7594
J . Phys. Chem. 1992, 96, 7594-7604
Semiemplrlcal Calculations of Dlhydrogenated Buckmlnsterfullerenes, C,& Nobuyuki Matsuzawa,*David A. Dixon,* and Tadamichi Fukunaga The Central Research and Development Department, E. I. du Pont de Nemours & Co. Inc., Experimental Station, P.O. Box 80328, Wilmington, Delaware 19880-0328 (Received: February 4, 1992)
The electronic structures of all possible C,H2 isomers in the singlet state with hydrogens external to the surface have been calculated with the PM-3 parameterization of the MNDO Hamiltonian. A localized polyene structure similar to that of c 6 0 is predicted for all of the C,H2 isomers in the lowest energy singlet state except for two. For these two singlet isomers, one is predicted to have partial charge-transfer character and the other to have a highly delocalized ?r system. The f i t ' s of the closed-shell singlet isomers are predicted to fall in the range of 776-836 kcal/mol as compared to a calculated value of 812 kcal/mol for Cm The M : ' s are predicted to increase by 8.5 kcal/mol for each double bond placed in a five-membered ring on hydrogenation. The calculated M f O ' s for the open-shell singlets and triplets are -8 15 kcal/mol and are essentially independent of the position of hydrogen attachment. If the number of double bonds in five-membered rings is less than 5 , the closed-shell singlet should be favored, whereas an open-shell structure is predicted for the other isomers. Only a few isomers are predicted to form exothermically by addition of H2to C, or by extraction of two hydrogens from alkanes. The trends in the HOMO and LUMO energies follow those of the M r O ' s . The values p, a,0, and -y for the closed-shell singlet state of the C,H2 isomers were calculated with a finite-field method. The values of a and y increases as Mfoincreases.
substituted internal to the ring. We have calculated the electronic structures of C,H,s' ( n = 1-5), C,F,'s ( n = 1-4), and C,Cl,s' Following theoretical predictions, a mass peak attributable to ( n = 1-4) for a specific substitution pattern at the MNDO/PM-3 C, (buckminsterfullerene)was observed in a carbon cluster beam level.2s However, a series of systematic calculations of the C&, in 1985.l The authors proposed that the peak corresponded to isomers has not yet been reported, although the structures of these c 6 0 with a structure of a truncated icosahedron (I,, symmetry). molecules would be a starting point for the investigation of the Subsequently, a significant amount of work2 on C, as well as Cm properties of substituted fullerenes. was done, but no macroscopic amounts of the molecules were There are three ways for adding an X-Y molecule such as isolated. Kratschmer et al.3 developed a method for preparing hydrogen to C,: (1) the addition across one of the double bonds, macroscopic amounts of the fullerenes, allowing these new forms which requires no other changes in the canonical bonding structure of carbon clusters, especially Ce to be studied by normal chemical of C,; (2) the conjugate addition of two atoms, which forces methods. Many different experiments including infrared3 and changes in the cannonical bonding structure of C, in order to Raman4 spectroscopy, NMR,5 scanning tunneling microscopy,6 maintain a closed-shell Kekuli structure; and (3) the hydrogenand X-ray diffraction7".' have been done to support and confirm olysis that accompanies cleavage of a bond in C., One of the the proposed icosahedral structure of C., Presently, new results likely results of process 3 could be the formation of a highly on C,, C70, and other fullerenes are being published weekly. symmetric isomer of C&, with Th or oh symmetry without the Substituted fullerenes will be an important area of research for C, framework remaining intact, which will be discussed in more discovering compounds with novel properties for new applications. detail in a future publication. In this study, we focus on the However, only a few derivatives of c60 with discrete structures hydrogenation products, C&,, derived from the first two addition have been reported. For example, the complex of C, and C70 1 and 2. with metals7J such as C , - O L ~ O ~a C,-platinum ,~~~~ c o m p l e ~ , ~ ~ processes, ~~ There are 23 ways of uniquely adding 2 hydrogens externally and a variety of doped systems containing boron? alkali on the face of C,. The CWH2isomers are listed in Table I and and transition metals12have been synthesized. Hydrogenation designated as follows. First, the carbon atoms are numbered as of Cm via Birch reduction has been shown to yield CmH36.13 indicated in a Schlegel diagram of CmZ6in Figure la. The Fluorination of c60 and C70 has also been reported,I4 and the diagram also shows a canonical Kekuld structure of CW The 30 formation of C60F36,C7oF40, and other fluorinated Ca's and C7,,'s double bonds are located at the edges shared by 2 fused 6-memwas codmed from mass spectrometry.Ib An oxide of C70,C7,0, bered rings (for example, the C1
