Stability of silicon-doped C60 dimers

A theoretical investigation on the structure, stability, and thermal behaviors of the smallest polymeric units, the dimers, formed from substitutionally Si-doped fullerenes is presented. A density functional based nonorthogonal tight-binding model has been employed for describing the interatomic interactions. The study focuses on those polymeric structures which involve Si-Si or Si-C interfullerene bonds. The binding energy of the dimers increases with their Si content from about 0.25 eV in C(60)-C(60) to about 4.5 eV in C(58)Si(2)-C(58)Si(2). Moreover, the C(59)SiC(59) dimer, linked through the sharing of the Si atom between the two fullerenes, has been also considered. Upon heating, the dimers eventually fragment into their constituent fullerene units. The fragmentation temperature correlates with the strength of the interfullerene bonds. C(58)Si(2)-C(58)Si(2) exhibits a higher thermal stability (fragmentation temperature of approximately 500 K) than the pure carbon C(60)-C(60) dimer (with a fragmentation temperature of approximately 325 K). Given the higher structural and thermal stabilities of the Si-doped fullerene dimers, the authors propose the use of substitutionally Si-doped fullerenes as the basic units for constructing new fullerene-based polymers.     

Stabilities of multiply charged dimers and clusters of fullerenes

The authors find even-odd variations as functions of r (...+[C60]2(r+)([C60C70](r+)) electron-transfer collisions. This even-odd behavior is in sharp contrast to the smooth one for fullerene monomers and may be related to even-odd effects in dimer ionization energies in agreement with results from an electrostatic model. The kinetic energy releases for dimer dissociations [predominantly yielding intact fullerenes [C60]2(r+)-->C60(r1+)+C60(r2+) in the same (r1=r2) or nearby (r1=r2+/-1) charge states] are found to be low in comparison with the corresponding model results indicating that internal excitations of the separating (intact) fullerenes are important. Experimental appearance sizes for the heavier clusters of fullerenes [C60]n(r+) (n>3 and r=2-5) compare well with predictions from a new nearest-neighbor model assuming that r unit charges in [C60]n(r+) are localized to r C60 molecules such that the Coulomb energy of the system is minimized. The system is then taken to be stable if (i) two (singly) charged C60 are not nearest neighbors and (ii) the r C60(+) molecules have binding energies to their neutral nearest neighbors which are larger than the repulsive energies for the (r-1) C60(+)-C60(+) pairs. Essential ingredients in the nearest-neighbor model are cluster geometries and the present results on dimer stabilities.     

Regioselective synthesis of 1,4-di(organo)[60]fullerenes through DMF-assisted monoaddition of silylmethyl Grignard reagents and subsequent alkylation reaction

Monoaddition of Grignard reagents, in particular tri(organo)silylmethylmagnesium chlorides, to [60]fullerene took place smoothly in the presence of dimethylformamide to produce (organo)(hydro)[60]fullerenes, C60R(1)H, in good yield (up to 93% isolated yield). The hydrofullerene was then deprotonated to generate the corresponding anion, C60R(-), which was then alkylated to obtain 58pi-electron di(organo)[60]fullerenes, C60R(1)R(2), in good to high yield (up to 93% overall yield). The two-step methodology provides a wide variety of 1,4-di(organo)[60] fullerenes bearing the same or different organic addends on the [60] fullerene core. By changing the addends, one can control the chemical and physical properties of the compounds at the molecular and bulk levels.     

C60与叠氮化合物的单加成反应

本文从实验以及理论研究两方面介绍了C60与叠氧化合物的单加成反应。依照叠氮化合物的不同,C60与叠氮合物的单加成反应可分为烷基叠氮化物与C60的反应,酰基叠氧化物与C60的反应以及苯基叠氮化与C60的反应三类,而反应产物则为C60亚氨基[6,6]闭环衍生物和C60亚氨基[5,6]开环衍生物两类,不同类型的反应具有不同的反应机理,某些C60亚氨基[5,6]开环衍生物可以转化为C60亚氨基[6,6]闭环衍生物。本文还介绍了碳纳米管与叠氮化合物的加成反应。     

Is This a Chemical Bond? A Theoretical Study of Ng2@C60 (Ng=He,Ne, Ar,Kr, Xe)

Quantum-chemical calculations using DFT (BP86) and ab initio methods (MP2, SCS-MP2) have been carried out for the endohedral fullerenes Ng2@C60 (Ng=He-Xe). The nature of the interactions has been analyzed with charge- and energy-partitioning methods and with the topological analysis of the electron density (Atoms-in-Molecules (AIM)). The calculations predict that the equilibrium geometries of Ng2@C60 have D3d symmetry when Ng=Ne, Ar, Kr, while the energy-minimum structure of Xe2@C60 has D5d symmetry. The precession movement of He2 in He2@C60 has practically no barrier. The Ng--Ng distances in Ng2@C60 are much shorter than in free Ng2. All compounds Ng2@C60 are thermodynamically unstable towards loss of the noble gas atoms. The heavier species Ar2@C60, Kr2@C60, and Xe2@C60 are high energy compounds which are at the BSSE corrected SCS-MP2/TZVPP level in the range 96.7-305.5 kcal mol(-1) less stable than free C60+2 Ng. The AIM method reveals that there is always an Ng--Ng bond path in Ng2@C60. There are six Ng--C bond paths in (D3d) Ar2@C60, Kr2@C60, and Xe2@C60, whereas the lighter D3d homologues He2@C60 and Ne2@C60 have only three Ng--C2 paths. The calculated charge distribution and the orbital analysis clearly show that the bonding situation in Xe2@C60 significantly differs from those of the lighter homologues. The atomic partial charge of the [Xe2] moiety is +1.06, whereas the charges of the lighter dimers [Ng2] are close to zero. The a2u HOMO of (D3d) Xe2@C60 in the 1A1g state shows a large mixing of the highest lying occupied sigma* orbital of [Xe2] and the orbitals of the C60 cage. There is only a small gap between the a2u HOMO of Xe2@C60 and the eu LUMO and the a2u LUMO+1. The calculations show that there are several triplet states which are close in energy to each other and to the 1A1g state. The bonding analysis suggests that the interacting species in Xe2@C60 are the charged species Xe2q+ and C60q-, where 1相似文献   

Cycloaddition reactions of acrylonitrile on the Si(100)-2 x 1 surface

Multi-reference as well as single-reference quantum mechanical methods were adopted to study the potential energy surface along three possible surface reaction mechanisms of acrylonitrile on the Si(100)-2 x 1 surface. All three reactions occur via stepwise radical mechanisms. According to the computed potential energy surfaces, both [4+2] and [2+2](CN) cycloaddition products resulting from the reactions of surface dimers with the C[triple bond]N of acrylonitrile are expected, due to the negligible activation barriers at the surface. Another possible surface product, [2+2](CC), requires a 16.7 kcal/mol activation energy barrier. The large barrier makes this route much less favorable kinetically, even though this route produces the thermodynamically most stable products. Isomerization reactions among the surface products are very unlikely due to the predicted large activation barriers preventing thermal redistributions of the surface products. As a result, the distribution of the final surface products is kinetically controlled leading to a reinterpretation of recent experiments. An intermediate Lewis acid-base type complex appears in both the [4+2] and [2+2](CN) cycloadditions entrance channels, indicating that the surface may act as an electrophile/Lewis acid toward a strong Lewis base substrate.     

Triazole-linked dendro[60]fullerenes: modular synthesis via a ‘click’ reaction and acidity-dependent self-assembly on the surface

A series of Fréchet-type dendron functionalized [60]fullerene derivatives that bear a 1,2,3-triazole linkage group, referred to as triazole-linked dendro[60]fullerenes, were prepared via a modular synthetic protocol based on a Cu-catalyzed [3+2] cycloaddition (‘click’) reaction. Electronic properties of these dendro[60]fullerenes were investigated by UV-vis spectroscopy and cyclic voltammetry. Interfacial supramolecular self-assembly behavior of these dendro[60]fullerenes was studied using atomic force microscopy (AFM). The resulting self-assemblies showed different nanoscale packing geometries and morphologies on the surface, which are controllable by parameters such as the generation of dendron, the nature of peripheral functionalities, and the experimental conditions (e.g., acidity) applied. Correlations between molecular structure and self-assembling outcome were surveyed and discussed. The results of this study suggest a new avenue to gain better ‘bottom-up’ control over the self-assembly of dendrimer-fullerene hybrid materials in terms of shape and size.     

Fullerene-acene chemistry: single-crystal X-ray structures for a [60]fullerene-pentacene monoadduct and a cis-bis[60]fullerene adduct of 6,13-diphenylpentacene

[structure: see text]. Crystal structures of the cis-bis[60]fullerene adduct of 6,13-diphenylpentacene (with the closest carbon atoms on adjacent fullerenes only 3.065(8) angstroms apart) and the [60]fullerene-pentacene monoadduct are in remarkably close agreement with the MM2 calculated structures.     

On the thermal stability of [60]fullerene cycloadducts: retro-cycloaddition reaction of 2-pyrazolino[4,5:1,2][60]fullerenes

2-Pyrazolino[4,5:1,2][60]fullerenes undergo a thermally induced retro-cycloaddition process whose efficiency is influenced by the nature of the C-substituent. C-Aryl-N-Aryl-2-pyrazolino[60]fullerenes (2a-d) poorly undergo a thermal retro-cycloaddition reaction even in the presence of a strong dipolarophile or a metal Lewis acid which, in contrast to other fullerene derivatives, shows their remarkable thermal stability. C-Alkyl-N-Aryl-2-pyrazolino[60]fullerenes (2e-f) show a different behavior, being more vulnerable to the presence of copper triflate and leading to the retro-cycloaddition product (pristine C60) in good yield.     

Theoretical study of the stability of multiply charged C70 fullerenes

We have calculated the electronic energies and optimum geometries of C(70) (q+) and C(68) (q+) fullerenes (q=0-14) by means of density functional theory. The ionization energies for C(70) and C(68) fullerenes increase more or less linearly as functions of charge, consistent with the previously reported behavior for C(60) and C(58) [S. Diaz-Tendero et al., J. Chem. Phys. 123, 184306 (2005)]. The dissociation energies corresponding to the C(70) (q+)-->C(68) (q+)+C(2), C(70) (q+)-->C(68) ((q-1)+)+C(2) (+), C(70) (q+)-->C(68) ((q-2)+)+C(+)+C(+), C(70) (q+)-->C(68) ((q-3)+)+C(2+)+C(+), and C(70) (q+)-->C(68) ((q-4)+)+C(2+)+C(2+) decay channels show that C(70) (q+) (like C(60) (q+)) is thermodynamically unstable for q>or=6. However, the slope of the dissociation energy as a function of charge for a given decay channel is different from that of C(60) (q+) fullerenes. On the basis of these results, we predict q=17 to be the highest charge state for which a fission barrier exists for C(70) (q+).     

Structure and electronic properties of highly charged C60 and C58 fullerenes

We present a theoretical study of the structure and electronic properties of positively charged C60(q+) and C58(q+) fullerenes (q = 0-14). Electronic energies and optimum geometries have been obtained using density-functional theory with the B3LYP functional for exchange and correlation. We have found that closed- and semiclosed-shell C60(q+) ions (q = 0, 5, and 10) preserve the original icosahedral symmetry of neutral C60. For other charges, significant distortions have been obtained. The C58(q+) fullerenes are, in general, less symmetric, being C58(8+) the closest to the spherical shape. Most C60(q+) fullerenes follow Hund's rule for spin multiplicity, while most C58(q+) fullerenes are more stable with the lowest spin multiplicity. The calculated ionization potentials for both kinds of fullerenes increase almost linearly with charge, except in the vicinity of C60(10+) and C58(8+). We have also explored the region of the potential-energy surface of C60(q+) that leads to asymmetric fission. Minima and transition states corresponding to the last steps of the fission process have been obtained. This has led us to conclude that, for 3 < or = q < or = 8, C2(+) emission is the preferred fragmentation channel, whereas, for higher q values, emission of two charged atomic fragments is more favorable. The corresponding fission barrier vanishes for q > 14.     

Density functional study of chemical stability and nitrogen encapsulation of C48N12 and C58N12

On the basis of calculations using density functional theory, we show that C58N12, just as C48N12, can be a stable N-dopant of C70. By considering many different isomers of the product, we find that the chemical stability of C48N12 and C58N12, with respect to oxygenation, is not significantly different from that of C70, thereby indicating that the N-dopant would not easily be oxygenated in air under normal conditions. In both C48N12O and C58N12O, many different isomers are expected, in which oxygenation occurs at different C-N bonds as well as at C-C bonds, among which specific C-N bonds are the most amenable to the reaction. Investigation of their hydrogenations shows that C48N12 is slightly more easily hydrogenated than C60, while C58N12 is less easily hydrogenated. In addition, we expect a regiospecificity in the hydrogenated products of C58N12, which prefers to react at equatorial sites, while C70 prefers reaction at polar sites. Meanwhile, comparison of the encapsulation energy of a nitrogen atom (=N en) in C60, C48N12, C70, and C58N12 shows that the N-doped fullerenes, particularly C58N12, can encase the atom much better than the undoped ones, allowing us to expect the existence of N@C48N12 and N@C58N12. Spin multiplicities are doublet for most of their stable structures. These observations correlate with the formation of N en-C bonds, which are not found in N@C60 and N@C70. Various isomers of the N-encapsulating fullerenes were identified. The relative stability of these isomers heavily depends on the number of substitutional nitrogen atoms around N en-C bonds.     

Organometallic derivatives of fullerenes: a DFT study of (eta2-Cx)[Pt(PH3)2]n (x = 60, 70, 84; n = 1-6)

To determine the relationship among curvature, patch type, and reactivity of the C-C site, a series of density functional calculations were performed on several substituted fullerenes. [6:6] pyracylene-type sites are the most reactive sites in all analyzed cages: C(60), C(70), and C(84). The binding energy between the Pt(PH(3))(2) unit and fullerene is almost independent of the size of the cage and of the number of metals coordinated on the fullerene surface. Contrarily, curvature and type of carbon-carbon bond are determinant for the coordination strength. The use of relatively large basis sets is necessary to have consistent energies.     

Radical reaction of [60]fullerene with phosphorus compounds mediated by manganese(III) acetate

Radical reaction of [60]fullerene with phosphonates or phosphine oxide mediated by manganese(III) acetate dihydrate in chlorobenzene under three different conditions afforded three different types of phosphorylated fullerenes: singly bonded fullerene dimers 2, hydrophosphorylated fullerenes 3, and acetoxylated fullerene derivatives 4. In addition, interconversions among the three types of phosphorylated fullerene derivatives have also been investigated. A possible reaction mechanism was proposed to explain experimental results.     

Application and evaluation of solvent-free matrix-assisted laser desorption/ionization mass spectrometry for the analysis of derivatized fullerenes

A variety of derivatized fullerenes have been studied by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Of particular emphasis has been the evaluation of a recently introduced solvent-free sample/target preparation method. Solvent-free MALDI is particularly valuable in overcoming adverse solvent-related effects, such as insolubility and/or degradation of the sample. The method was applied to fullerene derivatives susceptible to decomposition under insufficiently "soft" MALDI conditions. Analytes included the hydrofullerene: C(60)H(36), fluorofullerenes: C(60)F(x) where x = 18, 36, 46, 48 and C(70)F(x) where x = 54, 56, methano-bridged amphiphilic ligand adducts to C(60) and the [4 + 2] cycloadduct of tetracene to C(60). The new solvent-free sample preparation is established as an exceedingly valuable addition to the repertoire of preparation protocols within MALDI. The MALDI mass spectra were of very high quality throughout, providing a testimony that "soft" MALDI conditions could be achieved. Using the [4 + 2] cycloadduct of tetracene to C(60) as the model analyte for direct comparison with solvent-based MALDI, the solvent-free approach led to less fragmentation and more abundant analyte ions. Applying solvent-free sample preparation, different matrix compounds have been examined for use in the MALDI of derivatized fullerenes, including sulfur, tetracyanoquinodimethane (TCNQ), 9-nitroanthracene (9-NA) and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2- propenylidene]malononitrile (DCTB). DCTB was confirmed as the best performing matrix, reducing unwanted decomposition and suppression effects.     

On the Formation of Cyclobutane Pyrimidine Dimers in UV‐irradiated DNA: Why are Thymines More Reactive?¶

The reaction pathways for thermal and photochemical formation of cyclobutane pyrimidine dimers in DNA are explored using density functional theory techniques. Although it is found that the thermal [2 + 2] cycloadditions of thymine + thymine (T + T --> T x T), cytosine + cytosine (C + C --> C x C) and cytosine + thymine (C + T --> C x T) all are similarly unfavorable in terms of energy barriers and reaction energies, the excited-state energy curves associated with the corresponding photochemical cycloadditions display differences that--in line with experimental findings--unanimously point to the predominance of T x T in UV-irradiated DNA. It is shown that the photocycloaddition of thymines is facilitated by the fact that the S1 state of the corresponding reactant complex lies comparatively high in energy. Moreover, at a nuclear configuration coinciding with the ground-state transition structure, the excited-state energy curve displays an absolute minimum only for the T + T system. Finally, the T + T system is also associated with the most favorable excited-state energy barriers and has the smallest S2-S0 energy gap at the ground-state transition structure.     

Facile fullerene modification: FeCl3-mediated quantitative conversion of C60 to polyarylated fullerenes containing pentaaryl(chloro)[60]fullerenes

A facile, one-step reaction using inexpensive reagents has been developed for functionalization of [60]fullerene, where the reaction of C(60) with FeCl(3) in chlorobenzene proceeded at 25 °C with 100% conversion, yielding a mixture of polyarylated products containing pentaaryl(chloro)[60]fullerene, C(60)(C(6)H(4)Cl)(5)Cl (up to 29%) and other polyarylated fullerenes (number of aryl groups is in a range from 5 to 10).     

Reaction mechanism and chemoselectivity of intermolecular cycloaddition reactions between phenyl-substituted cyclopropenone ketal and methyl vinyl ketone

In this paper, the mechanisms of the intermolecular [3+2] and [1+2] cycloaddition reactions of 1,1/1,3-dipolar π-delocalized singlet vinylcarbenes, which is obtained from cyclopropenone, with an electron-deficient C═O or C═C dipolarophile, to generate five-membered ring products are first disclosed by the density functional theory (DFT). Four reaction pathways, including two concerted [3+2] cycloaddition reaction pathways and two stepwise reaction pathways (an initial [1+2] cycloaddition and then a rearrangement from the [1+2] cycloadducts to the final [3+2] cycloadducts), are investigated at the B3LYP/6-31G(d,p) level of theory. The calculated results reveal that, in contrast to the concerted C═O [3+2] cycloaddition reaction pathway, which is 7.1 kcal/mol more energetically preferred compared with its stepwise reaction pathway, the C═C dipolarophile favors undergoing [1+2] cycloaddition rather than concerted [3+2] cycloaddition (difference of 5.3 kcal/mol). The lowest free energy barrier of the C═O concerted [3+2] cycloaddition reaction pathway shows that it predominates all other reaction pathways. This observation is consistent with the finding that the C═O [3 + 2] cycloadduct is the main product under experimental conditions. In addition, natural bond orbital second-order perturbation charge analyses are carried out to explain the preferred chemoselectivity of C═O to the C═C dipolarophile and the origins of cis-stereoselectivity for C═C [1+2] cycloaddition. Solvent effects are further considered at the B3LYP/6-31G(d,p) level in the solvents CH(3)CN, DMF, THF, CH(2)Cl(2), toluene, and benzene using the PCM model. The results indicate that the relative reaction trends and the main products are insensitive to the polarity of the reaction solvent.     

Intense near-infrared optical absorbing emerald green [60]fullerenes

Synthesis of emerald green fullerenes (EF) C60[C(CH3)(CO2Et)2]6 and C60[C(CH3)(CO2-t-Bu)2]6 was performed by using hexaanionic C60 intermediate (C60-6) as a reagent in one-pot reaction for attaching six alkyl ester addends on one C60 cage. These EF compounds exhibit intense optical absorption over 600-940 nm, the longest optical absorption of the C60 cage among many [60]fullerene derivatives synthesized.     

Mechanochemical Reactions of Fullerenes under High-Speed Vibration Milling

Since a novel technique called "high-speed vibration milling" (HSVM) was first applied to the Reformatskytype reaction of C60 in 1996, [1] this technique has been applied to various kinds of fullerene functionalizations including the preparation of C120. [2] Most recent reactions of fullerenes under HSVM conditions will be discussed: (1)reaction of C60/C70 with N-alkylglycines and aldehydes (Scheme 1); (2) reaction of C60 with active methylene compounds in the presence of bases (Scheme 2); (3) reaction of C6o with diazo compounds (Scheme 3); (4) reaction of C6o with anthracene derivatives (Scheme 4).