Activation energy of photosynthetic oxygen evolution: An attempt at theoretical analysis

In this study, a first attempt is made to calculate theoretically the activation energies of the different steps of water oxidation. The analysis has led to the following conclusions about the process mechanism: (1) the barrier produced by the repulsion of unbound O-atoms during their mutual approach may be partly surmounted at the expense of the binding energy of water molecules by manganese ions; (2) the concerted transfer of electrons and protons involving the participation of bases stronger than water improves essentially the process energetics; (3) multi-electron steps require the use of several one-electron acceptors; (4) four-electron oxidation of water in a single elementary act is unlikely: (5) the most likely reaction path is the rate-determining two-electron water oxidation to hydrogen peroxide (the possibility of this process occurring in two consecutive one-electron steps is not clear), followed by two fast oxidation steps of H₂O₂ to HO₂ and further to O₂.     

Control of extremely fast competitive consecutive reactions using micromixing. Selective Friedel-Crafts aminoalkylation

Friedel-Crafts reactions of aromatic and heteroaromatic compounds with an N-acyliminium ion pool were studied. The reaction of 1,3,5-trimethylbenzene in a batch reactor gave rise to the selective formation of a monoalkylation product (69%). Presumably, the second alkylation is slower than the first alkylation because of the protonation of the monoalkylation product that decreases its reactivity. The reaction of 1,3,5-trimethoxybenzene, however, gave rise to the formation of both monoalkylation (37%) and dialkylation (32%) products. Disguised chemical selectivity due to faster reaction than mixing seems to be responsible for the lack of selectivity. The use of micromixing was found to be quite effective to solve this problem to increase the selectivity. The monoalkylation product was obtained in 92% yield together with a small amount of the dialkylation product (4%). The reaction with various aromatic and heteroaromatic compounds revealed that the low mono/dialkylation selectivity was observed only for highly reactive aromatics. In such cases, the use of micromixing was quite effective to improve the selectivity. On the basis of micromixing, the selective sequential dialkylation using two different N-acyliminium ions was achieved. CFD simulations using a laminar flow and finite-rate model are consistent with the experimental observations and clearly indicate the importance of mixing.     

Role of solvation energy in starch adsorption on oxide surfaces

The effect of potassium and sodium cations on the adsorption of starch onto hematite and quartz was investigated. The role of these ions was analyzed in terms of their water structure-making or -breaking capabilities. In the presence of Na⁺, a structure maker, the polymer adsorption density did not change compared to the adsorption levels observed in distilled water. However, in the solutions of K⁺, a structure-breaking cation, the adsorption density of starch significantly increased. Assuming hydrogen bonding and chemical interaction to be the driving adsorption mechanism, it was proposed that the starch–oxide interactions can be envisioned as the competition between chemical interaction/hydrogen bonding and solvation energy:

K⁺ reduces solvation energy by disturbing interfacial water structure and thus increases the free energy of adsorption, allowing the polymer to more closely approach the oxide surfaces. In contrast, Na⁺ which is indifferent to solvation energy does not interfere with the free energy of adsorption.     

An attempt at quantum thermal physics

Summary This paper outlines an alternative exposition of the structure of quantum thermodynamics which is essentially based on Carnot's theory where fluxes of caloric are identified with negative information fluxes. It is further assumed that the thermal energy evolved by thermal processes is identical with the electromagnetic zero-point background energy evolved by the destruction of information inscribed in a structural unit (qubit). Theoretical arguments on an elementary level are accompanied by illustrative examples.     

BTISA-catalyzed Friedel-Crafts bimolecular cyclization of cinnamic acid under superelectrophilic solvation conditions

Friedel-Crafts bimolecular cyclizations of cinnamic acid and cinnamoyl chloride with aromatic compounds in strong and superstrong acids in present of 1 mol% BTISA were investigated. It was demonstrated that catalytic amounts of this new superacid have essential effect on such type of reactions. Its use makes possible the preparation of indanones with quantitative yields.     

Lewis acid-nitromethane complex-promoted Friedel-Crafts reactions of PS-DVB-resins

Aminomethyl-polystyrene resins were prepared using FeCl₃-nitromethane and FeCl₃-benzophenone complexes as Friedel-Crafts catalysts. All the resins were highly loaded and functionalized with Rink amide linker. A comparative synthesis of the classic difficult sequence ACP (65-74) on the prepared resins by Fmoc/t-Bu chemistry is presented. The target peptide of highest purity (91%) was that prepared using FeCl₃-nitromethane.     

Friedel-Crafts reactions of 2,2-difluorocyclopropanecarbonyl chloride: unexpected ring-opening chemistry

The Friedel-Crafts reactions of 2,2-difluorocyclopropanecarbonyl chloride with various arenes did not lead to the straightforward formation of the expected aryl 2,2-difluorocyclopropyl ketones. Instead the reactions proceeded, to various degrees depending on the reactivity of the arene, via an apparent rearrangement of the initially formed acylium ion to form novel aryl 3-chloro-3,3-difluoropropyl ketones. The ring-opened product was formed exclusively, and therefore the reaction may be synthetically useful when relatively unreactive arene substrates such as benzene, toluene, and p-xylene are used. No conditions were found where ring-intact products could be formed exclusively when using substituted benzenes as substrates, with the very reactive substrate thiophene being most selective in that regard, favoring ring intact product 3 with a selectivity of 98:2. The regioselectivity of ring-opening was examined and compared with other related systems computationally.     

Role of radiolytically formed protonating agents in gas phase competitive reactions

Radiolytically formed O₂H⁺, N₂H⁺, and CO₂H⁺ ions were allowed to react with gaseous p-cymene. Dealkylation and isomerization reactions were observed with O₂H⁺ and N₂H⁺ ions, while only the first process occurred when CO₂H⁺ ions were employed. The results show that dealkylation is favored with respect to isomerization as the protonation exothermicity decreases.     

Thermally generated phenylcarbenium ions: acid-free and self-quenching Friedel-Crafts reactions

2-Benzyloxy-1-methylpyridinium triflate (1) serves as a stable precursor to a phenylcarbenium species as evidenced by its reactivity in Friedel-Crafts alkylations with electron-rich arenes.     

Deep cavitands provide organized solvation of reactions

Deep self-folding cavitands have been shown to stabilize the charged enolate intermediate of the alpha-deuteration of activated olefins by means of a network of organized solvation.     

Role of solvation in the energy stabilization inside the hydrophobic core of the protein rubredoxin

There are many forces that contribute to the stability of a protein; among these are dispersion interactions, hydrogen bonding, and solvation effects. In a recent work, Vondrasek et al. estimated the in vacuo stabilization energy of the hydrophobic core of the protein rubredoxin using high level ab initio methods (Vondrasek, J.; et al. J. Am. Chem. Soc. 2005, 127, 2615). In this work, we evaluate the effects of solvation on the stability of the hydrophobic core of this protein. Solvation calculations are made using the polarizable continuum method at the MP2/aug-cc-pVDZ level of theory. It is found that, in a protein-like environment (mimicked by a continuum solvent with a dielectric constant of approximately 4), the stability of rubredoxin's hydrophobic core is decreased by 40-50%. We also observed that the stabilization energy of the hydrophobic core is only slightly lower in a protein-like medium than in an aqueous one (DeltaGether-DeltaGwater approximately 1.0-3.5 kcal/mol).     

Synthesis of quaternary stereogenic centres via stereoselective intermolecular Friedel-Crafts reactions

Highly stereoselective Friedel-Crafts reactions have been performed using a chiral anthracene template to control the selectivity of the reaction. In the case of additions to fully substituted N-acyliminium ions, competitive elimination and condensation reactions were observed. Retro-Diels-Alder reaction of one of the reaction products led to a precursor that could be used for the construction of pyroglutamic acids bearing quaternary stereogenic centres.     

Chloroindate(III) ionic liquids: recyclable media for Friedel-Crafts acylation reactions

Chloroindate(III) ionic liquids are versatile reaction media for Friedel-Crafts acylation reactions; the system is catalytic and totally recyclable, using an aqueous workup, with no leaching of the indium into the product phase.     

Electrostatic contribution to the ion solvation energy: cavity effects

Ion solvation process has been analysed for the spherically symmetrical system where an ion is located inside a cavity surrounded by an isotropic nonlocal dielectric medium. It has been proven that for any dielectric properties of the medium, the electric field outside the cavity as well as the ion solvation energy depend only on the total ion charge but not of the particular distribution of the ion charge density inside the cavity. These characteristics remain unchanged if the charge is displaced from the external boundary of the cavity into it. Analytical formulas for them have been derived for a particular model of the nonlocal dielectric function. Comparison of results for the solvation energy on the basis of this new theory and of the conventional approach (disregarding the existence of the cavity) shows a significant difference between their predictions if the ion charge is displaced inside the ion cavity.     

Continuous medium theory for nonequilibrium solvation: I. How to correctly evaluate solvation free energy of nonequilibrium

Considering the influences of electrostatic potential Phi upon the change of solute charge distribution deltarho and rho upon the change deltaPhi at the same time, a more reasonable integral formula of dG = (1/2) integral (V) (rhodeltaPhi + Phideltarho)dV is used to calculate the change of the electrostatic free energy in charging the solute-solvent system to a nonequilibrium state, instead of the one of dG = integral (V) PhideltarhodV used before. This modification improves the expressions of electrostatic free energy and solvation free energy, in which no quantity of the intermediate equilibrium state is explicitly involved. Detailed investigation reveals that the solvation free energy of nonequilibrium only contains the interaction energy between the field due to the solute charge in vacuum, and the dielectric polarization at the nonequilibrium state. The solvent reorganization energies of forward and backward electron transfer reactions have been redefined because the derivations lead to a remarkable feature that these quantities are direction-dependent, unlike the theoretical models developed before. The deductions are given in the electric field-displacement form. Relevant discussions on the reliability of theoretical models suggested in this work have also been presented.     

Catalytic enantioselective Friedel-Crafts/Michael addition reactions of indoles to ethenetricarboxylates

[reaction: see text] The Friedel-Crafts reaction is an important reaction for the formation of new C-C bonds. Recently, catalytic enantioselective Friedel-Crafts reaction of alkylidene malonates has been reported. However, the substituents in alkylidene malonates are limited. To explore new substituents such as carboxyl and carbonyl groups, catalytic enantioselective Friedel-Crafts reactions of reactive ethenetricarboxylates and acyl-substituted methylenemalonates 1 were investigated. The reaction of 1 with indoles in the presence of catalytic amounts of chiral bisoxazoline copper(II) complex (10%) in THF at room temperature gave alkylated products in high yields and up to 95% ee. The enantioselectivity can be explained by the secondary orbital interaction on approach of indole to the less hindered side of the 1-Cu(II)-ligand complex.     

PCMSolver: An open-source library for solvation modeling

PCMSolver is an open-source library for continuum electrostatic solvation. It can be combined with any quantum chemistry code and requires a minimal interface with the host program, greatly reducing programming effort. As input, PCMSolver needs only the molecular geometry to generate the cavity and the expectation value of the molecular electrostatic potential on the cavity surface. It then returns the solvent polarization back to the host program. The design is powerful and versatile: minimal loss of performance is expected, and a standard single point self-consistent field implementation requires no more than 2 days of work. We provide a brief theoretical overview, followed by two tutorials: one aimed at quantum chemistry program developers wanting to interface their code with PCMSolver , the other aimed at contributors to the library. We finally illustrate past and ongoing work, showing the library's features, combined with several quantum chemistry programs.     

Role of magnetic forces in electrochemical reactions at microstructures

The effect of an external magnetic field (up to 0.8 T) on the anodic dissolution of microstructures has been investigated systematically. Copper and silver wires (100 microm in diameter) were embedded in epoxy resin and dissolved potentiostatically while a magnetic field was periodically switched on and off. A special feature of the thus prepared structures is that they show a smooth transition from an inlaid disk to a recessed disk electrode. An increase or a decrease of the limiting current density in the presence of B was found depending on the orientation of the magnetic field and the hydrodynamic conditions in the cell (natural or forced convection). The magnetic forces which are responsible for this are the Lorentz force and the gradient force. We propose a model which discusses the interaction of these forces with the natural and the forced convection to explain the experimental results.     

An improved estimation of water-organic liquid interfacial tension based on linear solvation energy relationship approach

An equation based on the linear solvation energy relationship (LSER) was proposed to predict the interfacial tension between organic liquid and water. The equation takes into account five parameters characterizing properties of the organic liquid molecule: excess molar refraction, solute dipolarity/polarizability, effective hydrogen bond acidity, effective hydrogen bond basicity, and the McGowan molar intrinsic volume. The proposed equation provides a better approximation of the interfacial tension than a similar one derived earlier by Freitas et al. (J. Phys. Chem. B 101 (1997, 7488-7493), which is based on seven terms.     

Organocatalytic asymmetric Friedel-Crafts alkylation/cascade reactions of naphthols and nitroolefins

The asymmetric Michael-type Friedel-Crafts reaction of naphthols and nitroolefins promoted by bifunctional thiourea-tertiary amine organocatalysts (up to 95% ee) was investigated; on simply extending the reaction time further cascade reactions could occur to generate enantiopure dimeric tricyclic 1,2-dihydronaphtho[2,1-b]furanyl-2-hydroxylamine derivatives.