Thermal Transformation of Ring-Opened [2+2] C60 Dimer into a Wide-Bridged C120 Isomer. A Computational Evaluation of Fulvalene-Naphthalene Rearrangements

Abstract

As a continuation of the studies on thermal transformation of the [2+2] C₆₀ dimer (1), the consequence of the pyracylene-rearrangement-like valence isomerization of the fulvalene partial structure at the bridge of the ring-opened product from 1, namely 2, was searched by dynamic reaction coordinate /AM1 semiempirical MO calculations. It is predicted that the fulvalene bridge of 2 rearranges into naphthalene partial structure by the concerted ‘in-plane’ mechanism to give a wide-bridged C₁₂₀ intermediate having twenty five-membered rings and two ten-membered rings (3). The computed energy of activation (145 kcal/mol) is 40 kcal/mol lower than those computed for pyracylene rearrangements. In contrast, the recently reported analogous rearrangement of indigo (13) to dibenzonaphthyridindione (14) is computed to occur by the stepwise ‘sp³’ mechanism.     

C60(NO2)2: Quantum-Chemical Evaluations of Structure, Energetics, and Vibrational Spectra

The complete PM3 structural optimizations are reported for all 23 positional isomers of C₆₀(NO₂)₂ and interesting symmetry reductions are found. There are two low energy structures while all the remaining species are separated by more than 10 kcal/mol from the computed ground state. In the ground state the two nitro groups are placed in para position on one hexagon. The next lowest structure is produced by addition to one 6/6 (double) bond of the cage. The species highest in energy are separated by more than 50 kcal/mol from the ground state. IR vibrational spectra are also predicted for the low-energy structures.     

Penultimate Intermediates to C60 and C70

Penultimate intermediates leading to C₆₀ and C₇₀ fullerenes contain one or two pairs of pentalene units and 32 to 55 kcal/mol less stable than the final products. Energy barriers of pyracylene rearrangements from these intermediates to the final products are computed to be 118 to 125 kcal/mol in height by MOPAC/AM1, which are comparable to those of preceding steps in the Stone-Wales rearrangement cascade.     

A 4/6/8 Structural Pattern: Computations on C48

An alternative structural pattern for fullerenes, inclusion of four-membe-red rings, is studied on the C₄₈ case. The truncated cuboctahedron-like structure is computed (four-, six -and eight-membered rings) at semiempirical level (MNDO, AM1, SAM1). Although the structure is a local energy minimum, it is located more than 1000 kJ/mol above a pentagon/hexagon C48 structure (1500 kJ/mol in the SAM1 method). Owing to a high symmetry (O_h) the computed IR spectrum is relatively simple. The inclusion of the four-membered rings can be particularly significant with heterofullerenes and possibly with odd-numbered fullerenes.     

AMI Computations on C59: Significant Eight- and Nine-Membered Rings

C₅₉ is computed by the AMI method as a particular, experimentally known odd fullerene, 19 isomers being treated. New rules are reported for the pentagon/hexagon pattern, and structures with other types of rings are considered, too. A part of the isomers is derived from the lowest known isomer of C₅₈- The supposed ground-state structure of C₅₉ contains an eight-membered ring, while the next lowest one possesses a nine-membered ring. The C₅₉ ground state exceeds C₆₀ in heat of formation per particle by about 10.4 kJ/mol/atom. The ground-state species should prevail in the C₅₉ equilibrium isomeric mixture even at high temperatures.     

A Computational Treatment of 35 IPR Isomers of C88

The complete sets of 35 isolated-pentagon-rule (IPR) isomers of C₈₈ is described by the SAM1 (Semi-Ab-initio Model 1) quantum-chemical method. The separation energetics is also computed at the HF/STO-3G, HF/3-21G, and HF/4-31G levels. The SAM1 and HF/4-31G data mostly agree within a few kJ/mol. As the SAM1 energetics does not reproduce the recent NMR observations, entropy contributions are included, too, being based on the harmonic-oscillator and rigid-rotator model. Considerable temperature effects on the relative stabilities in the system are found. The ground-state structure of C₈₈ is a C₃ isomer, however, with an increase of temperature a C₂ structure becomes important. At still higher temperatures a near C₂ species is dominant. The results can be viewed as a good agreement with the available observations, and they further expand the family of the IPR sets where the thermodynamic equilibrium treatment allows for a satisfactory support of observations.     

GROUP TRANSFER AND DIMERIZATION PROCESSES OF C60(CN)2 UNDER ATMOSPHERIC PRESSURE IONIZATION CONDITIONS

Neutral and anionic C₆₀(CN)₂ were investigated with electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) mass spectrometry. It was observed that its anions underwent cyano- group and oxygen transfer, and dimerization processes in ESI conditions to form C₆₀(CN)₃H^-, C₆₀(CN)₂(OH₂)^-, [C₆₀(CN)₃H]^-₂, [C₆₀(CN)₂(OH₂)]^-₂ and [(C₆₀)₂(CN)₂(OH)]^-. Meanwhile, neutral C₆₀(CN)₂, for which no signal was observed in ESIMS, showed a base peak corresponding to C₆₀(CN)₂Cl^- in APCIMS spectra with CHCl₃ used as solvent, while only a molecular ion peak corresponding to C₆₀(CN)^-₂ was observed for the toluene solution of neutral C₆₀(CN)₂ in the same conditions. Possible mechanisms for group transfer and dimerization were proposed based on these observations.     

Possible Reaction Mechanism for the Formation of C60 From Naphthalene: A Semi-Empirical and Ab Initio Molecular Orbital Study

Mechanism of C₆₀ formation from naphthalenes was studied by quantum chemical calculations. a patch up process of T-shape dimerization of naphthalenes followed by the intramolecular ring fusion, all through radical reactions, was proposed for a possible mechanism for the C₆₀ formation from naphthalenes. the reaction barrier was found to be less than 60-70 kcal/mol for each reaction step in this mechanism.     

Electronic structure and electrical transport characteristics of C60, 2C60 and 4C60 fullerene molecules

The extended H?ckel method and the Green s function method were used to calculate the electronic structure and electrical transport of Au electrode-C₆₀, 2C₆₀ or 4C₆₀ fullerene-Au electrode systems. Furthermore, their electronic structure and electrical transport characteristics were compared and analyzed. The results show that (i) owing to the contact with the Au electrodes, the C₆₀, 2C₆₀ and 4C₆₀ molecules change in their electronic structures significantly, and their energy gaps between LUMO and HOMO are narrow; (ii) the bonding between C₆₀, 2C₆₀ or 4C₆₀ fullerene and Au electrodes is partially covalent and partially electrovalent; and (iii) the conductance of the three fullerenes conforms to the order of C₆₀>2C₆₀>4C₆₀.     

Arylation of [60]Fullerene with Br2/FeCl3/PhH: Formation of C58 Derivatives via CO Loss

Heating a mixture of [60]fullerene, bromine, ferric chloride and benzene under reflux for 24 h products a range of phenylated [60]fullerene derivatives. The main product is C₆₀Ph₅H but other components identified by mass spectrometry (and in some cases separated by HPLC) are: C_6oPh_n(n = 4, 6, 8, 10, 12), C₆₀Ph_nO₂(n = 4, 6, 8, 10, 12), C₆₀Ph_nOH (n = 7, 9, 11), C₆₀Ph_nH₂ (n = 4, 10), C₆₀Ph₄H₄, C₆₀Ph₅H₃, C₆₀Ph_n0₂H (n = 5, 9), C₆₀Ph₄C₆H₄O₂, C₆₀Ph₉OH₃, and C₆₀Ph₁₁ O₃H₂. In the corresponding reaction with toluene, the crude reaction mixture contained C₆₀(MeC₆H₄)₄ as a main product; C₆₀(ClC₆H₄)₅H was obtained from the reaction with chlorobenzene. Formation of these derivatives is believed to involve radical bromination of the fullerene, followed by electrophilic substitution of the hatogenofullerene into the aromatic, accompanied in some case by hydrolysis, elimination and epoxide formation; oxidation may also introduce ketone/dioxetane functionality. The EI mass spectra of C₆₀Ph₄O₂ and C₆₀Ph₈O₂show degradation to C₅₈Ph_n (n = 0-8), having structures believed to be related to the pseudofollerenes C₆₈Ph_n (n = 0-8) reported recently. These results suggest that some derivatisations of fullerenes confer stability, due to the relief of strain.     

C32: Computations of Low-Energy Cages with Four-Membered Rings

C₃₂ cages built from four-, five-, six-, and seven-membered rings are computed. The computations are primarily performed with semiempirical quantum-chemical methods (AM1, PM3, SAM1), and altogether 199 cages are optimized. The energetics is further checked through ab initio HF SCF computations with the standard 3-21G basis set, and also by density functional theory at the B3LYP level in the standard 6-31G* basis set. All five levels of theory suggest a D_4d cage (two four-membered rings, eight pentagons, eight hexagons) as the lowest-energy structure. Temperature effects are treated in the terms of partition functions so that the entropy contributions are considered accordingly. The thermodynamic treatment points out five cages significantly populated at high temperatures. At very high temperatures the structure lowest in energy is not the most abundant isomer. There are just six conventional fullerenes C₃₂, built exclusively from pentagons and hexagons, however, only two of them show significant populations at high temperatures. The remaining three relatively stable cages contain at least one four-membered ring. No structure with a heptagon shows a non-negligible concentration at high temperatures. The study suggests that in the non-IPR region the quasi-fullerene cages with four-membered rings can in some cases be more important than the conventional fullerenes built from pentagons and hexagons only.     

Spontaneous Decay of Highly-Charged Fullerene Ions C60Z and C58z

Using isotope-resolved, two sector field mass spectrometric techniques we have identified and investigated quantitatively the energetics and kinetics (in particular the kinetic energy release, KER) of the spontaneous decay reactions C_60-2m^z+ → C_60-2m-p(z-1)+ ₊ C_p⁺ with m = 0 or 1, z ranging from 3 to 6 and p = 2 and 4. The obtained KER results are not compatible with the properties expected for a single-step fissioning reaction as described by the liquid drop model. Therefore the present data had to be interpreted by a different fragmentation mechanism. This novel reaction sequence, termed auto charge transfer (ACT) reaction, is initiated by the statistically driven neutral C₂ (or C₄₎ evaporation followed by an electron transfer process from the receding C₂ (or C₄) fragment to the remaining highly-charged fullerene ion thereby leading finally to the two charged reaction products observed in the exit channel of the decay reaction. Moreover, in the case that a C₂⁺ loss from C₆₀z+ is occurring in the first field-free region we have been able to demonstrate that it is possible to observe in the second field-free region a subsequent C₂ evaporation from the C₅₈(z-1)⁺ fragment ion.     

Ab Initio Quantum Chemical Methods for Investigations of Fullerene C60, CS2 and Tetrathiafulvalene Molecules and Their Complexes

Geometry and energy of formation of single molecules: fullerene C₆₀, CS₂ and tetrathiofulvalene (TTF) and their complexes: C₆₀ +CS₂ and C₆₀ +TTF were obtained using Hartree-Fock (HF) and Density Functional Theory methods in various basis sets. Weak chemical interactions were estimated enough well using HF/6-31G for a comparison of various geometrical conformations of these complexes. Energy of formation evaluation in charge-transfer complex C₆₀ +TTF is performing additionally calculating complex with far-separated molecules.     

Comparative In Vitro evaluation of cumulative release of the urinary antiseptics Nalidixic acid, Pipemidic acid, Cinoxacin, and norfloxacin from white beeswax Microspheres

The in vitro diffusion of nalidixic acid (1), pipemidic acid (2), cinoxacin (3), and norfloxacin (4) was studied. The transfer rate constants (k_d) from simulated gastro-intestinal juices to simulated plasma, throughout artificial wall lipid membranes, were defined. The k_d values suggested that the four drugs are absorbed both in gastric and intestinal environments in similar amounts. To obtain lack of gastric unwanted effects white beeswax microspheres containing 1, 2, 3, and 4 were investigated as a vehicle for the drug intestinal release; they were prepared by the meltable dispersion process using wetting agents. Discrete, reproducible free flowing microspheres were obtained. The drug content increased when the particle size growed; it ranged from 4% to 18%. More than 95% of the isolated microspheres were of particle size range 100-500 μm. The drug release was evaluated in vitro. Dissolution of entrapped active ingredients was greatly retarded allowing absorption only in the intestinal tract as result of microsphere formation.     

Computational Study on the Endohedral Isomers of C60H2

A computational study (MM3 and PM3) on the endohedral isomers of C₆₀H₂ has been carried out. The exohedral isomers were computed by MM3. A comparison between the computed properties for both isomers is also presented.     

Two Isomers of C60F48: Computed Inter-isomeric Equilibrium

C₆₀F₄₈ has been known to exist in two isomeric forms of D₃ and S₆ symmetries. However, the quantum-chemical calculations have not agreed on their stability order though a near-isoenergetic picture is otherwise always encountered. In order to clarify the situation, the entropy effects are evaluated for synthetic temperatures of about 500K. The entropy evaluations suggest that the D₃ isomer should be more stable in the potential energy by 2.05-2.55 kcal/mol (to which term the ab initio data are closer than the semiempirical ones).     

C60H18, C60H36 and C70H36 Fullerene Hydrides: Study by Methods of IR, NMR, XPS, EELS and Magnetochemistry

Fullerene hydrides of C₆₀H₁₈, C₆₀H₃₆ and C₇₀H₃₆ are studied by using IR, ¹H and ¹³C NMR, X-ray photoelectron and electron energy loss spectroscopies, and magnetochemistry. The comparison of IR and solid state ¹H and ¹³C NMR data for C₆₀H₃₆ with the theoretical ones allows the suggestion that fullerene hydride has a T symmetric structure and contains 4 isolated benzenoid rings located at tetrahedral positions on the surface of a closed skeleton of the molecule. The EELS revealed that the transition from fullerene to the hydride is accompanied by the decrease of the density of valence electrons. Magnetization measurements showed C₆₀H₃₆ to be a ferromagnet. The hydrogenated fullerenes were prepared by transfer hydrogenation procedures involving 9,10-dihydroanthracene. The compositions of the hydrides are determined by field desorption mass-spectral analysis.     

ISOLATION AND CHARACTERISATION OF C60(CF3)2

From the products of reactions of [60]fullerene with either K₂PtF₆ at 470 °C or AgF at 520 °C, we have isolated C₆₀(CF₃)₂, the simplest trifluoromethylfullerene, which gives a single ¹⁹F NMR line at -69.5 ppm. The HPLC retention time is less than that of C₆₀F₂ confirming the trend observed for other fluoro- vs. trifluoromethylfullerenes namely that the latter elute more rapidly. Other trifluoromethyl- containing species, C₆₀(CF₃)₄O, C₆₀F₅CF₃, C₆₀(CF₃)₄H₂, C₆₀(CF₃)₆H₂, and C₆₀(CF₃H)₃ were detected in the product mixture.     

Computations of metal-covered C60 and C70

Semiempirical quantum-chemical PM3 calculations are reported for a new class of exohedral metallo-fullerenes – metal-coated or metal-covered fullerenes: C₆₀M_n and C₇₀M_n. The exohedral species have been observed in gas phase, however, their geometrical and electronic structures are not known yet. Relatively-even metal-atom distributions over the fullerene rings are considered – such regular forms are computed for M = Be, Mg, Al. Three selected types of stoichiometries are treated in particular: C₆₀M₁₂/C₇₀M₁₂ (metal atoms above all pentagons), C₆₀M₂₀/C₇₀M₂₅ (metal atoms above all hexagons), and C₆₀M₃₂/C₇₀M₃₇ (metal atoms above all rings). If an odd number of electrons should result (Al), the related cation is computed, or one metal atom added or removed. This interesting arrangement above the rings is possible only for some types of atoms, while other elements are localized above bonds or atoms, or inside the cage, or even react and destroy the cage. Other limitation comes from the parametrization of the computational technique used – the PM3 semiempirical method is parametrized only for some selected metals. Metal-layer atomization heats are suggested as a stability measure. Structural characteristics are also presented and interesting reductions of the cage symmetry are found. Their relationship to Jahn–Teller effect is discussed. The metal covered fullerenes can represent models for metal catalysis in the nanotube synthesis and could eventually lead to new interesting materials.     

Ab initio calculation on fullerene C32 and Its Derivative

In this paper, we studied the equilibrium geometries, the electronic structures and the binding energies of fullerene C₃₂ as well as its hydrogenates and fluorides interactions by the SCF Hartree-Fock calculations using the latest version of GAMESS of ab initio programs. The fullerene C₃₂ is found to have a ¹A₂' closed-shell ground state in D_3d symmetry or a ¹E_U singlet state in D_3h symmetry. It is predicted to be easy to add some hydrogen or fluorine atoms to the C₃₂ cage and to produce a series of derivatives C₃₂H_2n,C₃₂F_2n (n=l, 3, 6), in which C₃₂H₂,C₃₂F₂ (D_3d), C₃₂H₆^a,C₃₂F₆^c(D_3h) are more stable. We suggest that this implies that the vertex of triplet pentagons of the C₃₂ cage is an activation site for addition.