The Structures And And Stabilities Of Fluorinated And Hydrogenated Fullerenes

Regular polyhedral clusters (CH)n have long been the subject of scientific curiosity. The discovery of fullerene (C60) had stimulated tremendous activity in this area and the structure and stability of polyhedral clusters were concerned by many studies. For Cn polyhedral clusters, it was found that the most stable isomers are comprised of five- and six-membered rings and the five-membered rings trend to be isolated as greatly as possible. The (BN)n polyhedral clusters, the number of valence electrons being equal to C2n clusters, are comprised of four- and six-membered rings, and the four-membered rings trend to be isolated. Both stable Cn and (BN)n polyhedral have cage-like structure.The hydrogenated (BN)n, namely (HBNH)n, as well as their analogy, (HGaNH)n polyhedral clusters, also being comprised of four- and six-membered rings as (BN)n, however, have tube-like structure. The distribution of four-membered rings in (HBNH)n and (HGaNH)n is totally different from that in (BN)n. In (HBNH)n, six four-membered rings cluster to two groups and locate at the ends of tube. All of CH, (HBNH)1/2, N and BCO have same number of valence electrons. The most stable Nn polyhedral clusters are comprised of three-, five- and six-membered rings, and also have tube-like structure. Stable (BCO)n polyhedral clusters are comprised of three- and six-membered rings. They also have tube-like structure with three-membered rings reside at the ends of tube.(CH)n polyhedral had received considerable attentions for a long time. However, for small (CH)n (n=4-24), previous studies mainly focused on the strain in their cage-like structures. For (CH)60, nearly all studies are based on the Ih C60H60 structure with isolated five-membered rings. We are interesting to know whether most stable large (CH)n cluster, like C60H60, has tube-like structure; whether (CF)n and (CH)n have same stable structures?By means of density functional method, the structures and stabilities of (CH)n and (CF)n (n=4-60), as well as (SiH)60 have been calculated and the relationship between structure and stability for these polyhedral clusters are investigated in detail. We also take an initial study about how the outer chemical bonds affect the endo atom in a C60 cage. The main contents and results are as follows.1. The stability of tube and cage C60F60The structure and stability of a set of (CF)60 isomers have been computed at the B3LYP/6-31G(d) density functional theory level. The most stable isomer (F4@C60F56) has tube-like structure with four endo C—F bonds and fused five-membered rings at the end of the tube, while the reported most stable cage structure (F8@C60F52) with eight endo C–F bonds is higher in energy by 94.6 kJ/mol. This is in contrast to the isolated pentagon rule for the stability of fullerenes. The mean bond dissociation energy of tube-like F4@C60F56 is larger than those of the experimental known C60F36, C60F48 and graphite fluoride. The relative energy per CF unit of tube-like F4@C60F56 to graphite fluoride (CF)n is 15.5 kJ/mol, which is smaller than that of C60 fullerene per carbon to graphite (about 37-42 kJ/mol).2. Tube and Cage C60H60: A Comparison With C60F60Tube C60H60 with fused five-membered rings is more stable than the cage isomer with isolated five-membered rings. Introduction of endo C–H bonds into tube isomer results in further stabilization, but the most stable tube structure with four endo C–H bonds is higher in energy than the most stable cage structure with ten endo C–H bonds by 310.5 kJ/mol. A comprehensive comparison of C60H60 with C60F60 has been made.3. Polyhedral Fluorocarbon (CF)n– Structures and EnergiesThe structures and energies of selected (CX)n (n = even, 6-60, X = F or H) polyhedra, computed at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d,p) level show (CH)n isomers to be generally less strained than the corresponding (CF)n analogs. For n = 6-24, both (CH)n and (CF)n favor the same geometric patterns. The tube isomers (t-(CX)n) of large (n = 26-60) species with fused five-membered rings at the tube ends are thermodynamically more stable than the cage isomers (c-(CX)n), deduced from the most stable carbon cages. The stabilities of tube isomers are determined by the distortion of the bond angles (angle strain) and the non-bonded F···F repulsive interactions. The latter have been quantified by means of homodesmotic equations. Four structural types of t-(CX)n structures are built on the basis of the number of fused five-membered rings; and the thermodynamically more stable t-(CX)n structures have more fused five-membered rings, in opposition to the isolated pentagon rule governing the stability of carbon fullerenes.4. Structure and Stability of Tube and Cage Si60H60Among all 1812 all-exo (SiH)60 fullerene isomers, the most stable isomer has a tube-like configuration (D5d) with twelve five-membered rings fused and located at the ends of the tube. The most stable cage isomer can admit ten to twelve endo Si–H bonds (H10@Si60H50 and H12@Si60H48), and they represent the most stable (SiH)60 isomers. It is found that (SiH)60 isomers have much lower angle and torsion strains than (CH)60 isomers.5. The structure and stability of X@C60F ,X@C60F2(X=N,H) and (H@C60)2The structure and stability of endohedral X@C60Fn (X = N, H; n = 1, 2) and (H@C60)2 are computed at the B3LYP level of density functional theory. The most stable N@C60F has one endo N–C bond, while N@C60F2 favors nitrogen atom at cage center. Both H@C60F and H@C60F2 favor isomers with one endo C–H bond. The most stable dimer structure, (H@C60)2, has one inter-cage C–C bond and two endo C–H bonds, and is stable below 650 K; but dissociate above 650 K.