Redox-induced hydrogen transfer from hydrofullerene C60H36 to fullerene C60

The possibility of hydrogen transfer from hydrofullerene C₆₀H₃₆ to electrogenerated radical anion C₆₀ ^.− or dianion C₆₀ ²⁻ in propylene carbonate-toluence (3∶2, v/v) was demonstrated by cyclic voltammetry. The process affords C₆₀H₂ as the product. The reaction found is the typical redox-induced process. Translated fromIzvestiya Akodemii Nauk. Seriya Khimicheskaya, No. 6, pp. 1136–1139, June, 1998.     

Bowl-shaped hydrocarbons related to C60

Ab initio studies at the HF/6-31G* and B3LYP/6-31G* levels are reported for two bowl-shaped hydrocarbons related to C₆₀: C₃₀H₁₂ and C₃₆H₁₂, of C₃ and C_3v symmetry, respectively. The former has an approximate heat of formation of 211 kcal/mol. Bowl-to-bowl interconversion may occur through a planar (C_3h) form of ca. 64 kcal/mol greater energy having one imaginary vibrational frequency. The larger C₃₆H₁₂ bowl has a calculated ΔH°_f of 265 kcal/mol. Its HF/6-31G*, B3LYP/6-31G*, and MM3 bond lengths are in good agreement with a recent X-ray structure. Chemical shifts for both compounds calculated by the GIAO method are in good agreement with the measured NMR spectra. The observed ¹³C chemical shifts increase with the extent of pyramidalization. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 189–194, 1998     

Rydberg electron transfer to C60 and C70

Experimental results are reported for the attachment of Ar* * (nl) Rydberg electrons to C₆₀ and C₇₀ over the range n = 19–270. In agreement with other Rydberg work we find that the rate coefficient for C ₆₀ ^? formation remains large (around 2·10^-8 cm³ s^-1) towards high n and is essentially constant for n > 40, thereby indicating the presence of an s-wave attachment process in contrast to interpretations of free electron attachment data postulating p-wave threshold behaviour. The rate coefficients for C ₇₀ ^? formation show a similar n-dependence as those for C ₆₀ ^? , but they are significantly larger. Possible mechanisms for s-wave attachment including formation of polarization-bound negative ion states are discussed. Regarding the threshold behaviour for the attachment of free electrons to C₆₀ we propose — based on an analysis of available free electron data — the presence of an s-wave (possibly resonance type) contribution near zero energy.     

Positron annihilation in C60 and K6C60

The positron density distributions in C₆₀ and K₆C₆₀ have been evaluated using the positron lifetime and Doppler-broadening spectroscopy for the annihilation radiation.In C₆₀, positrons are distributed in the interstitial sites between the C₆₀ molecules,which has been demonstrated by measurements of the temperature dependence of the Doppler-broadening of the annihilation radiation. On the other hand, the positron density distribution must be greatly changed in K₆C₆₀, because positrons are repelled by Coulomb interactions by the positively charged K atoms. It has been observed that there is an extremely short lifetime and a small Doppler-broadening component for the positron annihilation in K₆C₆₀. This component is considered to reflect the positron annihilation inside a C₆₀ molecule.     

How we drifted into peptide chemistry and where we have arrived at

The history of peptide chemistry in our group is described. It all started with the cyclic undecapeptide cyclosporin, the immunosuppressive compound, which is commercialised as Sandimmune^®/Neoral^® by Sandoz/Novartis, and which has revolutionized transplant medicine. The discovery that cyclosporin can be deprotonated to a hexalithio derivative, and thus C-alkylated on a sarcosine moiety, led us into a research project on peptide modifications. We defined structural prerequisites for the use of peptide enolates and for electrolytic decarboxylation of peptides. Parallel to these activities, the group was engaged in developing synthetic methodologies aimed at stereoselective preparations of α-, β-, and γ-amino acid derivatives (cf. diastereoselective alkylations, self regeneration of stereogenic centers, axially chiral enolates). A third avenue into peptide chemistry originated from our investigations on the biopolymer PHB (poly-3-hydroxybutanoic acid); the question arose ‘what happens upon replacement of chain-bound O by NH in the polyester?’ A brief summary is given of the results obtained in our ensuing discovery tour of β-peptides built of homologated proteinogenic amino acids. They form secondary structures with short chain lengths and they have unexpected physiological properties, rendering them candidates for peptidic drugs. The synthesis of β³-peptides is straightforward, and in the meantime most of the Fmoc-protected building blocks are commercial. The β²-homoamino acids are less readily available. Their preparation and the assembly of a β²-eicosapeptide with the twenty proteinogenic side chains are discussed herein. The reasons for the chosen sequence and the strategy of what turned out to be a 159-step synthesis are described. Full experimental details are given for the preparation of the dimeric Fmoc-β²hXaa(PG)-β²hXaa(PG)-OH building blocks used, for their solid-phase coupling to two β²-decapeptide segments, for the thioligation, and for the purification, isolation and spectroscopic characterization of the resulting 20mer. An outlook to future projects in the exciting field of β- and γ-peptide chemistry and biology is given.     

Band structure studies on crystalline C60, Ca3C60, and Ca5C60

The three-dimensional EHMO crystal orbital calculations for crystalline C₆₀, Ca₃C₆₀, and Ca₅C₆₀ are reported. The ground states of both undoped solid C₆₀ and partially doped Ca₃C₆₀ are found to be insulating with an indirect energy gap of 1.2 and 0.5 eV, respectively. In contrast, Ca₅C₆₀ forms a metallic conducting phase with a set of three half-filled bands crossing the Fermi level, which is found to be located close to a peak of the density of state. The character of crystal orbitals near the Fermi level for both Ca₃C₆₀ and Ca₅C₆₀ is completely carbonlike. In both cases, the Ca atoms are almost fully ionized and C₆₀ molecules form a stable negative charge state with six to 10 additional electrons. © 1995 John Wiley & Sons, Inc.     

Catalytic cycloaddition of diazoalkanes to fullerene C60

Cycloaddition to fullerene C₆₀ of monosubstituted diazomethanes generated in situ by oxidation of aldehyde hydrazones in the presence of Pd(acac)₂-2 PPh₃-4 Et₃Al as catalytic system resulted in selective formation of homofullerenes in which the alkyl substituent is located above the plane of the five-membered ring in C₆₀. Under analogous conditions, unsymmetrical disubstituted diazomethanes generated from the corresponding ketone hydrazones gave rise to mixtures of stereoisomeric 5,6-open adducts.     

Computer modeling of C2 cluster addition to fullerene C60

The reaction between C₂ cluster and C₆₀ fullerene resulting in C₂ insertion to C₆₀ with formation of closed C₆₂ cage (reaction of C₂ ingestion by C₆₀) was investigated by the semiempirical MNDO‐PM3 method. The geometries and energies of extremal points on the C₆₂ potential energy surface were calculated. Several reaction pathways leading to the formation of three different closed C₆₂ fullerenes were investigated. All insertion reactions proceed stepwise through intermediate adducts of different structures. The main reaction pathways were found to be addition of C₂ by its one side to the 6,6‐ or 5,6‐bond of C₆₀ with formation of primary unclosed C₆₂ adducts of “ball‐with‐fork” structures, lying in deep potential wells. Back reaction of C₂ detachment from primary adducts can compete with that of their transformation to the closed C₆₂ cages inasmuch as calculated activation barriers of the both reactions are comparable. Model calculations at the B3LYP/6‐31G* level, using C₃₂H₁₂ semisphere instead of C₆₀, confirmed the conclusion about two competitive pathways of the primary adducts transformation, C₂ detachment, and C₂ ingestion. The concerted insertion of C₂ to C₆₀ was realized only in the case of severe restrictions on starting geometry of the C₂ + C₆₀ system. The results of calculations explain recent experimental data on the formation of metastable adducts upon addition of C₂ to C₆₀, obtained using the time‐of‐flight mass spectrometer with laser desorption. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Contrasting photodynamics between C60-dithiapyrene and C60-pyrene dyads

The photodynamics of a C60-dithiapyrene donor-acceptor conjugate were compared with the corresponding C60-pyrene conjugate. The photoinduced charge separation and subsequent charge recombination processes were examined by time-resolved fluorescence measurements on the picosecond timescale and transient absorption measurements on the picosecond and microsecond timescales with detection in the visible and near-infrared regions. We have observed quite long lifetimes (i.e., up to 1.01 ns) for the photogenerated charge-separated state in a C60-dithiapyrene dyad without the need for i) a long spacer between the two moieties, or ii) a gain in aromaticity in the radical ion pair.     

Radioiodination of C60 Derivative C60(OH)xOy

In this paper water-soluble fullerene derivative C₆₀(OH)_xO_y was radioiodinated with the iodogen method. The labeling yield was determined by radio-TLC. The effects of pH value, reaction time, temperature and amount of the iodogen on the labeling yield were studied. The labeled product was purified by Sephadex G-25 column chromatography and then the stability of ¹²⁵I-C₆₀(OH)_xO_y was examined . The results showed that the radiochemical purity of ¹²⁵I-C₆₀(OH)_xO_y solution with benzylalcohol remained 82.7% after 43 hours.     

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.     

Crystal solvate of C60 with tricloroethylene,C60 · C2HCl3

Extraction of fullerenes from carbon soot by trichloroethylene has been studied. We have found that C₆₀ forms a solvate with trichloroethylene (C₆₀ · C₂HCl₃:a=31.31(1);b= 10.156(4);c=10.146(4) Å;V=3228.6 ų,Z=4,d _calc=1.752 g cm^–3, orthorhombic symmetry). Its thermal stability has been studied using TG and DSC. A phase transition of the first order at 167 K has been detected.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1248–1250, July, 1994.The authors are grateful to V. P. Bubnov and I. S. Krainskii for providing them with the samples of fulle-rene-containing carbon soot, and to M. G. Kaplunov and A. V. Zvarykina for assistance in the work.This work was carried out with the financial support of the Russian Foundation for Basic Research, Project Nos. 93-03-18705 and 93-03-5650.     

Photofragmentation of C60

Photo-ionisation and -fragmentation ofC ₆₀ by 15 ns excimer laser pulses at 308 nm and 193 nm as well as 0.8 ps laser pulses at 193 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is ejection ofC _n,n>2, as opposed to successiveC ₂ evaporation. Studies of the relative intensities of metastable fragmentation processes compared with direct fragmentation provide new insight into the fragmentation mechanism and provide a thermometer for the internal energy ofC ₆₀ ⁺ prior to fragmentation. The proposed mechanism is in agreement with measurements of the fragment ion kinetic energies. The results are compared with molecular dynamics simulations.     

C60 carbon cages

Arrangements of carbon atoms in cage-like structures with no dangling bonds are considered as possible novel allotropic forms of carbon. Five different C₆₀ cages, having certain favorable structural characteristics, are identified. Quantitative resonance-theoretic calculations are made and compared to simpe Hückel results. The favored structure is found to be the so-called Buckminsterfullerene structure.     

ChemInform Abstract: Electron Transfer to Triplet C60

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