Filip Uhlík

AM1 computations on seven isolated-pentagon-rule isomers of C80

Abstract The complete set of seven isolated-pentagon-rule isomers of C 80 is described by the AM1 quantum-chemical method, and their energetics is also checked by the SAM1 computations. Considerable temperature effects on the relative stabilities in the system are reported. The ground-state structure is a D 5d isomer but a supposed synthetic conditions a structure of a D 2 symmetry is the most populated one.

Electron pairing and chemical bonds. On the accuracy of the electron pair model of chemical bond

Abstract The accuracy of the Lewis electron pair model of the chemical bond is reinvestigated. It is shown that, contrary to previous pessimistic findings, the accuracy of this model is high enough to represent a good basis for the understanding and interpretation of molecular structure, not only for molecules well described by a classical structural formula with localised two-centre two-electron (2c2e) bonds, but also for molecules with more complex bonding patterns such as three- or multicentre bonding.

Computational modeling of the elemental catalysis in the Stone–Wales fullerene rearrangements

Abstract Catalytic effects on the kinetics of the Stone–Wales fullerene transformation are studied computationally. The catalytic agents are represented by free elements, neutral or charged. The computations are performed at semiempirical (PM3) and DFT (B3LYP/6-31G*//PM3) levels on a model bowl-shaped fragment C 34 H 12 . The semiempirical and DFT activation energies agree reasonably well. In all computed cases, the activation barrier is lowered compared with that of the uncatalyzed reaction. The kinetic barriers for the catalyzed rearrangements increase in the following order: N, H, O, P, S, B, Cl, C, F, Li, Se, Fe, Hg, Zn, Si, Sn, Ge, Mg, and Al. Nitrogen atoms are pointed out as especially potent catalytic agents. At the PM3 computational level, the isomerization kinetic barrier is reduced to 193, 110, and 342 kJ mol −1 for the N + , N, and N − species, respectively. If the activation barriers are re-computed at the B3LYP/6-31G*//PM3 level, they are changed to 76, 105, and 323 kJ mol −1 for the N + , N, and N − species, respectively. As small amounts of nitrogen (as well as other elements) are available in virtually any kind of fullerene synthesis, the study offers a computational support for kinetic feasibility of the Stone–Wales fullerene transformation.

Evidence for fullerenes in solid bitumen from pillow lavas of Proterozoic age from Mítov (Bohemian Massif, Czech Republic)

Abstract Andesitic pillow lavas containing biogenic, solid bitumen (SB) are a constituent of a Neoproterozoic volcanosedimentary sequence (Tepla-Barrandian unit, Bohemian Massif) in the Mitov area of the Czech Republic. A black shale formation that is crosscut by these andesitic basalts is 565 Ma old. Carbon disulfide extracts of two powdered samples of SB contain 0.2 and 0.3 ppm of C 60 , respectively, as determined by high-pressure liquid chromatography. The peak assignment based on retention time is fully supported by high-resolution electron ionization mass spectrometry (EI-MS). No C 70 was detected, nor was C 60 found in two other SB samples from this locality. Other investigated carbonaceous samples from Bohemia (coals and anthracites of Upper Paleozoic age and anthraxolite, graphitoids, and graphite of Upper Proterozoic age) did not contain fullerenes at concentrations above the detection limit of 0.01 ppm. The absence of C 60 in these samples was confirmed by EI-MS. The proposed mechanism of fullerene formation involves a primary algal phase, generation of a hydrocarbonaceous mixture in the course of thermal evolution of the sedimentary series, and their high-temperature transformation related to the extrusion of basalt. An important feature for fullerene conservation was the enclosure of fullerenes in SB with a structure similar to glasslike carbon, where the fullerene was protected against oxidation.

Computing the relative gas-phase populations of C60 and C70: Beyond the traditional ΔHf,298o scale

Abstract Computations and experiments have shown that the relative heat of formation (i.e., the heat of formation per carbon atom) of C 70 is lower than of C 60 . Moreover, various computations suggest that this is actually a general trend among fullerene cages. The relationship is particularly important for gas-phase fullerenes. Experiments have shown that C 60 is typically more populated than C 70 when produced in high-temperature gas-phase synthesis. It is not immediately obvious how to reconcile those two terms, or whether the relative heats of formation and the relative populations are in conflict or in agreement. This article deals with this problem, treating it as a general task of relative stabilities of gas-phase clusters of different dimensions (i.e., nonisomeric clusters) under different types of thermodynamic equilibria. The results are then applied to C 60 and C 70 and point out that the conventional standard pressure of 1 atm is considerably different from actual fullerene-synthesis conditions. Apparently, we should expect considerably lower cluster pressures in carbon-arc synthesis. At 1 atm, C 70 is more populated than C 60 , but at the conditions of a saturated carbon vapor the stability order is reversed in favor of C 60 so that an agreement with experiment is obtained already within the thermodynamic treatment. The pressure effects are modeled using the MNDO, AM1, PM3, and SAM1 quantum-chemical semi-empirical methods as well as the available experimental data. The computations consistently show that, if the pressure effects are considered, C 60 becomes more populated than C 70 . Relationships of the thermodynamic treatment to more sophisticated but impractical kinetic analysis are also discussed.

Mg@C72 MNDO/d evaluation of the isomeric composition.

Temperature development of the relative stabilities of isomers of Mg@C72 (which has not yet been isolated) is computed using the recently introduced MNDO/d method. Four isomers originally considered for the Ca@C72 case are treated: one isolated-pentagon-rule (IPR) structure, two structures with a pair of adjacent pentagons, and one cage with a heptagon. The IPR structure comes as the lowest in MNDO/d potential energy, being rather closely followed by the two structures with a pentagon-pentagon pair. On the other hand, the structure with a heptagon is located too high in potential energy to be of any experimental significance. The entropy contributions are evaluated by the MNDO/d-based partition functions so that the relative concentrations can be treated accordingly. The computations suggest that if Mg@C72 is isolated, it should be a mixture of either two or three isomers. The prediction depends on temperature prehistory. If preparation takes place at temperatures of approximately 1000 K, two isomers should be produced. If temperatures are increased to approximately 2000 K, there will already be three isomers with significant relative concentrations. The study supplies a further interesting example of the profound role of enthalpy-entropy interplay in stabilities of isomeric fullerenic structures.

Polyelectrolyte shells of copolymer micelles in aqueous solutions: A Monte Carlo study

Multimolecular micelles, formed by polystyrene-block-poly(methacrylic acid) in water, are studied by lattice Monte Carlo method. Electrostatic interactions are calculated in the mean-field approximation by solving the Poisson-Boltzmann equation. The model is parametrized according to available experimental data. The dependence of micellar size on pH and ionic strength is calculated and compared with experimental data. A special attention is devoted to the behavior in solutions with a low ionic strength.

La2@Cs(17 490)‐C76: A New Non‐IPR Dimetallic Metallofullerene Featuring Unexpectedly Weak Metal–Pentalene Interactions

Although all the pure-carbon fullerene isomers above C60 reported to date comply with the isolated pentagon rule (IPR), non-IPR structures, which are expected to have different properties from those of IPR species, are obtainable either by exohedral modification or by endohedral atom doping. This report describes the isolation and characterization of a new endohedral metallofullerene (EMF), La2@C76, which has a non-IPR fullerene cage. The X-ray crystallographic result for the La2@C76/[Ni(II)(OEP)] (OEP=octaethylporphyrin) cocrystal unambiguously elucidated the C(s)(17,490)-C76 cage structure, which contains two adjacent pentagon pairs. Surprisingly, multiple metal sites were distinguished from the X-ray data, which implies dynamic behavior for the two La(3+) cations inside the cage. This dynamic behavior was also corroborated by variable-temperature (139)La NMR spectroscopy. This phenomenon conflicts with the widely accepted idea that the metal cations in non-IPR EMFs invariably coordinate strongly with the negatively charged fused-pentagon carbons, thereby providing new insights into modern coordination chemistry. Furthermore, our electrochemical and computational studies reveal that La2@C(s)(17,490)-C76 has a larger HOMO-LUMO gap than other dilanthanum-EMFs with IPR cage structures, such as La2@D(3h)(5)-C78 and La2@I(h)(7)-C80, which implies that IPR is no longer a strict rule for EMFs.

Computing fullerene encapsulation of non-metallic molecules: N2@C60 and NH3@C60

Some endohedral fullerenes have been considered as possible candidate species for molecular memories. Recently, the encapsulation inside the fullerene cages has been extended from atoms to small molecules, for example the nitrogen molecule was placed inside the fullerene cage. The observed N2@C60 endohedral is computed in the paper together with NH3@C60, which was not yet observed. The computations are based on structural optimizations using density-functional theory (DFT) methods. In the optimized structures, the analytical harmonic vibrational analysis was carried out and the encapsulation energetics were evaluated using the second order Møller-Plesset (MP2) perturbation treatment. The lowest-energy structure has the N2 unit oriented towards a pair of parallel pentagons so that the complex exhibits D 5d symmetry. At the MP2 level, the encapsulation of N2 into C60 brings a potential energy gain of − 9.3 kcal/mol while that for NH3 is − 5.2 kcal/mol. The entropy term is also evaluated, yielding the standard Gibbs-energy change at room temperature for the encapsulation of N2 and NH3 of − 2.6 and 1.5 kcal/mol, respectively. Some computed structural and vibrational characteristics are also reported. Emerging broader landscape of future applications of such encapsulates in nanoscience and nantechnology is discussed.

Hiding and recovering electrons in a dimetallic endohedral fullerene: air-stable products from radical additions.

Fullerenyl radicals can be generated by addition of a free radical to a fullerene surface, by nucleophilic addition followed by one-electron oxidation, or by thermal dissociation of singly bonded fullerene dimers. However, fullerenyl radicals are usually very reactive and generally cannot be isolated. On the contrary, we have found that the reactions of the dimetallic endofullerenes, La2@Ih-C80 and La2@D5h-C80, with 3-chloro-5,6-diphenyltriazine resulted in mono-addition of the triazinyl radical to the fullerene cages to yield isolable fullerenyl radicals. The unusual stability of these fullerenyl radicals arises from the confinement of the unpaired electron to an internal, metal-metal bonding orbital. Accordingly, the fullerene cage protects the radical center from other reactive species. Furthermore, we demonstrate that the fullerenyl radical adduct of La2@Ih-C80 reacts with toluene to afford additional benzylation. Interestingly, the benzylated derivative is diamagnetic in solution, while it forms a paramagnetic dimer when crystallized.