Direct detection of ion pair formation and collapse in a migration reaction of a beta-phosphate radical

In solutions of trifluorotoluene or toluene containing 2,2,2-trifluoroethanol, the beta-phosphate radical heterolyzed to give a detectable ion pair, identified as a solvent-separated species. Rate constants for the radical fragmentation reaction forming the ion pair, for ion pair collapse, and for diffusive escape to free ions were measured. The kinetics and entropy of activation for fragmentation indicate that the rearrangement reaction occurs by a heterolysis pathway in all solvents. [reaction: see text]     

Extension of the Coulomb–Hole–Hartree–Fock theory to molecules

The Coulomb–Hole–Hartree–Fock method introduced by E. Clementi in the early 1960s and reparametrized more recently by S. Chakraworty and E. Clementi to compute the correlated electronic energy in atomic systems, is here extended to compute molecules. The new parametrization is obtained empirically by fitting first and second atomic ionization potentials from He to Ca and a few diatomic molecules. The present formulation makes use of either one or more determinants in order to ensure proper dissociation products, following the early proposal of G.C. Lie and E. Clementi in the context of density functional computations for molecular systems. The new formulation is tested against the dissociation energies of a large number of molecules and it is found satisfactory. © 1995 John Wiley & Sons, Inc.     

New Syntheses of Retinal and Its Acyclic Analog γ‐Retinal by an Extended Aldol Reaction with a C6 Building Block That Incorporates a C5 Unit after Decarboxylation. A Formal Route to Lycopene and β‐Carotene

Since the C₁₅ β‐end‐group aldehyde 10 ((β‐ionylidene)acetaldehyde), an excellent intermediate in the syntheses of retinoids, can be synthesized in many ways from β‐ionone, and since the corresponding acyclic C₁₅ ψ‐end‐group aldehyde 5 can easily be synthesized from citral ( 1 ) (Scheme 3), we applied the C₁₅+C₅ route to the syntheses of γ‐retinal ((all‐E)‐ 8 ) (Scheme 3) and retinal ((all‐E)‐ 13 ) (Scheme 4), and therefore, by coupling (2×C₂₀→C₄₀), to the preparation of lycopene ( 14 ) and β‐carotene ( 15 ) (Scheme 5). Our new syntheses of retinal ((all‐E)‐ 13 ) and γ‐retinal ((all‐E)‐ 8 use an extended aldol reaction with a C₆ building block that incorporates a C₅ unit after decarboxylation.     

Electron-transfer-catalyzed ligand substitution of carboxylato niobocene complex induced by electrochemical oxidation

Electrochemical reduction of niobocene dichloride (η⁵-C₅H₄SiMe₃)₂NbCl₂1 formulated as in the presence of 3,4-diaminobenzoic acid yields to the complex [Nb(η⁵-C₅H₄SiMe₃)₂(κ²-O,O-OOC(C₆H₃)(NH₂)₂)] 3. When CN(2,6-Me₂C₆H₃) formulated as xylylisonitrile (CNXylyl) is added to a complex 3 solution, a substitution reaction takes place to lead to the complex [Nb(η⁵-C₅H₄SiMe₃)₂(κ¹-O-OOC(C₆H₃)(NH₂)₂)(CN(2,6-Me₂C₆H₃)) 4 after 3 h. An alternative way to yield quantitatively and nearly instantaneously 4 consists in a previous oxidation of 3 in the presence of CNXylyl. Hence, we present here a new example of electron-transfer-catalyzed (ETC) ligand substitution of carboxylato niobocene complex induced by electrochemical oxidation. The structure of the complexes, the formation mechanism are described using electrochemical and spectroscopic data. Electrochemical simulation have been done to verify experimental results and to complete them with a kinetic study.     

The role of rotation in the vibrational relaxation of water by hydrogen molecules

Vibrational relaxation cross sections of the H(2)O(upsilon(2) = 1) bending mode by H(2) molecules are calculated on a recent high-accuracy ab initio potential-energy surface using quasiclassical trajectory calculations. The role of molecular rotation is investigated at a collisional energy of 3500 cm(-1) and it is shown that initial rotational excitation significantly enhances the total (rotationally summed) vibrational relaxation cross sections. A strong and complex dependence on the orientation of the water angular momentum is also observed, suggesting the key role played by the asymmetry of water. Despite the intrinsic limitations of classical mechanics, these exploratory results suggest that quantum approximations based on a complete decoupling of rotation and vibration, such as the widely used vibrational close-coupling (rotational) infinite-order-sudden method, would significantly underestimate rovibrationally inelastic cross sections. We also present some rationale for the absence of dynamical chaos in the scattering process.     

Can the Theoretical Fitting of the Proton‐Nuclear‐Magnetic‐Relaxation‐Dispersion (Proton NMRD) Curves of Paramagnetic Complexes Be Improved by Independent Measurement of Their Self‐Diffusion Coefficients?

The self‐diffusion (D_c) coefficients of various lanthanum(III) diamagnetic analogues of open‐chain and macrocyclic complexes of gadolinium used as MRI contrast agents were determined in dilute aqueous solutions (3–31 mM ) by pulsed‐field‐gradient (PFG) high‐resolution ¹H‐NMR spectroscopy. The self‐diffusion coefficient of H₂O (D_w) was obtained for the same samples to derive the relative diffusion constant, a parameter involved in the outersphere paramagnetic‐relaxation mechanism. The results agree with an averaged relative diffusion constant of 2.5 (±0.1)×10^?9 and of 3.3 (±0.1)×10^?9 m² s^?1 at 25 and 37°, respectively, for 'small' contrast agents (M_r 500–750 g/mol), and with the value of bulk H₂O (2.2×10^?9 and 2.9×10^?9 m² s^?1 at 25° and at 37°, respectively) for larger complexes. The use of the measured values of D_c for the theoretical fitting of proton NMRD curves of gadolinium complexes shows that the rotational correlation times (τ_R) are very close to those already reported. However, differences in the electronic relaxation time (τ_SO) at very low field and in the correlation time (τ_V) related to electronic relaxation were found.     

Screening by kinetic Monte Carlo simulation of Pt-Au(100) surfaces for the steady-state decomposition of nitric oxide in excess dioxygen

An ab initio-based kinetic Monte Carlo algorithm was developed to simulate the direct decomposition of NO over Pt and different PtAu alloy surfaces. The algorithm was used to test the influence of the composition and the specific atomic surface structure of the alloy on the simulated activity and selectivity to form N2. The apparent activation barrier found for the simulation of lean NO decomposition over Pt(100) was 7.4 kcal/mol, which is lower than the experimental value of 11 kcal/mol that was determined over supported Pt nanoparticles. Differences are likely due to differences in the surface structure between the ideal (100) surface and supported Pt particles. The apparent reaction orders for lean NO decomposition over the Pt(100) substrate were calculated to be 0.9 and -0.5 for NO and O2, respectively. Oxygen acts to poison Pt. Simulations on the different Pt-Au(100) surface alloys indicate that the turnover frequency goes through a maximum as the Au composition in the surface is increased, and the maximum occurs near 44% Au. Turnover frequencies, however, are dictated by the actual arrangements of Pt and Au atoms in the surface rather than by their overall composition. Surfaces with similar compositions but different alloy arrangements can lead to very different activities. Surfaces composed of 50% Pt and 50% Au (Pt4 and Au4 surface ensembles) showed very little enhancement in the activity over that which was found over pure Pt. The Pt-Pt bridge sites required for NO adsorption and decomposition were still effectively poisoned by atomic oxygen. The well-dispersed Pt(50%)Au(50%) alloy, on the other hand, increased the TOF over that found for pure Pt by a factor of 2. The most active surface alloy was one in which the Pt was arranged into "+" ensembles surrounded by Au atoms. The overall composition of this surface is Pt(56.2%)Au(43.8%). The unique "+" ensembles maintain Pt bridge sites for NO to adsorb on but limit O2 as well as NO activation by eliminating next-nearest neighbor Pt-bridge sites. The repulsive interactions between two adatoms prevent them from sharing the same metal atoms. The decrease in the oxygen coverage leads to a greater number of vacant sites available for NO adsorption. This increases the NO coupling reaction and hence N2 formation. The inhibition of the rate of N2 formation by O2 is therefore suppressed. The coverage of atomic oxygen decreases from 53% on the Pt(100) surface down to 19% on the "+" ensemble surface. This increases the rate of N2 formation by a factor of 4.3 over that on pure Pt. The reaction kinetics over the "+" ensemble Pt(56.2%)Au(43.8%) surface indicate apparent reaction orders in NO and oxygen of 0.7 and 0.0, respectively. This suggests that oxygen does not poison the PtAu "+" alloy ensemble. The activity and selectivity of the PtAu ensembles significantly decrease for alloys that go beyond 60% Au. Higher coverages of Au shut down sites for NO adsorption and, in addition, weaken the NO and O bond strengths, which subsequently promotes desorption as well as NO oxidation. The computational approach identified herein can be used to more rapidly test different metal compositions and their explicit atomic arrangements for improved catalytic performance. This can be done "in silico" and thus provides a method that may aid high-throughput experimental efforts in the design of new materials. The synthesis and stability of the metal complexes suggested herein still ultimately need to be tested.     

A stabilized formulation of IBX (SIBX) for safe oxidation reactions including a new oxidative demethylation of phenolic methyl aryl ethers

[reaction: see text] SIBX is a nonexplosive formulation of IBX that can be used as a suspension in a variety of standard organic solvents such as refluxing EtOAc and THF to oxidize safely alcohols into aldehydes and ketones. The use of hot THF is limited to the oxidation of allylic and benzylic alcohols. Most yields are comparable to those obtained with IBX or DMP. SIBX can also be used to perform oxygenative demethylation of 2-methoxyarenols into orthoquinones and catechols.     

Application of relaxation periods during electrodialysis of a casein solution: Impact on anion-exchange membrane fouling

Electrodialysis (ED) is a membrane process used on a large scale. However, one of the common problems is fouling of ion-exchange membranes stacked in the cell. The use of pulsed power, consisting in applying a constant current density during a fixed time of application (T_on) followed by a pause duration (T_off), was demonstrated recently as an effective fouling mitigation method for electrodialysis. Up until now, no work has investigated the potential of electrodialysis using pulsed electric field on protein fouling. The aim of the present work was to study the influence of pulsed electric field (PEF) with a low frequency square shaped periodic signal (T_on = 10 s–T_off = 10 s, T_on = 10 s–T_off = 40 s) in comparison with dc current during electrodialysis of a casein solution at different current densities (10, 20 and 30 mA/cm²) on membrane fouling. It appeared from these results that PEF, under certain conditions of pulse, would avoid fouling on anion-exchange membranes. For 10 s–40 s pulsed electric field conditions, no fouling was observed with any density, while for 10 s–10 s PEF conditions, fouling appeared only at current density over 10 mA/cm². dc current, whatever the current density conditions, led to a fouling on the diluate side of the AEM. Furthermore, when fouling occurred, magnitude layer thickness and dry weight increased with the applied current density. The nature of the fouling was identified as 97% protein. The protein fouling would be due to the dissociation of water molecules and/or heat increase at the anion-exchange membrane interface. The relaxation time of the pulse would limit both phenomena on the membrane.