Stable configurations of carbon clusters: Chains, rings, and fullerenes

Planar-to-tubular structural transition in boron clusters: B20 as the embryo of single-walled boron nanotubes

Experimental and computational simulations revealed that boron clusters, which favor planar (2D) structures up to 18 atoms, prefer 3D structures beginning at 20 atoms. Using global optimization methods, we found that the B20 neutral cluster has a double-ring tubular structure with a diameter of 5.2 A. For the B(-)20 anion, the tubular structure is shown to be isoenergetic to 2D structures, which were observed and confirmed by photoelectron spectroscopy. The 2D-to-3D structural transition observed at B20, reminiscent of the ring-to-fullerene transition at C20 in carbon clusters, suggests it may be considered as the embryo of the thinnest single-walled boron nanotubes.

Structure, stability, and spectra of C[sub 9]H[sub 3], C[sub 11]H[sub 3], and C[sub 13]H[sub 3] radicals

Density functional theory has been used to investigate the geometries, vibrational frequencies, rotational constants, and dipole moments of the C(9)H(3), C(11)H(3), and C(13)H(3) radicals. Vertical electronic transition energies of C(9)H(3), C(11)H(3), and C(13)H(3) are calculated by the time-dependent density functional theory. Present results show that the most stable arrangements of C(9)H(3), C(11)H(3), and C(13)H(3) are H(2)C(9)H, H(2)C(11)H, and H(2)C(13)H with a C(2v) symmetry, respectively. Such lowest-energy isomers have an obvious single and triple bond alternation carbon chain. Their isomers HC(4)(HC)C(4)H, HC(4)[C(C(2)H)]C(4)H, and C(C(4)H)(3) are predicted to have vibrational frequencies and vertical excitation energies in good agreement with experimental observations. HC(4)(HC)C(4)H, HC(4)[C(C(2)H)]C(4)H, and C(C(4)H)(3) have similar trigonal structure, which gives rise to the remarkably similar spectroscopic features as obtained experimentally. On the basis of present calculations, the isomers HC(4)(HC)C(4)H, HC(4)[C(C(2)H)]C(4)H, and C(C(4)H)(3) of C(9)H(3), C(11)H(3), and C(13)H(3) radicals are most likely the carriers of the observed spectra.

Highest electron affinity as a predictor of cluster anion structures

Small clusters have a range of unique physical and chemical phenomena that are strongly size dependent. However, analysis of these phenomena often assumes that thermodynamic equilibrium conditions prevail. We compare experimentally measured and ab initio computed photoelectron spectra of bare and deuterated silicon cluster anions produced in a plasma environment. We find that the isomers detected experimentally are usually not the ground-state isomers, but metastable ones, which indicates that cluster relaxation is strongly limited kinetically by a dwell time that is much shorter than the relaxation time. We show that, under these conditions, the highest electron affinity replaces the traditional lowest total energy as the appropriate criterion for predicting isomer structures. These findings demonstrate that a stringent examination of non-equilibrium effects can be crucial for a correct analysis of cluster properties.

Isomer-resolved ion spectroscopy

We demonstrate the isomer-resolved spectroscopy of gas-phase ions, with different geometries separated prior to spectroscopic probe using ion mobility techniques. Specifically, ring and chain isomers of carbon cluster anions with 10;-12 atoms have been separated by ion mobility/mass spectrometry and examined by photoelectron spectroscopy. This methodology should also apply to other ion spectroscopies, including IR photodissociation.

Isomers of C(20): an energy profile

Semiempirical calucaltions, at the PM 3 level, are used to geometrically optimize and determine the absolute energies (heats of formation) of a variety of C(20) isomers. Based on the geometrically optimized Cartesian coordinates of the ring and the bowl isomers, and the subsequent saddle-point calculation, a two-dimensional energy profile between these two isomers is generated. Performing geometry optimization on the Cartesian coordinates that correspond to energy minima within the ring-bowl profile, we have been able to identify several more isomers of C(20) that are predicted to be energitically stable. With these additional stable structures, we have identified pairs of isomers that lie adjacent to one another on the potential energy surface, as is evidenced by the form of their respective energy profiles. These adjacent pairs of isomers establish a step-wise transformation between the ring and the bowl. This process, which extends out over the three-dimensional surface, is predicted to require less energy than that of the direct, two-dimensional transformation predicted in the ring-bowl profile.

Theoretical identification of the smallest fullerene, C20

By using first-principles calculations, we calculate the vibronic fine structure in photoelectron spectra of C-20 clusters. Based on our results, we assign one of the recently observed spectra [H. Prinzbach et al., Nature (London) 407, 60 (2000)] to the fullerene structure and, therefore, confirm the experimental claim that the smallest fullerene is really synthesized.

Gas-phase production and photoelectron spectroscopy of the smallest fullerene, C20

Fullerenes are graphitic cage structures incorporating exactly twelve pentagons. The smallest possible fullerene is thus C20, which consists solely of pentagons. But the extreme curvature and reactivity of this structure have led to doubts about its existence and stability. Although theoretical calculations have identified, besides this cage, a bowl and a monocyclic ring isomer as low-energy members of the C20 cluster family, only ring isomers of C20 have been observed so far. Here we show that the cage-structured fullerene C20 can be produced from its perhydrogenated form (dodecahedrane C20H20) by replacing the hydrogen atoms with relatively weakly bound bromine atoms, followed by gas-phase debromination. For comparison we have also produced the bowl isomer of C20 using the same procedure. We characterize the generated C20 clusters using mass-selective anion photoelectron spectroscopy; the observed electron affinities and vibrational structures of these two C20 isomers differ significantly from each other, as well as from those of the known monocyclic isomer. We expect that these unique C20 species will serve as a benchmark test for further theoretical studies.