Showing papers on "Radical ion published in 1978"
TL;DR: In this article, the magnetic field dependence of the geminate recombination triplet yield of radical ion pairs generated via photoinduced electron transfer in polar solvents is investigated for the systems pyrene/N, Ndimethylaniline (Py/DMA), pyrene 3,5,dimethoxy•N,Ndimethynylinox•N and pyrene DMDMA, and the perdeuterated system Py•d10/dMA•d11.
Abstract: The magnetic field dependence of the geminate recombination triplet yield of radical ion pairs generated via photoinduced electron transfer in polar solvents is investigated for the systems pyrene/N,N‐dimethylaniline (Py/DMA), pyrene/3,5‐dimethoxy‐N,N‐dimethylaniline (Py/DMDMA), and the perdeuterated system Py‐d10/DMA‐d11. The magnetic field dependence characterized through its B1/2 value is found to be dependent on the sum of the hyperfine coupling constants in the radical pair in agreement with previous theoretical predictions. A drastic reduction of the B1/2 value is observed with the perdeuterated system. By means of measurements of the radical ion and triplet absorption signals with nanosecond time resolution, the influence of the solvent on the geminate singlet and triplet recombination yields is investigated. Complementary measurements of exciplex lifetimes and quantum yields are carried out in a series of solvents with different polarities in order to determine the rate constants of fluorescence e...
126 citations
61 citations
TL;DR: In this article, a columnar structure of a pure hydrocarbon radical cation has been established, and Patterson synthesis and difference Fourier analyses showed the compound to have a column-ar stacking of C₁₀H₈ units the long molecular axes of which are twisted alternately by 90° around a screw axis in c-direction.
Abstract: By anodic oxidation of naphthalene in H₂CCl₂/O.O2m Bu₄NPF₆ at -45 °C dark red-violet crystals of (C₁₀H₈)₂PF₆ can be obtained by electrocrystallisation. They are stable at low temperatures, however, decompose on warming. In solution and in the polycrystalline state these radical cation salts show only narrow e.p.r. signals without h.f.s. The specific conductivity of a polycrystalline pellet at room temperature was determined to be 0.12 ±0.046 Ohm⁻¹ cm⁻¹ . The structure determination of (C₁₀H₈)₂PF₆ yielded the tetragonal space group P4₂/n, Z = 2, a = b = 1156(2), c = 640(1) pm. Patterson synthesis and difference Fourier analyses showed the compound to have a columnar stacking of C₁₀H₈ units the long molecular axes of which are twisted alternately by 90° around a screw axis in c-direction, and the molecular planes of which are 320 pm apart. The PF₆⁻ions have four nearest C₁₀H₈ neighbours lying in pairs in parallel planes 63 pm above and below that plane of PF₆⁻ perpendicular to the c-axis and containing a PF₄ group. This is the first established case for a columnar structure of a pure hydrocarbon radical cation. (C₁₀H₈)₂AsF₆ is isomorphous.
54 citations
TL;DR: The application of the Dewar-Zimmerman rules to the interaction between radical cations, derived from 4n+2 systems, shows that a nucleophile orbital interacting suprafacially with the ion should correspond to an antiaromatic transition state and hence to a less favored pathway relative to competing ones, e.g., electron transfer as discussed by the authors.
45 citations
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38 citations
TL;DR: The radiative relaxation of the first excited states, A(π −1 ), of two C 6 H 6 + radical cations in the gaseous phase has been detected following excitation by low energy electron impact as mentioned in this paper.
35 citations
TL;DR: The hydroperoxide catalyzed oxidation of hydrogen or electron donors, unsaturated lipids or antioxidants involves the protein or porphyrin free radical of compound I and the ferryl iron of compound II.
34 citations
TL;DR: In this article, the authors studied photochemical electron transfer from bacteriopheophytin (BPh) to methyl viologen (MVC12) and to m-dinitrobenzene (m-DNB) in solution.
Abstract: . As a model for the primary reactions of photosynthesis, we studied photochemical electron transfer from bacteriopheophytin (BPh) to methyl viologen (MVC12) and to m-dinitrobenzene (m-DNB) in solution. Both MVC12 and m-DNB cause reductions in the lifetime of the first excited singlet state of BPh (BPh*), in the fluorescence quantum yield, and in the quantum yield of the triplet state, BPh +. The quenching of BPh* probably results from electron transfer, which generates short-lived radical pairs involving the BPh radical cation (BPh+) and the reduced form of the quencher. Electron transfer from BPh* is thermodynamically favorable, but that from BPhT is not. From the magnitude of the quenching, we calculate rate constants for electron transfer in collision complexes formed between BPh* and MVC12 or m-DNB. Measurements of the quantum yield of the free BPh+ radical indicate that about 3/4 of the [BPh+ MV+] radical pairs decay by reverse electron transfer, rather than dissociating to give the free radicals. Essentially all of the [BPh+m-DNB +] radical pairs must decay by reverse electron transfer, because free BPh+ cannot be detected in this case. From these data, we estimate the rate constants for the reverse electron transfer reactions. The higher probability of dissociation in the [BPh+ MV+] radical pair can be explained by coulombic repulsion.
The rate of the primary electron transfer reaction in photosynthetic bacteria is comparable to that of forward electron transfer in the BPh* collision complexes. Reverse electron transfer, however, is at least 103-times slower in the radical pair formed in the bacterial reaction center than it is in [BPh+m-DNB−], and more than 104-times slower than in [BPh+ MV+]. The explanation for this dramatic and crucially important difference remains unclear, but several possibilities are discussed.
33 citations
TL;DR: In this article, the 1,3,5-trifluorobenzene radical cation has been observed in the gas phase and it has been shown that a Jahn-Teller distortion is likely present.
30 citations
TL;DR: In this paper, it was shown that the radical cation of 1,3-butadiene reacts with vinyl methyl (or ethyl) ether in the gas phase through a [2+4] cycloaddition reaction.
Abstract: By use of deuterium labelling it is shown that the radical cation of 1,3-butadiene reacts with vinyl methyl (or ethyl) ether in the gas phase through a [2+4] cycloaddition reaction. This leads to an excited collision complex of ionized 4-methoxy- (or ethoxy) cyclohexene, which expels mthanol (or ethanol) through a 1,3 elimination, as does 4-methoxycyclohexene itself upon electron impact. The spectrum of daughter ions produced by collision induced decomposition of the resulting ion with m/e 80 has been compared with spectra produced by the collision induced decompositions of the m/e 80 ions given by 1,3-cyclohexadiene, 1,4-cyclohexadiene, 1,3,5-hexatriene and the [M–MeOH] ion from 4-methoxycyclohexene.
TL;DR: The fragment ion with m/e 27 from CH 3 NH 2 or CH 3 NC has the HNC + configuration rather than the tautomeric HCN + structure as discussed by the authors.
TL;DR: In this paper, a series of trivalent phosphorus compounds, alone or in the presence of 3,3-dimethyl-2-t-butylbut-1-ene, have been studied using e.r.s. spectroscopy.
Abstract: Radical cations generated during electrochemical oxidation of a series of trivalent phosphorus compounds, alone or in the presence of 3,3-dimethyl-2-t-butylbut-1-ene, have been studied using e.s.r. spectroscopy. The oxidations were carried out in nitrile or dichloromethane solvents and the cell containing the working electrode was situated in the cavity of the spectrometer. Oxidation of X3P alone gives rise to the spectrum of the phosphine dimer cation radical, (X3PPX3)+, produced by rapid reaction of the initially formed phosphinium radical cation, X3P+, with a further molecule of phosphine. Trialkyl phosphites did not give detectable concentrations of dimer cation radicals in nitrile solvents, probably because of preferential reaction of (RO)3P+ with the solvent. In the presence of 3,3-dimethyl-2-t-butylbut-1-ene the spectra of the dimer cation radicals were completely or partially quenched and replaced by those of the adducts But2ĊCH2PX3. The phosphorus hyperfine splittings of both (X3PPX3)+ and But2ĊCH2PX3 increased with the electronegativity of the ligands X, and these trends are interpreted in terms of changes in hybridisation at phosphorus.
TL;DR: In this article, the photoelectron spectra of (H3C)2P and R2N substituted benzenes and the high-resolution ESR spectra for their radical ions confirm the differing acceptor or donor effects of the two substituents and yield information on the different charge distribution in the molecular states of individual radical ions.
Abstract: 1,4-Bis(dimethylphosphino)benzol last sich selektiv sowohl zum Radikalanion wie auch zum Radikaltrianion reduzieren — eine Oxidation zum Radikalkation, dem Phosphor-Analogon von Wursters Blau, gelingt dagegen nicht. Die Photoelektronen-Spektren (H3C)2P and (H3C)2N substituierter Benzole und die hochaufgelosten ESR-Spektren ihrer Radikalionen veranschaulichen die unterschiedlichen Akzeptor- oder Donatorwirkungen der beiden Substituenten und geben Auskunft uber die verschiedenartige Ladungsverteilung in den Molekulzustanden der einzelnen Radikalionen.
Radical Ions, XXIII: R2P and R2N Substituted Benzenes: Charge Distribution in Their Cations, Anions, and Trianions
1,4-Bis(dimethylphosphino)benzene can be selectively reduced to the radical anion as well as to the radical trianion — in contrast, no oxidation is possible to the radical cation, the phosphorus analogue of Wursters blue. The photoelectron spectra of (H3C)2P and (H3C)2N substituted benzenes and the high-resolution ESR spectra of their radical ions confirm the differing acceptor or donor effects of the two substituents and yield information on the different charge distribution in the molecular states of the individual radical ions.
TL;DR: The spin trap, 5, 5-dimethyl-1-pyrroline-1oxide has been used to detect the OH radical produced by the Haber-Weiss reaction between superoxide ion and hydrogen peroxide as discussed by the authors.
Abstract: The spin trap, 5, 5-dimethyl-1-pyrroline-1-oxide has been used to detect the OH radical produced by the Haber-Weiss reaction between superoxide ion and hydrogen peroxide. In the presence of a small amount of H2O2, the OH radical adduct can be detected by electron spin resonance spectroscopy, while, H2O2 being added excessively, the HO2 radical adduct can be detected.
TL;DR: In this paper, a pulsed tunable dye laser was used to obtain a resonant Raman spectra for the radical anion of pyromellitic dianhydride (PMDA) in MTHF glass and the radical cation of naphthalene in alkyl halide glass at 77 K.
TL;DR: In this article, the spectra of the radical cation and anion of trans-stilbene in benzene and cyclohexane have been resolved using various electron and positive charge scavengers.
Abstract: The spectra of the radical cation and anion of trans-stilbene in benzene and cyclohexane have been resolved using various electron and positive charge scavengers. The decay of the ions is second order in both solvents. In cyclohexane only part of the yield of solvent cations is scavengeable by trans-stilbene; in benzene the stilbene cation is converted to the dimer in the presence of high stilbene concentrations. The electron scavengers N/sub 2/O and c-C/sub 6/F/sub 12/ compete directly with trans-stilbene for electrons and we deduce rate constants for the reactions of electrons with c-C/sub 6/F/sub 12/ and trans-stilbene of (1.2 +- 0.6) x 10/sup 12/ and (0.72 +- 0.5) x 10/sup 12/ M/sup -1/ s/sup -1/, respectively. SF/sub 6/ reduces the yield of trans-stilbene anion more efficiently than can be attributed solely to electron scavenging and it is shown that electron transfer from trans-stilbene anion to SF/sub 6/ occurs at a rate constant, k/sub 4/ = (3.9 +- 0.4) x 10/sup 9/ M/sup -1/ s/sup -1/. The presence of either aniline or n-butylamine causes enhancement of the yield of stilbene anion, the effect being particularly marked in the case of aniline. Possible explanations of the effect are considered. It is proposedmore » that the UV absorptions are due to radical species, which in cyclohexane solution are most probably formed by the addition of cyclohexyl radicals to the olefinic double bond of the stilbenes.« less
TL;DR: The formation of the radical cation and the radical anion of the SiH-, SiF-, SiCh3-Derivates were investigated using photo electron and electron spin resonance spectroscopy as mentioned in this paper.
Abstract: Durch Umsetzung des bei der Pyrolyse von CH3SiCl3 gebildeten 2,2,4,4,-6,6,8,8-Octachlor-2,4,6,8-tetrasilabicyclo[3,3,0]-oct-1(5)en mit LiAlH4 konnte der Grundkorper (Titelverbindung) isoliert werden. Es wird uber Untersuchungen zur Bildung des Radikalkations und des Radikalanions des Grundkorpers und der Si-methylierten sowie Si-fluorierten Derivate mit Hilfe der Photoelektronen-und Elektronenspinresonanz-Spektren berichtet.
Formation of Organosilicon Compounds. 71. 2,4,6,8,-Tetrasilabicyclo[3,3,0]-oct-1(5)ene; Formation of the Radical Cation and the Radical Anion of the SiH-, SiF-, SiCh3-Derivates
Reacting 2,2,4,6,6,8,8-Octachlor-2,4,6,8-tetrasilabicyclo[3,3,0]-oct-1(5)ene formed in the pyrolysis of CH3SiCl3 with LiAlH4 the basic compound (title compound) was isolated. The formation of the radical cation and the radical anion of the basic compound and its Si-methylated as well as its Si-fluorinated derivate were investigated using photo electron and electron spin resonance spectroscopy.
TL;DR: In this paper, the short-lived 1,4-dimethoxybenzene radical cation has been generated in acidic aqueous solution by oxidation of the parent molecule with cerium(IV) by performing the reaction under fast flow conditions.
Abstract: The short-lived 1,4-dimethoxybenzene radical cation has been generated in acidic aqueous solution by oxidation of the parent molecule with cerium(IV). By performing the reaction under fast flow conditions the free radical has been studied in a steady-state condition by resonance Raman spectroscopy. Molecular orbital and normal co-ordinate calculations have been employed in analysing the resulting spectra in terms of the changes in structure and bonding which accompany radical formation in this system.
TL;DR: The short-lived free radical cation of diazabicyclooctane has been studied in aqueous solution by resonance Raman spectroscopy as mentioned in this paper, showing that the radical was produced at ∼10−4 mol dm-3 concentration by chemical oxidation of the parent compound in a continous flow apparatus.
Abstract: The short-lived free radical cation of diazabicyclooctane has been studied in aqueous solution by resonance Raman spectroscopy. The radical was produced at ∼10–4 mol dm–3 concentration by chemical oxidation of the parent compound in a continous flow apparatus. The spectra of both parent and radical have been interpreted in detail through normal coordinate and molecular orbital calculations. These indicate that weaking of the C—C bonds results from electron removal, with a lesser increase in the C—N bond strengths. Evidence for strong solvent effects is presented.
TL;DR: In this paper, the acyloxy radical ion −O2CCHODCHODCȮ2 (I) was identified in irradiated Rochelle salt by combining the techniques of pulse radiolysis and EPR spectroscopy.
Abstract: The acyloxy radical ion −O2CCHODCHODCȮ2 (I) was identified in irradiated Rochelle salt by combining the techniques of pulse radiolysis and EPR spectroscopy. This primary radical gives hyperfine couplings due to 13C, 1H, and 23Na and has a broad optical absorption band with the peak at 405 nm. It decomposes by decarboxylation in a first order decay. The half life at 22 °C is 55 ns and the activation energy for decarboxylation is 7.5 kcal mol−1. The G value for the CO2 formation is 2.7 and the extinction coefficient of I is 900M−1 cm−1. No effects of the ferroelectricity of the salt on emaxG were found.
TL;DR: In this paper, the photoinduced formation of radical ion pairs and their geminate recombination to give singlet (ground state) and triplet product states was studied at different temperatures between 233 and 296 K with the system pyrene/N,N-dimethylaniline (DMA) in methanol.
TL;DR: In this paper, the same authors used a simple MO model to assign the coupling constants to pairs of equivalent protons to reproduce satisfactorily the experimental values for the radical anions and cations of dibenzo [b,f]pentalene (III) and its 5, 10-dimethyl derivative (IV).
Abstract: Proton hyperfine data have been determined for the radical anions and cations of dibenzo [b,f]pentalene (III) and its 5, 10-dimethyl derivative (IV). The assignment of the coupling constants to pairs of equivalent protons follows from a simple MO model, the use of which enables one to reproduce satisfactorily the experimental values. The proton hyperfine data, aHμ1 , for the radical anion III⊖. correlate fairly well with the π-charge populations ϵμ derived from 1H-NMR. spectra for the carbon centres μ in the dianion III2⊖. The analogous correlation is less good in the case of the radical cation III⊕. and the dication III2⊕., presumably due to the rough approximations involved in the evaluation of the numbers ϵμ for the latter species. The coupling constants aH5,10 for III⊖. and III⊕. are very close to the corresponding values aH4,6 for the radical ions of 1,3,5-tri-t-butylpentalene (II), in accord with the prediction of the MO model. A similarity is also found between the proton hyperfine data for III⊖. and those for the radical anions of structurally related 1,4-diphenyl-1,3-butadiene (V). On the other hand, there are striking changes in the coupling constants of the analogous protons on passing from III⊖. to the radical anions of dibenzo [a,e] cyclooctene (VI) and [16] annulene (VII), as a consequence of raising the symmetry from C2h to D2h and D4h, respectively.
TL;DR: In this paper, the photoreduction of 2-tert-butyl-p-benzoquinone and 2,5-and 2,6-di-tertsbutyl p-benzosquinone were studied by esr spectroscopy.
Abstract: The photoreduction of 2-tert-butyl-p-benzoquinone and 2,5- and 2,6-di-tert-butyl-p-benzoquinone were studied by esr spectroscopy. In the photolysis of 2,6-di-tert-butyl-p-benzoquinone, only one semiquinone radical was observed and its decay kinetics were monitored by esr. The reduction of the triplet 2-tert-butyl-p-benzoqinone leads to the formation of isomeric semiquinone radicals 1 and 2 in the primary photochemical processes, and this is then followed by a chemical equilibrium between the two isomeric radicals. The relative rates of formation of the two radicals cannot be monitored by conventional esr methods but the CIDEP results show that the formation of radical 2 is about five to seven times faster than that of radical 1. A secondary reaction of the phenoxy radical with a product benzohydrofuran 4 leads to the observation of a polarized radical 3 which derives all its polarization from the primary phenoxy radical.