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How short can a C−C single bond get? The bonding in a set of molecules that are related structurally to previously synthesized or theoretically examined systems with short C−C bonds is investigated. According to calculations, a single C−C bond could be compressed to 1.313 Å! To the best of our knowledge, this is the shortest single C−C bond reported to date. This shortening is a consequence of a change in the C−C−C bond angle, θ, to minimize strain in the cages and an effort to offset the tension in the surrounding bridges. [ABSTRACT FROM AUTHOR]

The link between structural preferences in the monomers, dimers, and extended solid-state structures of the group 2 dihalides (MX2: M = Be, Mg, Ca, Sr, Ba and X = F, Cl, Br, I) is examined theoretically. The question posed is how well are geometric properties of the gas-phase MX2 monomers and lower order oligomers "remembered" in the corresponding MX2 solids. Significant links between the bending in the MX2 monomers and the D2h/C3v M2X4 dimer structures are identified. At the B3LYP computational level, the monomers that are bent prefer the C3v, triply bridged geometry, while the rigid linear molecules prefer a D2h doubly bridged structure. Quasilinear or floppy monomers show, in general, only a weak preference for either the D2h or the C3v dimer structure. A frontier orbital perspective, looking at the interaction of monomer units as led by a donor-acceptor interaction, proves to be a useful way to think about the monomer-oligomer relationships. There is also a relationship between the structural trends in these two (MX2 and M2X4) series of molecular structures and the prevalent structure types in the group 2 dihalide solids. The most bent monomers condense to form the high coordination number fluorite and PbCl2 structure types. The rigidly linear monomers condense to form extended solids with low coordination numbers, 4 or 6. The reasons for these correlations are explored. [ABSTRACT FROM AUTHOR]

Halogen bonding (R—X⋯Y) is a qualitative analogue of hydrogen bonding that may prove useful in the rational design of artificial proteins and nucleotides. We explore halogen-bonded DNA base pairs containing modified guanine, cytosine, adenine and thymine nucleosides. The structures and stabilities of the halogenated systems are compared to the normal hydrogen bonded base pairs. In most cases, energetically stable, coplanar structures are identified. In the most favorable cases, halogenated base pair stabilities are within 2 kcal mol-1 of the hydrogen bonded analogues. Among the halogens X = Cl, Br, and I, bromine is best suited for inclusion in these biological systems because it possesses the best combination of polarizability and steric suitability. We find that the most stable structures result from a single substitution of a hydrogen bond for a halogen bond in dA:dT and dG:dC base pairs, which allows 1 or 2 hydrogen bonds, respectively, to complement the halogen bond. [ABSTRACT FROM AUTHOR]

Abstract: The potential energy surfaces of the Si4Li n (n =1–7) systems have been explored in detail and the global minimum structures were identified. The stability and growth patterns of the title clusters is rationalized in terms of the phenomenological shell model in which the tetrahedral Si4Li4 cluster is a closed shell (20 valence electron) highly stable cluster. Given that the structures reported herein are global minima on their corresponding potential energy surfaces, we propose them as good candidates for experimental detection. [Copyright &y& Elsevier]