Activation of metal-acyl oxygen atoms by triflic anhydride: a simple synthetic route to reactive cationic vinylidene complexes

(η⁵-Cyclopentadienyl) (acyl) (carbonyl) (tertiary phosphine) iron complexes react rapidly with trifluoromethanesulfonic acid anhydride to afford cationic vinylidene complexes, via the intermediacy of cationic carbene complexes. Spectroscopic and chemical characterizations of these vinylidene complexes are reported.     

Vacancy dynamics and reorganization on bromine-etched Si(100)-(2 x 1) surfaces

Halogen etching of Si(100) surfaces has long been considered to involve the selective removal of atoms from an essentially static surface. Here we show that vacancy sites produced by etching are mobile at elevated temperature and rearrange to form features that were considered to be the direct products of etching. We demonstrate that the etch features observed at different temperatures are not due to different mechanisms. Rather, kinetic etch products formed at low temperatures are transformed into thermodynamically more stable features at higher temperatures.     

Duftstoffe: Die Sprache der Pflanzen. Signalrezeption,Biosynthese und Ökologie

Plants respond with coordinated actions to threats from their environment. After being attacked by a herbivorous insect, plants are able to emit a complex blend of volatiles, which attract carnivorous arthropods that reduce the number of herbivores and thus benefit the plant. By modulating the emitted volatile spectrum, plants are able to transmit information on type and intensity of their infestation to their defenders. Using either various elicitors from the saliva secretion of the herbivore or differences in the spatio‐temporal pattern of wounding, the plant is able to discriminate between different herbivore species. Subsequently, a complex signalling network ensures the specific reaction to various stresses. Moreover, also plant roots are able to emit volatiles upon attack to attract the predators of root‐feeding insects. New studies suggest that plants may also be able to communicate with each other: By “eavesdropping” on their damaged neighbour, plants of the same or another species can use the information on a currently increased risk of being attacked by herbivores and, as a precaution, activate its own defences. However, most of these interactions between plants and other organisms have been studied mainly under laboratory conditions and we are only starting to understand the ecology of these mechanisms in nature.     

The capacitated multiple allocation hub location problem: Formulations and algorithms

In this paper we consider and present formulations and solution approaches for the capacitated multiple allocation hub location problem. We present a new mixed integer linear programming formulation for the problem. We also construct an efficient heuristic algorithm, using shortest paths. We incorporate the upper bound obtained from this heuristic in a linear-programming-based branch-and-bound solution procedure. We present the results of extensive computational experience with both the heuristic and the exact methods.     

Stereoselective cycloaddition of 1,3-cyclohexadiene on Si(100): a simple algorithm for product identification based on secondary orbital interactions

We consider the reaction of 1,3-cyclohexadiene (1,3-CHD) on Si(100) and show that the observed reactivity and stereoselectivity cannot be explained on the basis of thermodynamics. We postulate the existence of secondary orbital interactions (SOIs) and introduce a simple algorithm that examines all possible secondary interactions between the frontier orbitals of the molecule and the surface. We demonstrate using an orbital symmetry-based algorithm supported by DFT calculations that SOIs favor a particular molecular configuration, consistent with the experimental observations. The potential role of SOIs in controlling surface chemical reactions is discussed.     

A mechanophysical phase transition provides a dramatic example of colour polymorphism: the tribochromism of a substituted tri(methylene)tetrahydrofuran-2-one

Background

Derivatives of fulgides have been shown to have interesting photochromic properties. We have synthesised a number of such derivatives and have found, in some cases, that crystals can be made to change colour on crushing, a phenomenon we have termed “tribochromism”. We have studied a number of derivatives by X-ray crystallography, to see if the colour is linked to molecular structure or crystal packing, or both, and our structural results have been supported by calculation of molecular and lattice energies.

Results

A number of 5-dicyanomethylene-4-diphenylmethylene-3-disubstitutedmethylene-tetrahydrofuran-2-one compounds have been prepared and structurally characterised. The compounds are obtained as yellow or dark red crystals, or, in one case, both. In two cases where yellow crystals were obtained, we found that crushing the crystals gave a deep red powder. Structure determinations, including those of the one compound which gave both coloured forms, depending on crystallisation conditions, showed that the yellow crystals contained molecules in which the structure comprised a folded conformation at the diphenylmethylene site, whilst the red crystals contained molecules in a twisted conformation at this site. Lattice energy and molecular conformation energies were calculated for all molecules, and showed that the conformational energy of the molecule in structure IIIa (yellow) is marginally higher, and the conformation thus less stable, than that of the molecule in structure IIIb (red). However, the van der Waals energy for crystal structure IIIa, is slightly stronger than that of structure IIIb – which may be viewed as a hint of a metastable packing preference for IIIa, overcome by the contribution of a more stabilising Coulomb energy to the overall more favourable lattice energy of structure IIIb.

Conclusions

Our studies have shown that the crystal colour is correlated with one of two molecular conformations which are different in energy, but that the less stable conformation can be stabilised by its host crystal lattice.

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Graphical abstract Graphical representation of the structural and colour change in the tribochromic compound (III).

     

Studies in chemical ionization mass spectrometry part. XVI: Are the reactions of aliphatic cc double bonds with NO+ governed by remote functional groups?

It is shown that the characteristic fragments observed by Malosse and Einhorn (Adv. Mass Spectrom. 1369 (1986)) in the CI(NO) spectra of alkenyl acetates and related compounds (cleavage of the double bond and formation of an acyl ion), which had not been reported, for alkenes and alkenoic acids, for example, are actually formed from all straight-chain olefinic compounds (the only exceptions being where the double bond is too close to one of the ends of the chain). Their relative abundance may, however, vary from almost zero to 100%, and it is highly dependent on experimental parameters (e.g. the source temperature and even the type of instrument used). Apparent inconsistencies in the data reported in the literature could thus be resolved.     

Signal Substances and their Reception in the Sexual Cycle of Marine Brown Algae

Attraction and repulsion are responses to chemical stimuli which can be received and processed even by unicellular organisms without a morphologically defined nervous system. Chemical substances trigger off a chain of events which starts with a membrane-bound signal receptor and, after a sequence of regulatory and modulatory steps, ends in the modulation of a motor effector organ. Binding of the signal substances to the receptor produces conformational changes in which the receptor subunits are mutually dependent on one another, and also leads to chemical modification of the subunits and affects their molecular activity. These interactions, together with the characteristic type of movement, result in a physiological pattern of behavior which enables the flagellated sex cells (gametes) of marine brown algae to finally locate their partners. The simple but highly specific brown algae gamete systems have been investigated structurally and their biological activity analyzed. The signal substances are mainly highly unsaturated aliphatic or cyclic hydrocarbons with unsaturated side chains (general formula e. g. C₁₁H₁₄, C₁₁H₁₆, C₁₁H₁₈). These systems also serve as a simplifying model which helps in the understanding of complex ganglionic pathways in higher living organisms where the sense organs convey information from the surroundings to the central nervous system through nerve pathways. The information is then processed and answered, via efferent pathways, as movement.