Neutral C60 Research Articles

ChemInform Abstract: Potential-Dependent Surface Raman Spectroscopy of Buckminsterfullerene Films on Gold: Vibrational Characteristics of Anionic versus Neutral C60.

Abstract : Real time surface enhanced Raman spectra have been obtained for thin films of buckminsterfullerene (C60) on gold in acetonitrile during cyclic voltammetric potential excursions. At potentials where neutral C60 is stable, all ten normally active Raman bands are observed, along with three weaker features that are infrared active in the bulk phase vibrational spectra. Upon electroreduction to the C60 monoanion, several Raman-active bands downshift significantly in frequency, most prominently for the Ag mode (1460 cm-1), which decreases by almost 20 cm-1. Additionally, several Hg bands become more intense, and in two cases yield doublets upon C60 formation, most probably reflecting a diminution in molecular symmetry.

Radical Corannulene

Neutral diradical molecules are known to be useful probes for studying intra­molecular magnetic interactions in organic molecules. Here the authors describe a corannulene-based diradical 1 bearing two phenoxyl radical moieties. The air-stable crystalline material was ­investigated using X-ray crystallography and ESR spectroscopy revealing extensive spin delocalization onto the corannulene moiety (e.g. 3). The authors suggest that the above system is a useful ­intermediate between previously reported planar π-conjugated structures and neutral C60 in the triplet state.

Intensity-resolved IR multiple photon ionization and fragmentation of C60

The sequential absorption of multiple infrared (IR) photons by isolated gas-phase species can lead to their dissociation and/or ionization. Using the newly constructed "Free-Electron Laser for IntraCavity Experiments" (FELICE) beam line at the FELIX facility, neutral C(60) molecules have been exposed to an extremely high number (approximately 10(23)) of photons/cm(2) for a total time duration of up to 5 micros. At wavelengths around 20 microm, resonant with allowed IR transitions of C(60), ionization and extensive fragmentation of the fullerenes are observed. The resulting photofragment distributions are attributed to absorption in fragmentation products formed once C(60) is excited to internal energies at which fragmentation or ionization takes place within the duration of the laser pulse. The high IR intensities available combined with the large interaction volume permit spatially resolved detection of the ions inside the laser beam, thereby disentangling the contributions from different IR intensities. The use of spatial imaging reveals intensity dependent mass distributions that are substantially narrower than what has been observed previously, indicating rather narrow energy distributions. A simple rate-equation modeling of the excitation process supports the experimental observations.

Time-dependent delayed electron spectra: a direct measurement of total decay rate as a function of internal energy

We present a general method to measure internal energy dependant decay rate in the case of systems emitting thermal electrons. Our approach is based on the measurement of the time-dependent kinetic energy spectra of delayed electrons using time-resolved velocity map imaging spectrometry. We illustrate this method in the case of C60 molecules. Indeed electron spectra for C60 have been studied in great details in the past few years, allowing a complete analysis of the observed features. Moreover, C60 offers the opportunity to study competing decay mechanisms demonstrating that the technique may have broader applicability to other molecules. Using a model that includes all contributing decay channels relevant in our time delay range (namely delayed ionization and dissociation by C2 emission) we are able to derive quantitative information on the decay channels of the molecule. In the situation considered here, the time-dependent electron temperature extracted from the kinetic energy spectra is used to determine more precisely the rate constant for the dominant process, namely neutral C2 dissociation channel. In other words, the measurement of the cooling of an ensemble of C60 molecules as a function of time delay after heating provides a direct and quantitative access to its decay dynamics. This method may be used to map out the total rate for complex decay mechanisms.

Far above threshold confinement resonances in A@C60z atoms

It is established, that confinement resonances in the photoionization of an nl subshell of the atom A confined in either neutral or charged C60 may be significant even at photon energies far above the nl ionization threshold, contrary to the existing expectations. Reasons are given. Proof is presented.

Energetics and kinetics of Ti clustering on neutral and charged C60 surfaces

Using ab initio spin density functional theory, we investigate the energetics and kinetics of Ti clustering on both neutral and charged C(60) surfaces. We compare the formation energy of sparsely dispersed zero-dimensional (0D), compact single-layered two-dimensional (2D), and clustered three-dimensional (3D) Ti(N) configurations as a function of cluster size (N < or = 12) and further study the transformation kinetics between them. We find that 0D configuration is always less stable than that of 2D and 3D configurations and 0D to 2D transformation involves in a single Ti diffusion process with kinetic barrier of < or = 0.7 eV. On the other hand, there exists a critical cluster size (N(C)) of N(C) = 5, below which 2D layers are preferred to 3D clusters. Hole- or B-doping greatly enhance the Ti-fullerene interaction and lead to stronger dispersion of Ti atoms. Even so, for moderate charge doping (less than seven holes) the critical size of Ti atoms on neutral C(60) surprisingly remains unchanged or only slightly increases to N(C) = 6 by B-doping. However, we find that the formation of 3D clusters may be hindered by a high kinetic barrier related to the process of single Ti atoms climbing up a single Ti layer. This barrier is approximately 1 eV or even 1.47 eV for B-doped C(60) surfaces which is high enough to stabilize larger 2D structures (N > or = N(C)) at low temperatures. These findings may prove to be instrumental in stabilizing transition metal coated nanostructures and especially homogeneously Ti-coated fullerenes, which are believed to be a very promising material for hydrogen storage.

Crystal Structure and Disorder Phenomena of the Adduct of Neutral C60 and the Ionic Gold Complex (Ptol3)2AuNO3

AbstractThe adduct C60·(Ptol3)2AuNO3 has been crystallized from toluene solutions of C60 and (Ptol3)2AuNO3 as violet, trigonal plate‐shaped crystals. The compound was studied by single‐crystal x‐ray diffraction ($P{\bar 3}{\rm 1}c$, a = 1278.17(3), c = 2282.92(6) pm). The gold complex is dissociated into [(Ptol3)2Au]+ cations, which encapsulate the neutral C60 molecules, and nitrate anions. The intermolecular interactions between the fullerene molecules and the gold complex cations and the resulting disorder is investigated. The crystal structure of C60·(Ptol3)2AuNO3 is providing an instructive example, demonstrating that symmetry‐matching is not always sufficient for avoiding disorder.

Synthesis and Identification of Heterocyclic Derivatives of Fullerene C60: Unexpected Reaction of Anionic C60 with Benzonitrile

During the reaction of reduced C60 with benzyl bromide in benzonitrile, a novel cis-1 C60 adduct, 1,4-dibenzyl-2,3-cyclic phenylimidate C60 (1), was obtained rather than the expected product of 1,4-dibenzyl C60. The structure of compound 1 was analyzed by X-ray single-crystal diffraction, identifying the presence of a five-membered heterocycle at a [5,6] bond of C60. One of the heteroatoms is assigned as a nitrogen atom; however, the identity of the other heteroatom cannot be determined unambiguously by crystallography due to similarity between the nitrogen and oxygen atoms. A related compound (2) bearing the same heterocycle was obtained from anionic C60 benzonitrile solution when no benzyl bromide was added. The structure of compound 2 was determined by NMR, MALDI FT-ICR MS, and UV-vis. Results from MALDI FT-ICR MS for compound 2 show unambiguously that the second heteroatom is an oxygen atom, which is probably from traces of water in the solvent. Control experiments of the reactivity of the neutral, monoanionic, dianionic, and trianionic C60 have shown that the reactive species for the unexpected reaction is the C60 trianion.

IR spectroscopy of gas-phase C60−

We present the IR spectrum of the gas-phase C60(-) anion. The C60(-) anions are mass-selected and trapped in an ion cyclotron mass spectrometer cell. The spectrum is obtained by recording the yield of electrons following infrared multiple photon absorption and subsequent electron detachment of the anions. Two bands are observed that correlate to two of the four IR-allowed transitions in neutral C60. The results show that the higher frequency band is strongly shifted from its position in neutral C60 and is a sensitive marker for the charge state. The band positions and intensities are compared to results obtained by theory as well as to known bands in solid samples and good agreement is found.

Neutral and ionic complexes of C60with (ZnOEP)2·BPy coordination dimers

Coordination (ZnOEP)2·BPy dimers formed by zinc octaethylporphyrin and 4,4′-bipyridine co-crystallize with neutral C60 and the (TDAE˙+)·(C60˙–) ion-radical salt to form molecular and ionic multi-component complexes: {(ZnOEP)2·BPy}·(C60)2·(CHCl3)2 (1) and (TDAE˙+)2·{(ZnOEP)2·BPy}·(C60˙–)2·(C6H6)0.75·(C6H4Cl2)0.25 (2). The C60 molecules are arranged in layers in 1 with the interfullerene center-to-center distances of 10.04–10.17 Å and in linear chains in 2 with the uniform interfullerene center-to-center distance of 10.61 Å. EPR measurements show exchange interaction between C60˙– and TDAE˙+ ion-radicals in 2, which results in the appearance of one signal with g = 2.0027 and ΔHpp = 19.8 G at 295 K rather than two signals from individual ion-radicals. The EPR signal from 2 is split into two components below 30 K and its integral intensity decreases below 9.6 K, which is evidence of antiferromagnetic interactions of spins.

Interaction of Low-Energy Ions and Atoms of Light Elements with a Fluorinated Carbon Molecular Lattice

The mechanism of interaction of low-energy atoms and ions of light elements (H, H+, He, Li, the kinetic energy of the particles 2-40 eV) with C6H6, C6F12, C60, and C60F48 molecules was studied by ab initio MD simulations and quantum-chemical calculations. It was shown that starting from 6 A from the carbon skeleton for the "C6H6 + proton" and "C60 + proton" systems, the electronic charge transfer from the aromatic molecule to H+ occurs with a probability close to 1. The process transforms the H+ to a hydrogen atom and the neutral C6H6 and C60 molecules to cation radicals. The mechanism of interaction of low-energy protons with C6F12 and C60F48 molecules has a substantially different character and can be considered qualitatively as the interaction between a neutral molecule and a point charge. The Coulomb perturbation of the system arising from the interaction of the uncompensated proton charge with the Mulliken charges of fluorine atoms results in an inversion of the energies of the electronic states localized on the proton and on the C6F12 and C60F48 molecules and makes the electronic charge transfer energetically unfavorable. On the different levels of theory, the barriers of the proton penetration for the C6F12 and C60F48 molecules are from two to four times lower than those for the corresponding parent systems (C6H6 and C60). The penetration barriers of the He atom and Li+ ion depend mainly on the effective radii of the bombarding particles. The theoretical penetration and escaped barriers for the "Li+ + C60" process qualitatively explain the experimental conditions of synthesis of the Li@C60 complex.

Experimental Evidence for Modulations in the Relative Double-Photoionization Cross Section ofC60from Threshold up to 280 eV

The relative double-photoionization cross section of neutral C60 clusters was investigated using monochromatized synchrotron radiation between 18 and 283 eV. Our measurement of the double-to-single photoionization ratio reveals two modulating components that are superimposed on a smooth ratio curve from threshold (19.0 eV) up to 280 eV, when inner-shell excitations become possible. The maxima in the modulation can be related to geometrical dimensions of the C60 cluster.

Mechanisms of inelastic scattering of low-energy protons by C6H6, C60, C6F12, and C60F48 molecules

The mechanisms of inelastic scattering of low-energy protons with a kinetic energy of 2–7 eV by C6H6, C6F12, C60, and C60F48 molecules are studied using the methods of quantum chemistry and nonempirical molecular dynamics. It is shown that, for the C6H6 + proton and C60 + proton systems, starting from a distance of 6 A from the carbon skeleton, the electronic charge transfer from the aromatic molecule to H+ occurs with a probability close to unity and transforms the H+ ion into a hydrogen atom and the neutral C6H6 and C60 molecules into cation radicals. The mechanism of interaction of low-energy protons with C6F12 and C60F48 molecules has a substantially different character and can be considered qualitatively as the interaction between a neutral molecule and a point charge. The Coulomb perturbation of the system arising from the interaction of the noncompensated proton charge with the Mulliken charges of fluorine atoms results in an inversion of the energies of the electronic states localized, on the one hand, on the positively charged hydrogen ion and, on the other hand, on the C6F12 and C60F48 molecules. As a result, the neutral molecule + proton state becomes the ground state. In turn, this inversion makes the electronic charge transfer energetically unfavorable. Quantum-chemical and molecular-dynamics calculations on different levels of theory showed that, for fluorine derivatives of some aromatic structures (C6F12, C60F48), the barriers to proton penetration through carbon hexagons are two to four times lower than for the corresponding parent systems (C6H6, C60). This effect is explained by the absence of active π-electrons in the case of fluorinated molecules.

Photoionization of the fullerene ion C+60

The photoionization cross section of the fullerene ion C+60 has been calculated within a single-electron approximation and also by using a consistent many-body theory accounting for many-electron correlations. Electronic wavefunctions of the ground and excited states have been determined within the jellium model based on the local density approximation. A giant resonance in the photoionization cross section at the photon energy ∼25 eV is predicted. It is demonstrated that the resonance profile is much higher and narrower than in the case of the neutral fullerene C60.

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.

Excitation dynamics of Rydberg states in C60

The electron and nuclear dynamics of C60 fullerenes irradiated with femtosecond laser pulses are investigated with photoelectron and photoion spectroscopy. The focus of this work is the detailed exploration of the population mechanism of Rydberg levels within the excitation process of neutral C60. The effect of excitation wavelength, intensity, chirp, and polarization on the kinetic energy distribution of photoelectrons in single-pulse experiments gives first insight into the underlying processes. In combination with time-resolved two-color pump-probe spectroscopy depending on either pump, or probe pulse intensity, a more complete picture of the interaction can be drawn. The results point towards a very interesting but nevertheless complex behavior including four steps: (i) non-adiabatic multielectron excitation of the HOMO (hu) → LUMO+1 (t1g) transition; (ii) thermalization within the hot electron cloud on a time scale below 100 fs, followed by a coupling of energy to vibrational modes of the molecule via doorway state(s); (iii) population of electronically excited Rydberg states by multiphoton absorption, and (iv) single photon ionization from the excited Rydberg states. This excitation process results in a characteristic sequence of photoelectron lines in the photoemission spectra. The comparison of the experimental results with recent theoretical work gives convincing evidence that non-adiabatic multielectron dynamics (NMED) plays a key role for the understanding of the response of C60 to short-pulse laser radiation.

Vibrational cooling in a cold ion trap: Vibrationally resolved photoelectron spectroscopy of cold C60− anions

We demonstrate vibrational cooling of anions via collisions with a background gas in an ion trap attached to a cryogenically controlled cold head (10-400 K). Photoelectron spectra of vibrationally cold C60(-) anions, produced by electrospray ionization and cooled in the cold ion trap, have been obtained. Relative to spectra taken at room temperature, vibrational hot bands are completely eliminated, yielding well-resolved vibrational structures and a more accurate electron affinity for neutral C60. The electron affinity of C60 is measured to be 2.683+/-0.008 eV. The cold spectra reveal complicated vibrational structures for the transition to the C60 ground state due to the Jahn-Teller effect in the ground state of C60(-). Vibrational excitations in the two A(g) modes and eight H(g) modes are observed, providing ideal data to assess the vibronic couplings in C60(-).

Branching ratios of successive emission (up to three) of C2n+ (n = 1–5) fragments in asymmetrical fission processes of C60r+ ions (r = 4–6)

Highly charged C60 molecules are produced in collisions between neutral C60 and multiply charged ions within a large range of temperatures. Successive emission of one, two or three light monocharged fragments referred as one-, two- and three-step processes have been observed. The experimental mass branching ratios for the emission of one C2n+ fragment from C606+, C605+ and C604+ ions are compared with the theoretical values using a statistical model. From hotter C606+ ions, branching ratios for three-step processes have been measured and the data are in good agreement with an estimation using the branching ratios in one-step process.

Ion–Ion Collisions Involving Fullerene Ions

Fullerenes are a direct link between atoms with discrete electronic energy levels and solids with a band structure and a well defined surface. In collision experiments, both between energetic ions or electrons and neutral C60 molecules [Walch, B.; Cocke, C.L.; Voelpel, R.; Salzborn, E. Electron capture from C60 by slow multiply charged ions. Phys. Rev. Lett. 1994, 72, 1439–1442; Scheier, P.; Hathiramani, D.; Arnold, W.; Huber, K.; Salzborn, E. Multiple ionization and fragmentation of negatively charged fullerene ions by electron impact. Phys. Rev. Lett. 2000, 84, 55–58; Hathiramani, D.; Aichele, K.; Arnold, W.; Huber, K.; Salzborn, E.; Scheier, P. Electron‐impact induced fragmentation of fullerene ions. Phys. Rev. Lett. 2000, 85, 3604–3607], as well as between charged fullerene ions and neutral targets [Hvelplund, P.; Andersen, L.; Haugen, H.; Lindhard, L.; Lorents, D.C.; Malhotra, R.; Rouff, R. Dynamical fragmentation of C60 ions. Phys. Rev. Lett. 1992, 69, 1915–1918; Hvelplund, P.; Andersen, L.; Brink, C.; Yu, D.; Lorents, D.C.; Rouff, R. Charge transfer in collisions involving multiply charged C60 molecules. Z. Phys. D 1994, 30, 323; Rohmund, F.; Campbell, E.E.B. Resonant and non‐resonant charge transfer in C60 + + C60 and C70 + and C60 collisions. J. Phys. B: At. Mol. Opt. Phys. 1997, 30, 5293–5304; Shen, H.; Hvelplund, P.; Mathur, D.; Barany, A.; Cederquist, H.; Selberg, N.; Lorents, D.C. Fullerene–fullerene collisions: fragmentation and electron capture. Phys. Rev. A 1995, 52, 3847–3851], the fundamental processes of electron transfer, ionization, and fragmentation have been studied extensively. Here we report on our experiments on electron transfer in the collision systems C60 + + 3He2+ → C60 2+ + 3He+ and C60 + + C60 2+ → C60 2+ + C60 +. For the latter system we also report on an upper level for the fragmentation probability.

Calculations of electron-impact cross sections for the fragmentation and dissociative ionization of fullerenes using a semi-empirical method

We extended our previously developed semi-empirical method for the calculation of cross section functions (absolute value and energy dependence) for the electron-impact ionization of the neutral fullerenes C60 and C70, of the ionized fullerenes C60z+ (z=1–3), and of multiple ionization processes of the negatively charged fullerenes Cm− (m=60, 70, and 84) to the fragmentation and dissociative ionization of neutral and ionized C60 and C70. Measured cross sections for these processes were found to display a sharp peak in the cross section function at low impact energies which was attributed to a plasmon excitation at these energies [Phys. Rev. Lett. 85 (2000) 3604], whereas the high-energy shape of the cross sections exhibited the well-known energy dependence of a direct ionization process. The introduction of a “decay factor” in our model allowed us to separate the two contributions and we found in all cases that the plasmon contribution to the cross section exhibits a very similar shape.