P-benzosemiquinone Radical Anion Research Articles

Correlated ab Initio Spin Densities for Larger Molecules: Orbital-Optimized Spin-Component-Scaled MP2 Method

The performance of the orbital-optimized MP2 method (OO-MP2) and its spin-component-scaled variant is investigated for the prediction of atomic and molecular hyperfine coupling constants (hfccs). The calculated hfccs are compared to experimental as well as to CCSD(T) reference results. The OO-MP2 isotropic hfccs for a series of small radicals are almost of CCSD quality but are obtained with iterative O (N⁵) effort. The dipolar hfccs are less well predicted by the OO-MP2 methods, whereas spin-component scaling improves the description of the hyperfine structure. The spin contamination in the OO-MP2 wave function is drastically reduced compared to the standard unrestricted Hartree-Fock wave function. The applicability of the OO-MP2 to fairly large systems is demonstrated for the solvated p-benzosemiquinone radical anion, where calculations with almost 2000 basis functions have been performed.

Reactions of Hydrated Electron with Various Radicals: Spin Factor in Diffusion-Controlled Reactions

The reactions of hydrated electron (eaq-) with various radicals have been studied in pulse radiolysis experiments. These radicals are hydroxyl radical (*OH), sulfite radical anion (*SO3-), carbonate radical anion (CO3*-), carbon dioxide radical anion (*CO2-), azidyl radical (*N3), dibromine radical anion (Br2*-), diiodine radical anion (I2*-), 2-hydroxy-2-propyl radical (*C(CH3)2OH), 2-hydroxy-2-methyl-1-propyl radical ((*CH2)(CH3)2COH), hydroxycyclohexadienyl radical (*C6H6OH), phenoxyl radical (C6H5O*), p-methylphenoxyl radical (p-(H3C)C6H4O*), p-benzosemiquinone radical anion (p-OC6H4O*-), and phenylthiyl radical (C6H5S*). The kinetics of eaq- was followed in the presence of the counter radicals in transient optical absorption measurements. The rate constants of the eaq- reactions with radicals have been determined over a temperature range of 5-75 degrees C from the kinetic analysis of systems of multiple second-order reactions. The observed high rate constants for all the eaq- + radical reactions have been analyzed with the Smoluchowski equation. This analysis suggests that many of the eaq- + radical reactions are diffusion-controlled with a spin factor of 1/4, while other reactions with *OH, *N3, Br2*-, I2*-, and C6H5S* have spin factors significantly larger than 1/4. Spin dynamics for the eaq-/radical pairs is discussed to explain the different spin factors. The reactions with *OH, *N3, Br2*-, and I2*- have also been found to have apparent activation energies less than that for diffusion control, and it is suggested that the spin factors for these reactions decrease with increasing temperature. Such a decrease in spin factor may reflect a changing competition between spin relaxation/conversion and diffusive escape from the radical pairs.

Thermochromism of Metal Ion Complexes of Semiquinone Radical Anions. Control of Equilibria between Diamagnetic and Paramagnetic Species by Lewis Acids

Metal ion complexes of semiquinone radical anions exhibit different types of thermochromism depending on metal ions and quinones. Metal ion complexes of 1,10-phenanthroline-5,6-dione radical anion (PTQ(.-)) produced by the electron-transfer reduction of PTQ by 1,1'-dimethylferrocene (Me(2)Fc) in the presence of metal ions (Mg(2+) and Sc(3+)) exhibit the color change depending on temperature, accompanied by the concomitant change in the ESR signal intensity. In the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is in equilibrium, when the concentration of the PTQ(.-)-Mg(2+) complex (lambda(max) = 486 nm) increases with increasing temperature because of the positive enthalpy for the electron-transfer equilibrium. In contrast to the case of Mg(2+), electron transfer from Me(2)Fc to PTQ is complete in the presence of Sc(3+), which is a much stronger Lewis acid than Mg(2+), to produce the PTQ(.-)-Sc(3+) complex (lambda(max) = 631 nm). This complex is in disproportionation equilibrium and the concentration of the PTQ(.-)-Sc(3+) complex increases with decreasing temperature because of the negative enthalpy for the proportionation direction, resulting in the remarkable color change in the visible region. On the other hand, the p-benzosemiquinone radical anion (Q(.-)) forms a 2:2 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(2)-Q] with Q and Sc(3+) ions at 298 K (yellow color), which is converted to a 2:3 pi-dimer radical anion complex [Q(.-)-(Sc(3+))(3)-Q] with a strong absorption band at lambda(max) = 604 nm (blue color) when the temperature is lowered to 203 K. The change in the number of binding Sc(3+) ions depending on temperature also results in the remarkable color change, associated with the change in the ESR spectra.

The Origin of Base Catalysis in the •OH Oxidation of Phenols in Water

Time-resolved Raman and transient absorption studies on hydroxyl (•OH) radical reactions with aqueous 4-carboxyphenol (-O2C−PhOH), the substrate in p-hydroxybenzoate hydroxylase, have led to the first direct observation of the basic form of a dihydroxycyclohexadienyl (•OH-adduct of phenolate anion) radical and hydration-induced intramolecular electron transfer in this species. The base-catalyzed phenoxyl radical formation in this model system has been quantitatively described in terms of pKa of the phenolic proton (9.05) of the •OH-adduct and the rate of OH- elimination (2.9 × 106 s-1) from its basic form. A small fraction, 12(+2)%, of the phenoxyl radical is formed via water elimination from the •OH-adduct at the ipso position of the hydroxyl group (rate > 107s-1). About 3% •OH addition is seen at the carboxylic ipso position in basic solutions, which produces the p-benzosemiquinone radical anion (rate ∼ 106s-1). This work provides spectroscopic and kinetic evidence of the early chemical steps in the phenoxyl radical formation by •OH oxidation and establishes the precise relationship between the formation rate and pH. A relationship between the rates of OH- elimination from the •OH adducts of phenolate anions and pKa of the corresponding phenols is given.

Hydrogen Bond Geometries from Electron Paramagnetic Resonance and Electron−Nuclear Double Resonance Parameters: Density Functional Study of Quinone Radical Anion−Solvent Interactions

Density functional theory was used to study the impact of hydrogen bonding on the p-benzosemiquinone radical anion BQ(*-) in coordination with water or alcohol molecules. After complete geometry optimizations, (1)H, (13)C, and (17)O hyperfine as well as (2)H nuclear quadrupole coupling constants and the g-tensor were computed. The suitability of different model systems with one, two, four, and 20 water molecules was tested; best agreement between theory and experiment could be obtained for the largest model system. Q-band pulse (2)H electron-nuclear double resonance (ENDOR) experiments were performed on BQ(*-) in D(2)O. They compare very well with the spectra simulated by use of the theoretical values from density functional theory. For BQ(*-) in coordination with four water or alcohol molecules, rather similar hydrogen-bond lengths between 1.75 and 1.78 A were calculated. Thus, the computed electron paramagnetic resonance (EPR) parameters are hardly distinguishable for the different solvents, in agreement with experimental findings. Furthermore, the distance dependence of the EPR parameters on the hydrogen-bond length was studied. The nuclear quadrupole and the dipolar hyperfine coupling constants of the bridging hydrogens show the expected dependencies on the H-bond length R(O.H). A correlation was obtained for the g-tensor. It is shown that the point-dipole model is suitable for the estimation of hydrogen-bond lengths from anisotropic hyperfine coupling constants of the bridging (1)H nuclei for H-bond lengths larger than approximately 1.7 A. Furthermore, the estimation of H-bond lengths from (2)H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed.

Energy Requirements for Inverted CIDEP in Reactions between eaq- or Radical Anions and Phenoxyl Radicals

Inverted (A/E) chemically induced dynamic electron polarization (CIDEP) is observed for random encounters of radical pairs such as the phenoxyl radical together with either e a q - or the radical anion of benzoate. A number of experiments have been carried out to clarify the relationship of the energetics of the reaction of oxidizing and reducing radicals to CIDEP behavior. Steady-state and time-resolved ESR experiments were carried out to determine the sense of F-pair CIDEP for a number of different radical pairs consisting of a phenoxyl radical together with one of a series of radical anions of para-substituted benzoates. Inverted polarization (A/E) was found for e a q - or the radical anions of benzoate and 4-methyl, 4-methoxy, and 4-carboxybenzoates in the presence of phenoxyl or 4-methylphenoxyl radicals. Other radical anions such as those of 4-formylbenzoate showed normal polarization. The pair consisting of e a q - with the p-benzosemiquinone radical anion also showed inverted polarization, but normal polarization was seen for all radical anions paired with the semiquinone radical anion. The corresponding excited-state energies of the reaction product (phenolate anion or hydroquinone dianion) were determined from the emission spectra, and the energies of the radical pairs were estimated from electrochemical experiments. The inverted CIDEP is clearly the result of a change in the normal energy ordering of the radical-pair states so that the triplet state is lower. The observations are discussed in terms of the electron-transfer reaction within the radical pairs using the approach presented by Kobori et al. (Kobori, Y.; Sekiguchi, S.; Akiyama, K.; Tero-Kubota, S. J. Phys. Chem. A 1999, 103, 5416. Kobori, Y.; Akiyama, K.; Tero-Kubota, S. J. Chem. Phys 2000, 113, 465).

P-Benzosemiquinone radical anion on silver nanoparticles in water.

This communication reports the SERS observation of p-benzosemiquinone radical anion, produced on reduction of p-benzoquinone by Ag nanoparticles at the metal-water interface. The species is positively identified by comparison of the SERS spectrum with the resonance Raman spectra of the radical anion in aqueous solution. This is a rare SERS observation of a radical intermediate formed by surface reaction on nanosize silver particles in solution.

Electronic structure of para aminophenoxyl radical in water

The electronic structure of aqueous p-aminophenoxyl radical (H2NPhO•) has been examined by time-resolved resonance Raman spectroscopy and ab initio and density functional theories. The effects of hydrogen bonding and solvent reaction field on polarity of the radical have been visualized in terms of simple models. Calculations predict the dipole moment of the radical in its ground electronic state (2B1) to increase by 8(±2) D and the difference between the CN and CO bond lengths to decrease by ∼0.05 Å from gas phase to aqueous solution. This profound hydration effect converts the structure and chemical properties of H2NPhO• from a substituted phenoxyl radical in the gas phase to a semiquinone-like radical in water. The observation of vibrational modes enhanced in Raman by a non-Franck–Condon vibronic coupling mechanism has led to the identification of two very weakly absorbing electronic states of A22 symmetry in the 340–390 nm region, which borrow transition moment from close by strongly allowed electronic states of B12 symmetry at lower (∼440 nm) and higher (∼320 nm) energies. One of these transitions is parity forbidden (2B2g↔2B1g) in p-benzosemiquinone radical anion (PhO2−•) and p-phenylenediamine radical cation (Ph(NH2)2+•) and this is the first experimental evidence on energy location (3.44 eV) of this transition in an isoelectronic radical. The experiment and theory are combined to estimate the CO and CN bond lengths in H2NPhO• as ∼1.263 and ∼1.34 Å, respectively, in liquid water and ∼1.245 and ∼1.37 Å in the gas phase.

Coordination of semiquinone and superoxide radical anions to the zinc ion in SOD model complexes that act as the key step in disproportionation of the radical anions.

Reactions of imidazolate-bridged CuII-ZnII heterodinuclear and CuII-CuII homodinuclear complexes, [CuIIZnII(bdpi)(CH3CN)2](ClO4)3 x 2CH3CN (1) and [CuII2(bdpi)(CH3CN)2](ClO4)3 x CH3CN x 3H2O (2) (Hbdpi = 4,5-bis(di(2-pyridylmethyl)aminomethyl)imidazole), with the p-benzosemiquinone radical anion (Q*-) have been examined to provide mechanistic insight into the role of the ZnII ion in copper-zinc superoxide dismutase (Cu,Zn-SOD). The addition of less than one equivalent of Q*- to a deaerated solution of 1 or 2 in propionitrile at -80 degrees C results in the oxidation of Q*- accompanied by the appearance of a new absorption band at 585 nm due to the CuI-Q complex (3 or 4), the absorbance of which increases linearly with the increase in Q*- concentration. Both the resonance Raman spectra of 3 and 4 exhibit a strong resonance-enhanced Raman band at 1580 cm(-1), which can be assigned to a CO stretching vibration in the CuI-Q complexes. Further addition of Q*- to a deaerated solution of 3 in propionitrile results in the reduction of Q*-, whereas no reduction of Q*- occurs with 4, which does not contain the ZnII ion. Thus, the coordination of Q*- to the ZnII ion is essential for the reduction of Q*- by the CuI ion in 3. The coordination of O2*- and Q*- to the ZnII ion has been confirmed by the electronic and ESR spectra of the O2*- and Q*- complexes with mononuclear ZnII complexes, [ZnII[MeIm(Py)2](CH3CN)](ClO4)2 (5) and [ZnII[MeIm(Me)2](H2O)](ClO4)2 (6) (MeIm(Py)2 = (1-methyl-4-imidazolylmethyl)bis(2-pyridylmethyl)amine, MeIm(Me)2 = (1-methyl-4-imidazolylmethyl)bis(6-methyl-2-pyridylmethyl)amine). The binding energies of O2*- with the ZnII ion in 5 and 6 have been evaluated from the deviation of the g values of the ESR spectra from the free spin value.

A Theoretical Determination of the Low-lying Electronic States of thep-Benzosemiquinone Radical Anion

The low-lying electronic states of the p-benzosemiquinone radical anion are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. Vertical excitation energies at the optimized geometries of the anion and the neutral molecule are computed. In light of the present theoretical results, the information provided by different spectroscopic techniques, such as UV/vis absorption, excitation, fluorescence, electron photodetachment, and electron scattering, is rationalized. CASSCF force fields are employed to compute vibronic intensities of the two lowest-energy π−π* transitions in order to solve controversial assignments and to give an interpretation of the available resonance Raman data.

Effect of Hydrogen Bonding on the Vibrations of p-Benzosemiquinone Radical Anion

The vibrational bands experimentally observed for aqueous p-benzosemiquinone radical anion are analyzed by use of electronic structure calculations. To clarify the effects of solvation, calculations on the isolated semiquinone anion are compared to those on supermolecule clusters having two to four water molecules strongly hydrogen-bonded at the semiquinone oxygen atoms. The results allow satisfactory assignment of all experimentally known aqueous vibrational bands, some being assigned here for the first time. The clusters considered give reasonable accounts both of the observed frequencies in water and of the frequency shifts between acetonitrile and water that arise from hydrogen bonding.

π-Donor Substituent Effects on Calculated Structures, Spin Properties, and Vibrations of Radical Anions ofp-Chloranil,p-Fluoranil, andp-Benzoquinone

Vibrational and electronic properties of p-benzosemiquinone radical anions are important indicators of charge transfer in ion pairs, charge-transfer salts, and electron-transfer reactions. To provide missing data for radical anions of p-chloranil, p-fluoranil, and p-benzoquinone, and to test the capabilities of various computational methods, calculated structures, spin properties, and harmonic vibrational frequencies and mode assignments are reported based on molecular orbital, density functional, and hybrid Hartree−Fock/density functional computations. Qualitative structural changes upon reducing the neutral quinones to their semiquinone radical anions are predictable from the nodal structure of p-benzoquinone's LUMO. Furthermore, trends in calculated CO and CC bond distance changes for the three molecules follow a pattern expected from the π-bonding abilities of the substituents. The most accurate calculated spin densities and isotropic hyperfine coupling constants show qualitative agreement with experimentally derived values for p-benzosemiquinone radical anion, whereas predicted spin densities and hyperfine coupling constants are reported for the other two species. Although calculated vibrational frequencies display small average absolute differences from experimentally measured frequencies (24 cm-1 for p-chloranil's radical anion), for p-chloranil's radical anion calculations generally fail to reproduce several important details of the vibrational spectra.

Reduction Potentials and Kinetics of Electron Transfer Reactions of Phenylthiyl Radicals: Comparisons with Phenoxyl Radicals

The reduction potentials relative to the standard hydrogen electrode (SHE) for a number of para-substituted phenylthiyl radicals (E°(p-XC6H4S•/p-XC6H4S-)) have been derived from pulse radiolytic studies of electron transfer equilibria which compare their values to those of radicals of known reduction potentials. A ladder combining the reduction potentials for both phenylthiyl and phenoxyl radicals has been established. These reduction potentials have been shown to be self-consistent and are intermediate between those of p-benzosemiquinone radical anion at 0.02 V and phenoxyl radical at 0.79 V. The reduction potential decreases as the electron donating power of the para substituent rises. The substituent effect is, however, much weaker for the phenylthiyl radicals than for their oxygen analogs. These observations demonstrate that the electronic interaction between the sulfur atoms and the aromatic ring system is much less than that which occurs with oxygen atoms. Examination of the rates of electron transf...

Ab initio structures of para-substituted phenoxy radicals (XC6H4O.bul.; X = H, F, Cl, OH, NH2, and O-) and substituent effects on molecular vibrations

The structures of the ground-state phenoxy and para-substituted phenoxy radicals (XC 6 H 4 O . :X=H, F, Cl, OH, NH 2 , and O - ) were calculated by the unrestricted natural orbital-complete active space (UNO-CAS) method using the 6-31G + basis set. The results indicate that the p-aminophenoxy radical is not a special phenoxy radical; its structure is more like those of the other neutral phenoxy radicals rather than the p-benzosemiquinone radical anion. Presumably, the difference between the structural features of the p-aminophenoxy radical and the p-benzosemiquinone radical anion is due to significant delocalization of the unpaired electron of the latter onto the substituent, which results in equal CO distance of the radical anion

Spectra, ionization constants, and rates of oxidation of 1,4-dimercaptobenzene and properties of the p-mercaptophenylthiyl and p-benzodithiyl anion radicals

In basic solution, p-dimercaptobenzene, which is found to have pK a 's of 6.0 and 7.7, is rapidly oxidied by azide radical (k=7.4×10 9 M -1 s -1 ) to p-benzodithiyl radical anion, the sulfur analog of p-benzosemiquinone radical anion. This radical has very strong absorption bands at 390 and ∼700 nm. In acidic solutions, CO 2 .- abstracts a SH hydrogen atom (k=3.1×10 9 M -1 s -1 ) to form the p-mercaptophenylthiyl radical, which has absorption maxima at 350 and 585 nm and a pk a , of 2.7

Low-energy, doublet states of p-benzosemiquinone radical anion from SCF-CI and CASSCF calculations

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTLow-energy, doublet states of p-benzosemiquinone radical anion from SCF-CI and CASSCF calculationsRalph A. WheelerCite this: J. Phys. Chem. 1993, 97, 8, 1533–1537Publication Date (Print):February 1, 1993Publication History Published online1 May 2002Published inissue 1 February 1993https://doi.org/10.1021/j100110a013RIGHTS & PERMISSIONSArticle Views106Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (587 KB) Get e-Alerts

Structure, bonding and vibrational modes of the p-aminophenoxyl radical and p-benzosemiquinone radical anion from ab initio molecular orbital calculations

Download options Please wait... Article information DOI https://doi.org/10.1039/FT9938900665 Article type Paper Download Citation J. Chem. Soc., Faraday Trans., 1993,89, 665-670 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Structure, bonding and vibrational modes of the p-aminophenoxyl radical and p-benzosemiquinone radical anion from ab initio molecular orbital calculations K. S. Raymond and R. A. Wheeler, J. Chem. Soc., Faraday Trans., 1993, 89, 665 DOI: 10.1039/FT9938900665 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Kevin S. Raymond Ralph A. Wheeler

Structure of the p-aminophenoxyl radical

Experimental results cited here clearly indicate that the electronic and vibrational structures of p-aminophenoxyl radical are close to those of the isoelectronic p-benzosemiquinone radical anion and p-phenylenediamine radical cation. Recently published ab initio calculations by Raymond and Wheeler (J. Chem. Soc., Faraday Trans., 1993, 89, 665), which predict a benzene-like structure for the p-aminophenoxyl radical, grossly underestimate the Raman frequencies of the Wilson 8a (ring stretch) and 7a (CO/CN stretch) modes. In particular, the latter, which is observed at 1434 cm–1, demonstrates that the p-aminophenoxyl radical has a semiquinone-like structure. The referenced calculation also places the unpaired spin population almost entirely on the oxygen atom. This conclusion is inconsistent with the spin distribution exhibited in the observed EPR hyperfine constants which demonstrate quite conclusively that there is a significant fraction of the unpaired spin on the ring and also on the amino nitrogen atom. The delocalization of the unpaired spin manifest in both the Raman and EPR spectra is also reflected in the reduction potential of the radical (0.21 V vs. NHE) which is lower than that of bona fide phenoxyl radicals by ca. 0.6 V. It is clear that ab initio calculations at the level used by Raymond and Wheeler do not give a realistic picture of the electronic structure of p-aminophenoxyl-like radicals.

Resonance enhancement of the non-totally symmetric vibrations in the raman spectra of para-benzosemiquinone radical anion

Resonance Raman spectra of p-benzosemiquinone radical anion excited in the 340–430 nm region are reported. Prominent bands, which are not present in the 430 nm spectrum, are observed at 1472 and 1271 cm −1 in the 370 nm spectrum. These bands are attributed to the non-totally symmetric (b 1g) modes, ν 8b (CC stretch) and ν 3 (CH bend), respectively. These vibrations are resonance-enhanced because of vibronic coupling between the 370 nm ( 2A u← 2B 3g) and 315 nm ( 2B 1u← 2B 3g) transitions.

Resonance Raman spectra of p-benzosemiquinone radical and hydroquinone radical cation

The time-resolved resonance Raman spectrum of the p-benzosemiquinone radical (HOC/sub 6/H/sub 4/O), excited at 415 nm and observed on the microsecond time scale in the pulse radiolysis of moderately acidic solutions of hydroquinone (pH 2-4), is reported. The nu/sub 7a/ CO stretching mode appears prominently in the spectrum at 1511 cm/sup -1/, showing that the CO bond is very similar to that in phenoxyl where this vibration is observed at 1505 cm/sup -1/. Strongly resonance enhanced bands are also observed at 1613 and 1162 cm/sup -1/ and are assigned respectively to the nu/sub 8a/ ring stretching and nu/sub 9a/ CH bending modes. The strong enhancement of these latter vibrations, which are very much weaker in the corresponding spectrum of the phenoxyl radical, and the similarity of the ring stretching frequency (nu/sub 8a/) to that in p-benzosemiquinone radical anion (1620 cm/sup -1/) indicate that there is a strong interaction of the OH p(..pi..) electron pair with the phenoxyl radical's ..pi..-electron system. The coefficient for the acid-catalyzed dehydroxylation of the OH adduct of hydroquinone is estimated from the pH and time dependences of the Raman signals of the semiquinone to be 5.4 x 10/sup 8/ M/sup -1/ s/sup -1/. In strongly acidicmore » solutions (e.g., approx. 2.3 M H/sub 2/SO/sub 4/) a second transient species, identified as the hydroquinone radical cation, is observed. The vibrational features of this radical are qualitatively similar to those for p-benzosemiquinone radical anion with the nu/sub 8a/ ring stretching mode very strongly resonance enhanced and observed at 1644 cm/sup -1/. The nu/sub 7a/ CO stretching mode is observed at 1426 cm/sup -1/ but is very much weaker than in the corresponding spectrum of the radical anion.« less