Fused five-membered rings determine the stability of C60F60

The 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 (6, 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 (2, F8@C60F52) with eight endo C-F bonds is higher in energy by 22.6 kcal/mol. This is in contrast to the isolated pentagon rule for the stability of fullerenes. The mean bond dissociation energy of 6 is larger than those of the experimental known C60F36, C60F48, and graphite fluoride. The relative energy per CF unit of 6 to graphite fluoride (CF)n is 3.7 kcal/mol, which is smaller than that of C60 fullerene per carbon to graphite (about 9-10 kcal/mol).     

Quantum chemical study of the reactivity of C60HR and C60(CHR) derivatives

In the present work a quantum chemical study of a series of substituted hydrofullerenes, C(60)HR, and a series of methanofullerenes, C(60)(CHR), is presented. Their reactivity and geometrical, energetic, electronic, and magnetic properties, as well as the influence of the substituent, are discussed. As a probe of the reactivity, the acidic properties of these fullerene derivatives were predicted, based on the calculated deprotonation energies, with a previously set up scheme. The electronic delocalization upon deprotonation was described, and the global (magnetizabilities) and local aromaticity (nucleus-independent chemical shifts) was analyzed and compared with respect to the group properties for the series of functional groups. The geometries of both acidic and basic forms were fully optimized at the AM1 level, and all property calculations were performed at the HF/3-21G and the B3LYP/6-31G* level of theory.     

C24团簇结构与稳定性的理论研究

采用量子化学HF, B3LYP和MP2方法,选用6-31G^*, 6-311G^*, cc-pVDZ和cc-pVTZ基组,对C₂₄团簇的6种异构体进行了优化,并对它们的几何构型、振动频率、核独立化学位移(NICS)和稳定性进行了讨论,比较C₂₄团簇各种异构体的稳定性.研究表明:在6-311G^*和cc-pVDZ水平上,B3LYP方法给出的稳定性大小顺序分别为c＞f＞b＞e＞a＞d和c＞b＞f＞a＞e＞d, MP2方法给出的稳定性大小顺序为b＞c＞a＞e＞f＞d.     

C_(59)F_(2n)HN(n=1,2)异构体的结构与稳定性的理论研究

分别用MNDO和AM1两种半经验方法,对C59F2nHN (n = 1, 2) 的异构体进行几何构型全优化,结合频率分析及HF/6-31G单点能计算,确定了各异构体的基态结构及其相对稳定性。计算结果表明,C59HN的F加成物的立体选择性规律与C60的不同,最稳定异构体不是1-2加成物。C59F2HN的最稳定异构体是1-4加成的6, 18-或12, 15-异构体; C59F4HN的最稳定异构体是1-4,1-4加成的6, 18; 12, 15-异构体,其能量远小于其它各异构体的能量。N原子取代碳笼骨架C原子后,改变了碳笼F加成物的立体选择性规律。     

In the first proven SN2' fullerene reaction,both C3 and C1 C60F36 hydrolyse to C1 isomers of C60F35OH that eliminate HF to give epoxides C60F34O; C60F36O oxides are shown to be ethers,and a fourth isomer of C60F36 exists

On standing in organic solvents containing traces of water, C3 and C1 isomers of C60F36 slowly convert to C1 isomers of C60F35OH. Both fluorofullerenols eliminate HF during EI mass spectrometry to give C60F34O epoxides, one fullerenol being much less stable than the other to the extent that the mass spectrum shows only the epoxide. Both C60F35OH isomers have C1 symmetry, one being identified by the remarkable linear relationship between chemical shifts in its 19F NMR spectrum and those in the spectrum of C1 C60F36; the spectrum of the other shows the pattern of C3 C60F36 rendered asymmetrical by the replacement of one F by OH. The reactions are facilitated by the presence of isolated double bonds, and provide the first proven examples of an SN2' reaction of a fullerene derivative. Our observation explains why only a limited number of fluorines are readily replaced in C60F36 and why C60F18 is by contrast much more resistant to hydrolysis. We have isolated also a pure isomer of C60F36O, which is shown to be an oxahomofullerene (ether) apparently derived from C1 C60F36, and an impure fraction comprising a fourth isomer of C60F36, a trifluoromethyl derivative of C60F36, a second isomer of C60F36O, and an unknown species of 1392 u.     

Electrochemical, spectroscopic, and DFT study of C60(CF3)n frontier orbitals (n = 2-18): the link between double bonds in pentagons and reduction potentials

The frontier orbitals of 22 isolated and characterized C(60)(CF(3))(n) derivatives, including seven reported here for the first time, have been investigated by electronic spectroscopy (n = 2 [1], 4 [1], 6 [2], 8 [5], 10 [6], 12 [3]; the number of isomers for each composition is shown in square brackets) fluorescence spectroscopy (n = 10 [4]), cyclic voltammetry under air-free conditions (all compounds with n 相似文献   

NMR Properties of Polylithiated C60

Endohedral, ¹³C, ⁷Li, and nucleus‐independent (NICS) chemical shifts are reported for selected Li_nC₆₀ isomers (n = 6, 12, 18) at the GIAO (gauge‐including atomic orbitals)‐SCF/DZP//BP86/3–21G level. Li₆C₆₀ closely resembles C₆₀^6– in terms of NMR criteria for aromaticity, as evidenced by an exceptionally high endohedral shielding. In contrast, nonaromaticity is indicated for Li₁₂C₆₀, based on a positive endohedral chemical shift. NICS and δ(endo) values very similar to those of Li₁₂C₆₀ are obtained for Li₁₈C₆₀. According to population analysis, indeed the same number of electrons are transferred to the fullerene cage in both cases. Endohedral chemical shifts, accessible via ³He NMR of the corresponding endohedral helium compounds, could thus be a valuable indicator for the extent of reduction of the C₆₀ molecule. Energetic estimates suggest that in the bulk, Li₁₂C₆₀ should be unstable with respect to decomposition into Li₆C₆₀ and lithium metal.     

Structure and reactivity of C54q+ (q = 0, 1, 2 and 4) fullerenes

Using density functional theory we have studied the structural properties of eleven C54 isomers that appear in the C60 fragmentation. We have evaluated the relative stability of the different isomers with respect the most stable one, which corresponds to the structure with the minimum number (four) of adjacent pentagons. On average, the length of a bond shared by pentagons and/or hexagons increases in the order hexagon-hexagon, hexagon-pentagon and pentagon-pentagon. However, we have found that the central bond in the confluence of four hexagons, i.e. a pyrene substructure, is anomalously large, becoming in some cases the largest one. We have also evaluated the nucleus-independent chemical shifts (NICS) at the center of every individual ring in the most stable isomers. For the chlorine derivatives, our calculations show that the favorite position for chlorine addition are the bonds shared by pentagons.     

Synthesis, structure and theoretical study of mixed fluoro-trifluoromethyl derivatives of C60. Molecular structures of C60F18(CF3)6 and C60F16(CF3)6

A series of novel mixed C60Fn(CF3)m compounds has been produced by trifluoromethylation of C60F18 with CF3I in ampoules at 380-420 degrees C. Two of these compounds, C60F18(CF3)6 and C60F16(CF3)6, have been characterized by X-ray crystallography, which has revealed addition of six CF3 groups to the C3v-C60F18 for the former and replacement/elimination of two outermost F atoms in the latter. Quantum chemical calculations have been employed to predict the most stable possible isomers of C60F16/18(CF3)6 in order to rationalize the experimental results.     

Th-C60F24

The regioselective fluorination of Th-C60Br24 with XeF2 in anhydrous HF at 25 degrees C produced Th-C60F24, which is isostructural with Th-C60Br24. The compound, the first example of a fluorofullerene with a noncontiguous pattern of F-bearing sp3 C atoms, was characterized by EI and ESI mass spectrometry and by 19F NMR (single resonance at delta -144 for all 24 F atoms), 13C NMR (two resonances at delta 145.9 (C=C) and delta 83.5 (C-F; JCF = 212 Hz), and FTIR spectroscopy. DFT calculations revealed that at least two other isomers of C60F24 are more stable than Th-C60F24 by >/=280 kJ mol-1, demonstrating that Th-C60F24 is a kinetically stable fluorofullerene.     

Photoelectron spectroscopy and electronic structures of fullerene oxides: C60Ox- (x = 1-3)

We report a photoelectron spectroscopy (PES) study on a series of fullerene oxides, C60Ox- (x = 1-3). The PES spectra reveal one isomer for C60O-, two isomers for C60O2, and multiple isomers for C60O3-. Compared to C60, the electronic structures of C60Ox are only slightly perturbed, resulting in similar anion photoelectron spectra. The electron affinity of C60Ox was observed to increase only marginally with the number of oxygen atoms, x, from 2.683 eV for C60, to 2.745 eV for C60O, and 2.785 eV/2.820 eV for C60O2 (two isomers). We also carried out theoretical calculations, which confirmed the observed isomers and showed that all the fullerene oxides are in the form of epoxide. The PES and theoretical calculations, as well as molecular orbital analysis, indicate that addition of oxygen atoms to the C60 cage only modifies the local carbon network and leave the rest of the fullerene cage largely intact geometrically and electronically.     

Structures, stabilities, and electronic and optical properties of c(58) fullerene isomers, ions, and metallofullerenes.

The 1205 classical isomers of fullerene C58, as well as one quasi-fullerene C58 isomer with a heptagonal ring (labeled as Cs:hept) have been investigated by the quantum chemical methods PM3, HCTH/3-21G, and B3LYP/6-31G(d). Isomer C3v:0001, which has the lowest number of adjacent pentagons, is predicted to be the most stable isomer, but the quasi-fullerene isomer Cs:hept is only 2.50 kcal mol-1 higher in energy. Systematic investigations of the electronic properties of C3v:0001 and Cs:hept find that the C3v:0001 isomer has high vertical electron affinity (3.19 eV). The nucleus-independent chemical shifts (NICS) value at the center of Cs:hept (-5.1 ppm) is more negative than that of C60 (-2.8 ppm). The NICS value at the center of the heptagonal ring in Cs:hept (-2.5 ppm) indicates weakly aromatic character. In contrast, the C58(6-) and C58(8-) ions of the C3v:0001 and Cs:hept geometries possess large aromatic character, with NICS values between -14.0 and -26.2 ppm. To clarify the thermodynamic stabilities of C58 isomers at different temperatures, the entropy contributions are taken into account on the basis of the Gibbs energy at the B3LYP/6-31G(d) level. The C3v:0001 isomer prevails in a wide range of temperatures, and the Cs:hept isomer is also an important component around 2800 K. The IR spectra of C58 isomers are simulated to facilitate experimental identification of different isomers. In addition, the electronic spectra and the second-order hyperpolarizabilities are predicted by ZINDO and the sum-over-states model. The static second-order hyperpolarizability of the C3v:0001 isomer is 96.5 % larger than that of C60, and its second-order hyperpolarizabilities at external field frequencies are at least nine times larger than those of C60.     

On the structure and relative stability of C50 fullerenes

The complete set of 271 classical fullerene isomers of C50 has been studied by full geometry optimizations at the SAM1, PM3, AM1, and MNDO quantum-chemical levels, and their lower energy structures have also been partially computed at the ab initio levels of theory. A D(5h) species, with the least number of pentagon adjacency, is predicted by all semiempirical methods and the HF/4-31G calculations as the lowest energy structure, but the B3LYP/6-31G* geometry optimizations favor a D3 structure (with the largest HOMO-LUMO gap and the second least number of adjacent pentagons) energetically lower (-2 kcal/mol) than the D(5h) isomer. To clarify the relative stabilities at elevated temperatures, the entropy contributions are taken into account on the basis of the Gibbs energy at the HF/4-31G level for the first time. The computed relative-stability interchanges show that the D3 isomer behaves more thermodynamically stable than the D(5h) species within a wide temperature interval related to fullerene formation. According to a newly reported experimental observation, the structural/energetic properties and relative stabilities of both critical isomers (D(5h) and D3) are analyzed along with the experimentally identified decachlorofullerene C50Cl10 of D(5h) symmetry. Some features of higher symmetry C50 nanotube-type isomers are also discussed.     

Variable-temperature 19F NMR and theoretical study of 1,9- and 1,7-C60F(CF3) and C(s-) and C1-C60F17(CF3): hindered CF3 rotation and through-space J(FF) coupling

Milligram amounts of the new compounds 1,9- and 1,7-C60F(CF3) (ca. 85:15 mixture of isomers) and C60F3(CF3) were isolated from a high-temperature C60/K2PtF6 reaction mixture and purified to 98 mol % compositional purity by two-dimensional high-performance liquid chromatography using Buckyprep and Buckyclutcher columns. The previously observed compounds C60F5(CF3) and C60F7(CF3) were also purified to 90+ mol % for the first time. Variable-temperature 19F NMR spectra of the mixture of C60F(CF3) isomers and the previously reported mixture of C(s)- and C1-C60F17(CF3) isomers demonstrate for the first time that fullerene(F)n(CF3)m derivatives with adjacent F and CF3 substituents exhibit slow-exchange limit hindered CF3 rotation spectra at -40 +/- 10 degrees C. The experimental and density functional theory (DFT) predicted deltaH++ values for CF3 rotation in 1,9-C60F(CF3) are 46.8(7) and 46 kJ mol(-1), respectively. The DFT-predicted deltaH++ values for 1,7-C60F(CF3), C(s)-C60F17(CF3), and C1-C60F17(CF3) are 20, 44, and 54 kJ mol(-1), respectively. The (> or = 4)J(FF) values from the slow-exchange-limit 19F spectra, which vary from ca. 0 to 48(1) Hz, show that the dominant nuclear spin-spin coupling mechanism is through-space coupling (i.e., direct overlap of fluorine atom lone-pair orbitals) rather than coupling through the sigma-bond framework. The 2J(FF) values within the CF3 groups vary from 107(1) to 126(1) Hz. Collectively, the NMR data provide an unambiguous set of (> or = 4)J(FF) values for three different compounds that can be correlated with DFT-predicted or X-ray diffraction derived distances and angles and an unambiguous set of 2J(FF) values that can serve as an internal standard for all future J(FF) calculations.     

Infrared, Raman, and DFT vibrational spectroscopic studies of C60F36 and C60F48

IR and Raman spectra of two fluorofullerenes, C60F48 and C60F36, are thoroughly studied. Assignment of the experimental spectra is provided on the basis of density functional theory (DFT) computations. Perfect correspondence between experimental and computed spectra enabled us to confirm that the major isomer of C60F48 has D3 symmetry. It was found that as-synthesized samples of C60F36 consist mainly of C3 and C1 isomers in ca. 2:1 ratio and 2-3% of T-symmetric structures. Extensive AM1 and DFT computations have shown that all three structures are the most stable isomers of C60F36. Previous structural assignment of the C3 isomer (Gakh, A. A.; Tuinman, A. A. Tetrahedron Lett. 2001, 42, 7137-7139) was confirmed by the vibrational data.     

Unusual addition patterns in trifluoromethylation of [60]fullerene

From pyrolytic trifluoromethylation of [60]fullerene with CF3CO2Ag at 300 degrees C we have isolated ca. sixty C60(CF3)n isomers (numbers in parentheses) as follows: n = 2(1), 4(8), 6(13), 8(21), 10(11), 12(5), 14(4), twenty-one of which have been characterised by 19F NMR. Compounds with addition levels up to n = 20 have also been identified. With increasing value of n, yields decrease and the separation of compounds of similar HPLC retention time but different addend levels becomes more difficult. Many of the 19F NMR spectra show combinations of quartets and septets (the latter tending to be more downfield) due to 'linear' addend arrays. The spectra are consistent with addition across both 6:6- and 5:6-ring junctions [double (1.2) and single (1.6) bonds, respectively], giving corresponding coupling constants for adjacent addends of ca. 14.5 and 12.0 Hz respectively, the differences being attributable to the different 1.2- and 1.6-bond lengths. The 13C NMR spectrum of C60(CF3)2 shows the CF3 groups are in either a 1.4- or 1.6-relationship; the UV-vis band appears at 442 nm. Other unsymmetrical tetra-adducts are comprised of isolated pairs of CF3 groups. The exceptionally large number of derivatives and isomers, (much greater than in any other fullerene reaction), no dominant product, and unusual addition pattern indicates that thermodynamic stability is not of primary importance in governing product formation. EI mass spectrometry of trifluoromethylfullerenes is characterised by loss of CF3 groups, the more highly addended compounds also showing fragmentation by CF2 loss, attributable to steric compression. The CF3 group shows strong IR bands at ca. 1260 and 1190 cm-1. The compounds are stable to aq. acetone, which contrasts to the behaviour of fluorofullerenes. Trifluoromethylation by the Scherer radical (C9F19.) gave addition of up to eight CF3 groups, together with hydrogen in some products. During EI mass spectrometry of some of these, loss of HF attributable to CF3 and H adjacency can occur, giving CF2-containing derivatives.     

Substitution Effect in NICS Values in Tropylium Ion Derivatives: An Ab Initio Study

Hartree–Fock (HF) and hybrid density functional theory (B3LYP) calculations were performed on tropylium ion and 19 of its mono- and diheteroatomic derivatives. The aromaticity in this class of compounds is evaluated based on the nucleus independent chemical shift (NICS) values. The NICS values are calculated at the center of the rings NICS (0) and at 1Å above the molecular plane NICS (1). The geometry optimization and NICS calculations were carried out at the HF/6–311+G?? and at the B3LYP/6–311+G (2d, p) density functional level, respectively. These calculations in the effects of heteroatoms such as N, B, P, and Si are considered on aromaticity, molecular properties, NICS values, and structural parameters.     

Prediction of low-energy isomers of large fullerenes from C132 to C160

To predict energetically favored isomers, we used a topological scheme as a prescreening tool to select candidate isomers for each fullerene from C(106) to C(160). Comparison with the PM3 and tight-binding (TB) potential calculated results and few published data for the low-energy isomers of C(106) to C(130) indicates that the prescreening approach is feasible. For each fullerene from C(132) up to C(160), the selected 1000 candidate isomers were further optimized by PM3 and TB potential. The analysis of the semiempirical PM3 and TB results of C(106) to C(160) provides some qualitative features of the large fullerenes. Furthermore, calculations at the B3LYP/6-31G*//B3LYP/3-21G level of theory were carried out on the top ten PM3 and TB low-energy isomers of C(132) to C(160) to accurately predict the stable isomers, and the HOMO-LUMO gap, the ionization energy, and electron affinity of the lowest-energy isomers were also investigated at the same level.     

Influence of electron doping on the hydrogenation of fullerene C60 : a theoretical investigation

The influence of electron attachment on the stability of the mono- and dihydrogenated buckminsterfullerene C(60) was studied using density functional theory and semiempirical molecular orbital techniques. We have also assessed the reliability of computationally accessible methods that are important for investigating the reactivity of graphenic species and surfaces in general. The B3LYP and M06L functionals with the 6-311+G(d,p) basis set and MNDO/c are found to be the best methods for describing the electron affinities of C(60) and C(60)H(2) . It is shown that simple frontier molecular orbital analyses at both the AM1 and B3LYP/6-31G(d) levels are useful for predicting the most favourable position of protonation of C(60)H(-) , that is, formation of the kinetically controlled product 1,9-dihydro[60]fullerene, which is also the thermodynamically controlled product, in agreement with experimental and previous theoretical studies. We have shown that reduction of exo- and endo-C(60)H makes them more stable in contrast to the reduction of the exo,exo-1,9-C(60)H(2) , reduced forms of which decompose more readily, in agreement with experimental electrochemical studies. However, most other dihydro[60]fullerenes are stabilized by reduction and the regioselectivity of addition is predicted to decrease as the less stable isomers are stabilized more by the addition of electrons than the two most stable ones (1,9 and 1,7).     

C40异构体的结构和稳定性的理论研究

利用Gaussian98程序,采用密度泛函(DFT)方法中的B3LYP,选用6-31G基组对富勒烯(Fullerene)C40的6种异构体[D5d,Td,D2h,C3v,D2(Ⅰ),D2(Ⅱ)]进行了几何构型优化,其中,对于Td对称性的C40由于易发生Jahn-Teller畸变,则降低其对称性为D2d,再进行优化.对它们的平衡几何和电子结构进行了比较具体的分析,同时,根据计算得到的总能量推断出这6种异构体的稳定性顺序是D2(Ⅰ)>D5d>Td>C3v>D2h>D2(Ⅱ).