Showing papers on "Radical ion published in 1993"

Effects of the electron-withdrawing power of substituents on the electronic structure and reactivity in oxoiron(IV) porphyrin π-cation radical complexes

Abstract: The effects of the electron-withdrawing power of the substituents bound to a porphyrin ring on the electronic structures and the reactivities of oxoiron(IV) porphyrin π-cation radical complexes were studied by using 2,7,12,17-tetramethyl-3,8,13,18-tetraarylporphyrins (aryl=(1) mesityl, (2) 2-chloro-6-methylphenyl, 2,6-dichlorophenyl, or 2,4,6-trichlorophenyl). The electronic structures of oxoiron(IV) porphyrin π-cation radicals were investigated by low-temperature UV-visible absorption spectroscopy and proton NMR measurements.

Mechanism of oxidation of aliphatic thioethers to sulfoxides by hydroxyl radicals. The importance of molecular oxygen

Abstract: The sulfoxide yield in the . OH radical-induced oxidation of dimethylsulfide in aqueous solution is dramatically enhanced by admission of molecular oxygen. Various possible mechanistic pathways leading to sulfoxide can now be eliminated from speculation as a result of a detailed study on the influence of sulfide concentration, pH, isotope effects, radical cation stability and deprotonation kinetics, and electron-transfer reactions from transient radicals to special additives. The major route to sulfoxide is demonstrated to involve a 2σ/1σ * three-electron-bonded radical cation, (>S.-.S S(OH)S-<

Quenching mechanism in a highly exothermic region of the Rehm-Weller relationship for electron-transfer fluorescence quenching

Abstract: The fluorescence quenching mechanism in a highly exothermic region of the Rehm-Weller relationship is shown to be a long-distance electron transfer for producing the geminate radical ion pair with fluorescer radical cation in an electronically excited state and quencher radical anion in the ground state

Solvent effects on photoinduced electron-transfer reactions

Abstract: Dependence of the fluorescence-quenching rate constant k q , effective quenching distance r q , and free radical yield Φ R on the free enthalpy change ΔG f of full electron transfer has been studied in dichloromethane by using anthracenecarbonitriles as electron-accepting fluorescers and amino- and methoxybenzenes as electron-donating quenchers. On the basis of the present data in dichloromethane and the previous data in acetonitrile, the solvent effects on the electron-transfer fluorescence-quenching mechanism and on the back electron-transfer reaction within the geminate radical ion pairs are elucidated

Electrochemical and Electronic Properties of Neutral and Oxidized Soluble Orthogonally-Fused Thiophene Oligomers.

Abstract: : The chemical and electrooxidation of spiro-fused thiophene trimers, SIII, and spiro-fused thiophene heptamers, SVII, with terminal alpha-Si(CH3)3 and Beta-CH3 groups were studied in Ch2Cl2 at room temperature. The heptamer segments in SVII oxidize stepwise to produce the mono radical cation, the bis(radical cation). the radical cation/dication, and the bis(dication), respectively. The existence of these intermediate ions was confirmed by vis/ near-IR and ESR analyzed. The presence of anodic peaks at different potentials for the removal of the first and second electron from SIII and SVII suggests that some interaction exists between the radical cation and the neutral oligomers. However, once both oligomers are there appears to be no significant electronic interactions between tbe orthogonally positioned heptamer cations except possibly some weak magnetic coupling. With SIII, the trimer segments oxidize at fairly anodic potentials, and the four sequential intermediates are not observed because the less stable cations probably dimerize to form a bis(spire structure) containing a thiophene hexamer cation.

Intramolecular hydrogen transfer as the key step in the dissociation of hydroxyl radical adducts of (alkylthio)ethanol derivatives

Abstract: The reaction of the hydroxyl radical with dimethyl sulfide (DMS), 2-(methylthio)ethanol (2-MTE), 2,2'-dihydroxydiethyl sulfide (2,2'-DHE), and 3,3'-dihydroxydipropyl sulfide (3,3'-DHP) has been investigated in H[sub 2]O and D[sub 2]O. As an initial step hydroxyl radicals add to the sulfur moiety. These hydroxyl radical adducts subsequently decay via a thioether concentration-dependent and a thioether concentration-independent pathway. The hydroxyl radical adduct of DMS dissociates into a sulfur radical cation and HO[minus] in the thioether concentration-independent pathway (k[sub H]/k[sub D] = 2.09), whereas a rate-limiting proton transfer from water operates in the thioether concentration-dependent mechanism (k[sub H]/k[sub D] = 5.40), as deduced from the measured solvent kinetic isotope effects. In contrast the hydroxyl radical adducts of 2-MTE and 2,2'-DHE decompose via elimination of water, formed through a rapid intramolecular hydrogen transfer from the adjacent hydroxyl groups. This mechanism leads to the formation of (alkylthio)ethoxy radicals. The latter undergo [alpha],[beta]-fragmentation into formaldehyde and [alpha]-thioether radicals as well as hydrogen abstraction from a [delta]-methylene group, analogous to a hydrogen transfer in the Barton reaction, leading to [alpha]-thioether radicals. 37 refs., 5 figs., 2 tabs.

Photoinduced electron transfer (PET) in organic synthesis

Abstract: PET processes between ground and excited states of donor-acceptor pairs lead to the formation of radical ion pairs which upon disproportionation gives neutral radicals and/or ions required for synthetic purposes. A host of new reactions has been uncovered, the nature of which is governed by the chemical properties and reactivity profiles of these reactive intermediates. Our objective in this chapter will be to enlist critical examples from the last ten years dealing with the radical ion reactions of synthetic importance initiated by photoexcitation of donor-acceptor pairs.

Generation of large radical ions from oligometallocenes by matrix‐assisted laser desorption ionization

Abstract: Non-polar polymers containing ferrocene, ferrocenylnaphthalene, and ruthenocenylnaphthalene groups in their repeating units were studied by matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry. Sample preparation for these polymers utilized tetrahydrofuran as solvent and several new matrices such as dithranol, 9-nitroanthracene, and quinizarin (all of these are anthracene derivatives). By comparing the mass spectra of oligometallocenes recorded with different matrices at different wavelengths (337 nm and 2.94 μm) and laser desorption ionization mass spectra recorded without matrix, it could be verified that the major analyte-related peaks in the MALDI mass spectra corresponded to the radical molecular ions. Radical ions have not been seen in the MALDI mass spectra of biopolymers such as proteins, peptides, and carbohydrates. Radical formation was demonstrated for samples in the mass range (1 kDa–13 kDa). Even in the presence of potential cationization sites such as methyl ester groups in the repeating units of some polyferrocene samples, MALDI mass spectra were dominated by radical ions of the analyte. Two possible mechanisms for radical formation in MALDI are discussed. Comparison of results with different matrices suggested that the distribution of masses observed in the mass spectra and characterized by the polydispersity index was independent of the matrix, but significant differences (5%) in the average molecular weights of the mass distributions were found.

Evidence of stable hydroxyl radicals and other oxygen radical species generated by interaction of hydrogen peroxide with magnesium oxide

Abstract: Three distinct radical species are formed and stabilized in the solid matrix upon the treatment of magnesium oxide with hydrogen peroxide in aqueous solution leading to the transformation of the solid into magnesium peroxide. The species in question are the O 2 - superoxide radical ion, the O - radical ion, and the OH hydroxyl radical. The latter radical is characterized by orthorhombic g and A tensors whose principal values, due to the ionicity of the trapping matrix, are slightly different from those previously observed for OH in irradiated ice or similar systems. In comparison to the OH radical trapped in ice, the hydroxyl radical observed in the present case (as well as the two companion radicals) is stable up to 473 K

Laser flash photolysis determination of absolute rate constants for reactions of bromine atoms in solution

Abstract: The photodecomposition of vicinal dibromides at 266 nm produces bromine atoms with a quantum yield of ∼2.0. This results from an efficient primary photocleavage of a C-Br bond, followed by rapid elimination of a second bromine atom from radicalsofthe type RCH-CH 2 Br. This cleavage occurs with a lifetime of <20 ns at room temperature. Bromine atoms react with bromine ions with a rate constant of 1.6×10 10 M -1 s -1 to yield Br 2 .- , an easily detectable and long-lived radical ion. This reaction can be used as a probe in order to determine absolute rate constants for other reactions of bromine atoms

The .beta.-distonic ion from the reaction of pyridine radical cation and ethene: a demonstration of high-pressure trapping in Fourier transform mass spectrometry

Abstract: The adduct ion formed in the reaction of pyridine radical cation and ethene exhibits different chemical reactivities and fragmentation behavior than the isomeric ethylpyridine radical cations. The most characteristic features of the adduct ion are the collisionally activated loss of C 2 H 4 to give back the pyridine radical cation and the second ionization to produce a doubly charged ion. A characteristic reaction is with dioxygen to form a new adduct, which may have the structure of a distonic peroxy radical cation. The reactive site for attachment of C 2 H 4 is the nitrogen atom of the pyridine, giving rise to the β-distonic ion adduct, C 5 H 5 N + -CH 2 C.H 2 , which can be differentiated from the isomeric a-distonic ion, C 5 H 5 N + -C.HCH 3

Picosecond dynamics of the photoreduction of benzophenone by DABCO

Abstract: Picosecond absorption spectroscopy is used to examine the dynamics of the quenching of the first excited singlet and triplet states of benzophenone by 1,4-diazabicyclo [2.2.2]octane (DABCO) in acetonitrile. At 1 M DABCO, the quantum yield for the formation of the singlet radical ion pair is 0.35 and the triplet radical ion pair is 0.65. The time evolution of the triplet radical ion pair absorption spectrum, with λ max 720 nm, is monitored onto the 4-ns time scale. There is no shift in the absorption band with time which is contrary to previous observations. It is proposed that the triplet radical ion pair is formed as a solvent-separated radical ion pair

The C4H8.bul.+ potential energy surface. 2. The (C2H4)2.bul.+ complex cation and its reaction to the radical cations of cyclobutane and 1-butene

Abstract: The reaction of ethylene (Et) and its radical cation (Et .+ ), which has received much attention from the experimental side, was studied at th eQCISD(T)/6-31G * //UMP2/631G * level of theory. According to these calculations the primary product of this reaction is a π complex cation (Et---Et] .+ , where the two Et molecules are linked by a single bond of ≃1.9 A and which is bound by 18.2 kcal/mol relative to Et+Et .+ , in good agreement with experiment

Fragmentations by photoinduced electron transfer. Fundamentals and practical aspects

Abstract: Removal of an electron from a bonding orbital or addition of an electron to an antibonding orbital of a diamagnetic molecule activates the resulting radical ion for fragmentation. Such reactive radical ions may be generated by photoinduced electron transfer (PET). There are two alternative ways to accomplish the transfer of an electron: (1) the local excitation of a donor or an acceptor which is well described by the empirical Weller equation and (2) the excitation of the charge-transfer complexes according to the Mulliken theory. The fragmentation reaction competes with back-electron transfer (BET) within the photogenerated radical ion pairs. The back electron transfer is well described by the Marcus theory. In most PET systems the rate of BET decreases with the increasing exergonicity and the rate is faster within contact ion pairs than with solvent separated ion pairs. The exergonicity of BET as well as ion pair solvation and spin multiplicity are predetermined by the method of ion-pair generation. These factors, in addition to the rate of cleavage, are critical in determining the overall efficiency of the PET fragmentation.

Photo-CIDNP investigation of the deprotonation of aminium cations

Abstract: The photoreactions of triallylamine and triethylamine with a variety of triplet sensitizers containing carbonyl functions were studied in acetonitrile-d 3 by using pseudo-steady-state measurements of chemically induced dynamic nuclear polarization (CIDNP). These reactions are hydrogen abstractions formally, but they are known to proceed in two steps: electron transfer from the amine to the excited sensitizer, followed by deprotonation of the resulting aminium cation to give an α-aminoalkyl radical. From the CIDNP intensity patterns in the reaction products, we determined the intermediates (radical ion pairs or pairs of neutral radicals) that give rise to the observed polarizations

Electron paramagnetic resonance investigation of the interaction of CO with the surface of electron-rich magnesium oxide: evidence for the CO– radical anion

Abstract: Carbon monoxide is easily reduced at the surface of electron- rich magnesium oxide prepared by contacting the solid with vapours of low-ionisation- energy metals (such as magnesium or alkali metals). The process has been followed by electron paramagnetic resonance (EPR) spectroscopy. The first step of the interaction, occurring at about 100 K, is the formation of adsorbed CO– radical anions characterised by partial electron transfer from the solid to the molecule. At higher temperatures, this species reacts further with CO and surface electrons yielding long- chain diamagnetic oxocarbon anions, which are also formed on bare MgO via the C2O–2 ethylenedione radical ion. A fraction of the C2O–2 radical ions are stabilised at the surface and are observed even at room temperature.

2,2'-bithienyl derivatives : EPR investigation of their radical ions in solution, electrochemical properties, and crystal structure

Abstract: 5,5'-Dimethyl-, -diisopropyl-, -di-tert-butyl-, and -bis(trimethysilyl)-2,2'-bithienyl have been prepared (1, 2, 3, and 4 respectively). The radical cations of 1, 2, and 3 have been generated in fluid solution. The electron paramagnetic resonance (EPR) spectra have been analyzed with the help of Huckel/McLachlan and MINDO/3 calculations and show the presence of cis and trans isomers, although X-ray diffraction studies of 1 and 4 have identified only the trans isomer in solid state. MINDO/3 predicts the observation of both isomers of 1 .+ on the EPR time-scale by estimating the barrier to ring rotation. This value is much higher than in the parent molecule, which fits in with the observation of presumably only one isomer by nuclear magnetic resonance (NMR)

A novel mechanism for reactions of thiirane with the thiirane radical cation. An experimental and ab initio study

Abstract: An experimental and computational study of the ion-molecule reactions initiated by the reaction of SC 2 H 4 .+ with SC 2 H 4 has been carried out. The dimer ion is observed in a hybrid drift tube/ion source under chemical ionization conditions at low temperatures and high pressures. However, the ion intensity is much lower than expected for a product containing a stable two-center three-electron (2c-3e) S. . .S bond. Above room temperature, ion intensities for m/z 92 S 2 C 2 H 4 .+ , m/z 124 S 3 C 2 H 4 .+ , m/z 156 S 4 C 2 H 4 .+ , and m/z 184 S 4 C 4 H 8 .+ dominate

The C4H8.bul.+ potential energy surface. 1. The cyclobutane radical cation

Abstract: The potential energy surface of the cyclobutane radical cation (CB .+ ) has been explored at the QCISD-(T)/6-31G * //UMP2/6-31G * level of theory. Thereby it was found that the first-order Jahn-Teller rhombic and rectangular structures are more stable than the long-bond trapezium structure reported to be the global minimum on the CB .+ surface in previous semiempirical and ab initio SCF calculations. In agreement with ESR experiments, a rhombic structure very flexible to ring puckering was found to be the most stable one

Hyperfine anisotropy and mobility of organic radical cations in zeolites

Abstract: Quantitative simulations of EPR powder spectra of organic radical cations in zeolites are reported. The anisotropy of the hypefine interactions and of the g-factor tensors is interpreted in terms of the mobility of the species. The radical cation formed by spontaneous oxidation of tetramethylethene in activated HZSM-5 is shown to rotate about the axis perpendicular to the molecular plane at room temperature, but it is immobile at 100 K. The radical cation derived from 2,5-dimethylhexa-1,5-diene in H-mordenite can execute only librational motion on a timescale of ca. 50 ns.

Side-chain oxidation of .alpha.-substituted 4-methoxytoluenes by potassium 12-tungstocobalt(III)ate. The effect of .alpha.-substituents on the formation and deprotonation of the intermediate cation radicals

Abstract: A kinetic study of the side-chain oxidation of α-substituted 4-methoxytoluenes (4-MeOPhCH 2 R, R=H, Me, OH, OMe, OAc, CN) by K 5 Co III W 12 O 40 .11H 2 O (Co(III)W) in AcOH/H 2 O (55:45) has been carried out. The reactions follow complex kinetics, suggesting that both the electron transfer and the radical cation deprotonation steps influence the reaction rate. A careful kinetic analysis has allowed the determination of the values of k 1 , the rate constant for the electron transfer step, and of k -1 /k 2 , the ratio between the rate constant of the back electron transfer and the one of the deprotonation of the cation radical

Preparation, characterization, and reaction of novel dioxoruthenium(VI) porphyrin cation radical complexes

Abstract: Novel dioxoruthenium(VI) porphyrin cation radicals were prepared in the stoichiometric oxidation of Ru[sup VI]TMP(O)[sub 2] (1) and Ru[sup VI]OEP(O)[sub 2] (2) with phenoxathiin hexachloroantimonate. [sup 2]H-NMR, UV-vis, and ESR were measured to determine the electronic structure of the oxidation products. The oxidation product (3) of 1 shows a broad Q band and a less intense and blue-shifted Soret band, suggestive of the porphyrin cation radical formation. Further, the single ESR signal for 3 at g = 2.002 at 77 K is a clear demonstration of one-electron oxidation of the porphyrin ring to form a free radical complex. The presence of two oxo ligands in 3 was confirmed in the reaction of 3 with Ph[sub 3]P under argon to give 2 mol equiv of Ph[sub 3]P[double bond]O. Very similar results were also obtained when 2 was oxidized under the same condition, and the oxidation product (4) was also assigned to be a dioxoruthenium(VI) prophyrin cation radical. The chemical shifts of 3 and 4 were determined by deuterium NMR using partially deuterated 3 and 4. On the basis of paramagnetic shifts, the radical orbitals of 3 and 4 were determined to be A[sub 1u]. Further, oxidation reactions of diphenyl sulfide and somemore » olefins were carried out with high valent ruthenium complexes (1-4), and the results show that 3 and 4 exhibit greater reactivity than 1 and 2; however, the enhancement of the reactivities are less than those expected for the complexes bearing A[sub 2u] radical orbitals. 27 refs., 5 figs., 2 tabs.« less