anti-Diradical Formation in 1,3-Dipolar Cycloadditions of Nitrile Oxides to Acetylenes.

By means of high level quantum chemical calculations (B2PLYPD and CCSD(T)), the mechanisms of the reaction of nitrile oxides with alkenes and alkynes were investigated. We were able to show that in the case of alkenes, regardless of the chosen substituents, the concerted mechanism is always energetically favored as compared to a two-step process, which runs through an anti-diradical species. In the case of alkynes, the concerted mechanism is favored only for the reaction of alkyl-substituted acetylenes.

Encapsulated Guests in the Smallest Spaces: Shrinking Guests by Compression and Investigations under Solvent-Free Conditions.

Noncovalent interactions play a pivotal role in a variety of biological and chemical processes. The experimental determination and quantum chemical calculations of the forces driving these interactions are of utmost importance. Of special interest are interactions of molecules in small spaces which show phenomena different from conventional behavior in solution.

Light and chemically driven molecular machines showing a unidirectional four-state switching cycle.

The imitation of macroscopic movements at the molecular level is a key step in the development of nanomachines. The challenge is the synthesis of molecules that are able to transform external stimuli into a direction-controlled mechanical movement. The more complex such motion sequences are, the more difficult is the construction of the corresponding nanomachine.

From eight-membered 10π electron sulfur-nitrogen cycles to bicycles and cages: a theoretical approach.

A simple way of rationalizing the structures of cyclic, bicyclic, and tricyclic sulfur-nitrogen species and their congeners is presented. Starting from a planar tetrasulfur tetranitride with 12π electrons, we formally derived on paper a number of heterocyclic eight-membered 10π electron species by reacting the 3p orbitals of two opposite sulfur centers with one radical each, or by replacing these centers by other atoms with five (P) or four (Si, C) valence electrons. This led to planar aromatic 10π electron systems, nonplanar bicyclic structures with a transannular S-S bond, and tricyclic structures by bridging the planar rings with an acceptor or donor unit.

4,4'-Bipyridine as a unidirectional switching unit for a molecular pushing motor.

A chirality switch in which the intrinsic chirality of a 4,4'-bipyridine is combined with a metal-ion-induced switching principle is described. In the uncomplexed state the 4,4'-bipyridine unit, which is linked to an S,S,S,S-configured cyclic imidazole peptide, is P-configured. The addition of zinc ions leads to a rotation around the C-C bond axis of the 4,4'-bipyridine and the M isomer of the metal complex is formed.

Strongly underestimated dispersion energy in cryptophanes and their complexes.

Cryptophanes, composed of two bowl-shaped cyclotriveratrylene subunits linked by three aliphatic linker groups, are prototypal organic host molecules which bind reversibly neutral small guest compounds via London forces. The binding constants for these complexes are usually measured in tetrachloroethane and are in the range of 10(2)-10(3) M(-1). Here we show that tetrachloroethane is--in contrast to the scientific consensus--enclosed by the cryptophane-E cavity.

A very stable complex of a modified marine cyclopeptide with chloroform.

Noncovalent interactions play a pivotal role in molecular recognition. These interactions can be subdivided into hydrogen bonds, cation-π interactions, ion pair interactions and London dispersion forces. The latter are considered to be weak molecular interactions and increase with the size of the interacting moieties.