Phenol Radical Cations Research Articles

Ultrafast proton transfer of the aqueous phenol radical cation.

Proton transfer (PT) reactions are fundamental to numerous chemical and biological processes. While sub-picosecond PT involving electronically excited states has been extensively studied, little is known about ultrafast PT triggered by photoionization. Here, we employ femtosecond optical pump-probe spectroscopy and quantum dynamics calculations to investigate the ultrafast proton transfer dynamics of the aqueous phenol radical cation (PhOH˙+). Analysis of the vibrational wave packet dynamics reveals unusually short dephasing times of 0.18 ± 0.02 ps and 0.16 ± 0.02 ps for the PhOH˙+ O-H wag and bend frequencies, respectively, suggestive of ultrafast PT occurring on the ∼0.1 ps timescale. The reduced potential energy surface obtained from ab initio calculations shows that PT is barrierless when it is coupled to the intermolecular hindered translation between PhOH˙+ and the proton-acceptor water molecule. Quantum dynamics calculations yield a lifetime of 193 fs for PhOH˙+, in good agreement with the experimental results and consistent with the PT reaction being mediated by the intermolecular O⋯O stretch. These results suggest that photoionization can be harnessed to produce photoacids that undergo ultrafast PT. In addition, they also show that PT can serve as an ultrafast deactivation channel for limiting the oxidative damage potential of radical cations.

Microsolvation of the 5-hydroxyindole cation (5HI+) with nonpolar and quadrupolar ligands: Infrared photodissociation spectra of 5HI+-Ln clusters with L = Ar and N2 (n ≤ 3)

Solvation of biomolecules and their building blocks has a strong influence on their structure and function. Herein we characterize the initial microsolvation of the 5-hydroxyindole cation (5HI+) in its 2A″ ground electronic state with nonpolar and quadrupolar ligands (L=Ar, N2) by infrared photodissociation (IRPD) spectroscopy of cold and mass-selected 5HI+-Ln (n≤3) clusters in a molecular beam and dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). The isomer-selective OH and NH stretch frequency shifts (ΔνOH/NH) disentangle the competition between H-bonding to the acidic OH and NH groups and π-stacking to the conjugated bicyclic aromatic π-electron system, the intermolecular interaction strengths, and the cluster growth sequence. For 5HI+-Arn, H-bonding and π-stacking strongly compete, indicating that dispersion forces are important for the interaction of 5HI+ with nonpolar ligands. In contrast, for 5HI+-(N2)n clusters, the H-bonds are much stronger than the π-bonds and largely determine the initial solvation process. In all clusters, the OH…L bonds are stronger than the NH…L bonds followed by the π-bonds. The interaction of 5HI+ with N2 is roughly twice stronger than with Ar, mainly due to the additional quadrupole moment of N2. The nature and strength of the individual interactions are quantified by the noncovalent interaction approach. Comparison of 5HI+-L with the corresponding neutral clusters reveals the strong impact of ionization on the total and relative interaction strengths of the H-bonds and π-bonds. In addition, comparison of 5HI+-L with corresponding clusters of the phenol, indole, and pyrrole radical cations illustrates the effects of substitution of functional groups and the addition of aromatic rings to the various subunits of 5HI on the intermolecular potential.

Biocatalytic trifluoromethylation of unprotected phenols.

Organofluorine compounds have become important building blocks for a broad range of advanced materials, polymers, agrochemicals, and increasingly for pharmaceuticals. Despite tremendous progress within the area of fluorination chemistry, methods for the direct introduction of fluoroalkyl-groups into organic molecules without prefunctionalization are still highly desired. Here we present a concept for the introduction of the trifluoromethyl group into unprotected phenols by employing a biocatalyst (laccase), tBuOOH, and either the Langlois' reagent or Baran's zinc sulfinate. The method relies on the recombination of two radical species, namely, the phenol radical cation generated directly by the laccase and the CF3-radical. Various functional groups such as ketone, ester, aldehyde, ether and nitrile are tolerated. This laccase-catalysed trifluoromethylation proceeds under mild conditions and allows accessing trifluoromethyl-substituted phenols that were not available by classical methods.

Proton-Coupled Electron Transfer in a Hydrogen-Bonded Charge-Transfer Complex.

A proton-coupled electron transfer (PCET) reaction in a hydrogen-bonded charge-transfer (CT) complex of 4-([2,2'-bipyridin]-4-yl)phenol (bpy-phenol) with a F- ion has been investigated by ultrafast time-resolved transient absorption spectroscopy. The phenolic receptor molecule, bpy-phenol, binds to the F- ion through a hydrogen bond and senses the F- ion via the Stokes-shifted CT band. Upon photoexcitation, CT from the phenol residue to the bpy residue promotes proton transfer from the phenol radical cation (ArOH•+) to the fluoride ion at ultrafast time scales of <150 fs (instrument response function limited) and 3 ps, separately. The fast and slow proton-transfer times are linked to two different types of hydrogen-bonding networks between the phenol residue and fluoride ion. Crystalline water in the fluoride salt hydrates mediates the proton-transfer reaction. This work demonstrates the participation of a hydrogen-bonded water bridge within a PCET reaction in a water-restricted environment.

One‐Step Selective Hydroxylation of Benzene to Phenol

AbstractRecent developments in the one‐step thermal and photochemical catalytic hydroxylation of benzene to phenol using various oxidants are described, focusing on the catalytic mechanisms. Hydroxylation of benzene with hydrogen peroxide is catalyzed by homogeneous metal complexes and heterogeneous metal oxides to produce phenol when high‐valent metal‐oxo complexes are reactive species The selective hydroxylation of benzene to phenol is achieved by using mesoporous silica‐alumina as the catalyst support, which prohibits further oxidation of phenol due to the strongly acidic sites. Photocatalytic oxidation of benzene with dioxygen and water is made possible by using organic photocatalysts via photoinduced electron transfer from benzene to the excited states of the photocatalysts, followed by the hydroxylation of the benzene radical cation with water to yield phenol selectively. The further oxidation of phenol is prohibited because of the fast back electron transfer from the one‐electron reduced species of the photocatalyst to the phenol radical cation, whereas back electron transfer from the one‐electron reduced species to the benzene radical cation is slowed down because of the large driving force of the back electron transfer, which is in the Marcus inverted region.

Substituent Effects in the Absorption Spectra of Phenol Radical Species: Origin of the Redshift Caused by 3,5‐Dimethoxyl Substitution

The ground-state equilibrium geometries, electronic structures and vertical excitation energies of methyl- and methoxyl-substituted phenol radical cations and phenoxyl radicals have been investigated using time-dependent density-functional theory (namely TD-B3LYP) and complete-active-space second-order perturbation theory (CASPT2). The "anomalous" large redshifts of the absorption maxima of the phenol radical species observed in the ultraviolet-visible spectral region upon di-meta-methoxyl substitution are reproduced by the calculations. Furthermore, these "anomalous" shifts which were unexplained to date can be rationalized on the basis of a qualitative molecular orbital perturbation analysis.

Role of Hydrogen Bonding in the Photophysical Properties of Isomeric Tetrapyridylporphyrins in Aprotic Solvent

Extensive photophysical properties of isomeric tetra-2-pyridylporphyrin (TpyP(2)), tetra-3-pyridylporphyrin (TpyP(3)), and tetra-4-pyridylporphyrin (TpyP(4)) have been studied in the presence of a series of phenols of increasing hydrogen bonding power in dichloromethane solution by employing UV/vis spectroscopy; steady-state, time-resolved fluorescence spectroscopy; and transient absorption spectroscopic techniques. The change of absorption spectra of all three porphyrins as a function of different phenol concentrations established the preference of hydrogen bonded complex formation to the peripheral pyridyl nitrogen rather than the pyrrole nitrogen of the porphyrin macrocycle. The fluorescence behaviors of the porphyrins which were observed upon addition of different phenols point to a marked dependence on the nature of the added phenols. Phenols with an electron withdrawing group do not quench the fluorescence of porphyrins, whereas phenols with an electron donating group quench the singlet porphyrin both in static and dynamic pathways. A remarkable difference in quenching behaviors of singlet excited porphyrin by 4-methylphenol (4-MePhOH) and 4-MeOPhOH/4-EtOPhOH (4-EtOPhOH = 4-ethoxyphenol) are observed. The quenching of singlet excited porphyrins by 4-MePhOH is attributed to be purely static in nature, and the H-bond provides a strong nonradiative channel to singlet excited porphyrins. However, the quenching of singlet excited porphyrins by 4-MeOPhOH/4-EtOPhOH is mostly dynamic, and it is ascribed to be the reductive quenching of single excited porphyrins. Picosecond transient absorption study with TpyP(2) and 4-MeOPhOH provides the evidence of porphyrin radical anion and phenol radical cation of equal lifetime, which indicates the fact that electron transfer occurs from phenol to singlet excited porphyrin. The temperature effect on dynamic quenching by 4-MeOPhOH/4-EtOPhOH and kinetic deuterium isotope effect established the reaction to be a photoinduced concerted proton coupled electron transfer.

Ion chemistry in gaseous discharges at atmospheric pressure

The role of ions in plasma chemistry is briefly reviewed on the basis of recent literature reports. In addition, different chemical processes induced by gaseous discharges at atmospheric pressure are discussed here, with particular emphasis on the elucidation of some aspects of the ion chemistry occurring in benzene/air dielectric barrier and corona discharges. Through a multi-technique approach relying on gas chromatographic, mass spectrometric, spectroscopic and computational analyses, we have been able to reveal subtle mechanistic aspects involved in the ionic processes leading to the synthesis of covalent adducts. In particular, we report here the experimental conditions and the thermochemical reasons whereby we can access the synthesis (i) of benzenediazonium ion from the reaction of phenylium ion with N2, (ii) of protonated biphenyl ions from electrophilic aromatic attack of phenylium ion on C6H6, (iii) of phenol radical cations C6H5OH+. via O radical addition on the benzene ring followed by ionization of the neutral product and finally (iv) of biphenyl-oxonium ions C12H11O+ and hydroxylbiphenyl-oxonium ions as end-products of the electrophilic attack of phenol ions on benzene and phenol, respectively. Finally, intriguing chemical paths involved in the corona discharge ionization of acetonitrile are also discussed.

Proton Exchanges between Phenols and Ammonia or Amines: A Computational Study

Density functional theory calculations at the B3LYP/6-31+G(d,p) level of theory have been performed to explore proton exchanges between phenols and ammonia or amines, which can be used to account for previous NMR experiments. For the parent phenol-NH(3) system, a transition state with a symmetric phenolate-NH(4)(+)-like structure, which lies about 35 kcal mol(-1) in energy above the hydrogen-bonded complex, has been successfully located. An intrinsic reaction coordinate (IRC) analysis indicates that the proton exchange is a concerted process, which can be roughly divided into four continuous subprocesses. A series of para-substituted phenol-NH(3) systems have been considered to investigate the substituent effect. Whereas introduction of an electron-withdrawing group on the phenol appreciably reduces the barrier, an opposite effect is observed for an electron-donating group. Moreover, it has been disclosed that there exists a good linear correlation between the activation barriers and the interaction energies between the phenols and NH(3), indicating the important role of proton transfer (or hydrogen bonding) in determining the proton exchange. Also considered are the proton exchanges between phenol and amines and those for some sterically hindered systems. The results show that the phenol tends to exchange hydrogen with the amines, preferably the secondary amines, and that the steric effect is favorable for the proton exchange, which imply that, as the IRC analysis suggested, besides the proton transfer, the flip of the ammonium-like moiety may play a significant role in the course of proton exchange. For all of these systems, we investigated the solvent effects and found that the barrier heights of proton exchange decrease remarkably as compared to those in a vacuum due to the ion pair feature of the transition state. Finally, we explored the phenol radical cation-NH(3) system; the barrierless proton transfer and remarkably low barrier (5.2 kcal mol(-1)) of proton exchange provide further evidence for the importance of proton transfer in the proton exchange.

Concerted proton-electron transfer in the oxidation of hydrogen-bonded phenols.

Three phenols with pendant, hydrogen-bonded bases (HOAr-B) have been oxidized in MeCN with various one-electron oxidants. The bases are a primary amine (-CPh(2)NH(2)), an imidazole, and a pyridine. The product of chemical and quasi-reversible electrochemical oxidations in each case is the phenoxyl radical in which the phenolic proton has transferred to the base, (*)OAr-BH(+), a proton-coupled electron transfer (PCET) process. The redox potentials for these oxidations are lower than for other phenols, predominately from the driving force for proton movement. One-electron oxidation of the phenols occurs by a concerted proton-electron transfer (CPET) mechanism, based on thermochemical arguments, isotope effects, and DeltaDeltaG(++)/DeltaDeltaG degrees . The data rule out stepwise paths involving initial electron transfer to form the phenol radical cations [(*)(+)HOAr-B] or initial proton transfer to give the zwitterions [(-)OAr-BH(+)]. The rate constant for heterogeneous electron transfer from HOAr-NH(2) to a platinum electrode has been derived from electrochemical measurements. For oxidations of HOAr-NH(2), the dependence of the solution rate constants on driving force, on temperature, and on the nature of the oxidant, and the correspondence between the homogeneous and heterogeneous rate constants, are all consistent with the application of adiabatic Marcus theory. The CPET reorganization energies, lambda = 23-56 kcal mol(-)(1), are large in comparison with those for electron transfer reactions of aromatic compounds. The reactions are not highly non-adiabatic, based on minimum values of H(rp) derived from the temperature dependence of the rate constants. These are among the first detailed analyses of CPET reactions where the proton and electron move to different sites.

50-fs Photoinduced Intramolecular Charge Separation in Triphenylmethane Lactones

Upon excitation of their structural subunits, phenolphthalein and malachite green lactone undergo ultrafast intramolecular charge separation with formation of a long-lived charge-transfer state (intramolecular “radical ion pair”). The kinetics of the primary charge separation in these molecules in aprotic solvents has been studied using femtosecond pump−probe spectroscopy. The phenol radical cation is formed by electron transfer within 50 fs after excitation of phenolphthalein to the S1 state. In malachite green lactone the products of charge separation are observed 150 fs after excitation to the S2 state. The results demonstrate that electron transfer in these molecules occurs faster than the time scale of inertial solvation dynamics. An intramolecular vibrational mode to promote charge separation is indicated.

One-electron oxidation of a hydrogen-bonded phenol occurs by concerted proton-coupled electron transfer.

The hydrogen-bonded phenol 2-(aminodiphenylmethyl)-4,6-di-tert-butylphenol (HOAr-NH2) was prepared and oxidized in MeCN by a series of one-electron oxidants. The product is the phenoxyl radical in which the phenolic proton has transferred to the amine, *OAr-NH3+. The reaction of HOAr-NH2 and tris(p-tolyl)aminium ([N(tol)3]*+) to give *OAr-NH3+ + N(tol)3 has Keq = 2.0 +/- 0.5, follows second-order kinetics with k = (1.1 +/- 0.2) x 105 M-1 s-1 (DeltaG = 11 kcal mol-1), and has a primary isotope effect kH/kD = 2.4 +/- 0.4. Oxidation of HOAr-NH2 with [N(C6H4Br)3]*+ is faster, with k congruent with 4 x 107 M-1 s-1. The isotope effect, thermochemical arguments, and the dependence of the rate on driving force (DeltaDeltaG/DeltaDeltaG degrees = 0.53) all indicate that electron transfer from HOAr-NH2 must occur concerted with intramolecular proton transfer from the phenol to the amine (proton-coupled electron transfer, PCET). The data rule out stepwise paths that involve initial electron transfer to form the phenol radical cation *+HOAr-NH2 or that involve initial proton transfer to give the zwitterion -OAr-NH3+. The dependence of the electron-transfer rate constants on driving force can be fit with the adiabatic Marcus equation, yielding a large intrinsic barrier: lambda = 34 kcal mol-1 for reactions of HOAr-NH2 with NAr3*+.

Phenol radical cations and phenoxyl radicals in electron transfer from the natural phenols sesamol, curcumin and trolox to the parent radical cations of 1‐chlorobutane

AbstractThe free electron transfer from sesamol, curcumin and trolox to solvent (1‐chlorobutane) radical cations was studied. The solutes (ArOH) react with BuCl⋅+ at diffusion‐controlled rates (∼1010 dm3 mol−1 s−1) resulting, simultaneously, in phenol‐type radical cations (ArOH⋅+) and phenoxyl radicals (ArO⋅) with an ArOH⋅+/ArO⋅ ratio &gt; 1;1. It was found that the kinetics are markedly affected by the different substituents on the aromatic ring of ArOH⋅+. The various ArOH⋅+ react with triethylamine with rate constants of ∼109 dm3 mol−1 s−1, whereas ethanol and dioxane are ineffective as cation scavengers. The effect of substituents on the electron transfer and the ArOH⋅+/ArO⋅ ratio is discussed on the basis of experimental data and quantum chemical calculations. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Antithetical product situation in the femtosecond and nanosecond photoionization of sterically hindered phenols in non-protic solvents

The photoionization of sterically hindered phenols in acetonitrile or n-butyl chloride solution gives apparently contradictory results performing biphotonic ionization with femtosecond as well as nanosecond UV lasers: in the first case phenol radical cations were found whereas in the latter one only phenoxyl radicals were detected. This is explained by non-resonant and resonant ionization pathways. Furthermore, the deprotonation rate of the phenol radical cations in acetonitrile in the presence of water traces could be determined as to be diffusion controlled ((3 to 7 )×10 9 dm 3 mol −1 s −1) which could not be observed till now because of its extreme p K a value of ≪−1.

Hydrogen Bonding of Phenols or Their Radical Cations with Water or Ammonia: Substituent Effects and the Influence on Phenol Oxidation

We did UB3LYP/6-31++g** and ROMP2/6-311++g** calculations on the hydrogen bonding of para-substituted phenols and their radical cations with water and ammonia. It was found that the magnitudes of the proton affinities increase in the order water (165 kcal/mol) < ammonia (204 kcal/mol) ≈ phenoxyl radicals (193−235 kcal/mol) < phenolate anions (321−352 kcal/mol). The slopes of the proton affinities against the substituent σp constants are about 22 and 15 kcal/mol for phenoxyl radicals and phenolate anions. It was also found that the slopes of the binding energies against the substituent σp constants decrease in the order phenol−water complex (1.1 kcal/mol) < phenol−ammonia complex (1.4 kcal/mol) < phenol radical cation−water complex (4.1 kcal/mol) < phenol radical cation−ammonia complex (9.3 kcal/mol). The structure of the substituted phenol radical cation−ammonia complex was found to rely on the proton affinity of the corresponding phenoxyl radical. When the proton affinity is larger than 214 kcal/mol, the non-proton-transferred form is the only minimum on the potential energy surface. When the proton affinity is smaller than 210 kcal/mol, the proton-transferred form is the only minimum. The only complex for which both the proton-transferred and non-proton-transferred forms are minima was found for p-hydroxylphenol radical cation. On the other hand, all the phenol radical cation complexes with water have the non-proton-transferred form as the only minimum on the potential surface. Hydrogen bonding to ammonia was found to lower the adiabatic oxidation potentials of phenols by 0.5−1.2 eV. Hydrogen bonding to water was found to lower the adiabatic oxidation potentials of phenols by 0.4−0.6 eV. In general, a phenol substituted with a more electron-withdrawing group shows larger reduction in the adiabatic oxidation potential when complexed to water or ammonia.

Monitoring of the Heterogroup Twisting Dynamics in Phenol Type Molecules via Different Characteristic Free-Electron-Transfer Products

Phenol radical cations as well as phenoxyl radicals were observed as direct products of free electron transfer from phenol type solute molecules to solvent parent radical cations generated by ionizing irradiation. It is shown that the finding of the two species in comparable amounts can be explained by a nuclear-structure dependent solute cation dissociation behavior: Quantum-chemical calculations indicate that for phenol as solute primarily its conformers with perpendicular C−OH axis orientation to the aromatic ring tend to prompt deprotonation after ionization. A quite similar behavior could be predicted also for the heteroanalogous thiophenols and selenophenols. Quite generally considerable changes in the electron distribution of the ground-state molecules with the twisting angle of the −OH, −SH and −SeH groups could be calculated, with the greatest differences between “parallel” and “perpendicular” conformations. On the assumption that the fast electron-transfer projects the equilibrium solute conformer distribution onto the solute cation conformer one it is demonstrated that the experimental findings are compatible with a simple solute-cation internal relaxation model. By applying the quantum-chemically calculated conformer interconversion barrier heights it can be understood that radical fraction among the direct products increases if phenols are substituted by thiophenols or those by selenophenols, as observed in experiment.

Nucleophilic effects on the deprotonation of phenol radical cations

Considering dependence on their electronic structure, in non-polar surroundings phenol radical cations exhibit lifetimes between 100 and 500 ns and decay by deprotonation forming phenoxyl radicals. Adding various quenchers of different nucleophilicity, we determined a set of rate constants for the acceleration of deprotonation i.e. the reaction of phenol radical cations with the nucleophiles. The data are basically discussed in terms of Reichardt's polarity scale, also considering effects like competing electron transfer aided by comparing the differences of calculated adiabatic ionization potentials as well as spin densities and local charges on heteroatoms for phenols and the quenchers.

Electron transfer from phenolic compounds to parent n-butyl chloride radical cations—a quantum chemical study of product transient formation and stability

The Density Functional Theory (DFT) B3LYP and the semiempirical PM3 quantum chemical methods were used to describe the mechanism of electron transfer from aromatic solutes to n-butyl chloride radical cations. The influence of the electronic parameters, the distribution of the Mulliken charges for the radical cations, and the equilibrium geometries of the ground state and the radical cation on the chemical reactivity were analyzed. Pulse radiolysis experiments had shown that phenol radical cations and phenoxyl radicals appear as direct products of the electron transfer, indicating a product distribution determined by encounter geometry. The quantum chemical approaches introduced enabled both the geometry, the energy and the molecular orbitals of an encounter complex to be calculated, explaining the rapid phenoxyl radical formation by a second reaction channel. The experimentally observed lifetimes of the phenol radical cations were correlated with quantum chemical calculated spin and Mulliken charge distributions. It was found that the influence of the solvent type approximated by the Onsager reaction field model and the DFT method showed no essential differences in the atomic spin and Mulliken charge distributions. The semiempirical PM3-calculated parameters tally well with those observed at the B3LYP/6-31G(d) level. Furthermore, a reliable trend correlation between the atomic spin density on the oxygen atom and the lifetime of the radical cations was found. The configuration interaction with single excitation approximation at the semiempirical PM3 level was applied to calculate the low-energy electronic transitions of the aromatic transients. Finally, the electronic transitions are calculated for both the radical cations and the deprotonated neutral radicals.

Stability of phenol and thiophenol radical cations – interpretation by comparative quantum chemical approaches

The deprotonation kinetics of phenol-type radical cations, formed via a very efficient electron transfer in the pulse radiolysis of non-polar solutions, for example n-chlorobutane, is governed mainly by electronic effects due to the nature of the phenol substituents, whereas steric effects are of minor importance; thiophenols, which are sulphur analogues of phenols, exhibit a similar behavior. Comparative quantum chemical calculations show that the calculated spin densities at the hetero atoms correlate well with the experimentally determined radical cation lifetimes. Not only the Density Functional Theory (DTF) B3LYP but also the semiempirical quantum chemical model PM3 can be applied for the open shell systems mentioned.

Free electron transfer from several phenols to radical cations of non-polar solvents

Electron-transfer reactions from phenols to parent radical cations of solvents were studied using pulse radiolysis. Phenols bearing electron-withdrawing, electron-donating and bulky substituents were investigated in non-polar solvents such as cyclohexane, n-dodecane, n-butyl chloride and 1,2-dichloroethane. The experiments revealed the direct, synchronous formation of phenoxyl radicals and phenol radical cations in all cases and in nearly the same relative amounts. This was explained by two competing electron-transfer channels which depend on the geometry of encounter between the parent solvent radical cations and the solute phenol molecules. The mechanism is analysed at a microscopic level, treating diffusion as a slow process and the local electron transfer as an extremely rapid event. Furthermore, the effect of various phenol substituents and solvent types on the electron-transfer mechanism and on the decay kinetics of the solute phenol radical cations was analysed. The results were further substantiated using a quantum chemical approach.