Initial Radical Cation Research Articles

Thermodynamic and kinetic models for the photolytic and hydrolytic degradation of picaridin: A computational and theoretical study

The popularity of insect repellents presents the opportunity for continuous human and environmental exposure to such chemical mixtures. Consequently, the effects of their chemical components on human health and environmental safety is significantly reliant on their stability and hence, residence times under typical environmental conditions. Hence, for the first time, a thorough density functional theoretical (DFT) study on some possible hydrolytic and photolytic degradation pathways is presented and applied in the derivation of a comprehensive kinetic model, allowing evaluation of the residence time of picaridin; a very popular insect repellent. These results show that picaridin undergoes very slow hydrolysis via two pathways which present activation free energy barriers (ΔG‡) of ca. 225 and ca. 247 kJ mol−1 due to the asynchronous formation of four-membered cyclic activated complexes in both cases. Similarly, their photolytic transformations occur via two pathways, both of which are limited by the formation of the initial radical cation.

Primary Radical Cations in Irradiated Ethylene Carbonate

To search for and elucidate the nature of radical cations arising in irradiated liquid ethylene carbonate (EC), the effect of the external magnetic field on the radiation-induced fluorescence of fluorophore solutions in EC has been studied. Using quantum-chemical calculations, it has been shown for the first time that primary EC radical cations are rapidly transformed into ion–molecular complexes containing, in addition to the initial radical cation, one or more solvent molecules. In the complexes, two particles are oriented to each other by carbonyl groups, on which almost all of the spin density is localized. This result shows that the prevailing concepts describing the primary stages of liquid carbonate radiolysis should be revised, and the possibility of the formation of such ion–molecular complexes should be taken into account when studying the oxidative decomposition of carbonates used in organic electrolytes.

Initial radical cation pathway in the Mo2Cl10-mediated dehydrogenative arene coupling.

Experimental (EPR) and theoretical (DFT) evidence is provided for radical cation formation as initial step in the Mo2Cl10-mediated dehydrogenative arene coupling. The initial electron transfer from methoxyarenes to molybdenum proceeds via an inner sphere mechanism.

Psoralen and Coumarin Photochemistry in HSA Complexes and DMPC Vesicles†

The photochemistry and photophysics of several psoralens and coumarins have been examined in human serum albumin (HSA) complexes and dimyristoylphosphatidylcholine (DMPC) vesicles. Fluorescence spectroscopy indicates that there are multiple binding sites with polarities that are intermediate between those of acetonitrile and water for the substrates complexed to HSA. In the case of the 6,7-dimethoxycoumarin-HSA complex, laser flash photolysis experiments provide evidence for the formation of radical cation in addition to triplet. Radical cations are not detected for other coumarin-HSA complexes, either due to a lower yield of formation or to rapid reaction of an initial radical cation with adjacent amino acids. Fluorescence spectra for coumarins indicate that they are primarily solubilized in the polar headgroup region in DMPC vesicles. Consistent with this, radical cations generated by photoionization are detected in transient experiments. For dimethoxycoumarins the radical cation is long-lived, indicating rapid exit from the vesicle and decay in the aqueous phase. However, 4,5',8-trimethylpsoralen and 7-ethoxy-4-hexadecylcoumarin radical cations are much shorter-lived, presumably due to rapid decay by electron recombination in the vesicle. The results for both HSA complexes and vesicles indicate that radical ions may play a role in psoralen and coumarin photochemistry in a cellular environment.

Anodic Olefin Coupling Reactions Involving Ketene Dithioacetals: Evidence for a “Radical‐Type” Cyclization.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Anodic olefin coupling reactions involving ketene dithioacetals: evidence for a ‘radical-type’ cyclization

A series of anodic coupling reactions between ketene dithioacetal groups and enol ethers have been studied in order to probe why some of the cyclizations are successful while other closely related attempts fail. It has been found that the success of the cyclization reactions strongly depends on the location of substituents on the olefins. The reactions are highly sensitive to substituents on the terminating olefin but not to substituents on the initial radical cation. This behavior is consistent with what has been observed previously with radical cyclization reactions.

Electron and oxygen transfer in polyoxometalate, H(5)PV(2)Mo(10)O(40), catalyzed oxidation of aromatic and alkyl aromatic compounds: evidence for aerobic Mars-van Krevelen-type reactions in the liquid homogeneous phase.

The mechanism of aerobic oxidation of aromatic and alkyl aromatic compounds using anthracene and xanthene, respectively, as a model compound was investigated using a phosphovanadomolybdate polyoxometalate, H(5)PV(2)Mo(10)O(40), as catalyst under mild, liquid-phase conditions. The polyoxometalate is a soluble analogue of insoluble mixed-metal oxides often used for high-temperature gas-phase heterogeneous oxidation which proceed by a Mars-van Krevelen mechanism. The general purpose of the present investigation was to prove that a Mars-van Krevelen mechanism is possible also in liquid-phase, homogeneous oxidation reactions. First, the oxygen transfer from H(5)PV(2)Mo(10)O(40) to the hydrocarbons was studied using various techniques to show that commonly observed liquid-phase oxidation mechanisms, autoxidation, and oxidative nucleophilic substitution were not occurring in this case. Techniques used included (a) use of (18)O-labeled molecular oxygen, polyoxometalate, and water; (b) carrying out reactions under anaerobic conditions; (c) performing the reaction with an alternative nucleophile (acetate) or under anhydrous conditions; and (d) determination of the reaction stoichiometry. All of the experiments pointed against autoxidation and oxidative nucleophilic substitution and toward a Mars-van Krevelen mechanism. Second, the mode of activation of the hydrocarbon was determined to be by electron transfer, as opposed to hydrogen atom transfer from the hydrocarbon to the polyoxometalate. Kinetic studies showed that an outer-sphere electron transfer was probable with formation of a donor-acceptor complex. Further studies enabled the isolation and observation of intermediates by ESR and NMR spectroscopy. For anthracene, the immediate result of electron transfer, that is formation of an anthracene radical cation and reduced polyoxometalate, was observed by ESR spectroscopy. The ESR spectrum, together with kinetics experiments, including kinetic isotope experiments and (1)H NMR, support a Mars-van Krevelen mechanism in which the rate-determining step is the oxygen-transfer reaction between the polyoxometalate and the intermediate radical cation. Anthraquinone is the only observable reaction product. For xanthene, the radical cation could not be observed. Instead, the initial radical cation undergoes fast additional proton and electron transfer (or hydrogen atom transfer) to yield a stable benzylic cation observable by (1)H NMR. Again, kinetics experiments support the notion of an oxygen-transfer rate-determining step between the xanthenyl cation and the polyoxometalate, with formation of xanthen-9-one as the only product. Schemes summarizing the proposed reaction mechanisms are presented.

Probing M−O Bond Cleavage in Silicon and Titanium Bisenolate Radical Cations

In this study, several novel sterically congested silicon and titanium bisenolates of exceptional hydrolytic stability have been synthesized and characterized. The structure of the titanium bisenolate T2 could be determined by X-ray structure analysis. Preparative one-electron oxidation of the sterically shielded metal bisenolates S1-S4 and T1-T2 led to formation of the benzofurans B1 and B2. As the benzofuran formation is strong evidence for an initial M-O bond cleavage process at the stage of the initial radical cation we were able to measure the kinetics of this scission process using cyclic voltammetry techniques (first-order rate constants range at k(f) = 0.1-90 s(-)(1)). These data indicate that both silicon and titanium bisenolates are interesting candidates for oxidative intramolecular carbon-carbon bond formation processes.

Calibration of Intramolecular Radical Cation Probes Based on Arylalkene Cycloadditions1

The radical cations of three probe molecules (1−3) that undergo intramolecular cyclobutanation or Diels−Alder reactions have been generated by both photoionization and photosensitized electron transfer. In each case, flash photolysis experiments indicate formation of an initial radical cation with λmax at ∼600 nm that is localized on the arylalkene moiety. The radical cations of 1 and 3 have decay rate constants of 1.2 × 109 and 3.0 × 108 s-1 and can only be observed by picosecond spectroscopy. These cyclization rate constants agree reasonably well with previous estimates based on product and Stern−Volmer quenching studies using a triarylaminium salt as the electron transfer catalyst. The transient experiments are consistent with cyclization of 1•+ to give a cyclobutane radical cation that has a lifetime of ∼100 ns and decays by either reduction to cyclobutane or rearrangement to a hexatriene radical cation, λmax at 500 nm (10), but does not revert to 1•+. Consistent with this, product studies under photo...

Synthesis, Electrooxidation, and Characterization of Bis(diphenylamine)naphthalene Diimide

We have synthesized a new poly(imide) precursor for solution electrodeposition onto conducting surfaces. The new monomer, N,N‘-bis[p-phenylamino(phenyl)]-1,4,5,8-naphthalenetetracarboxylic diimide (DNTD), contains a central naphthalene diimide moiety flanked by two dimerizable diphenylamine groups. DNTD was oxidatively electrodeposited onto Au-, Pt-, and In-doped SnO2 surfaces from DMSO, CH3CN, and CH2Cl2. The cyclic voltammetry is consistent with initial radical cation formation of diphenylamine groups, and then para C−C coupling of radicals to form dimers and higher order oligomers. IR spectroscopy was used to determine the average degree of polymerization and confirm para coupling. The resulting material shows electrochemically reversible 1e-/ and 2e-/monomer unit reduction waves corresponding to the naphthalene tetracarboxylic diimide radical anion and dianion. Also a quasireversible 1e-/ and 2e-/monomer unit oxidation corresponding to the oxidation of diphenylbenzidine unit is shown. Visible−NIR spec...

Regioselective Rearrangement of Bridgehead-Methyl-Substituted Radical Cations Derived from Bicyclo[2.1.0]pentanes and 2,3-Diazabicyclo[2.2.1]hept-2-enes through Photoinduced Electron Transfer and Radiolytic Oxidation: Product Distribution and Matrix ESR Studies

Cyclopentane-1,3-diyl radical cations were generated from the 1-methyl- and 1,4-dimethyl substituted bicyclo-[2.1.0]pentanes 1b,c through photoinduced electron transfer (PET) and radiolytic oxidation. The unsymmetrical bridgehead-substituted bicyclopentane 1b rearranged spontaneously and exclusively to the 3-methylcyclopentene 3b under PET conditions. ESR studies showed similarly that 3b[sup [sm bullet]+] was the only final oxidation product of 1b; the initial radical cation 1b[sup [sm bullet]+] was not detected because it rearranges rapidly and stereoselectively by a 1,2-hydrogen shift to 3b[sup [sm bullet]+], even at 80 K, and no trace of the more stable 1-methylcyclopentene radical cation 3a[sup [sm bullet]+] was observed. This contra-thermodynamic regioselectivity is rationalized in terms of essential localization of positive charge at the tertiary center as the reaction proceeds in the 1,3-diyl radical cation 1b[sup [sm bullet]+]. The symmetrical dimethyl derivative 1c rearranged much more reluctantly than 1b despite its lower oxidation potential, and this is attributed to the greater persistence of radical cation 1c[sup [sm bullet]+] through its reluctance to undergo a 1,2-H shift. The oxidation of these azoalkanes generates highly reactive transients, presumably diazenyl radical cations, which readily denitrogenate and undergo 1,2-H shifts in either a consecutive or concerted manner to form olefin radical cations. 25 refs., 6 figs., 2more » tabs.« less