Radical Pair Mechanism Polarization Research Articles

Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair

The radical pair mechanism (RPM) of chemically induced dynamic electron polarization (CIDEP) is theoretically analyzed to determine what intermolecular separations (r eff) effectively contribute to the CIDEP generated from diffusive, separated radical-ion pairs (RIP) in terms of the chargetransfer interaction in the singlet-triplet energy splitting (J) by taking into account the distance-dependent electronic coupling and reorganization energy. The diffusion-model analysis reveals that the hyperfine-dependent RPM polarization (P RPM) phase is varied with the driving force (−ΔG CR) for the charge-recombination (CR) process and that the boundary −ΔG CR between the opposite phases coincides well with the total reorganization energy around the diffusible separation distance,r eff=1.2 nm, between the ion radicals. For the first time, ther eff is well described by the exponent parameter (β) in the distance-dependent electronic coupling, suggesting that the RPM CIDEP detection can be applied to characterize the electronic coupling in individual solvent-separated RIP systems. It has been concluded that, in contrast to the spin exchange interaction of the neutral radical pairs, the characteristic long-range charge-transfer interaction enables us to utilize the simple diffusion-model analysis to successfully evaluate ther eff and theP RPM in homogeneous liquid polar solvents.

Dynamics of Spin-Correlated Radical Pairs in Non-Ionic Surfactant Solutions

Time-resolved (CW) electron paramagnetic resonance spectroscopy (TREPR) has been used to investigate the diffusional dynamics of radical pairs created in micelles made from nonionic, poly(ethylene glycol) (PEG)-based surfactants (Brij-35, Triton X-100, and Cremophor EL). Photoexcitation of solubilized perdeuterated benzophenone (BP) at 308 nm leads to hydrogen atom abstraction by triplet BP from the surfactant chains. Spectral simulation and comparison to other PEG radicals shows that H-atom abstraction is taking place in the PEG outer shell of Brij-35 and Triton X-100 micelles rather than in the alkyl chain core. The TREPR spectra exhibit a temperature-dependent superposition of chemically induced electron spin polarization (CIDEP) patterns. In Brij-35 solutions, radical pair mechanism (RPM) multiplet polarization is predominant at room temperature, but strong spin-correlated radical pair (SCRP) polarization is observed at temperatures above 40 °C. In Triton X-100, the triplet mechanism (TM) dominates at all temperatures and delay times, with some evidence of S-T - SCRP polarization at longer delay times. The results are discussed qualitatively in terms of a spectral exchange model that uses spin product operators. Using known micellar dimensions, the diffusion coefficient of the radicals in the micelle interior and their escape rates can be estimated. Radicals of identical structure produced from pure PEG and aqueous PEG solutions show ordinary RPM polarization at all temperatures, and these spectra are used to confirm the photochemical mechanism and to provide characterization data (hyperfine couplings) for the radicals. The BP/Cremophor EL system was TREPR silent most likely because of triplet quenching impurities.

Photocleavage reaction of bromine substituted aromatic acyl compounds studied by CIDEP and transient absorption spectroscopy

The photocleavage of the CBr bond in bromoacetylnaphthalene is investigated by transient absorption and time resolved EPR spectroscopy. In the transient absorption of 2-bromo-2′-acetylnaphthalene, the absorption band observed at λmax ~440 nm is assigned to the triplet state of the parent molecule. After decay of the triplet absorption, a long lived absorption band is observed at λmax ~380 nm, which is assigned to naphthoylmethyl radical. The yield of this radical is not dependent on the concentration of oxygen even though the absorption band of the triplet state was quenched by addition of oxygen. Thus we conclude that the spin multiplicity of the precursor molecule is singlet. The CW time resolved EPR spectrum shows a typical E*/A CIDEP pattern of three hyperfine lines of the naphthoylmethyl radical. This result suggests some contribution from triplet precursor molecules. However, a careful analysis of the time profile of the CIDEP intensity observed by FT-EPR revealed that the polarization is generated from the radical pair mechanism (RPM) from the encountered pair of two free naphthoylmethyl radicals and the radical-triplet pair mechanism. RPM polarization by the geminate radical pair, formed by the Br atom and the naphthoylmethyl radical, is not observed. This fact indicates that large spin-orbit coupling (Δg and/or fast spin relaxation by g anisotropy) spoils the RPM polarization. The finding is in contrast to the recent observation of RPM polarization in the Cl cleavage reaction of 1-(chloromethyl)naphthalene.

Quantitative time-resolved EPR CIDEP study of the photodecomposition oftrans-azocumene in solution

The cumyl radical system, which is created after laser flash irradiation oftrans-azocumene in benzene solution at room temperature, is investigated using time-resolved EPR spectroscopy. From the quantitative analysis of EPR time-profiles at different microwave powers the spin relaxation timesT 1=3.5±0.3 μs andT 2=2.5±0.1 μs are evaluated as well as the magnitude of the chemically induced electron polarization (CIDEP), which is generated by the radical pair mechanism (RPM). The geminate RPM polarization is found to be considerably smaller than the F-pair one, 32±2 and 48±5 in units of the Boltzmann polarization, respectively. This is attributed to an initial radical separation in the geminate pair, caused by the cleavage reaction. Besides cleavage, the photoexcitedtrans-azocumene also decays via isomerization to the thermally unstablecis-isomer, the lifetime of which is found to be 14±3 μs at 293 K in benzene, three times longer than in cyclohexane. The quantum yield of free radicals, escaping from the primary cage, is determined as 0.28±0.06 for the decay of the excitedtrans-azocumene and 0.18±0.04 for the thermal cleavage of thecis-isomer. The self-termination of cumyl radicals proceeds with a rate constant 2k t=7±1)·108 M−1s−1 in benzene at RT.

A Time-Resolved CIDEP Study of the Laser Photoreduction of Pyruvic Acid by Lactic Acid

Abstract In addition to the well-known initial phototriplet polarization, unusual Chemically Induced Dynamic Electron Polarization (CIDEP) behavior in solutions of neat pyruvic acid/lactic acid has been attributed to ST_ Radical Pair Mechanism (RPM) polarization in combination with absorptive initial Triplet Mechamism (TM) polarization and ST0 RPM polarization. Significant ST_ RPM contributions to such carbon centered radicals with small hyperfine splittings have rarely been observer in solution at room temperature.

A time-domain EPR study of the SO−3 combination reaction: Radical pair CIDEP without singlet–triplet mixing

The radical ion ⋅SO−3 was studied by in situ pulsed radiolysis of aqueous alkaline sodium sulfite solutions in a pulsed EPR spectrometer. Initial radical concentrations on the order of 5×10−4 M were generated, and detailed measurements of the decay of ⋅SO−3 magnetization were performed on the microsecond time scale. It was found that the second order decay of both longitudinal and transverse magnetization lags behind the second order decay of radical population when the reaction rate approaches the relaxation rates 1/T1,2. The observations may be explained in terms of the spin-pairing requirements for formation of a singlet reaction product. The spin pairing gives rise to a ‘‘passive’’ form of radical pair mechanism (RPM) chemically induced dynamic electron polarization (CIDEP) which has previously gone unrecognized. RPM polarization therefore occurs in ⋅SO−3 encounters despite the lack of hyperfine dependent singlet–triplet mixing between radical reencounters, which is necessary for the accepted RPM mechanism. The implications of these observations for the interpretation of time-resolved EPR experiments are discussed.