Photoinduced electron transfer in a triad that can be assembled/disassembled by two different external inputs. Toward molecular-level electrical extension cables

We have designed, synthesized, and investigated a self-assembling supramolecular system which mimics, at a molecular level, the function performed by a macroscopic electrical extension cable. The system is made up of three components, 12+, 2-H3+, and 3. Component 12+ consists of two moieties: a [Ru(bpy)3]2+ unit, which plays the role of an electron donor under light excitation, and a DB24C8 crown ether, which fulfills the function of a socket. The wire-type component 2-H3+ is also composed of two moieties, a secondary dialkylammonium-ion center and a bipyridinium unit, which thread into the DB24C8 crown-ether socket of 12+ and the 1/5DN38C10 crown-ether socket 3, respectively. The photochemical, photophysical, and electrochemical properties of the three separated components, of the 12+ superset 2-H3+ and 2-H3+ subset 3 dyads, and of the 12+ superset 2-H3+ subset 3 triad have been investigated in CH2Cl2 solution containing 2% MeCN. Reversible connection/disconnection of the two plug/socket systems can be controlled independently by acid/base and redox stimulation. The behavior of the various different dyads and triad has been monitored by light absorption and emission spectroscopies, as well as by electrochemical techniques. In the fully connected 12+ superset 2-H3+ subset 3 triad, light excitation of the [Ru(bpy)3]2+ unit of component 12+ is followed by electron transfer (k = 2.8 x 108 s-1) to the bipyridinium unit of component 2-H3+, which is plugged into component 3. Possible schemes to obtain improved molecular-level electrical extension cables are discussed.     

An LCAO calculation of the band structure of cadmium oxide

An LCAO method has been used to calculate the energy band structure of cadmium oxide along certain symmetry lines in the Brillouin zone. A satisfactory account of the observed direct and indirect band gaps is obtained.     

Investigation of the electronic structures of vinyl silicon compounds by the free electron method

The two compounds, Me₄Si₂ (C₂H₃)₂ and Me₈Si₂ (C₂H₃)₂ have been studied by the anti-symmetrized free electron molecular orbital method. Electron delocalisation over the whole chain via the silicon atoms occurs and a satisfactory account of the electronic spectra may be obtained.

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Zusammenfassung Me₄Si₂ (C₂H₃)₂ und Me₈Si₄ (C₂H₃)₂ wurden nach der MO-Methode des freien Elektronengases behandelt. Delokalisierung der Elektronen über die Si-Atome der Kette wird festgestellt. Die berechneten Spektren sind zufriedenstellend.

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Résumé On a étudié par la méthode des orbitales moléculaires d'électrons libres avec antisymétrisation les composés Me₄Si₂(C₂H₃)₂ et Me₈Si₄(C₂H₃)₂. Il se produit une délocalisation électronique le long de toute la chaÎne par l'intermédiaire des atomes de silicium, et l'on peut rendre compte d'une manière satisfaisante des spectres électroniques.

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We wish to thank the S.R.C. for a maintenance grant to one of us (D.R.A.).     

Ethene adsorption, dehydrogenation and reaction with Pd(110): Pd as a carbon 'sponge'

The interaction of ethene with the Pd(110) surface has been investigated, mainly with a view to understanding the dehydrogenation reactions of the molecule and mainly using a molecular beam reactor. Ethene adsorbs with a high probability over the temperature range 130 to 800 K with the low-coverage sticking probability dropping from 0.8 at 130 K to 0.35 at 800 K. The adsorption is of the precursor type, with a weakly held form of ethene being the intermediate between the gas phase and strong chemisorption. Dehydrogenation begins at approximately 300 K and is fast above 350 K. If adsorption is carried out at temperatures up to approximately 380 K, adsorption saturates after about 0.25 monolayer have adsorbed, but above approximately 450 K, adsorption continues at a high rate with continuous hydrogen evolution and C deposition onto the surface. It appears that, in the intermediate temperature range, the carbonaceous species formed is located in the top layer and thus interferes with adsorption, whereas the C goes subsurface above 450 K, the adsorption is almost unaffected, and the C signal is significantly attenuated in XPS. However, the deposited carbon can easily be removed again by reaction with oxygen, thus implying that the carbon remains in the selvedge, that is, in the immediate subsurface region probably consisting of a few atomic layers. No well-ordered structures are identified in either LEED or STM, though some evidence of a c(2 x 2) structure can be seen. The Pd surface, at least above 450 K, appears to act as a "sponge" for carbon atoms, and this effect is also seen for the adsorption of other hydrocarbons such as acetaldehyde and acetic acid.     

Synthesis of an analogue of the marine polypropionate tridachiahydropyrone

[reaction: see text] A novel approach to the formation of the unusual pyrone-containing ring systems as found in polypropionate metabolites from the Tridachia family of marine molluscs has been developed. This approach includes an intramolecular cyclization of a beta-keto acid onto a cyclohexenone ring to afford a fused, bicyclic pyrone.     

Valence-bond crystal in a pyrochlore antiferromagnet with orbital degeneracy

We discuss the ground state of a pyrochlore lattice of threefold orbitally degenerate S=1/2 magnetic ions. We derive an effective spin-orbital Hamiltonian and show that the orbital degrees of freedom can modulate the spin exchange, removing the infinite spin-degeneracy characteristic of pyrochlore structures. The resulting state is a collection of spin-singlet dimers, with a residual degeneracy due to their relative orientation. This latter is lifted by a magnetoelastic interaction, induced in the spin-singlet phase space, that forces a tetragonal distortion. Such a theory provides an explanation for the helical spin-singlet pattern observed in the B spinel MgTi2O4.     

Acetaldehyde adsorption and catalytic decomposition on Pd(1 1 0) and the dissolution of carbon

The reaction of acetaldehyde with the Pd(1 1 0) surface has been studied using a molecular beam reactor, TPD and LEED. Below 270 K acetaldehyde sticks to the surface with a high initial probability (∼0.8), but no gas phase products evolve. When the reaction is run at >270 K, hydrogen evolves into the gas phase early in the reaction together with methane in a non-steady-state fashion, but above 300 K there is a very efficient steady-state catalytic reaction at the surface; this reaction is the decarbonylation of acetaldehyde to produce methane and carbon monoxide in the gas phase. This behaviour continues up to about 400 K. However, when acetaldehyde is dosed at 423 K, the reaction rate slowly evolves through a maximum to a very low catalytic rate. Upon carrying out reactor experiments at 473 K and above, the reaction mechanism changes to total dehydrogenation, and CO and H₂ are produced at high steady-state rate, not withstanding the fact that carbon is continually being deposited onto the surface. This carbon does not appear to affect the reaction, which takes place on a surface with a c(2 × 2)-C layer present, since the extra carbon is lost from the reaction zone by diffusion into the bulk of the crystal.