σ/σ- and π/π-Interactions Are Equally Important: Multilayered Graphanes

Synthesis and transformation of linear adamantane assemblies inside carbon nanotubes

We report the assembly and thermal transformation of linear diamondoid assemblies inside carbon nanotubes. Our calculations and observations indicate that these molecules undergo selective reactions within the narrow confining space of a carbon nanotube. Upon vacuum annealing of adamantane molecules encapsulated in a carbon nanotube, we observe a sharp Raman feature at 1857 cm(-1), which we interpret as a stretching mode of carbon chains formed by thermal conversion of adamantane inside a carbon nanotube. Introduction of pure hydrogen during thermal annealing, however, suppresses the formation of carbon chains and seems to keep adamantane intact.

A benchmark comparison of σ/σ and π/π dispersion: the dimers of naphthalene and decalin, and coronene and perhydrocoronene

The stacking interaction between π systems is a well-recognized structural motif, but stacking between σ systems was long considered of secondary importance. A recent paper points out that σ stacking can reach the energy of chemical bonds and concludes that "σ/σ and π/π interactions are equally important" (Fokin, A. F.; Gerbig, D.; Schreiner, P. R. J. Am. Chem. Soc. 2011, 133, 20036). Our analysis shows that strong dispersion interaction requires rigid subsystems and good fits of their repulsive potential walls, conditions which are satisfied for both graphenes and larger graphanes (perhydrographenes). Comparison of the dimerization energies of decalin and perhydrocoronene with those of the naphthalene and coronene dimers at the coupled cluster (CC) CCSD(T) level confirms the substantial σ-stacking energies in graphanes but shows lower binding energies than do the B97D calculations of Fokin et al. Graphane dimerization energies are substantially lower at the CC level than the corresponding π-stacking energies: the value for perhydrocoronene is only 67% of the value for coronene, and the difference increases with system size. Our best estimate for the dimerization energy of naphthalene is 6.1 kcal/mol. Spin-component scaled MP2 is unbalanced: it gives only 70% of the CCSD(T) binding energy in σ dimers. The B3LYP-D3 method compares very well with CC for both σ and π dimers at the van der Waals minimum but underestimates the binding at larger distances. We used the largest possible atomic basis for these systems with 64-bit arithmetic, half-augmented-pVDZ, and the results were corrected for basis set incompleteness at the MP2 level.

Stable alkanes containing very long carbon-carbon bonds

The metal-induced coupling of tertiary diamondoid bromides gave highly sterically congested hydrocarbon (hetero)dimers with exceptionally long central C-C bonds of up to 1.71 Å in 2-(1-diamantyl)[121]tetramantane. Yet, these dimers are thermally very stable even at temperatures above 200 °C, which is not in line with common C-C bond length versus bond strengths correlations. We suggest that the extraordinary stabilization arises from numerous intramolecular van der Waals attractions between the neighboring H-terminated diamond-like surfaces. The C-C bond rotational dynamics of 1-(1-adamantyl)diamantane, 1-(1-diamantyl)diamantane, 2-(1-adamantyl)triamantane, 2-(1-diamantyl)triamantane, and 2-(1-diamantyl)[121]tetramantane were studied through variable-temperature (1)H- and (13)C NMR spectroscopies. The shapes of the inward (endo) CH surfaces determine the dynamic behavior, changing the central C-C bond rotation barriers from 7 to 33 kcal mol(-1). We probe the ability of popular density functional theory (DFT) approaches (including BLYP, B3LYP, B98, B3LYP-Dn, B97D, B3PW91, BHandHLYP, B3P86, PBE1PBE, wB97XD, and M06-2X) with 6-31G(d,p) and cc-pVDZ basis sets to describe such an unusual bonding situation. Only functionals accounting for dispersion are able to reproduce the experimental geometries, while most DFT functionals are able to reproduce the experimental rotational barriers due to error cancellations. Computations on larger diamondoids reveal that the interplay between the shapes and the sizes of the CH surfaces may even allow the preparation of open-shell alkyl radical dimers (and possibly polymers) that are strongly held together exclusively by dispersion forces.

Graphane/fluorographene bilayer: considerable C-H···F-C hydrogen bonding and effective band structure engineering

Systematic density functional theory (DFT) computations revealed the existence of considerable C-H···F-C bonding between the experimentally realized graphane and fluorographene layers. The unique C-H···F-C bonds define the conformation of graphane/fluorographene (G/FG) bilayer and contribute to its stability. Interestingly, G/FG bilayer has an energy gap (0.5 eV) much lower than those of individual graphane and fluorographene. The binding strength of G/FG bilayer can be significantly enhanced by applying appropriate external electric field (E-field). Especially, changing the direction and strength of E-field can effectively modulate the energy gap of G/FG bilayer, and correspondingly causes a semiconductor-metal transition. These findings open new opportunities in fabricating new electronics and opto-electronics devices based on G/FG bilayer, and call for more efforts in using weak interactions for band structure engineering.

Al4(2-); the anion-π interactions and aromaticity in the presence of counter ions

The influence of presence of counter ions and π-complexation with benzene on the bonding and magnetic properties of Al(4)(2-), the most studied all-metal cluster, is studied here. It is shown that complexation by either counter ions or benzene decreases the delocalization index between Al atoms and the magnitude of bond magnetizability, that is a Quantum Theory of Atoms in Molecules, QTAIM, -based magnetic index of aromaticity. Benzene forms two types of π-complexes with the Al(4) framework; CH-π (T-shaped) complexes and parallel π-π stacking (PPS) complexes. It is shown that variation in the π-charge of the Al(4) framework affects the relative stability of the T-shaped/PPS complexes. Free Al(4)(2-) forms a stable T-shaped anion-π complex with benzene but in the presence of cations, formation of PPS complexes is more favourable, energetically. It is suggested that this property could be used for designing molecular switches and tuneable anion sensors.