Chemically Induced Electron Polarization Research Articles

Analysis of EPR-time profiles of transient radicals with unresolved spectra

A method for analysing EPR-time profiles of transient radicals in solution with unresolved hyperfine structure is proposed. It is based on considering the magnetic field integral of the magnetization, i.e., the total EPR signal intensity, instead of single components of overlapping EPR transitions. For a radical system involving chemical kinetics, chemically induced electron polarization (CIDEP), and spin relaxation, an analytical solution is found for the evolution of the integral magnetization in the Laplace domain. The solution in the time domain is given for the case of negligible saturation, i.e., ω 1 2 T 1 T 2≪1. The formulae presented are suitable to avoid equivocal multi-parameter fits when analysing the results of time-resolved continuous-wave EPR experiments for the observables, which characterize the chemical kinetics, CIDEP, and electron spin relaxation of radical systems.

Electron Spin Polarization after Photolysis of AIBN in Solution: Initial Spatial Radical Separation

Transient 2-cyano-2-propyl radicals are generated by laser flash photolysis of 2,2‘-azobisisobutyronitrile (AIBN) in solvents of various viscosities. Their spin magnetization is followed by time-resolved EPR spectroscopy and analyzed in terms of modified Bloch equations accounting for chemical reaction, Heisenberg spin exchange, and rise and decay of chemically induced electron polarization (CIDEP). The results are: (i) The cross-section for spin exchange exceeds that for radical termination by a factor of 1.8. (ii) No geminate net polarization is found, implying that eventually formed primary diazenyl radicals decay on the picosecond time scale. (iii) The ratio of F-pair and geminate pair polarization increases with viscosity from |PF/PG| = 1 at η = 0.6 cP to |PF/PG| = 4.7 at η = 18 cP. According to the diffusion theory of CIDEP, this means that the initial spatial distance ri of the radicals escaping the geminate cage is independent of the viscosity. This finding is at variance with the cage effect, wh...

DNP and CIDEP Study of Cross-Relaxation Processes in Short-Lived Radicals in Solution

Dynamic nuclear polarization (DNP) and chemically induced electron polarization (CIDEP) of transient radicals in solution are measured at L- and X-band frequencies to investigate their electron−nuclear cross-relaxation. It is demonstrated experimentally and theoretically, that a strong multiplet-type electron spin polarization leads to a multiplet-type DNP spectrum with the same relative line intensities as they are observed in the CIDEP spectrum. DNP spectra are found in cases such as this to be separable from the always superimposed SNP (stimulated nuclear polarization) spectra via measurements at different microwave powers. Examination of radicals (CH3)2XC• with X = CH3, OH, D, and C(O)CH3 shows the existence of efficient electron−nuclear flip-flop (Δm = 0) cross-relaxation for t-butyl and 2-hydroxy-2-propyl radicals, whereas for 2-acetyl-2-propyl radicals DNP is absent. The results indicate that phase inversions, observed in time-resolved CIDEP spectra of t-butyl and 2-hydroxy-2-propyl radicals, are a...

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.

CIDEP measurement of the electron spin-lattice relaxation time of triplet benzophenone in liquid solution

The relaxation of chemically induced electron polarization (CIDEP) in photoexcited triplet benzophenone is measured in competition with triplet quenching by 2,2′-azobis[isobutyronitrile] (AIBN). The spin polarization, transferred to the resulting 2-cyano-2-propyl radicals, is determined by time-resolved ESR spectroscopy for different AIBN concentrations. The rate constant for quenching is found to be k Q = 4.4 × 10 9 M −1 s −1, and 3 T 1 = 0.8 ns is obtained for the electron spin-lattice relaxation time of triplet benzophenone in 1,2-epoxypropane at room temperature.

Electron spin polarization in radical-triplet pairs. Size and dependence on diffusion

Using laser flash irradiation and time-resolved EPR spectroscopy, the chemically induced electron polarization (CIDEP) is investigated, which is generated by interaction of persistent 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals with triplet excited benzophenone in 1,2-epoxypropane solution. Net emission superposed by a weak emission/absorption multiplet type polarization in the TEMPO radical system is observed and analyzed quantitatively at temperatures between 193 and 298 K. With decreasing temperature the relative diffusion coefficient of the particles decreases from 5.9×10 -5 to 0.9×10 -5 cm 2 /s, and the absolute values of net and multiplet CIDEP increase from 0.6 to 9.0 and from 0.07 to 0.26, respectively, in units of Boltzmann polarization

A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution

Transient alkyl radicals and triplet-state molecules are generated simultaneously in solution by laser flash irradiation, and chemically induced electron polarization (CIDEP) of the radicals is studied by time-resolved ESR spectroscopy. The presence of triplet-state molecules is found to induce emissive ESR polarizations. The phenomenon is attributed to triplet-state quenching by radicals as well as to quartet-doublet mixing and splitting in radical-triplet encounters.

Electron spin polarization of free radicals induced by electron-nuclear cross relaxation

Transient radicals •C (CH 3) 2X are generated in solution by laser flash photolysis, and chemically induced electron polarization (CIDEP) is investigated by time-resolved ESR spectroscopy. The sign reversal of multiplet polarization at longer times, observed for t-butyl and 2-hydroxy-2-propyl radicals is studied as a function of temperature and solvent and is explainable by efficient electron-nuclear cross relaxation due to modulation of the nonplanarity at the radical centre.

Cross relaxation in free radicals with chemically induced electron and nuclear polarization

Electron-nuclear cross relaxation in free radicals with chemically induced nuclear polarization (CIDNP) is discussed in the light of some unexpected chemically induced electron polarization (CIDEP) effects. Sign rules are deduced for the transfer of net CIDNP to net CIDEP and multiplet CIDNP to multiplet CIDEP. Experiments indicating that cross relaxation is not responsible for the unusual CIDEP observations are described.

On the theory of chemically induced electron polarization (CIDEP). II. Potential forced diffusion

A formula is developed for the spin polarization developed by radicals simultaneously diffusing in a spherically symmetric potential field and precessing in a magnetic field; general initial conditions are included, and two boundary conditions: zero rate and infinite rate of reaction at the point of contact. The special case of simple diffusion and infinite rate of reaction is also discussed and compared to the unreacting case treated in the first paper of this series. Asymptotic formulas are given that allow a simplification of the formulas when the exchange forces are very large. This takes the form of multiplying the spin polarization for simple diffusion by simple functions of the potential, allowing a very easy estimate of the effect of potential forced diffusion.

On the theory of chemically induced electron polarization (CIDEP): Vector model and an asymptotic solution

It is shown that the stochastic–Liouville model of CIDEP can be cast into the form of a ’’Bloch-type’’ equation with diffusion. This leads to a generalized vector model of the radical pair mechanism of chemically induced magnetic polarization, which gives a clear picture of the qualitative features of both CIDNP and CIDEP. For the case of simple Brownian motion of the two radicals the differential form of the stochastic–Liouville equations of CIDEP is readily converted into a single integral equation. In the limit of slow singlet–triplet mixing by the magnetic spin Hamiltonian, and with an exchange interaction that decays exponentially with radical separation, this integral equation may be solved exactly. The electron polarization then takes the form of a superposition of an infinite number of ’’contact exchange modes’’.

ChemInform Abstract: CHEMICALLY INDUCED ELECTRON POLARIZATION PART 6, DEPENDENCE OF SPIN POLARIZATION OF 1,4‐BENZOSEMIQUINONE AND 9,10‐ANTHRASEMIQUINONE RADICALS ON THE ORIENTATION OF THE EXCITING POLARIZED LIGHT

Chemischer InformationsdienstVolume 6, Issue 30 Preparative Organic Chemistry ChemInform Abstract: CHEMICALLY INDUCED ELECTRON POLARIZATION PART 6, DEPENDENCE OF SPIN POLARIZATION OF 1,4-BENZOSEMIQUINONE AND 9,10-ANTHRASEMIQUINONE RADICALS ON THE ORIENTATION OF THE EXCITING POLARIZED LIGHT B. B. ADELEKE, B. B. ADELEKESearch for more papers by this authorK. Y. CHOO, K. Y. CHOOSearch for more papers by this authorJ. K. S. WAN, J. K. S. WANSearch for more papers by this author B. B. ADELEKE, B. B. ADELEKESearch for more papers by this authorK. Y. CHOO, K. Y. CHOOSearch for more papers by this authorJ. K. S. WAN, J. K. S. WANSearch for more papers by this author First published: July 29, 1975 https://doi.org/10.1002/chin.197530158Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume6, Issue30July 29, 1975 RelatedInformation

Chemically induced electron spin polarization in electron-transfer reactions

Chemically induced electron polarization (CIDEP) has been observed for the durosemiquinone radical anion generated in the flash photolysis of solutions of duroquinone in the presence of various amines. The initial polarization has been measured directly by using a fast response time-resolved E.S.R. spectrometer. The magnitude of polarization is shown to depend on amine concentration and identity, and the solvent medium. Conventional nanosecond flash photolysis has been used to measure duroquinone triplet lifetimes under various conditions. The results are discussed in terms of the triplet mechanism and the radical pair mechanism.