Double-layer structure and mechanism of electrode reactions: I. Nitromethane reduction on mercury from aqueous solutions

Kinetic measurements of CH₃NO₂ electroreduction in aqueous solutions of pH values ranging from 1 to 9 were carried out with the polarographic technique under different experimental conditions. In particular, different concentrations of the supporting electrolytes NaClO₄ and NaCl, which are nonspecifically adsorbed at the potentials investigated, or else of the specifically adsorbed electrolyte KI, were employed to change the potential _d at the outer Helmholtz plane and/or the charge density q_i at the inner Helmholtz plane. The effects of these changes between pH 4 and 9 are consistent with a rate-determining protonation step $C{H}_{3}N{O}_2^{?}\stackrel{(+H^{+})}{\to }C{H}_{3}N{O}_{2}H$ taking place within the compact layer. As pH is gradually decreased from 4 to 1, double-layer effects are satisfactorily interpreted by assuming that the control of the overall electrode process passes from the above protonation step to the electron-transfer step CH₃NO₂+e→CH₃NO₂^?, again occurring within the compact layer.     

Structure and stability of (CuNO) isomers

Quantum-mechanical calculations have been performed on various isomers of the (CuNO)⁺ system. A ²Π ground state is found for the linear CuNO⁺ and CuON⁺ isomers and a ²A′ state for the bent CuNO⁺ and CuON⁺ isomers. Energy calculations indicate that the linear CuNO⁺ structure is the most stable, the bent CuNO⁺ and CuON⁺ and the linear CuON⁺ structures are at 0.86 eV, 0.99 eV and 1.04 eV above this respectively. In the CuNO⁺ → CuON⁺ interconversion between the linear isomers, three transition states are involved, whereas the bent CuNO⁺ isomer is found to be an intermediate species. The isomerization barriers, dissociation energies, equilibrium geometries and vibration frequencies are given for all isomers in their ground and first excited states.     

An efficient synthesis of chalcones based on the Suzuki reaction

A general method for the synthesis of chalcones based on the Suzuki reaction either between cinnamoyl chlorides and phenylboronic acids or between benzoyl chlorides and phenylvinylboronic acids is described.     

Organic synthesis with sulfones XXXII. A convenient route to α,β -unsaturated sulfones from saturated sulfones.

α-sulfonyl litiated anions are oxidized by cupric carboxylates into β, β-unsaturated sulfones. Primary sulfones lead to pure trans-vinylic sulfones.     

Tetrathiafulvalene (TTF)‐Bridged Resorcin[4]arene Cavitands: Towards New Electrochemical Molecular Switches

We report the synthesis of novel resorcin[4]arene‐based cavitands featuring two extended bridges consisting of quinoxaline‐fused TTF (tetrathiafulvalene) moieties. In the neutral form, these cavitands were expected to adopt the vase form, whereas, upon oxidation, the open kite geometry should be preferred due to Coulombic repulsion between the two TTF radical cations (Scheme 2). The key step in the preparation of these novel molecular switches was the P(OEt)₃‐mediated coupling between a macrocyclic bis(1,3‐dithiol‐2‐thione) and 2 equiv. of a suitable 1,3‐dithiol‐2‐one. Following the successful application of this strategy to the preparation of mono‐TTF‐cavitand 3 (Scheme 3), the synthesis of the bis‐TTF derivatives 2 (Scheme 4) and 19 (Scheme 5) was pursued; however, the target compounds could not be isolated due to their insolubility. Upon decorating both the octol bowl and the TTF cavity rims with long alkyl chains, the soluble bis‐TTF cavitand 23 was finally obtained, besides a minor amount of the novel cage compound 25a featuring a highly distorted TTF bridge (Scheme 6). In contrast to 25a , the deep cavitand 23 undergoes reversible vase → kite switching upon lowering the temperature from 293 to 193 K (Fig. 1). Electrochemical studies by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) provided preliminary evidence for successful vase → kite switching of 23 induced by the oxidation of the TTF cavity walls.     

A planar microfabricated nanoelectrospray emitter tip based on a capillary slot

We report on the fabrication and testing of planar nib-like structures for nanoelectrospray ionization-mass spectrometry (nanoESI-MS) applications. The micro-nib structures were fabricated on silicon substrates using the negative photoresist SU-8; they include capillary slots with widths of 8 and 16 microm. A suitable wafer cleaving step made the nib-like structures overhang the edge of a silicon substrate to provide a robust interface for nanoESI-MS applications; this freeing of the nib tip from the wafer surface created a point-like structure that is essential to establish an electrospray. The micro-nib sources were successfully tested on an LCQ Deca XP+ ion trap mass spectrometer using peptide samples at concentrations down to 1 microM. The high voltage was applied using a platinum wire inserted in the sample reservoir upstream to the capillary slot. A Taylor cone was clearly seen at the nib tip. The micro-nibs performed well at voltages as low as 0.8 kV; such performances are state-of-the-art with respect to current micromachined ESI-MS interfaces and are conditions comparable to those used for standard emitter tips. In addition, we clearly observed the influence of the micro-nib slot width on the ionization performances: the narrower the slot, the better the performances.     

Photophysical Properties of Gilvocarcins V and M and Their Binding Constant to Calf Thymus DNA

Abstract— Absorption and emission techniques were used to characterize the ground (S₀), singlet (S|) and triplet states (T₁) of gilvocarcin V (GV) and gilvocarcin M (GM) in different solvents. Aggregation of GV with dimerization constant equal to 7800 M^?1is observed in 10% dimethyl-sulfoxide (DMSO)/water. The photophysical properties of the S, state of these molecules are more sensitive to changes in solvent characteristics than the corresponding ground states. The absorption of visible light by GV and GM results in a higher dipole moment of the excited state causing a red shift in the fluorescence spectra with increasing solvent polarity. The fluorescence quantum yield remains practically unchanged with changes in solvent properties unless water is present as a co-solvent. Both φ_f and φ_f values corresponding to GV in DMSO are larger than those of GM, whereas in 10% DMSO/H₂O the opposite is observed. Thus, GV is more susceptible to other deactivation pathways besides emission in the presence of water than GM. The relative phosphorescence quantum yield (φ_p= 0.03) and the triplet energy (E_T= 52 kcal/mol) of GV and GM are similar. The S₀-S₁ energy difference is 63 kcal/mol for GV, whereas for GM it is 67. Thus, the singlet-triplet energy difference is 11 and 15 kcal/mol, respectively. The PM3/CI calculated electronic structures of these compounds are consistent with the observed photophysical properties. The dark binding constants of GV to calf thymus DNA ([1.1–0.08] × 10⁶M^?1) are about an order of magnitude larger than those of GM ([0.24–0.018] × 10⁶M^?1) at different ionic strengths (0–2.00 M NaCl). Also, the number of gilvocarcin molecules bound per base pair is smaller for GM than for GV. These differences in dark DNA binding parameters between GV and GM could have implications in the large photocytotoxic ability of GV as compared to GM.     

Photophysics and photochemistry of imipramine, desimipramine, and clomipramine in several solvents: a fluorescence, 266 nm laser flash, and theoretical study

Imipramine (IPA) and its derivatives are used widely for the treatment of depression and other mental disorders. Although there are more than 20 FDA-approved antidepressant drugs, the search continues for better compounds with fewer deleterious side effects and higher efficacy. Over the past decade, several classes of antipsychotic drugs have been developed, which-in spite of their structural diversity-share an ability to modulate neurotransmission and to produce undesirable side effects. Phototoxicity is one of the most important side effects noted in treatment with tricyclic antidepressants (TCAs), but its mechanism has not yet been elucidated. To develop new knowledge regarding the relationship between the structure and the photophysics of these TCAs, we measured the photophysical properties of IPA, desimipramine (DIPA), and clomipramine (CIPA) in different solvents. The electronic configurations for the ground and the first excited singlet states were calculated using the AM1/RHF/CI and the AM1/RHF/HE semiempirical quantum theoretical methods, respectively. The ground-state properties are solvent-independent, while the emission maxima are red-shifted with increasing solvent polarity/polarizability. However, the fluorescence quantum yield is relatively low in all of the tested solvents (phif<0.02). The primary transient intermediates produced by 266 nm high-intensity laser photolysis are the solvated electron and the corresponding radical cation, with a negligible contribution of triplet-triplet absorption. The properties determined for the primary transients generated with a 266 nm laser flash are consistent with the photodamaging effects generated through a limited radical mechanism.     

On the curvature of free boundaries with a Bernoulli-type condition

In this paper we study the classical external Bernoulli problem set in an annular domain Ω$\Omega$ of the plane.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part IV. Carbon Tetrachloride electroreduction in the presence of Cl−, Br−, SCN−, and N3−

The rate constant for CCl₄ electroreduction on mercury, once corrected for diffuselayer effects according to Frumkin, still depends on the charge density q_i due to specifically abdsorbed supporting ions. Thus, in the presence of the adsorbed anions Cl^?, Br^?, SCN^? and N₃^?, the logarithm Φ of the rate constant corrected for diffuse-layer effects decreases linearly with |q_i|. The slopes of the various Φ vs. q_i plots are in fairly good agreement with the theoretical treatment of ref. 9, which accounts for the electrostatic interactions between the activated complex for the electrode reaction and the neighbouring adsorbed electroinactive ions within the compact layer. An analogous behaviour is observed in the reduction of CBr₄.