Memory effects based on intermolecular photoinduced proton transfer

We have identified a strategy to communicate a chemical signal between two independent molecular components. One of them is a photoactive merocyanine that switches to a spiropyran, releasing a proton, when stimulated with visible light. The other is a 4,4'-pyridylpyridinium monocation that captures the released proton, producing an electroactive 4,4'-bipyridinium dication. Under the irradiation conditions employed, the photoinduced transformation requires ca. 15 min to reach a photostationary state. In the dark, the ensemble of communicating molecules reequilibrates to the original state in ca. 5 days. These processes can be monitored following the photoinduced enhancement and thermal decay, respectively, of the current for the monolectronic reduction of the 4,4'-bipyridinium dication. The pronounced difference in time scale for the current enhancement and decay steps can be exploited to implement a memory element with a bit retention time of 11 h. A bit of information can be written optically in the chemical system and it can be read electrically and nondestructively. The memory can be reset, extending its permanence in the dark beyond the bit retention time. A binary logic analysis of the signal transduction operated by the communicating molecules reveals the characteristic behavior of sequential logic operators, which are the basic components of digital memories.     

Structure of closed finitely starshaped sets

A set is finitely starshaped if any finite subset of is totally visible from some point of . It is well known that in a finite-dimensional linear space, a closed finitely starshaped set which is not starshaped must be unbounded. It is proved here that such a set must admit at least one direction of recession. This fact clarifies the structure of such sets and allows the study of properties of their visibility elements, well known in the case of starshaped sets. A characterization of planar finitely starshaped sets by means of its convex components is obtained. Some plausible conjectures are disproved by means of counterexamples.     

Theoretical evaluation of the substrate-assisted catalysis mechanism for the hydrolysis of phosphate monoester dianions

Quantum chemistry methods coupled with a continuum solvation model have been applied to evaluate the substrate-assisted catalysis (SAC) mechanism recently proposed for the hydrolysis of phosphate monoester dianions. The SAC mechanism, in which a proton from the nucleophile is transferred to a nonbridging phosphoryl oxygen atom of the substrate prior to attack, has been proposed in opposition to the widely accepted mechanism of direct nucleophilic reaction. We have assessed the SAC proposal for the hydrolysis of three representative phosphate monoester dianions (2,4-dinitrophenyl phosphate, phenyl phosphate, and methyl phosphate) by considering the reactivity of the hydroxide ion toward the phosphorus center of the corresponding singly protonated monoesters. The reliability of the calculations was verified by comparing the calculated and the observed values of the activation free energies for the analogous S_N2(P) reactions of F⁻ with the monoanion of the monoester 2,4-dinitrophenyl phosphate and its diester analogue, methyl 2,4-dinitrophenyl phosphate. It was found that the orientation of the phosphate hydrogen atom has important implications with regard to the nature of the transition state. Hard nucleophiles such as OH⁻ and F⁻ can attack the phosphorus atom of a singly protonated phosphate monoester only if the phosphate hydrogen atom is oriented toward the leaving-group oxygen atom. As a result of this proton orientation, the SAC mechanism in solution is characterized by a small Brønsted coefficient value (β_lg=−0.25). This mechanism is unlikely to apply to aryl phosphates, but becomes a likely possibility for alkyl phosphate esters. If oxyanionic nucleophiles of pK_a<11 are involved, as in alkaline phosphatase, then the S_N2(P) reaction may proceed with the phosphate hydrogen atom oriented toward the nucleophile. In this situation, a large negative value of β_lg (−0.95) is predicted for the substrate-assisted catalysis mechanism.     

Structural Analysis of the Michael-Michael Ring Closure (MIMIRC) Reaction Products

A representative number of decalin and hydrindane derivatives 2a–l were prepared in 11–91% yield by means of a cascade reaction of cyclohexanone/cyclopentanone enolates and methyl acrylate through a Michael–Michael ring closure (MIMIRC) process. The relative stereochemistry of the four stereogenic centers formed in all products was determined by analyzing the vicinal coupling constants from the ¹H NMR and X-ray crystallography. Such a stereochemical outcome was corroborated by conformational analysis supported by DFT calculations and simulating the ¹H NMR spectra of representative products. All products showed the same relative stereochemistry at C-1 and C-8a, while at C-3 and bridgehead carbon C-4a, configurational changes were observed. The present results provide some insights about the scope and limitations of the triple cascade reaction between cycloalkanone enolates with methyl acrylate. This synthetic protocol is still a simple and very practical alternative to generate decalin and hydrindane derivatives with great structural diversity.     

Visible-light promoted photoprocesses on aqueous gallic acid in the presence of riboflavin. Kinetics and mechanism

Within the frame of possible precursory photoreactions in the generation of humic substances, the visible-light promoted interaction between riboflavin (Rf), a native photosensitizer in aqueous systems, and gallic acid (GA), a polyphenol naturally formed after lignin degradation, was investigated. A systematic kinetic and mechanistic study was conducted under aerobic conditions in aqueous media, through visible-light continuous photolysis, polarographic detection of oxygen uptake, stationary and time resolved fluorescence spectroscopy, time resolved near-IR phosphorescence detection and laser flash photolysis techniques. GA is degraded relatively fast in pH 7 aqueous solutions, where singlet molecular oxygen (O₂(¹Δ_g)), superoxide radical anion (O₂^?) and hydrogen peroxide (H₂O₂) – all three species photogenerated from triplet excited Rf – participate in the photoprocess. The general conclusion is that in natural waters GA can undergo spontaneous phototodegradation under environmental conditions. Radical species generated in the presence of Rf can participate in condensation or polymerization reactions promoting the natural synthesis of humic products.     

Controlled Zn‐Mediated Grafting of Thin Layers of Bipodal Diazonium Salt on Gold and Carbon Substrates

A controlled, rapid, and potentiostat‐free method has been developed for grafting the diazonium salt (3,5‐bis(4‐diazophenoxy)benzoic acid tetrafluoroborate (DCOOH)) on gold and carbon substrates, based on a Zn‐mediated chemical dediazonation. The highly stable thin layer organic platforms obtained were characterized by cyclic voltammetry, AFM, impedance, XP, and Raman spectroscopies. A dediazonation mechanism based on radical formation is proposed. Finally, DCOOH was proved as a linker to an aminated electroactive probe.     

Reaction Mechanism in Ionic Liquids: Kinetics and Mechanism of the Aminolysis of 4‐Nitrophenyl Acetate

In this work, we report a kinetic study of the reactions of the title compound with secondary alicyclic amines at different temperatures in acetonitrile and several ionic liquids (ILs). From this study, we have described that the reactions in MeCN and in the studied ILs occur by a concerted mechanism and that the activation parameters are sensitive to the structural properties of the ILs. The pKa values of all amines used in both MeCN and ILs were determined.     

Ion-Molecule Clustering in Differential Mobility Spectrometry: Lessons Learned from Tetraalkylammonium Cations and their Isomers

Differential mobility spectrometry (DMS) can distinguish ions based upon the differences in their high- and low-field ion mobilities as they experience the asymmetric waveform applied to the DMS cell. These mobilities are known to be influenced by the ions’ structure, m/z, and charge distribution (i.e., resonance structures) within the ions themselves, as well as by the gas-phase environment of the DMS cell. While these associations have been developed over time through empirical observations, the exact role of ion structures or their interactions with clustering molecules remains generally unknown. In this study, that relationship is explored by observing the DMS behaviors of a series of tetraalkylammonium ions as a function of their structures and the gas-phase environment of the DMS cell. To support the DMS experiments, the basin-hopping search strategy was employed to identify candidate cluster structures for density functional theory treatment. More than a million cluster structures distributed across 72 different ion-molecule cluster systems were sampled to determine global minimum structures and cluster binding energies. This joint computational and experimental approach suggests that cluster geometry, in particular ion-molecule intermolecular separation, plays a critical role in DMS. Figure

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Weighted eigenvalue problems for quasilinear elliptic operators with mixed Robin–Dirichlet boundary conditions

We investigate the existence of principal eigenvalues type problems with weights for the quasilinear operator −_Δp+V_ψp$-{\mathrm{\Delta }}_p+V{\psi }_p$ with mixed weighted Robin–Dirichlet boundary conditions in a bounded regular domain. We also give some results on the existence of nonprincipal eigenvalues.     

Determining targets for multi-stage adaptive tests using integer programming

This paper considers a multi-stage adaptive test (MST) where the testlets at each stage are determined prior to the administration. The assembly of a MST requires target information and target response functions for the MST design. The targets are chosen to create tests with accurate scoring and high utilization of items in an operational pool. Forcing all MSTs to have information and response function plots to be within an interval about the targets will yield parallel MSTs, in the sense that standardized paper-and-pencil tests are considered parallel. The objective of this paper is to present a method to determine targets for the MST design based on an item pool and an assumed distribution of examinee ability. The approach is applied to a Skills Readiness Inventory test designed to identify logical reasoning deficiencies of examinees. This method can be applied to obtain item response theory targets for a linear test as this is a special case of a MST.