Stable junctions between a single carbon chain and two single-wall carbon nanotubes were produced via coalescence of functionalized fullerenes filled into a single-wall carbon nanotube and directly imaged by in situ transmission electron microscopy. First principles quantum chemical calculations support the observed stability of such molecular junctions. They also show that short carbon chains bound to other carbon structures are cumulenes and stable semiconductors due to Peierls-like distortion. Junctions like this can be regarded as archetypical building blocks for all-carbon molecular electronics.

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The crystal structure of the hexaborides MB 6 of alkali, alkaline-earth, and rare-earth metals displays a network of interconnected B 6 octahedra, while isolated B 6 H 6 units occur in the molecular crystal K 2 [B 6 H 6 ]. For the case of a total charge transfer of two electrons from the metal atoms, the B 6 units serve as classical examples of electron deficient clusters. QTAIM and ELI-D analyses of chemical bonding based on solid state DFT/APW+lo quantum chemical calculations were performed. Consistent with Wade's rules, a number of about seven endohedral bonds for the octahedral units is recovered from the 2013 delocalization index (DI). Detailed analyses of two-and three-center delocalization indices yield a clear two-center character of the exohedral B-B bonds and a mixed two-and three-center character of the endohedral B-B bonds. The picture obtained by topological analysis of ELI-D is in agreement with the DI results. With changing effective electron transfer from the metal atoms, electronic saturation of the intra-cluster bonding is found, which leads to the notion of generally two-valent rare-earth atoms in MB 6 hexaborides. 2014The partitioning of position space into basins of ELI-D offers the possibility to determine regions of different bonding characteristics. The projection of the orbital contributions to the total ELI-D (partial ELI-D) [15] reveals the possibility to compare the results for the molecule [B 6 H 6 ] 2-, a molecular crystal K 2 [B 6 H 6 ], and finally for crystalline hexaborides. Results and DiscussionThe optimized structures of the hexaborides are in good agreement with numerous previous studies: [11,12,17] the exohedral B-B distance is significantly shorter than the endohedral distance. The lattices are characterized by a linear relationship between the metal-octahedral face distance and the volume of the unit cell (Figure 2, top). Figure 2. Optimized MB 6 structures, black experimentally observed and red hypothetical MB 6 representatives. Top: metal-octahedral face distance vs. unit cell volume. Bottom: endohedral (diamonds) and exohedral (circles) B-B distances vs. unit cell volume.Notably, a limiting volume range for the experimentally observed MB 6 representatives is evident. It is found between 67 Å 3 and 77 Å 3 , which correlates with a small window for realizable metal-octahedral face distances between 2.5 and

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