trajectory. This work does not supplant the work of Dixon et aL7xs Instead it complements their studies which have provided a clear understanding of the energetics of reactivity and adduct stability.
Computational Method The Cm geometry utilized in the molecular mechanics modelling was optimized using the M M f force field (HyperChem Inc.) to normal convergence limits. The geometries of the various c 6 0 adducts were obtained in a similar manner. The model for c 6 0 used in the extended Huckel studies was obtained from X-ray crystallographic data.5 Both models provide a fullerene model with “ambiguous” aromaticity.22 The C-C bond lengths are nearly identical for both, yet symmetry differences affect the reaction chemistry as demonstrated previously. As shown in Figure 1, two distinct bond types are observed in the fullerene icosahedron. One fuses a five- and a six-membered ring and thus is called the 5-6’ linkage. Similarly, the 6-6’ linkage is one that fuses two six-membered
0022-3654/95/2099-8556$09.00/00 1995 American Chemical Society
Halogenation and Hydrogenation of
J. Phys. Chem., Vol. 99, No. 21, 1995 8557
c60
M.dcl-
F
8’
? Figure 2. Possible reaction pathways for Cm: (a) addition reactions of anthracene, phenanthrene, and indene which are Cm moieties and representative of para, 6-6’ ortho, and 5-6’ ortho product structures. Here, the uncertain product of indene is obtained from an anti-addition pattern. The O polyenes of anthracene, phenanthrene and indene. N-bromosuccinimide (NBS) represents monosubstitution. (b) C ~ moieties:
P
I
/tI
Figure 3. Structures of CaH, C~OF, and CsoBr.
rings. Owing to the %fold symmetry in the icosahedron, these linkages are distinct. Both of these bond types were considered in the reaction studies. As for the fullerene adducts, the bond lengths of C-X, where X = H, F, and Br, were taken from typical halo me thane^,^^ and the bond angles for the C-C-X bonds at the addition sites were taken from literature data.7ss-26For monosubstitution,there is no site preference for addition; however, the addition of the incoming second addendum will be affected by electronic and steric effects induced by the initial substitution. The mechanism for Cm reactivity may parallel the addition pathways now well understood for aromatic species. These pathways describe the production of ortho, meta, and para isomers for aromatic species in terms of both free radical and molecular addition. These pathways are shown in Figure 2. For the icosahedral geometric considerations, two isomers are possible for ortho substitution, the 5-6’ and the 6-6’. In all previous work, only the 6-6‘ site has been considered in comparing stabilities of ortho, meta, and para isomers. Since the primary focus was the steric or substituent size effect on the energy of the isomer, consideration of 5-6‘ and 6-6‘ sites may not have appeared necessary. Our own previous work5
on fullerenes has shown that orbital mechanisms, especially at the frontier, are critical in defining substitution chemistries of C ~ as O the HOMO is bonding along the 6-6’ linkage and antibonding along 5-6’, while the converse is observed in the LUMO. One related example is the addition of metal addenda to the Cm cage. The antibonding character at the 6-6’ site of the LUMO promotes back-bonding in the formation of certain transition metal fullerene analogues. Thus for addition chemistry, the energy and the symmetry of the frontier states are important considerations. Furthermore, as halogen-halogen interaction becomes important for ortho substitution, this effect will be modulated through bond interactions with Cm regardless of reaction mechanism. The electronic structure calculations have relied on applications of the extended Huckel method. The application of the extended Huckel formalism has already demonstrated significant utility in describing hydrocarbon chemistry and has reliably produced experimental trends in fullerene chemistry. The method developed by Hoffmann et al.24is well suited for this study as relative energy trends are valid and o and n bonding terms are well described. Frontier orbital effects are well characterized also through applications of the EH method. The
Wang and Jansen
8558 J. Phys. Chem., Vol. 99, No. 21, 1995
i
TABLE 1: MM+ Calculation Results: Energy Contributions dihedral stretch- electrospecies total“ bond angle strain Vdw bend static Cm
267.63 3.77
89.83 154.31 24.46 -4.74
0.00
(a) H Adducts, X = H HC60 H2Ca(0-56) HzCa(0-66’) H2Ca(m) HzCa(p) C60
BrZCSOfortho
296.63 326.63 327.11 326.12 326.00
ss’)
FC6o 357.30 FzCm(0-56‘) 444.25 &C,50(0-66’) 445.71 F&o(m) 448.51 F2C60(p) 448.81
4.20 95.97 177.21 23.58 5.14 102.45 199.35 24.49 5.28 102.99 198.75 24.95 4.64 102.51 200.21 23.40 4.51 101.55 201.08 23.45 (b) F Adducts, X = F 4.96 101.92 231.92 23.34 8.01 116.21 299.21 23.83 8.05 116.16 300.60 23.91 6.70 115.72 308.09 22.95 6.07 112.93 311.62 23.11
(c) Br Adducts, X = Br BrCm 350.52 5.06 97.71 230.40 22.56 Br2Cm(0-56’) 434.81 8.38 113.10 293.58 24.07 Br2Cso(O-66’) 438.14 9.14 112.20 296.35 25.40 Br$&(m) 433.81 7.10 107.80 303.57 21.33 Br2Csa(p) 435.55 6.37 104.72 308.75 21.39 rrrcro(rt.)
66‘~
Figure 4. Distortion of the Cm shell after addition reaction.
parameters used in this study are typical valence state ionization energies and are listed in Table 4.
Results and Discussion (a) Molecular Mechanics Analysis. Two starting models were used for C ~ in O this work. The fiist is one in which alterating bond distances were selected by defining two types of carbon bonds, that is, each carbon as sp2, in the fullerene unit, the so-called localized geometry. A delocalized model was generated by defining all atom types as aromatic. Both of these starting geometries, Le., the “localized” and “delocalized”, produced essentially the same final structure after minimi~ation.~ The total energies differed by less than 0.5 kcallmol. This is somewhat surprising as different parameter sets are involved for the two. However, the rigidity of the cage as a closed polyhedron is critical in determining the final geometry; most other molecular species are not so constrained. From this starting point, the distortion in the fullerene cage was assessed as a function of substitution. The structures of CmH, C d , and C60Br are shown in Figure 3. It is not surprising to note that the primary structural effect of a single addendum is localized at the substitution site, inducing strain in bond angle and dihedral angle terms. All other energy parameters are not as significantly affected. The increases in total energy are not terribly high and can be attributed to these two energy terms primarily. The second addition is not as benign to the fullerene cage. Severe distortions are noted, in particular for ortho substitution in either the 5-6’ or 6-6‘ site. This is shown for bromine addition in Figure 4. Not surprisingly, these effects are greater for the halogenated species than for the hydrogenated ones. The first hints of site preference are observed for ortho, meta, and para substitution. For hydrogenation, the meta and para isomers possess nearly the same stability. The two ortho isomers are considerably higher in energy, 20.6 kcal. For fluorine, the ortho isomers appear the most stable; however, it is the 5-6’ isomer that appears to be the most stable. This isomer is favored by almost 1.5 kcal over the 6-6’ isomer, while the preference for bromine appears to be for meta sites by 1 kcal over the 5-6’ ortho site. These data are displayed in Table 1. The 6-6’ site was not favored for any disubstituted species, though for
+
0.00 0.00 0.00 0.00 0.00
-4.85 -5.82 -6.11 -5.82 -5.55
0.00 2.81 3.10 0.88 0.62
-5.21 -6.21 -6.98 -6.75 -6.29
0.00
+
1.89 2.03 0.76 0.61
+
Total energy (kcaYmol)= bond angle dihedral strain Vdw Here, the total energy follows the same trend traces in binding energy indicated from the extended Hiickel calculation due to the zero energy of the addendudaddenda. a
aracso(ortho
-4.33 -4.81 -4.86 -4.65 -4.59
+ stretch-bend + electrostatic.
TABLE 2: Extended Hiickel Calculation Results: Binding Energies of XCm and X2Cm binding energy (eV) species molecular free radical(s) (a) H Adducts, X = H +2.71 - 1.36 -0.35
-0.88 -1.98
- 1.25 -2.81 -2.56 - 1.74 -2.21
(b) F Adducts, X = F +2.98
+1.03 +3.27 +5.22
$2.43 +0.67 +2.41 +4.96 +3.30
(c) Br Adducts, X = Br +2.38 +7.86 $11.31 +4.71 1.40
+
+1.32 +6.18 $8.78 +2.58 +0.95
hydrogen and fluorine substitution it was the most competitive isomer with the 5-6’ ortho isomer. Dixon’s work suggested that the 6-6’ site was the site of either molecular or free radical addition for fluorine and that the 1,4-para site was the low energy site for bromine. The results presented here which contrast molecular mechanics modelling with heats of formation from those obtained by semiempirical molecular orbital methods suggest that further study into isomer preference is both warranted and necessary. (b) Extended Htickel Analysis. The mode of addition has been analyzed in this study by comparison of the energy of fragment species. The computed binding energy of two X’and XI moiety with the fullerene cage allows for the calculation of the stabilization effect of any X-X interaction. Thus the energy is expressed as the difference between the fullerene complex and its X2 and Cm constituents. For free radical addition, the constituents are two x’and the Cm. The differences between these energy considerations are produced by residual covalent
Halogenation and Hydrogenation of
J. Phys. Chem., Vol. 99, No. 21, 1995 8559
c60
Figure 5. Energy diagrams of orbitals of interest in X, X2, ortho 6-6’ X2Cm and where X = F and Br. In order to observe the orbital interaction, the energy scale has been changed, and thus the degeneracy of the energy state is varied as compared to Fignre 6. TABLE 4: Extended Hickel Parameters
TABLE 3: Overlap Populations for XZCSO bond type
5 -6‘
6-6’
C-F C-C(5-6’) C-C (6-6’) C-C (ring)”
0.340 0.598 0.801 0.890
Fz-C6n 0.346 0.750 0.506 0.856
C-Br
0.240 0.630 0.877 0.898
Brz-C6n 0.278 0.798 0.850 0.901
c-c
(5-6‘)
C-C (6-6’) C-C (ring“)
Para
meta
0.367 0.796 0.987 0.970
0.365 0.781 0.975 0.981
0.277 0.780 1.001 0.967
0.254 0.763 0.988 0.980
atom H C
orbital, H,i (eV) 1s 2s
F
2s 2P 4s
2P Br
a The (ring) is the average overlap population for fulvalene ring structure of addition site.
bonding in the X2 species which may stabilize or destabilize the fullerene adduct. The results for these calculations are provided in Table 2. In all cases, molecular addition energies are the highest, implying that the addition of a molecular analogue may not be realistic. The high energy results from losses in bonding that occur between the X units in the diatomic unit upon interaction with c60. All other compensatory energy terms are equivalent for the free radical and molecular addition, Le., cluster C-C and C-X bonding; loss of bonding in the X-X unit suggests that molecular addition is not the most favored process. The overlap populations computed for hydrogenation and halogenation show effectively no residual bonding between species at the cluster except in the case of 6-6’ ortho-bromine addition. The complete loss of bonding and X-X stabilization in the case of Br addition can be explained in terms of simple orbital effects. These effects are shown in Figure 5. Here the energies of states for X and X2 are compared with orbital energies of Cm in a simple interaction diagram. In the case of fluorine, it is readily seen that atomic states of fluorine are positioned well below frontier states of c60. Substitution with a single fluorine causes a destabilization of the HOMO of Cm as these states interact with those of fluorine centered at a considerably lower energy. In addition, the low energy bonding orbitals are displaced to higher energy as a result of their interaction with fluorine, and thus C-C 0 and n bonding becomes compromised in the sites adjacent to fluorine substitution. This can be seen in an analysis of the overlap populations provided in Table 3. For molecular addition of fluorine, the data in Figure 5 show that orbital interactions between molecular F2 and Cm are
4P
E1
Ci
-13.6 -21.4 -11.4 -40.0 -18.1 -27.01 -12.44
1.3 1.625 1.625 2.425 2.425 2.588 2.131
minimal. The HOMO of F2 is at a very low energy relative to the frontier states of Cm and its LUMO, an antibonding 0 state, is positioned at an energy almost equivalent to that of the HOMO of c 6 0 . Thus, before significant orbital interaction can occur along the reaction trajectory, charge transfer from Cw to F2 comprises the bonding in the diatomic species producing free atoms, i.e., providing the free radical mechanism. For free radical addition, the fullerene may be predisposed to certain addition chemistries if the F atoms are added sequentially. An analysis of orbital states in the F-Cm moiety formed by initial addition of F shows that the frontier states are nearly noded at the two ortho positions, that is, those completing the 5-6’ and 6-6‘ linkages in the cluster. The energies of these states are shifted to higher energy than those of the “free” Cm, and thus the frontier states are somewhat inactive toward further addition of fluorine at the ortho sites. In addition, these states are significantly antibonding along the cluster periphery. In fact, the frontier orbital density tends to localize at meta and para substitution sites. Addition of fluorine at these sites is expected to destabilize the c 6 0 further by populating initially unoccupied antibonding states. There is some preference for the 5-6‘ site as antibonding interactions between fluorine addenda remain minimal. Though the reaction trajactory favors initial formation of the 1-4 isomer, isomerization is expected to produce the lower energy 1-2 isomer. An analysis of the frontier orbitals of the 1-4 isomer shows that these orbitals of p-CmF2 are noded at the 5-6’ site but not at the 6-6’ site, suggesting an orbital based mechanism for isomerization. Therefore, though the 5-6’ site appears to be slightly lower in energy than the 6-6’ site, there is no “pathway” for this isomerization to occur, and thus the 6-6’ isomer may predominate. The cluster orbitals affected most predominately by fluorine addition are those below the frontier. In general, they are more strongly n bonding along the 6-6’ linkage than along the 5-6‘
8560 J. Phys. Chem., Vol. 99, No. 21, 1995
Wang and Jansen
I
--
‘
-I
‘ \
-
\ \ \
I
‘
\
I I
‘-1
-I -I
-I
-3
-3
--I
.l2.0
-
\
I
\
11.0. --I
\
I
m9 -I
m 4 -5 -2
-1
-1
-2
\ \ \ \
‘-5 HOMO -5 -5
-4
MI10
Figure 6. Energy diagram of
C60
and its brominated adducts.
linkage and contribute to the stability of the fullerene cluster to a greater degree. Therefore, interaction with fluorine p-states of considerably lower energy destabilizes the bonding along the 6-6’ linkage. Similarly, the bonding along 5-6’ is destabilized; however, the cluster bonding is less affected by the addition of fluorine. Thus the primary driving force for site preference appears to be related to the retention of bonding within c 6 0 and not the formation of the C-F bond. For bromine the situation differs owing to the molecular orbital energies. For molecular bromine, the HOMO is positioned at an energy compatible with interaction with the frontier states of c60. This orbital is doubly degenerate, x antibonding, and thus of appropriate symmetry to interact at the 6-6’ site of c60. This interaction occurs through the LUMO states of C ~ O which are antibonding along the 6-6‘ linkage and bonding along 5-6‘. This interaction destabilizes cluster bonding in c 6 0 but restores some bonding in the molecular BrZ moiety as reflected in comparisons of the overlap population for isolated and bound diatomic species. Here molecular addition motifs may be observed. The 6-6’ ortho substitution site is the least favored owing to orbital factors which comprise cluster bonding as described previously. Similarly, the 5-6‘ site is destabilized, albeit to a less significant degree. The favored site appears to be the 1-4 para site. In this site, there is no residual Br-Br interaction. The correlation diagram provided in Figure 6 suggests that initial substitution occurs between atomic states of Br with one of the LLJh40 states of c60. These states of Br are positioned below the tl, states, and the bonding orbital that forms through interaction with Br is lowered in energy. Though this orbital is x-antibonding along the 6-6’ bond, owing to the halogen electronegativity or, more simply, orbital energies, the localization favors Br and the antibonding character is diminished somewhat. The C-Br bonding is significant and the orbital is stabilized. The halogen tends to be para directing in the case of Br. The HOMO and LUMO of the Br-C60 complex, though antibonding in nature, are localized at the 1-4 para site. This site did not appear active for fluorine substitution as an orbital energy match was not sufficient for covalent bonding. However, in the case of Br addition, the p-states of Br are energetically compatible with the frontier states of the monosubstituted fullerene moiety, and, thus, the initial substitution directs the Br to the para site. Since
the localization favors bromine, the new populated orbital is not strongly antibonding with the carbon cluster. It seems that this preference is defined by orbital effects but accommodates the steric effects satisfactorily. By these arguments, the preference for hydrogen substitution should be somewhat intermediate. Both the atomic and molecular states are positioned at an energy well below the frontier states of c60. Owing to the symmetry and nature of the orbital states, the C-H interaction is at a relative minimum compared to C-F and C-Br. The overall result is that no strong preference is observed for hydrogenation. The 5-6’ site appears the most favored as was the case for fluorine. The 1-4 site also appears active for the addition of hydrogen while the 1-3 meta site is the least favored. These affects are likely the result of orbital localization effects. In addition to the orbital effects discussed above, considerable charge localization occurs with the addition of a single substituent that further aids in understanding stability effects and site preference. Initially neutral in c60, the carbon at the substitution site is considerably oxidized by the addition of any halogen, with the effect being the greatest for fluorine. In addition to the substitution site being affected, the carbon at the adjacent site along the 5-6’ site is significantly oxidized as well, making this site more appealing for fluorination. There is no such “oxidation” at the adjacent carbon along the 6-6‘ direction or at the 1-3 meta site. In fact, the carbon atoms in these sites become slightly reduced upon addition of a single fluorine atom. The carbon at the 1-4 para site is slightly oxidized upon monosubstitution;however, the overall magnitude of the charge at this site is small. These observations help to further rationalize the addition chemistry for fluorine in which the anticipated frontier orbital interactions are defected by poor energy compatibility. These charges are displayed for a fragment of F-C60 in Figure 7. In this figure, the carbon fragment is simply “z-clipped” from the full structure of Cm for presentation; the full icosahedron has been used in the calculation of these charges. This effect is modulated by the electronegativity of the addenda. As the electronegativity of the substituent decreases, the charge localization and “oxidation” of the fullerene cluster is muted. Thus “electrostatic” preferences for site directed chemistry are reduced.
Halogenation and Hydrogenation of Cm
J. Phys. Chem., Vol. 99, No. 21, 1995 8561
\o. 5 8 ~ 0 . 0 1 s
(2) Matsuzawa, N.; Fukunaga, T.; Dixon, D. A. J . Phys. Chem. 1992, 96, 10747 and references therein. (3) (a) Fagan, P. J.; Chase, B.; Calabrese, J. C.; Dixon, D. A,: Harlow, R.: Krusic, P. J.; Matsuzawa, N.; Tebbe, F. N.; Thom, D. L.; Wasserman, E. (p 75); (b) Olah, G. A,: Bucsi, I.; Aniszfeld, R.; Prakash, G. A. S. (p 65). In Fullerenes: Kroto, H. W., Fisher, J. E., Cox, D. E., Eds.; Pergamon Press: New York, 1993 and references therein. (4) (a) Hawkins, J. M.; Meyer, A,; Lewis, T. A,; Loren, S. (Chapter 6). (b) Wudl, F.; Hirsch, A,; Khemani, K. C.; Suzuki, T.; Allemand, P.M.; Koch, A,; Eckert, H.; Srdanov, G.; Webb, H. M. Chapter 11, (c) Fagan, P. J.; Calabrese, J. C.; Malone, B. (Chapter 12). In Fullerenes Hammond, G. S., Kuck, V. J., Eds.; ACS Symposium Series 481; American Chemical Society: Washington, DC 1992 and references therein. ( 5 ) (a) Chen, F.; Singh, D.: Jansen, S. A. J . Phys. Chem. 1993, 97, 10958 and references therein. (b) Wang, S.-H.; Chen, F.; Kashani, M.; Malaty, M.; Jansen, S. A.; Fann, Y.-C. J . Phys. Chem., in press. ( 6 ) Wang, Z.; Shen, E.; Wang, L. Chem. Res. Chin. Univ. 1991, 7, 227. (7) Dixon, D. A.; Matsuzawa, N.: Fukunaga, T.; Tebbe, F. N. J . Phys. Chem. 1992, 96, 6107 and references therein. (8) Matsuzawa, N.; Dixon, D. A,; Krusic, P. J. J . Phys. Chem. 1992, 96, 8317 and references therein. (9) Seshadri, R.; Govindaraj, A,; Nagarajan, R.; Pradeep, T.; Rao, C. N. R. Tetrahedron Lett. 1992, 33, 2069. (10) Wudl, F. In Buckministerfullerenes; Billups, W. E., Ciufolini, M. A., Eds; VCH: New York, 1993; Chapter 13. (11) Tebbe, F. N.; Harlow, R. L.; Chase, D. B.; Thom, D. L.; Campbell, G. C., Jr.: Calabrese, J. C.: Heron, N.: Young. R. J., Jr.: Wasserman. E. Science 1992, 256, 822. (12) Taylor, R.; Langley, G. J.; Brisdon, A. K.; Holloway, J. H.: Hpoe, E. G.; Kroto, H. W.; Walton, D. R. M. J . Chem. SOC.,Chem. Commun. 1993, 875. (13) Okino, F.; Touhara, H.; Seki, K.; Mitsumoto, R.: Shigematsu, K.; Achiba, Y. Fullerene Sci. Technol. 1993, 1, 425. (14) Hirsch, A,; Lamparth, I.; Karfunkel, H. R. Angew. Chem., Znt. Ed. Engl. 1994, 33, 437. (15) Birkett, P. R.; Hitchcock, P. B.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Nature 1992, 357, 479. (16) (a) Disch, S. L.; Schlmen, J. N. Chem. Phys. Lett. 1986, 25, 465. (b) Reference 2 and references therein. (17) Stewart, J. J. P. J . Comput.-Aided Mol. Des. 1990, 4, 1. (18) Newton, M. D.: Stanton, J. M. J . Am. Chem. SOC.1986, IO8,2469. (19) Bakowies, D.; Thiel, W. J . Am. Chem. SOC.1991, 113, 3704. (20) Shibuya, T.-I.; Yoshitachi, M. Chem. Phys. Lett. 1986, 137, 13. (21) Feng, J.; Li, J.; Wang, Z.; Zemer, M. C. Int. J . Quantum Chem. 1990, 37, 599. (22) Aihara, J.-I.: Hosoya, H. Bull. Chem. SOC.Jpn. 1993.66, 1955 and references therein. (23) Douglas, B.; McDaniel, D.: Alexander, J. Concepts and Models of Znorganic Chemistry; John Wiley: New York, 1994; pp 89-91. (24) (a) Hoffmann, R. J . Chem. Phys. 1963, 39, 1397. (b) Hoffmann, R.; Lipscomb, W. M. J . Chem. Phys. 1962, 36, 3179. (c) Hoffmann, R.: Lipscomb, W. M. J . Chem. Phys. 1962,37, 2872. (d) Sanda, P. N.; Dove, D. B.; Ong, H. L.; Jansen, S. A.: Hoffmann, R. Phys. Rev. A 1989, 39, 2653. (e) Vuckovic, D. L.; Jansen, S. A,; Hoffmann, R. Langmuir 1990, 6, 732. (f) Otamiri, J.; Andersson, A,; Jansen, S. A. Langmuir 1990, 6, 365. (25) (a) Cahill, P. A.: Henderson, C. C.; Rohlfing, C. M.; Assink, R. A. Angew. Chem., Int. Ed. Engl. 1994.33, 786. (b) Cahill, P. A,; Henderson, C. C. Science 1993, 259, 1885. (26) (a) Koruga, D.; Hameroff, S.: Withers, J.; Loutfy, R.; Sundaresharn, M. Fullerene C60: History, Physics, Nonobiology, Nanotechnology; Elsevier Science Publishers: North-Holland, 1994: Chapter 3 for electron density figures of HOMO and LUMO: Chapter 4 for bond angles of carbon R bonds. (b) Haddon, R. C. Acc. Chem. Res. 1988, 21, 243. (c) Gaddon, R. C.: Brus, L. E.; Raghavachari, K. Chem. Phys. Lett. 1986, 125, 459.
-
‘I
0,020
-0.012
Figure 7. Charges for CmF,
Conclusions In this work we have presented an orbital reasoning for site preferences in the substitution chemistry of Cm. This reasoning provides a clear understanding of the mechanisms of halogenation and hydrogenation which are consistent with other modelling studies. In the case of fluorine substitution, electron transfer effects defeat molecular addition motifs, and the energy incompatibility of the frontier orbitals drives the preference for the 5-6’ and 6-6’ ortho sites. Though orbital mechanisms suggest 1,4 addition for F, the most stable isomer appears to be the 5-6’ or 6-6’ ortho site. This work parallels the results presented by Cahill et al.,25in which the kinetic product appears to be formed, 1,4 isomer, and is fairly rapidly converted to the thermodynamically stable 1,2 isomer. In the case of bromine substitution where a good match occurs in the frontier states, the addition is guided by the orbital interactions. Since the frontier states of Br-C60 are strongly localized at the 1,4 site, this becomes the stable isomer for subsequent addition. The effects for hydrogenation are intermediate between fluorine and bromine, and therefore the energy differences in the substitutional isomers are not as extreme. We are pleased that the orbital models discussed here agree well with previously published work. In addition, this work complements and further extends our understanding of the cage chemistry of C a . The analysis of orbital effects and reactivity should provide a foundation for further exploitation of the fullerene shell and contribute to rational design objectives for carbon clusters. The next obvious question in the course of the study is: “where and how do the third, fourth, ... addenda bind to CM?’
Acknowledgment. The authors gratefully acknowledge financial support for this work from the AFOSR, Grant No. F49620-93-1-0018, the Swern Fellows program, and Sun Oil for support of this work. References and Notes (1) Kratschmer, W.; Lamb, D. L.; Fostiropoulos, K.; Huffman, D. R. Nature, 1990, 347, 354.
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