Showing papers on "Radical ion published in 1994"

Photophysical and photochemical properties of fullerenes

Abstract: On irradiation with visible and UV light, fullerenes C60 and C70 give high yields of their triplet states. Fluorescence is very weak from both, and phosphorescence has been observed only from C70. The triplet states are also formed efficiently by energy transfer from triplet sensitizers with energies above 35 kcal/mol. Triplet fullerenes transfer energy efficiently to oxygen, giving singlet molecular oxygen, but they react and quench singlet oxygen very inefficiently, making them excellent photosensitizers. The fullerenes C60 and C70 are easily reduced but very difficult to oxidize. Electron transfer to triplet C60 and C70 occurs readily from electron donors to produce the radical anions. C60 can also be oxidized by excited high-potential photosensitizers to the radical cation. Some dihydrofullerenes also give high yields of the triplet state and singlet oxygen on irradiation. Photoreaction of C60 with electron-rich compounds appears to be an efficient route to difunctionalized adducts.

ENDOR and special triple resonance studies of chlorophyll cation radicals in photosystem 2.

Abstract: Electron nuclear double resonance (ENDOR) and special triple (ST) resonance spectroscopies have been used to study the cation radicals of the primary donor, P680, and two secondary donor chlorophylls (Chl) in photosystem 2 (PS2). Two different preparations were employed, Tris-washed PS2 membranes and PS2 reaction centers (D1-D2-I-Cytb559 complex). One secondary donor Chl a cation radical, Chl1.+, was generated in the Tris-washed preparation, while the P680.+ radical cation and a further Chl a cation radical, Chl2.+, were produced in the reaction center preparation. The ENDOR spectrum of the primary donor radical cation of photosystem 1 (P700.+) is also presented for comparison. Hyperfine coupling constants for methyl groups have been measured for all three PS2 radical species and assigned by comparison with previously published spectra of Chl a radicals in vitro. Electron spin densities were calculated from these hyperfine couplings. Comparison of ENDOR spectral features with those of Chla.+ in vitro indicates similar values for Chl1.+ and Chl2.+ radicals but an apparent reduction in unpaired electron spin density for P680.+. It has been proposed from the more detailed studies of purple bacterial reaction centers that such a reduction in spin density can be interpreted as a delocalization over two Chl a molecules. Our calculations therefore suggest that P680.+ is a weakly coupled chlorophyll pair with 82% of the unpaired electron spin located on one chlorophyll of the pair at 15 K. Environmental or geometrical changes to the chlorin ring structure to give a novel monomeric primary donor are also possible.(ABSTRACT TRUNCATED AT 250 WORDS)

Two-photon excitation of alkyl-substituted magnesium phthalocyanine: radical formation via higher excited states

Abstract: The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t 4 -PcMg) in solution and microheterogeneous systems (liposomes and micelles) were investigated. Radical cation formation occurs in chloroform during UV excitation in the presence of an electron acceptor (CBr 4 ). The same result is achieved by two-step absorption in the singlet manifold using pulsed excitation at λ exc =670 nm, which is of interest from the viewpoint of photon delivery through the therapeutic window of tissues. To obtain a deeper insight into the photophysics leading to radical cation formation via the higher excited singlet state, the transient spectra and singlet—singlet absorption cross-sections were determined. In addition to strong excited state absorption within the spectral range of the Q x -band, relatively large absorption cross-sections were also found in regions with low ground state absorption. The importance of these transitions for an effective two-colour excitation regime is discussed with regard to new start mechanisms for photodynamic laser tumour theraphy.

Radical ions - where organic-chemistry meets materials sciences

Abstract: Radical ions generated by electron transfer reactions are known as important intermediates in organic chemistry. On the other hand, their formation, recombination and transport in organic materials is responsible for a series of attractive physical properties. Radical ion formation is often accompanied by structural changes being well understood in small organic molecules, which also constitute repeating units of intensively studied macromolecules. Therefore, an approach is described herein to compare and combine the structural and energetic description of monomeric and oligomeric radical ions with that of partially oxidized or reduced polymeric materials.

Pulse radiolysis study of the reactions of SO˙–4 with some substituted benzenes in aqueous solution

Abstract: The reactions of SO˙–4 with several substituted benzenes having the general formula, C6H5–nXnY (where X = F, Cl or Br and Y = CH3, CH2Cl, CHCl2, CF3 or OCH3), have been investigated in aqueous solution by pulse radiolysis. The transient absorption spectra exhibit maxima at 315–330 nm and additional peaks at 270–290 nm with chlorotoluenes and weak peaks around 400 nm with chlorobenzene and 3-chlorofluorobenzene. Only in the case of 3-chloroanisole is the observed spectrum different, exhibiting two distinct peaks at 290 and 475 nm. The second-order rate constants for the reaction of SO˙–4 range from about 108 for 2-chlorobenzotrifluoride to 1010 dm3mol–1s–1 for 3-chloroanisole. It is concluded from the Hammett treatment (ρ+=– 1.6) that the reaction mechanism involves both direct electron transfer and addition–elimination reactions. The intermediate radical cation is hydrolysed to give the corresponding ˙OH adduct absorbing at 315–330 nm except in the case of 3-chloroanisole where it is stabilized. The formation of a benzyl-type radical by direct H abstraction by SO˙–4 from the CH3 group and/or deprotonation of the radical cation is an additional process whose extent is determined by the relative position of the CH3 group, the order being para > ortho≈meta with monochlorotoluenes. The transient species absorbing around 400 nm is assigned to the phenoxyl-type radical. The differences in reaction mechanism between SO˙–4 and ˙OH attack are discussed.

Charge-Transfer Excitation of Electron Donor-Acceptor Complexes in Zeolite Matrixes: Direct Observation of Radical ion Pairs from Picoseconds to Milliseconds by Diffuse-Reflectance Laser Flash Photolysis

Abstract: Laser excitation of charge-transfer complexes between pyridinium-analogous acceptors and arene donors incorporated into zeolite-Y and zeolite-L generates transient radical ion pairs that can be monitored using time-resolved diffuse-reflectance spectroscopy techniques. At least three types of radical ion pairs have been distinguished kinetically with lifetimes from picoseconds to milliseconds, depending on donor and acceptor strength and on the zeolite type. As cause for the complex decay kinetics, the interactions between the radical ion pairs and the zeolite framework leading to spatial separation are discussed. Furthermore, the unique effects of coadsorbed water are described

On the vibronic structure of the absorption spectra of radical cations of some polycyclic aromatic hydrocarbons

Abstract: The vibronic structure of electronic absorption spectra of four polyacenic radical cations is investigated by means of quantum chemical calculations. We use the semiempirical quantum consistent force field/π electron method (QCFF/PI) augmented by a configuration interaction (CI) scheme that includes all excited configurations having at most one electron in the originally empty molecular orbitals and the ab initio restricted open shell Hartree–Fock (ROHF) method. Displacement parameters for the totally symmetric modes are calculated for different electronic transitions and the resulting Franck–Condon structure is compared with the experimental spectra. The analysis reaffirms the initial interpretation of the absorption spectrum of the cation of naphthalene and reinterprets the recently observed spectrum of the cation of anthracene. First ever analysis of the absorption spectrum of the radical cation of pyrene and perylene is given. For the former, we demonstrate that the unusually large frequency value of ...

Catalysed isomerization of simple radical cations in the gas phase

Abstract: Interaction with polar neutral molecules can cause α-distonic radical cations and conventional molecular ions to interconvert in the gas phase, in spite of substantial energy barriers for the unassisted isomerization; the interconversion is particularly facile when the proton affinity of the neutral molecule is close to that of the deprotonated radical cation.

Umpolung of ketones via enol radical cations

Abstract: For several tautomeric systems (ketones/enols, imines/enamines and others) a distinct reversal of the stability order is observed when going from the neutral compounds to the radical cations, the first use of which in a new preparative α-Umpolung reaction has been documented for keto/enol systems. The present review provides a critical evaluation of the chemistry of enol radical cations in solution with a special emphasis on the Umpolung reaction and the intermediates thereof. Other enol type of radical cations are discussed with respect to their potential to provide α-carbonyl radical and α-carbonyl cation intermediates. Hence, this article does not constitute a comprehensive summary on all enol type of radical cation reactions. All potentials in this review are referenced versus SCE, unless noted otherwise. Potentials measured against the ferrocene/ferrocenium couple were converted to SCE by adding 0.334 V.

Thianthren‐Radikalkation‐Tetrachloroaluminat

Abstract: Structures and Molecular Properties of Charge-Perturbed Molecules, 53[1,2]. — Thianthrene Radical Cation Tetrachloroaluminate One-electron oxidation of thianthrene, which exhibits a reversible half-wave potential at + 1.3 V and an irreversible one at + 1.65 V, under aprotic conditions with AlCl3/H2CCl2 yields the violet radical cation salt [H4C6S2C6H4⊕][AlCl4⊖]. Its single-crystal structure analysis at 90 K reveals an interplanar angle between the phenyl rings of 174° and demonstrates that the neutral sulfur heterocyclic molecule, bent by 128°, is considerably flattened upon one-electron removal. Semiempirical enthalpy of formation hypersurfaces allow to rationalize the experimental results by a double-minimum potential for the bent neutral molecule with a rather low inversion barrier and a single minimum one for a planar radical cation. Other radical cation properties such as the spin distribution deduced from its ESR spectrum or the visible absorption maxima near = 11000 and at 18250 cm-1 corresponding to π π* and π σ* transitions are also satisfactorily reproduced by PM3 calculations partly including configuration interaction based on the experimental structure.

Iminoethenethiones, Rn=c=c=s - Characterization by Neutralization-Reionization Mass-Spectrometry and G2(Mp2) Theory

Abstract: (Methylimino)ethenethione (2) and iminoethenethione (4) are stable molecules on the microsecond time scale of neutralization-reionization mass spectrometry experiments. The corresponding radical cations were generated by fragmentation of thiazolopyrimidinedione molecular ions 1(.+) and 3(.+). Iminoethenethione (4) does not tautomerize to thioformyl cyanide (H-CS-CN, 5) under the wall-less conditions of the MS experiment, but it does so under FVP conditions when generated from isoxazolones 6. Thioformyl cyanide was unequivocally identified by IR and mass spectra. The structures and stabilities of 2, 4, and 4(.+) were investigated by ab initio calculations at the G2(MP2) level of theory. Both 2 and 4 are predicted to have a singlet ground state, in contrast to O double bond C double bond C double bond S, for which a triplet state is preferred. The singlet-triplet gaps are approximately 40 kJ mol(-1). In agreement with experimental findings, both iminoethenethiones are calculated to be thermodynamically and kinetically stable species, lying in energy wells with at least a 100 kJ mol(-1) barrier to dissociation into HNC (or CH3NC) + CS. The IR and UV spectra and ionization energies of 2 and 4 are predicted. The iminoethenethione radical cation (4(.+)) is found to be the global minimum on the C2HNS.+ potential energy surface and stable toward all possible fragmentations: the most favorable fragmentations into H-. + NCCS+ and HNC + CS.+ are in accord with the mass spectrometric observations.

Photoreduction of methyl viologen mediated by tris(bipyridyl)ruthenium(II) in inert colloidal suspensions

Abstract: The photoreduction of methyl viologen, covalently attached to a tris(bipyridyl)ruthenium(II) chromophore, in suspensions of positively charged alumina-coated colloidal silica particles, via N-phenylglycine electron donor, is reported. In the tris(bipyridyl)ruthenium(II) complexes, the central metal atom is coordinated to two 4,4[prime]-dicarboxy-2,2[prime]-bipyridine ligands and thus carries a net 2-charge at pH 5.0. Coadsorption of the N-phenylglycine and ruthenium chromophore to the colloidal particles results in rapid production of reduced viologen following visible laser flash excitation of the tris(bipyridyl)ruthenium(II) complex. The yield of the radical was dependent upon the concentration of the electron donor added to the system, but its multiphasic decay was independent of both the N-phenylglycine concentration and the initial concentration of viologen radical cation. Furthermore, both the yield and decay kinetics of the viologen radical cation were relatively independent of the number of intervening methylene units between the ruthenium complex and viologen electron acceptor. 18 refs., 5 figs., 2 tabs.

Addition and cycloaddition reactions via photoinduced electron transfer

Abstract: This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exciplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given: reactions of radical cations with nucleophiles; reactions of radical anions with electrophiles; reactions of radical cations and radical anions with neutral radicals; radical-radical coupling reactions; addition and cycloaddition reactions via triplexes; three-component addition reactions.

Inverted spin trapping. Part III. Further studies on the chemical and photochemical oxidation of spin traps in the presence of nucleophiles

Abstract: Inverted spin trapping, denoting the reaction between the radical cation of a spin trap (ST˙+) and a nucleophile (Nu–), has been further explored with respect to the photochemical generation of ST˙+ and the use of 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) and 3,3,5,5-tetramethyl-1-pyrroline 1-oxide (TMPO) as ST. It is shown that photoexcitation of N-benzylidene-tert-butylamine N-oxide (PBN) in the presence of a weak electron acceptor, tetrabutylammonium 12-tungstocobaltate(III), and a nucleophile gives the corresponding spin adducts, PBN–Nu˙, most likely via the intermediate radical cation, PBN˙+. This species could also be generated by sensitized photoxidation and made to react with Nu–.The cyclic N-oxides DMPO and TMPO are more difficult to oxidize than PBN(by 0.2–0.3 V), They both engage in inverted spin trapping, using both thermal and photochemical oxidation, but in a more restricted way than for PBN.

Highly Conductive 1 : 1 Radical Cation Salts of Anthra[1,9-cd : 4,10-c′d′]bis[1,2]dichalcogenoles

Abstract: Various radical cation salts of 2,3-dimethyl-tetrathioanthracene (DMTTA) and -tetraselenoanthracene (DMTSA) with NO3−, BF4−, ClO4−, PF6−, AsF6−, and Br− counter anions were prepared by an electrocrystallization technique. It has been found that these salts have either 1 : 1 or 2 : 1 stoichiometry depending on the volumes of the anions, but all are electrically conductive regardless of the stoichiometries. In particular, the NO3 and BF4 salts of DMTSA, in spite of 1 : 1 stoichiometry, recorded unusually high room temperature conductivities of 440 and 450 S cm−1, respectively, and had metallic behaviors down to around 200 K. In addition, the other salts were semiconductors with very low activation energies. X-Ray crystallographic analyses showed that the crystal structures of all the salts consist of one-dimensional stacking columns of the donor molecules, but the two metallic salts have more favorable uniform stacking columns in which there are not only effective π electronic interactions but also strong h...

Isomerization of Olefin Radical Cations in ZSM-5 Zeolites

Abstract: Variable-temperature EPR was used to investigate reactions of olefin radical cations generated radiolytically in nonacidic and acidic ZSM-5 zeolites. The olefin radical cations undergo isomerization reactions even at 4 K. Radical cation reactions are presumably driven by the exothermicity of charge transfer, which is not efficiently quenched by the vibrational modes of the zeolite lattice. The observation of H-addition type radicals indicates Bronsted acid-catalyzed rearrangements prior to irradiation on the more acidic zeolites. 72 refs., 7 figs., 2 tabs.

Photoionization of naphthalene, trans-stilbene and anthracene adsorbed on the external and internal surfaces of zeolites

Abstract: Laser photolysis of anthracene, naphthalene and trans -stilbene on zeolite NaA and in zeolite NaX produces electrons trapped in Na n n + clusters, singlet and triplet excited states and radical cations of the arenes. Only Na 4 3+ is observed in NaX where the arene is located inside the supercage; however, other trapped species, such as Na 3 2+ , are also observed in NaA where the arene is located solely on the external surface. Thermal decomposition of trans -stilbene in NaX produces an unknown brown product, which shortens the lifetime of the electrons trapped in the sodium ionic clusters. The photoionized electrons from arenes on NaA are trapped by sodium cation clusters inside the sodalite cage next to the surface and close to the arene radical cation. No oxygen quenching of the trapped species, Na 3 2+ and Na 4 3+ , in the arene—NaA system is observed due to the exclusion of O 2 by the small entry pore of the sodalite cage (approximately 2.6 A).