Anthracene Radical Cations Research Articles

Substituent and Solvent Effects on the Rate Constants for the Reactions of 9-Substituted Anthracene Radical Cations with Trinitromethanide.

Substituent and Solvent Effects on the Rate Constants for the Reactions of 9-Substituted Anthracene Radical Cations with Trinitromethanide.

Radical cations from aryl-silanes, -germanes and -digermanes

During irradiation of trimethyl(p-tolyl)silane 1b with (CF3CO2)2Hg in CF3CO2H, an EPR spectrum due to 1b˙+ is observed. The spin density in the transient radical cation resembles that of 1b˙–. In contrast to this, the analogous germane 3b gives the radical cation of p-bitolyl. After irradiation of tetrakis-(pmethoxyphenyl)silane 2d, the radical cation 2d˙+ is identified by ENDOR. However, the radical cation of p-bi(methoxyphenyl) is formed with p-methoxyphenyl(trimethyl)silane 1d and tetrakis(p-methoxyphenyl)germane 4d.The reaction of phenyl-silanes and -germanes 1a, 4a, 5a with Alcl3 in CH2Cl2 or CHCl2CH3 yields the radical cations of anthracene, I, or 9, 10-dimethylanthracene, II. Treatment of para-substituted phenyl-silanes, -germanes and -digermanes 1–5 with AICl3 in CH2Cl2 leads to 2,6-disubstituted, and with AICl3 in CHCl2CH3 to 2,6,9,10-tetrasubstituted, anthracene radical cations III–VII. The first step of the reaction is an electrophilic ipso-substitution of the silyl or germyl residue followed by a condensation and an oxidation. With hexamesityldigermane 5e, intermolecular methyl transfer takes place to give the radical cations of 1,2,3,4,5,6,1,8-octamethyl- and 1,2,4,5,6,8-hexamethyl-anthracene VIII and IX.

Multiphoton processes in cyclohexane and trans-decalin and the formation of high-mobility cations

The dependence of the absolute efficiencies of production of free electrons and HM[sub +] (positive ions with anomalously high mobility) on the intensity of 248- and 308-nm laser pulses has been measured for neat cyclohexane and trans-decalin and for solutions containing aromatic compounds. In the neat solvents, the yields of electrons and HM[sub +] have the same intensity dependence; for ionization of these two alkanes, two photons are required at 248 nm and three photons at 308 nm. In solutions containing aromatic solutes, where the major fraction of the light absorption is by the solute, yields of both free electron and HM[sub +] are markedly higher, and at both 248 and 308 nm the intensity dependences indicate two photons are required for ionization but that three photons are required to create HM[sub +]. This is consistent with the explanation, based on previously reported product analysis studies from this laboratory, that the aromatic solute is ionized when its excited state, created by the first photon, absorbs a second photon and the radical cation absorbs a third photon, which enables it to react with the solvent, creating HM[sub +]. Examination of previously reported results on anthracene in 2-propanol supports a similar explanationmore » for the observed decrease in the quantum yield of the anthracene radical cation with increasing intensity. 39 refs., 12 figs., 4 tabs.« less

Mass spectral and theoretical (AM1) study of cations derived from janusene. Evidence for interannular proton transfer

A previous failure to find clear evidence for protonation of janusene in superacid solutions, coupled with evidence for radical cation formation, led to the current mass spectral and theoretical study. The janusene radical cation JAN .+ generated by EI/MS undergoes cycloreversion to give the anthracene radical cation with high selectivity over the dibenzobarrelene cation. Cycloreversion of tetrafluorojanusene radical cation F 4 JAN .+ is also selective toward AN .+ formation, with lesser amounts of F 4 N .+ , DBB .+ , and F 4 DBB .+ cations being formed in order of decreasing relative abundance. In constrast, protonated janusenium ion (JAN.H) + and (F 4 JAN.H) + undergo cycloreversion with a preponderance of (DBB.H) + as the charged fragment. Acetylation (with MeCO + ), trimethylsilylation (with TMS + ), and trifluoroacetylation (with CF 3 CO + ) of JAN and F 4 JAN, and fragmentation of their resulting cations, were also studied by tandem mass spectrometry. Remarkably, the acylated cations also gave predominantly (DBB.H) + as the cycloreversion cation, thus indicating interannular proton transfer. The stabilities of protonated and oxidized janusenes, and several model compounds, were calculated by the AM1 molecular orbital method. For JAN, β-facial protonation is preferred over lateral protonation , whereas in F 4 JAN preference for facial versus lateral protonation is less. The AM1 calculations lend support to the concept of rapid interannular proton transfer between facial rings, which may help to explain the observation of broad NMR lines in attempts to protonate janusene in superacid

ChemInform Abstract: Electronic and Vibrational Spectra of Matrix Isolated Anthracene Radical Cations: Experimental and Theoretical Aspects.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Electronic and vibrational spectra of matrix isolated anthracene radical cations: Experimental and theoretical aspects

Radical cations of anthracene have been formed by vapor phase electron impact followed by trapping in an argon matrix at 12 K. Visible/ultraviolet and infrared absorption spectra of the anthracene cations in an argon matrix have been run. Significant differences in the infrared band intensities between neutral and cationic anthracene have been observed. The effects of photolysis and added CCl4 have been studied and their influence on the infrared band intensities correlated with known visible bands attributable to the anthracene cation. Theoretical calculations using Pariser–Parr–Pople and intermediate neglect of differential overlap methodologies with high level multireference perturbation configuration interaction, specifically modified for spectroscopic applications, have been performed. Both approaches predict the previously observed photoelectron spectrum well. For the optical absorption, the match with the experimental spectrum is also good, but there are notable differences between the two predictions. Ab initio restricted open shell Hartree–Fock calculations for the cation vibrational frequencies show excellent correspondence with the observed infrared bands. The theoretical infrared intensities show some remarkable differences between the neutral and ionized species. For example, the CH in-plane bending modes and CC in-plane stretching vibrations are predicted to increase by several orders of magnitude upon ionization. Experimental infrared intensities have been estimated and certain bands do show a significant increase in the cation band intensities over the neutrals. The implication of these findings for the hypothesis that polycyclic aromatic hydrocarbon cations are responsible for the unidentified infrared emission bands from interstellar space is discussed briefly.

Electron-transfer reactions of aromatic radical ions with nucleophilic molecules

Reactions of pyrene, anthracene and triphenylene radical cations with electron-rich species have been established as being electron transfer in nature. The reaction rates of these systems were controlled by collision through diffusional processes rather than by formation of an intermediate such as a charge-transfer complex.Electron-transfer rates from the donor molecules to Py, An, Tr radical cations did not show significant decrease in the range –1.7 ⩽ΔG⩽ 0.2. The ΔG dependence of kET was simulated by using the quantum mechanical Marcus' energy-gap law with a relatively large coupling constant value for the electron-tunnelling interaction.

Products of the Reactions between Substituted Anthracene Radical Cations and Nitrogen-Centered Nucleophiles.

Products of the Reactions between Substituted Anthracene Radical Cations and Nitrogen-Centered Nucleophiles.

On the Reaction between Anthracene and the Nitrosonium Ion. A Simple Method for the Preparation of 9,10-Anthraquinone and Some Comments on its Formation in Aromatic Nitration.

A summary of the possible electron-transfer reactions between anthracene and the nitrosonium ion, and a convenient and simple method for the synthesis of 9,10-anthraquinone from anthracene, NaNO 2 and CH 3 OH in CH 2 Cl 2 /CF 3 COOH are presented. The suggestion that the formation of 9,10-anthraquinone in the N 2 O 4 nitration of anthracene originates from trapping of the anthracene radical cation by water is demonstrated to be invalid, e.g., by experiments involving H 2 18 O. Instead, 9,10-anthraquinone is suggested to be formed in the N 2 O 4 -based and in other related systems via follow-up reactions to an initial 1,4-addition across the 9,10 positions of anthracene

Novel photoinduced electron transfer reactions between naphthalene and triphenylsulphonium salts

Irradiation of solutions of naphthalene and triphenylsulphonium trifluoromethanesulphonate initiates an electron transfer reaction which gives 1- and 2-phenylnaphthalenes, 2- and 4-phenylthiobiphenyls, and diphenyl sulphide. The phenylnaphthalenes are formed by an in-cage reaction between phenyl radical and naphthalene radical cation, whereas diphenyl sulphide is an escape product. A second cage process, oxidation of diphenylsulphide by naphthalene radical cation, generates diphenylsulphinyl radical cation; reaction of this intermediate with phenyl radical gives the phenylthiobiphenyl isomers. However, irradiation of anthracene and triphenylsulphonium triflate yields only phenylated anthracenes and diphenylsulphide, because anthracene radical cation cannot oxidize diphenylsulphide.

Data-aquisition system for cyclic voltammetry at a microelectrode and application to fast electrode kinetics

A data-aquisition system for fast micro-electrode voltammetry is described. The computer-controlled system can be used at scan rates up to 10 000 V s −1. The response of the system was examined by monitoring fast electrochemical reactions. A heterogeneous rate constant of 1.1 cm s −1 was obtained for oxidation of ferrocene in acetonitrile. Oxidation of anthracene in acetonitrile was also studied; the enthalpy of activation was evaluated as 13 kJ mol −1 for the reaction of the anthracene radical cation with acetonitrile.

Pulse radiolysis study of salt effects on reactions of aromatic radical cations with Cl–. Rate constants in the absence and presence of quaternary ammonium salts

The effect of quaternary ammonium salts on the decays of the radical cations of biphenyl, trans-stilbene, anthracene and pyrene generated by pulse radiolysis in chlorohydrocarbons has been investigated. The decays, which are due to neutralization reactions with Cl–, are retarded by the addition of salts having non-nucleophilic PF–6, BF–4 and ClO–4, whereas the radical cations are rapidly quenched by salts having I– and BPh–4. The retarding effect of the salts is attributed to the formation of ion pairs between the reacting ions and the counter-ions from the salts. The rate constants for the neutralization reactions in 1,2-dichloroethane have been determined for the free-ion and ion-paired states; the latter state is attained by the addition of Bu4NPF6. The rate constants for the reactions of the radical cations except for Py·+(Py = pyrene) are in the ranges (1.3–1.9)× 1011 dm3 mol–1 s–1 for the free ions and (2.8–7.0)× 1010 dm3 mol–1 s–1 for the ion pairs. The rate constant determined in the absence of the salt for Py·+ is one order of magnitude smaller than those for the others. The salt effect is also smallest on the reaction of Py·+. The charge delocalization of the large aromatic radical cation may be responsible for the exceptional results for Py·+. The largest salt effect was observed on the reaction of Ph2CH+, a charge-localized carbenium ion investigated for comparison. The solvent effect on the neutralization reactions is also discussed.

Effect of Melt Composition on the Reactions of the Anthracene Radical Cation in SbCl3-Rich Melts: Chlorination versus Aryl-Aryl Coupling

In SbCl/sub 3/-AlCl/sub 3/-BPCl melts (BPCl = N-(1-butyl)pyridinium chloride) reactions of anthracene (AN) oxidized either coulometrically or with Sb(V) were highly selective in a way that depended on melt composition. In basic 60-19-21 m/o melts the only significant products were 9-chloroanthracene (CA) and 9,10-dichloroanthracene (DCA) with CA constituting >99% of the yield for partial oxidations up to 80%, while in acidic 60-21-19 m/o melts the only products found in significant amounts were condensed AN's, principally 2,9'-bianthracene. In SbCl/sub 3/-10 m/o BPCl oxidative chlorination occurred without condensation, while in SbCl/sub 3/-6 m/o KCl both chlorination and condensation took place. These results are discussed in terms of the effect of the acid/base properties of SbCl/sub 3/-rich melts on the manifold of reaction pathways available to AN radical cations. It is proposed that the half-regeneration pathway (an ECE process) is the primary chlorination route with a Cl/sup -//SbCl/sub 3/ complex we denote Cl/sup -/(cpx) serving as the nucleophile. In basic melts the concentration of Cl/sup -/(cpx) is substantial and chlorination reactions are dominant over condensation reactions while in acidic melts the concentration of Cl/sup -/(cpx) is enormously suppressed so that chlorination reactions become much slower and condensation predominates. 22 refs., 6 figs., 1more » tab.« less

Electronic Spectra of Radical Cations and Their Correlation with Photoelectron Spectra

Electronic absorption spectra of the radical cations of 1.2,7.8-dibenzochrysene, 1.2,3.4-dibenzanthracene, 1.2,5.6-dibenzanthracene and 1.2,7.8-dibenzanthracene in boric acid are measured. The transition energies of the cations are calculated using the Longuet-Higgins-Pople and Wasilewski type Open-Shell-SCF-MO methods with limited CI. A comprehensive discussion of the absorption bands is given in the light of the calculations and the photoelectron spectral data for the hydrocarbons. The correspondence between the optical and photoelectron spectra is found to be extremely good with almost negligible matrix shifts, thus highlighting the role of photoelectron spectroscopy in understanding the optical spectra of the radical cations and vice versa. Finally, a correlation diagram for the observed electronic transitions for the radical cations of anthracene and its benzologs is presented from which it is inferred that the optical A-transitions have some sort of correlation with the first IP's of the respective hydrocarbons.

Numerical analysis of EPR spectra. 2. The use of significance plots

The interpretation of an EPR spectrum is sometimes aided by calculating a “significance plot,” which may be calculated from a digitized EPR spectrum in the same way that a power spectrum may be calculated from a time series. The theoretical form of significance plots derived from ideal first-order spectra is discussed. Using the EPR spectrum of anthracene radical cations as an example, a method is described whereby a significance plot may be analyzed to determine the hyperfine splitting constants.

The phase composition and reduction of CoO-Al2O3, MoO3-Al2O3, and CoO-MoO3-A12O3 catalysts

1. The method of photoelectron spectroscopy has been used to show that the Co and Mo ions of CoO-MoO3-Al2O3 catalysts interact with the γ-Al2O3 lattice, the Mo ions blocking the penetration of the Co ions. Formation of metallic Co promotes extensive Mo ion reduction. 2. Thermal treatment of the CoO-MoO3-Al2O3 catalyst in air tends to enrich the surface with respect to Mo ions, while reduction enriches the surface with respect to Co ions. 3. Oxidational treatment of the CoO-MoO3-Al2O3 and MoO3-Al2O3 catalysts is required for formation of anthracene radical-cations. A description is given of an oxidizing center structure which accounts for the reduction in acceptor activity of the Mo6+ ions in the ternary system.

Remarks on anisotropic rotational diffusion effects in the ESR spectra of radical cations generated on an acidic surface

The linewidth variations in the ESR hyperfine spectra of the radical cations of anthracene, perylene, and tetracene generated by adsorption on an acidic surface (silica–alumina) are analyzed in terms of the Freed–Fraenkel theory as modified to include anisotropic diffusion effects. The analysis is based on computer simulation of the observed experimental spectra. In the case of anthracene and perylene quite acceptable agreement is obtained if the diffusion tensor is chosen to have axial and near-axial symmetry, respectively, although a small amount of modulation of the isotropic hyperfine interactions may also be present in each case. The results are reviewed in terms of current theories of the nature of the adsorbent–adsorbate interaction on this type of surface. Similar calculations applied to tetracene failed to produce agreement between the simulated and experimental spectra and an additional spin relaxation mechanism involving electron transfer was included in the calculation. Although agreement was somewhat improved thereby, discrepancies remained. The tetracene results are reviewed in terms of the known redox behavior of the system.

Radical Cation of [2.2] (9,10)Anthracenophane as a Model for Dimeric Anthracene Radical Cations

Oxidation of aromatic hydrocarbons can lead to dimeric radical cations. The radical cation of anthracenophane (1) provides an indication of the structure of such paramagnetic species.

Photochemical and fluorescence properties of anthracene radical cation

Photochemical and fluorescence properties of anthracene radical cation

Concentration dependent relaxation times in organic radical solids

The two-pulse electron spin echo envelope decays have been studied in rigid glass matrices of sulfuric acid and sulfuric acid-d2 containing the randomly oriented radical cations of anthracene and anthracene-d10, respectively. The sources of the time dependent local fields responsible for the shapes of the two-pulse echo envelope decays and the phase memory times have been identified. In the anthracene cation: sulfurie acid system, where the phase memory times are independent of the radical concentration, these local fields arise from spin flips of the protons on the matrix molecules which are a result of spin-spin relaxation within this proton spin system. In the deuterated system, where the phase memory times are strongly dependent on the radical concentration, the time dependent local fields have three sources: (1) the surrounding A electron spins which are flipped by the microwave pulses; (2) the surrounding B electron spins which are flipped as a result of spin-spin and spin-lattice relaxation processes within the electron spin system; and (3) the matrix deuterium spins which are flipped as a result of spin-spin relaxation processes within the matrix deuterium spin system. The electron spin-lattice relaxation times T1 have been measured at 77°K as a function of radical concentration C using an echo method. The data for both systems fit the expression 1/T1= 1.2× 107C2+28(± 5) where T1 is in second−1 and C in mole per liter. The lack of any dependence on the nature of the radical nuclei indicates that the concentration independent term is due to a spin-robit, orbit-lattice mechanism. The concentration dependent term is attributed to either a Wailer mechanism involving a modulation of the electron magnetic dipole-dipole interaction by the lattice vibrations or to the participation of clusters of radicals (pairs or triads) in the spin-lattice relaxation process. The shape of the recovery of the two-pulse echo following an adiabatic fast passage is multiexponential and its exact form is dependent on τ, the interpulse time. This is interpreted as indicating the presence of regions with different radical concentrations which probably form when the matrix solidifies.