Phenol Radical Cations Research Articles

Generation, Characterization, and Deprotonation of Phenol Radical Cations

A variety of methoxy- and methyl-substituted phenol radical cations have been generated and characterized by laser flash photolysis in solution under ambient conditions. The radical cations were generated by either photosensitized electron transfer using 1,4-dicyanonaphthalene with biphenyl as a cosensitizer in acetonitrile or by direct excitation of the phenol at 266 nm. The phenol radical cations have absorption maxima between 410 and 460 nm, with the exception of the 3,5-dimethoxyphenol which absorbs at 580 nm. The assignment of the observed transients to phenol radical cations is based on their spectral similarity to matrix spectra for the same species and to the corresponding methoxybenzene radical cations, as well as their characteristic reactivity. In the presence of small amounts of water the radical cations are not detected and the phenoxyl radical is the only observed transient in the photosensitized electron transfer. For those phenols for which authentic phenoxyl spectra are not available, the...

Vibrational projection analysis: New tool for quantitatively comparing vibrational normal modes of similar molecules

A new method for quantitatively comparing calculated vibrational modes is described that relies on projecting the vectors describing the normal modes of one molecule onto those of a basis molecule. The procedure virtually automates the assignment of vibrational modes from one molecule to a second, structurally similar one. We illustrate the concept by using the classical Wilson modes of benzene as a basis for describing normal modes of the monosubstituted benzene derivatives phenol, phenol-d5, and phenol radical cation. These examples demonstrate the method's power for accurately assigning and comparing the normal modes of molecules perturbed by chemical substitution, isotopic substitution, or oxidation state. The vibrational projection analysis method—a special case of vector projection analysis—is compared and contrasted with total energy distribution analysis, perhaps the most commonly used technique for quantitatively comparing vibrational modes, and is found superior in each case when comparing normal modes. Vibrational projection analysis need not be limited to single molecules and quantum calculations, because normal modes may be obtained for much larger systems using molecular mechanics or molecular dynamics techniques. The method should therefore prove useful for interpreting the normal modes of ordered periodic solids and structures perturbed by noncovalent contacts, including proteins and polymers. The method may also prove useful in evaluating new computational methods by allowing direct, quantitative comparison of the vibrational modes obtained from different techniques. The power of this technique will make vibrational projection analysis a valuable tool for computational chemists as well as those using calculations to complement vibrational spectroscopic measurements. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1663–1674, 1998

Generation and spectroscopic and kinetic characterization of methoxy-substituted phenoxyl radicals in solution and on paper

A number of methoxy-substituted phenoxyl radicals have been generated and characterized by laser flash photolysis techniques in solution and on paper. The radicals have been produced by three routes in solution: hydrogen abstraction from phenols by tert-butoxyl radical, photolysis of α-aryloxyacetophenones, and direct excitation of phenols. Most of the phenoxyl radicals studied have a characteristic absorption near 400 nm; the ortho-substituted radicals have an additional broad absorption in the visible in non-hydroxylic solvents (e.g., 650 nm for 2-methoxyphenoxyl radical). The relative intensities of the 400 nm and the long-wavelength bands vary with substitution pattern and solvent, with the long wavelength band being much weaker in hydroxylic solvents. Direct excitation of phenols provides an alternate source of phenoxyl radicals in some cases. However, for 4-methoxyphenol there are overlapping absorptions due to triplet phenol and phenol radical cation, both of which absorb in the 400–460 nm region. The 4-methoxyphenol radical cation is formed in a biphotonic process and decays by deprotonation to give the phenoxyl radical. The 4-methoxyphenoxyl radical can also be generated on paper from either the ketone or phenol precursor and decays with complex kinetics that depend on the precursor loading and laser intensity. There is no evidence for the formation of significant amounts of long-lived ketone or phenol triplets on paper. The decay kinetics for the phenoxyl radical are modified by the addition of the radical scavenger dithioerythritol. The results indicate both static and dynamic components to the radical scavenging process. The generation of phenoxyl radicals on paper in the presence and absence of scavengers provides the basis for the development of a new method for screening potential photoyellowing inhibitors and understanding their mode of action. Keywords: phenoxyl radicals, paper photoyellowing, laser flash photolysis, triplets, radical cations.

Charge transfer to a semi-esterified bifunctional phenol

The charge transfer from solvent radical cations of n-butyl chloride and cyclohexane to 2-butyl-6(3′-t-butyl-2′-hydroxy-5′-methylbenzyl)-4-methyl-phenylacrylate (GM) yields in the first step phenoxyl radicals as well as acrylate radical cations of this semi-acrylated bifunctional phenol. Subsequently an intramolecular charge transfer from the acrylate radical cation to the phenol group takes place. Because of the instability of phenol radical cations, under our experimental conditions (nanosecond pulse radiolysis, non-polar solvents, room temperature) phenoxyl radicals are the only observable products of phenol ionization.

Density-Functional-Derived Structures, Spin Properties, and Vibrations for Phenol Radical Cation

Phenol radical cation (PhOH•+) structures, proton hyperfine coupling constants, atomic spin densities, vibrational frequencies, and deuterium isotopic vibrational frequency shifts were calculated by using spin-unrestricted Hartree−Fock (UHF), second-order Møller−Plesset perturbation (UMP2), as well as density-functional (DF) and hybrid Hartree−Fock/density-functional (HF/DF) methods. Of the methods tested, the three-parameter, hybrid HF/DF method incorporating the gradient-corrected correlation functional of Lee, Yang, and Parr (B3LYP) appears to yield the most accurate overall results. Calculated CC bond distances for PhOH•+ display pronounced quinoidal features, and the CO bond distance (1.313 Å) implies significant double-bond character. Unscaled vibrational frequencies calculated by using the B3LYP method differ from experiment by an average absolute magnitude of only 15 cm-1 and are therefore more accurate than uniformly scaled UMP2-derived frequencies. Our assignment of the CO stretching mode to a calculated frequency of 1522 cm-1 and the strong mixing between the CO stretching mode 7a and a CH bending mode at lower frequency (19a) suggests that isotopic frequency shifts be measured to distinguish clearly the CO stretching and CH bending modes in radicals related to PhOH•+. PhOH•+ displays a spin density distribution reminiscent of an odd-alternant hydrocarbon, but with large spin density on its oxygen-bearing C1 atom. Of the methods tested, the gradient-corrected BP86 and BLYP and hybrid B3LYP methods yield the most accurate proton hyperfine coupling constants for PhOH•+, whereas calculated spin densities on the carbon and oxygen atoms are much less sensitive to the particular DF method used. The B3LYP method also gives an accurate proton affinity for PhO• (208.5 kcal/mol), within 2.9 kcal/mol of experiment (205.6 ± 0.3 kcal/mol).

Phenol-assisted photolytic and radiolytic radical cation formation of a sterically hindered tertiary amine

With 266 nm laser pulses a compound whose molecular structure incorporates a sterically hindered phenol group and two sterically hindered N-methyl piperidine groups was ionized in a two-photon process. From the intermediate phenol radical cation the change is transferred to the amine moiety, generating tertiary amine radical cations. This phenomenon is likely to proceed via an intramolecular charge transfer through space.

Resonant two-photon ionization of phenol in methylene chloride doped solid argon using 248 nm KrF laser and 254 nm Hg lamp radiation, a comparative study. The UV/VIS absorption spectrum of phenol radical cation

Resonant two-photon ionization (TPI) of phenol (PhOH) has been successfully achieved in methylene chloride (CH 2Cl 2) doped solid argon using a KrF laser and a Hg resonance lamp. The result constitutes the first-time TPI of a typically organic molecule in this medium using an excimer laser as well as the first-time spectroscopic identification of PhOH +•. A qualitative model is proposed which is consistent with both the unexpected photostability of PhOH +• and the incomplete running of the TPI process in the applied medium.

Polar effects in the decomposition of bis(3-alkoxyaroyl) peroxides. Synthesis of 8-alkoxy-6H-dibenzo[b,d]pyran-6-ones

The reaction of four substituted bis(3-alkoxybenzoyl) peroxides (1b-e) in neat phenols (2a-e) affords mainly 8-alkoxy-6H-dibenzo[b,d]pyran-6-ones (7) and ortho-benzoyloxylation products (4) of the phenol. Diaroyl peroxides without electron-releasing meta substituents afford essentially products 4. A mechanism involving monoelectronic oxidation of the phenol by the peroxide and biaryl coupling by preferential addition of the phenol radical cation to the ortho positions to the alkoxy group of the diaroyl peroxide is suggested

Esr and cidep studies of the photooxidation of phenols in single crystals and in solutions

Photooxidation of single crystals of 2,6-di-tert-butyl-4-methylphenol and 2,6-di-tert-butylphenol, respectively, led to localized pair-wise trapping of triplet phenoxyl radical pairs. Electron ejection is not implicated in the primary photooxidation of phenols in single crystals, but in liquid solutions time-resolved CIDEP observations for the 2,4,6-tritert-butylphenol in hexaftuorobenzene suggest an initial formation of a phenol radical cation from the photoexcited triplet state.

Effects of near degeneracy in the frontier orbitals on the electron spin resonance spectra of radicals related to 1,2,3-trihydroxybenzene (pyrogallol)

2,3-Methylenedioxyphenoxyl was generated in solutions containing 0–9 mol dm–3 sulphuric acid. Large variations in the e.s.r. spectra, due to progressive formation of the phenol radical cation with increasing acid concentration, are shown to be caused by a difference in the symmetry properties of the odd electron orbital in the neutral and charged species. Near degeneracy of the pair of frontier orbitals of the system, which are symmetrical and antisymmetrical with respect to a plane bisecting C(2) and C(5) in the substrate, causes this effect, which may also confirm the tentative assignment of coupling constants for 3-hydroxy-o-benzosemiquinone made by Carrington and Smith.

Determination of acid dissociation constants of some phenol radical cations. Part 2

Determination of acid dissociation constants of some phenol radical cations. Part 2

Calculations of g-factors of phenoxyl radicals, phenol radical cations and alkyl aryl ether radical cations

Using the semi-empirical method presented by Stone, experimental g-factors of some oxygen-substituted aromatic radicals have been correlated with the energy-level coefficient (λ) of the odd electron Hückel molecular orbitals, and with the total spin density (Σρ0) on the oxygen atoms. Parameters obtained from plots of Δgobserved/Σρ0 against λ were used to directly estimate g-factors of the radicals studied.

Determination of acid dissociation constants of some phenol radical cations

The variation of the ratio of acid–base forms of some phenol radical cations with acid strength is followed by e.s.r. spectroscopy. The acid dissociation constants (K) are given in the form of –log K where [graphic omitted] h and 55.5W are the number of moles of sulphuric acid and water, respectively, used to make up 1 dm3 of solution and n= 1 or 2 depending upon the structure of the radical. The implications of this relationship are discussed and it is shown that application of the method to acid–base data for carbon protonation of phenols produces a deviation from linearity of the same order as that observed when H0 is used.

Calculation of substituent effects on the spin distribution in radicals formed by electron-loss from phenols and from alkyl aryl ethers

Using the McLachlan S.C.F. method, the spin distributions in some phenoxyl radicals, phenol radical cations, and some alkyl aryl ether radical cations were calculated using a full set of basis π-electron orbitals. This made it possible to estimate the aliphatic proton splittings directly, also to explain trends in spin distribution caused by electron withdrawing groups, such as carbonyl.

Determination of the acidity constants of some phenol radical cations by means of electron spin resonance

Phenoxyl radicals were generated in solutions containing from 0 to 75 % sulphuric acid. The changes observed in the e.s.r. spectra over this range are shown to be due to progressive formation of phenol radical cations. In favourable cases plots of the e.s.r. parameters against acidity enabled us to determine the pKa of the radical cations, which varied from –2.0 (phenol radical cation) to –0.8 (hydroquinone radical cation). Changes in the coupling constants in going from a phenoxyl radical to the corresponding phenol radical cation are usually relatively small, but the g-values change from 2.004 4 ± 0.000 4 to 2.003 2 ± 0.000 4. The free energy changes involved in competing oxidative processes are discussed and the data support the view that the C—O group in phenoxyl resembles that in ketones.

The electron spin resonance spectra of alkyl aryl ether radical cations

Radical cations of alkyl aryl ethers may be generated in a flow system by means of oxidation of the ethers by cerium(IV) in concentrated sulphuric acid solution. The spin distributions and g values are similar to these in corresponding phenol radical cations. The 1,3-benzodioxoles give rise to particularly intense spectra which characteristically have triplet splittings close to 2.0 mT, which can be explained in terms of the molecular orbital theory of hyperconjugation. The relatively marked stability of the cations from the 1,3-benzodioxoles made it possible to obtain intelligible data from some complex natural products containing this group. Evidence from the spectra for various types of conformational isomerism is discussed.