CIDEP effect

Deviations from Boltzmann intensities were first noted in 1963 [72] they are known as CIDEP [73,74]. These effects can be caused by two different mechanisms, each giving rise to CIDEP spectra with different characteristic intensity patterns. CIDEP effects can have two sources they may be transferred to the intermediates from their immediate precursor, typically a triplet state (triplet mechanism, TM), or generated early during their existence (radical pair mechanism, RPM) [75]. 

Direct recording of the kinetics on an absolute time scale can be achieved by the time-resolved e.s.r. method. However, special attention has to be paid to CIDEP effects which may distort the observation of chemical kinetics. When the rate of radical decay is... 

Ag of the primary radical pair is usually very small, and therefore a pure net CIDEP effect due to the radical-pair mechanism... 

The Theory of the CIDEP Effect. — Applications to the Study of Chemical Reactions and Magnetic Properties. The Chemically Induced Dynamic Electron Polarization (CIDEP Effect). 

Photoionization. - 2.1.1 Sulfite anion. The photoionization of the sulfite anion SOf was studied by Fessenden et a few years ago. The steady-state EPR spectra of the hydrated electron and the sulfite radical-anion SOs show no CIDEP effects and indicate the lifetime of the hydrated electron to be about 100 ps. The sulfite radical-anion is often used as g factor standard in photolysis EPR experiments [ [(SOs") = 2.00316]. Bussandri et al. studied the laser photolysis of sulfite ions in basic solution by FT EPR with very high time resolution. They observed the FT EPR spectra of hydrated electrons and sulfite radical-anions with absorption/emission (A/E) pattern caused by the radical pair mechanism (RPM CIDEP) with the electron line in emission [fif(eaq ) = 2.00044 at room temperature] and the sulfite radical-anion in absorption. In the time profiles of both lines, oscillations of the EPR intensities were observed in the first 300 ns. This coherent oscillation in both radicals is the first direct EPR observation of zero quantum coherence in freely diffusing radicals. Previously zero... 

The photoionization of phenol, p-cresol and tyrosine in basic aqueous solution was investigated by CW TR EPR and FT EPR." " These studies are of interest because it is expected that the question of the spin multiplicity of the reactive state can be answered by analysing CIDEP effects. In basic aqueous solution (pH > 10), the photoexcitation of phenolates initiates the electron ejection reaction ... 

Photo-induced Electron Transfer. Electron transfer is one of the most fundamental and widespread reactions in nature and has been extensively studied. In addition to the optical absorption spectroscopy widely used, TR EPR has become established as an appropriate method to study electron-transfer processes. In most of these investigations CIDEP effects are observed. The spin-polarization effects originate in the spin selectivity of chemical and physical processes involved in free-radical formation and decay, as well as in the spin-state evolution in transient paramagnetic precursors. For this reason, CIDEP constitutes a unique probe of the mechanistic details of electron-transfer processes. 

Experimental investigations of the CIDEP effects and their interpretations have been reported for the systems benzil/various amines, aromatic ketones/triethylamine, " 2-chloroxanthone in 2-propanol,benzo-phenone and anthracenequinone in micellar solutions, sesamol in aqueous solution, and hydrogen peroxide with dimethyl sulfoxide. ... 

Spin-polarized mobile hydrogen and deuterium atoms were detected in silica glass containing 1200 ppm of OH or OD groups by the time-resolved pulsed EPR technique.The EPR spectra of the H/D atoms indicate the occurrence of CIDEP in reactions of H/D atoms with radiolytically induced metastable spin centres. The CIDEP effects in silica glass exhibit unusual polarization patterns, temperature dependence and dependence on hfs constants. A Monte Carlo lattice model is introduced to simulate the RPM spin polarization in amorphous silica. Although the model does not explain all the features observed, it indicates where peculiarities of CIDEP in disordered solids may originate. 

TR EPR is a powerful experimental technique in studying transient paramagnetic species on the nanosecond and microsecond time scales. The time-resolved EPR spectra of transient free radicals and radical-pairs are mostly influenced by spin polarization effects. These CIDEP effects provide unique experimental evidence of radical precursors and dynamic processes in primary radical pairs. The application of TR EPR to various photochemical and biochemical problems... 

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