The Theoretical Study On Structures And Stabilities Of Derivatives Of Smaller Fullerenes And Quasi-Fullerenes

Since the discovery and macroscopic scaled synthesis of C60, the fullerenes have aroused tremendous scientific interest because of their novel and unusual physical and chemical properties, and were extensively studied. Geometries and stabilities of fullerenes are governed by the well-known isolated pentagon rule (IPR), and the Ih C60 is the smallest carbon cage fulfilling this rule. The smaller fullerenes C2n (2n < 60) violating IPR, inevitable suffer large strain energies and show high lability. Besides that, quasi-fullerenes violating the classical definition, contain rings of other size, for example 4 or 7, are expected to suffer extra local strain or/and further loss ofπdelocalization, and thus highly unstable. Most of smaller fullerenes and quasi-fullerenes are not experimentally approachable at present, and therefore, theoretical studies are significant for investigating their structures, stabilities and properties. In this thesis, derivatives of smaller fullerenes and quasi-fullerenes have been studied by quantum chemical method. The main results are summarized as follows.1. The endohedral complexes formed by D3 C32 and 13 atoms ranging fromⅠA toⅤA, including H, Li, Na, K, Be, Mg, Ca, B, Al, C, Si, N, and P, as well as 9 cations ranging fromⅠA toⅢA, including H+, Li+, Na+, K+, Be2+, Mg2+, Ca2+, B3+, and Al3+, were theoretically investigated using density functional theory at the B3LYP/6-31G(d) level. Theoretical studies reveal that size of all endohedral fullerenes is slightly enlarged due to encapsulation, and cage containing atom is larger than the cage containing corresponding cation. Thermodynamic stability of neutral endohedral complex is determined by electronegativity of the guest atom, and only Li, Na, and Ca atoms can be stably put into the cage.2. The endohedral complexes formed by D5h C50 and H, alkali metals, and alkaline-earth metals, as well as charged states of the endohedral complexes, were theoretically investigated using B3LYP/6-31G(d) method. Theoretical studies reveal that the encapsulated H hardly affects the D5h C50, but the encapsulated metals significantly distort the cage. As the extra charges on the complexes increase, the structural distortions decrease for Li, Na, K, Mg, Ca series derivatives, but increase for Be series derivatives. Both EHOMO and ELUMO of complexes are determined by the number of extra charges on them. The D3 endohedral complexes are found to change in the same way as the D5h isomers.3. PM3, HCTH/3-21G, and B3LYP/6-31G(d) methods were orderly used for searching the most stable C2-C52H2 isomer. C2-C52X2 (X = F and Cl) isomers being isostructural with the first 19 most stable C2-C52H2 isomers were considered as candidates of the most stable one, and optimized at the B3LYP/6-31G(d) level. Theoretical studies reveal that the first 19 most stable C2-C52X2 (X = H, F, and Cl) isomers are energetically favorable, and the attaching sites of the first 5 most stable isomers all locate the pentagon-pentagon fusions of C2-C52. The additional reactions release the strain energy of C2-C52, and enhance its chemical stability. Relative stability of C2-C52X2 (X = H, F, and Cl) isomers is determined by both attaching sites and properties of addends. The C2-C52X2 (X = H, F, and Cl) complexes are potential targets in experiments. 4. To investigate the influence of size and polarity of endohedral molecules on fullerene derivatives, three H2@C58Hn, six CO@C58Hn and six LiH@C58Hn (n = 0, 18) complexes were optimized using density functional theory at the B3LYP/6-31G(d) level. The results show that C58Hn (n = 0, 18) cages destabilize nonpolar H2 and weakly polar CO, and stabilize strongly polar LiH. For cage encapsulating H2 or CO, size of endohedral molecule is the main factor determining its orientation inside the cages. Orientation of LiH inside C58Hn (n = 0, 18) cages is determined by the dipole-induced dipole attractive interaction between them, which is especially significant in LiH@C58H18 complexes.5. AM1, HF/STO-3G, B3LYP/3-21G, and B3LYP/6-31G(d) methods were employed orderly to search the most stable hept-C62F2 isomer. The hept-C62X2 (X = Cl and Br) isomers, which are isostructural with the first 5 most stable hept-C62F2 isomers, were chosed as candidates of the most stable one, and optimized at the B3LYP/6-31G(d) level. From the results of the first 5 most stable hept-C62X2 (X = F, Cl, and Br) isomers, it is found that halogenations release the strain energy of hept-C62, and enhance its chemical stability. All 5 most stable hept-C62X2 (X = F, Cl, and Br) halogenated complexes are energetically favorable, and their thermodynamic stability decreases following with the increase of size of addends. The hept-C62F2 complexes are potential targets in experiments with high thermodynamic stability.In all, theoretical investigations in the thesis reveal that structures and stabilities of endohedral complexes of smaller fullerenes are influenced by factors such as electronegativity, polarity, size of endohedral spcies, and capability of fullerenes. The encapsulation usually can enhance the electronic structures of smaller fullerenes, but cannot release their strain energies effectively. That is the reason why experimental isolation for the endohedral complexes of smaller fullerenes is difficult. However, additional reactions of H2/halogens can significantly release the strain energies of smaller fullerenes and quasi-fullerenes, and enhance their chemical stability. Therefore, the corresponding hydrofullerenes and halofullerenes show high thermodynamic stability, and are potential targets in experiments.