Picosecond kinetic study of the photoinduced homolysis of benzhydryl acetates: the nature of the conversion of radical pairs into ion pairs

The conversion of benzhydryl acetate geminate radical pairs to contact ion pairs following photoinduced homolysis in solution is studied using picosecond pump-probe spectroscopy. The dynamics for the decay of the geminate radical pairs into contact ion pairs is modeled within a Marcus-like theory for nonadiabatic electron transfer. A second decay channel for the geminate radical pairs is diffusional separation to free radicals. The kinetics of this latter process reveals an energy of interaction between the two radicals in the geminate pair.     

Photo-Fries rearrangements of phenyl phenylacylates in polyethylene films: comparison of reactivity and selectivity with 1-naphthyl phenylacylates

The fates and kinetics of recombination of singlet radical pairs generated by photolyses of three phenyl phenylacylates have been examined in unstretched and stretched polyethylene films. Comparisons with results from photolyses of analogous 1-naphthyl phenylacylates in the same media lead to the conclusions that (1) phenoxy is less reactive overall than 1-naphthoxy toward a common phenylacyl radical but (2) the constrained cages in which the radical pairs reside exert greater control over the movements of the 1-naphthoxy/phenylacyl pairs. The reasons for these observations are discussed in the context of the shapes and van der Waals volumes of the radical pairs, the void volumes of sites in native polyethylene films, and the electronic properties of the aryloxy radicals.     

Enantioselective cyclization of alkene radical cations

[reaction: see text] Enantiomerically enriched beta-(diphenylphosphatoxy)nitroalkanes undergo radical ionic fragmentation, induced by tributyltin hydride and AIBN in benzene at reflux, to give alkene radical cations in contact radical ion pairs. These contact ion pairs are trapped intramolecularly by amines to give pyrrolidines and piperidines with significant enantioselectivity ( approximately 60% ee), indicative of cyclization competing effectively with equilibration within the ion pairs. Use of an intramolecular N-propargylamine as a nucleophile provides an enantiomerically enriched pyrrolizidine skeleton via a tandem polar/radical crossover sequence.     

Photo-oxidation of diglycine in confined media Relaxation of longitudinal magnetization in spin correlated radical pairs

Time-resolved electron paramagnetic resonance spectra (X-band) of correlated radical pairs created in AOT reverse micelles are presented and simulated using the microreactor model. They are discussed in terms of the two-site model with a particular emphasis on longitudinal relaxation mechanisms. The geminate radical pair is created by photo-oxidation of dyglicine by the excited triplet states of an anthraquinone salt. The strong chemically induced electron spin polarization observed is due to three mechanisms: TM, RPM, and SCRPM. Relative contributions from these mechanisms depend on the water pool volume and the time of observation. There are three types of longitudinal relaxation in radical pairs. The first is relaxation of the RPM induced longitudinal magnetization in spin correlated radical pairs. The second is the longitudinal relaxation in radical pairs which are not correlated (with a zero value of the double quantum coherence). In such pairs, the generation of longitudinal magnetization due to RPM is impossible, and the spin-selective recombination of the pairs is ineffective. Under all experimental conditions, the first type of relaxation is slower than the second type. For both, the physical mechanism leading to relaxation is modulation of the Heisenberg electron spin exchange interaction. This is an internal relaxation process. The third relaxation type occurs in radical pairs due to ordinary longitudinal relaxation in non-interacting radicals. Normally, relaxation of the third type is the slowest of the three. This explains time and micelle size dependence of the relative contribution of RPM into TREPR spectra. It seems reasonable to suggest that the creation of the initial spin state populations is partially adiabatic.     

Determination of radical re-encounter probability distributions from magnetic field effects on reaction yields

Measurements are reported of the effects of 0-23 mT applied magnetic fields on the spin-selective recombination of Py*- and DMA*+ radicals formed in the photochemical reaction of pyrene and N,N-dimethylaniline. Singlet <--> triplet interconversion in [Py*- DMA*+] radical pairs is probed by investigating combinations of fully protonated and fully deuterated reaction partners. Qualitatively, the experimental B1/2 values for the four isotopomeric radical pairs agree with predictions based on the Weller equation using known hyperfine coupling constants. The amplitude of the "low field effect" (LFE) correlates well with the ratio of effective hyperfine couplings, aDMA/aPy. An efficient method is introduced for calculating the spin evolution of [Py*- DMA*+] radical pairs containing a total of 18 spin-1/2 and spin-1 magnetic nuclei. Quantitative analysis of the magnetic field effects to obtain the radical re-encounter probability distribution f (t )-a highly ill-posed and underdetermined problem-is achieved by means of Tikhonov and maximum entropy regularization methods. The resulting f (t ) functions are very similar for the four isotopomeric radical pairs and have significant amplitude between 2 and 10 ns after the creation of the geminate radical pair. This interval reflects the time scale of re-encounters that are crucial for generating the magnetic field effect. Computer simulations of generalized radical pairs containing six spin-1/2 nuclei show that Weller's equation holds approximately only when the radical pair recombination rate is comparable to the two effective hyperfine couplings and that a substantial LFE requires, but is not guaranteed by, the condition that the two effective hyperfine couplings differ by more than a factor of 5. In contrast, for very slow recombination, essentially any radical pair should show a significant LFE.     

Radical pair in crystalline dibenzoyl peroxide evidence for triplet ground states

Zero-field splitting (zfs) tensors and g tensors are reported for two phenyl-benzoyloxyl radical pairs (PB5K and PB15K) and two benzoyloxyl-benzoyloxyl pairs (BB10K and BB25K) in photolyzed single crystals of dibenzoyl peroxide. The g tensors show that in three of the pairs a benzoyloxyl radical has undergone in-plane rotation by about 40°. In PB15K this motion relieves steric repulsion from a CO₂ molecule, and it is suggested that motion in the BB pairs relieves repulsion between radical oxygens in a ground-state triplet radical pair. Spin-orbit coupling is necessary to explain the zfs of PB5K. This explanation requires that the triplet of the electronically excited radical pair lie below the corresponding singlet by an amount which is significant in comparison to the excitation energy. The source of singlet-triplet splitting is discussed briefly and simple VB calculations are shown not to support the above interpretation, although the STO3G and 4-31G basis sets employed are very likely inadequate. Atomic coordinates are reported for crystalline dibenzoyl peroxide.     

Marked influences on the adenine-cytosine base pairs by electron attachment and ionization

The reverse wobble and the reverse Hoogsteen adenine-cytosine mispairs regarding their radical cations and anions are studied with the hybrid three-parameter B3LYP density functional method and 6-31+G(d), 6-311+G(2df,2p) basis sets. Hydrogen bonding mispairs are remarkably influenced by electron attachment and ionization. Only one stronger hydrogen bond N6-H (in adenine)...N3 (in cytosine) exists in the radical pair, while the strengths of two N-H...N hydrogen bonds in the neutral pair are comparable. Geometrical coplanarity is found for the neutral and cationic pairs, in contrast to the anionic pairs in which the cytosine moiety exhibits significant deformation due to electron attachment. Dissociation energies for the neutral and radical pairs are slightly higher than those of the adenine-thymine pairs but much smaller than those of the guanine-cytosine pairs. Valence-bound anions of these two adenine-cytosine pairs are thermodynamically stable by 0.1-0.2 eV with respect to the neutral pairs. On the basis of the comparison between the experimental data of the solvated clusters and the calculated values, these two pairs can be quantitatively equivalent to the clusters in which each base is solvated by five water molecules.     

Chemically Induced Dynamic Nuclear Polarization VIII: Thermal decomposition of dibenzoyl peroxides in solution [1]

Thermolysis of di-(4-chlorobenzoyl)-peroxide was carried out in the probe of an NMR. spectrometer. The dependence of product distribution and chemically induced dynamic nuclear polarization (CIDNP.) patterns on the concentrations of radical trapping agents gives evidence that the CIDNP. effects are caused by singlet-triplet transitions, dependent on nuclear spin, in transient benzoyloxy/phenyl radical pairs and by the selective formation of radical products from singlet state pairs.     

The Origin of Magnetic Field Dependent Recombination in Alkylcobalamin Radical Pairs

Magnetic field effect studies of alkylcobalamin photolysis provide evidence for the formation of a reactive radical pair that is born in the singlet spin state. The radical pair recombination process that is responsible for the magnetic field dependence of the continuous-wave (CW) quantum yield is limited to the diffusive radical pair. Although the geminate radical pair of adenosylcob(III)alamin also undergoes magnetic field dependent recombination (A. M. Chagovetz and C. B. Grissom, J. Am. Chem. Soc. 115, 12152–12157, 1993), this process does not account for the magnetic field dependence of the CW quantum yield that is only observed in viscous solvents. Glycerol and ethylene glycol increase the microviscosity of the solution and thereby increase the lifetime of the spin-correlated diffusive radical pair. This enables magnetic field dependent recombination among spin-correlated diffusive radical pairs in the solvent cage. Magnetic field dependent recombination is not observed in the presence of nonviscosigenic alcohols such as isopropanol, thereby indicating the importance of the increased microviscosity of the medium. Paramagnetic radical scavengers that trap alkyl radicals that escape the solvent cage do not diminish the magnetic field effect on the CW quantum yield, thereby ruling out radical pair recombination among randomly diffusing radical pairs, as well as excluding the involvement of solvent-derived radicals. Magnetic field dependent recombination among alkylcobalamin radical pairs has been simulated by a semiclassical model of radical pair dynamics and recombination. These calculations support the existence of a singlet radical pair precursor.     

The solvent cage effect: is there a spin barrier to recombination of transition metal radicals?

This investigation explored whether there is a spin barrier to recombination of first- and second-row transition metal-centered radicals in a radical cage pair. To answer this question, the recombination efficiencies of photochemically generated radical cage pairs (denoted as FcP) were measured in the presence and absence of an external heavy atom probe. Two methods were employed for measuring the cage effect. The first method was femtosecond pump-probe transient absorption spectroscopy, which directly measured FcP from reaction kinetics, and the second method (referred to herein as the "steady-state" method) obtained FcP from quantum yields for the radical trapping reaction with CCl4 as a function of solvent viscosity. Both methods generated radical cage pairs by photolysis (lambda = 515 nm for the pump probe method and lambda = 546 nm for the steady-state method) of Cp'2Mo2(CO)6 (Cp' = eta(5)-C5H4CH3). In addition, radical cage pairs generated from Cp'2Fe2(CO)4 and Cp*2TiCl2 (Cp* = eta(5)-C5(CH3)5) were studied by the steady-state method. The pump-probe method used p-dichlorobenzene as the heavy atom perturber, whereas the steady-state method used iodobenzene. For both methods and for all the radical caged pairs investigated, there were no observable heavy atom effects, from which it is concluded there is no spin barrier to recombination.     

光诱导反应机理研究 ⅩⅦ.ESR波谱法研究多氯甲烷对降冰片二烯的光敏化加成反应机理

在以前的研究工作中^[1]我们已经证明芳胺可以有效地光敏化多氯甲烷对双环[2,2,1]-2,5-庚二烯的加成反应。该反应具有百分之百的区域选择性,在我们所选择的实验体系中只得到了3.5-加成物,而没有2.3-加成物。这一结果不同于典型的热诱导自由基加成反应^[2]。     

EPR spectral and laser photolysis study of radical pairs and their annihilation in quinone-doped pyrocatechol monocrystals

Conclusions The possibility of studying radical pairs in monocrystals by laser photolysis has been demonstrated. Comparison of the optical absorption spectra and EPR signals indicated that the radical pairs in 3,6-di-tert-butylpyrocatechol monocrystals with added 3,6-di-tertbutyl-o-benzoquinone at 77 K are comprised from the corresponding semiquinone radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1166–1167, May, 1988.     

Measurement of rate processes of free radicals in homogeneous and micellar solutions by cidnp

CIDNP is used to study rate processes of free radicals in both homogeneous and micellar solution. An estimate of the lifetime of the phenyl-acetyl radical at ambient temperature (τ__co?10^?7 sec) produced during photolysis of dibenzyl ketone is made based on quantitative CIDNP measurements and computer simulations. Observation of CIDNP in micellar solution is shown to be consistent with an isotropic medium which restricts diffusion on a short time scale, allowing for an increased tendency toward cage reaction. In the case of t-butyl/pivaloyl radical pairs, escape of the radical fragments from the micelle is shown to be competitive with decarbonylation of the pivaloyl radical Likewise, CIDNP is consistent with product yield results which show the enhanced tendency of triplet born benzyl radical pairs to undergo cage reaction when they are sequestered in a micelle.     

On the question of reversibility in the intramolecular addition of radicals to aldehydes to form cycloalkanols

Experiments with two pairs of epimeric alcohols establish conclusively that the intramolecular addition of a primary radical to an aldehyde to give a cycloalkanoxyl radical is not a reversible process.     

Production of free radicals and triplets from contact radical pairs and from photochemically generated radical ions

The quantum yields of triplets and free radicals (or radical ions) that escaped recombination in photochemically created primary radical pairs (or radical ion pairs) are calculated. As the products of monomolecular photodissociation, the neutral radicals appear at contact, while the ions are initially distributed over the space due to distant photoionization (bimolecular electron transfer) in the liquid solution. The diffusional dependence of the quantum yields is shown to be different when recombination starts from contact or from separated reactants. The experimental data for recombination of ionized perylene with aromatic amine counterions is well fitted with the noncontact initial distribution provided the recombination is also noncontact and even more distant than ionization.     

ESR and cidep studies of the photochemical processes in benzoyl formic acid single crystal

The photolysis of a single crystal of benzoyl formic acid at 77 K yielded the ESR observation of three trapped radical pairs, and a transient CIDEP of the diffused ketyl radical.     

Formation of radical pairs in the photolysis of 2, 6-di(tert-butyl)-l,4-benzoquinonediazide in single crystals of 2,6-di(tert-butyl)-4-methylphenol

In the photolysis of single crystals of 2,6-di(tert-butyl)-4-methylphenol with the addition of 2,4-di(tert-butyl)-l,4-benzoquinonediazide it was found that they form carbenes and radical pairs with r₁=6.0 å, which are secondary products of the photolysis of the quinonediazide and which form as a result of transfer of a hydrogen atom from the phenol to the carbene. The high thermal stability of these radical pairs makes it possible to use them as two-spin probes. Also found were radical products that are annealed by light with wavelength greater than 560 nm and are hypothetically classified as ion-radical pairs with r_av=8.9 å.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 560–564, March, 1991.     

Frustrated Radical Pairs: Insights from EPR Spectroscopy

Progress in frustrated Lewis pair (FLP) chemistry has revealed the importance of the main group elements in catalysis, opening new avenues in synthetic chemistry. Recently, new reactivities of frustrated Lewis pairs have been uncovered that disclose that certain combinations of Lewis acids and bases undergo single‐electron transfer (SET) processes. Here an electron can be transferred from the Lewis basic donor to a Lewis acidic acceptor to generate a reactive frustrated radical pair (FRP). This minireview aims to showcase the recent advancements in this emerging field covering the synthesis and reactivities of frustrated radical pairs, with extensive highlights of the results from Electron Paramagnetic Resonance (EPR) spectroscopy to explain the nature and stability of the different radical species observed.