Chemically induced electron polarization CIDEP

Blattler C, Jent F and Paul H 1990 A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution Chem. Phys. Lett. 166 375-80... 

Recording of the ESR spectrum under simultaneous irradiation of the sample by an electron beam was used in the first experiments [7], In later studies pulsed ESR was used in pulse radiolysis experiments [8]. This detection method has a high time resolution of ca. 10 ns for studies of reaction kinetics. Radicals formed by steady state photolysis were also studied at an early stage. Pulsed ESR is commonly used with laser flash photolysis to obtain high time resolution in modern applications [9]. For studies of radicals formed in chemical reactions a common method is by mixing of the reagents in a flow system before entering the ESR cavity. In these in sim experiments the ESR lines can occur in emission rather than absorption because the radicals are not in thermal equilibrium immediately after formation. A chemically induced electron polarization, CIDEP, occurs. 

Spin dynamic studies, including saturation and inversion recovery, Hahn echo and stimulated echo decay, as well as Carr-Purcell-Meiboom-Gill sequences, can be performed, yielding relaxation times. When rapid reaction kinetics are being measured, information is also gained on transient phenomena such as chemically induced electron polarization (CIDEP). 

Time-resolved laser flash ESR spectroscopy generates radicals with nonequilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963, are known as chemically induced dynamic electron polarization (CIDEP). Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively. ... 

A related technique is called chemically induced dynamic electron polarization (CIDEP). For a review, see Hore Joslin McLauchlan Chem. Soc. Rev, 1979, 8, 29-61. 

The first discovery of chemically induced dynamic electron polarization (CIDEP) was made by Fessenden and Schuler in 1963 (58). These authors observed the abnormal spectra of the H atoms produced during the irradiation of liquid methane. The low-field line in the esr spectrum was inverted compared to the corresponding high-field line. The related chemically induced dynamic nuclear polarization effect (CIDNP) was reported independently four years later by Bargon et al. (22) and by Ward and Lawler (134). Because of the wider application of nmr in chemistry, the CIDNP effect immediately attracted considerable theoretical and experimental attention, and an elegant theory based on a radical-pair model (RPM) was advanced to explain the effect. The remarkable development of the radical-pair theory has obviously brought cross-fertilization to the then-lesser-known CIDEP phenomenon. 

A study of chemically induced dynamic electron polarization, CIDEP (see Section 12.3.3) on F and G pairs of radicals formed under photolysis of a common termo- and photoinitiator 2,2 -azobis(2-methylpropionitrile) (AIBN) led to a tentative conclusion that initial spatial separation of 2-cyano-2-propyl radicals does not depend upon viscosity However, it is plausible that the diamagnetic dinitrogen molecule formed under photolysis of AIBN (and is invisible by ESR) separates further from a contact RP under photolysis in solvents of lower viscosity. The problem of initial spatial separation and mutual orientation ofradicals under photolysis still waits experimental elucidation. 

Laser flash photolysis of Pis in the cavity of an ESR spectrometer is often accompanied by chemically induced dynamic electron polarization, CIDEP, that is, by the formation... 

While it is difficult to ensure the rehable observation of the radical processes during the mixing of reagents, these are easily detected in the photoinduced reactions of aryl-substituted compounds of Ge and Sn. Laser pulse photolysis experiments show that direct photoionization of Ph3Ge(Sn) anion results in the neutral radical . The apphcation of the Chemically Induced Dynamic Electron Polarization (CIDEP) method has allowed the detection of polarized emission signal of the radicals, thus leading to a conclusion that direct photoionization of Ph3Ge(Sn) anion occurs from the triplet state (equation 7). ... 

In 1963, Fessenden and Schuler [1] found during irradiation of liquid methane (CH4 and CD4) at 98 K with 2.8 MeV electron that the low-field signals (al and bl) for both hydrogen and deuterium atoms appeared inverted (emissive signals) and that the central deuterium atom signal (b2) was very weak as shown in Fig. 4-1. Although the cause of such anomalous ESR spectra was not clear at that time, similar anomalous ESR signals have been observed in many reactions and have been called Chemically Induced Dynamic Electron Polarization (CIDEP)". CIDEP should be due to non-equilibrium electron spin state population in radicals. 

Chemically Induced Dynamic Electron Polarization (CIDEP)... 

Appropriate modification of the ESR spectrometer and generation of free radicals by flash photolysis enables time-resolved (TR) ESR spectroscopy [22]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They can be enhanced as much as one-hundredfold and appear as absorption, emission, or a combination of both. Effects of this type are a result of chemically induced dynamic electron polarization (CIDEP) these spectra indicate the intermediacy of radicals whose sublevel populations deviate substantially from equilibrium populations. Significantly, the splitting pattern characteristic of the spin-density distribution of the intermediate remains unaffected thus, the CIDEP enhancement not only facilitates the detection of short-lived radicals at low concentrations, but also aids their identification. Time-resolved ESR techniques cannot be expected to be of much use for electron-transfer reactions from alkanes, because their oxidation potentials are prohibitively high. Even branched alkanes have oxidation potentials well above the excited-state reduction potential of typical photo-... 

Laser-photoinduced electron transfer in the three tetraphenylporphyrin-bound viologens 34—36 in reversed micelles led to radical pairs whose chemically induced dynamic electron polarization (CIDEP) spectra at room temperature proved dependent on the length of the spacer. 37... 

Although EPR signals related to hydrocarbon cations radicals generated by electrochemical oxidation or chemical oxidation can be readily detected, only a few examples have been reported for cation radicals that are produced by irradiation of solutions of electron donors and an acceptor. Because electron spin polarization offers the advantage of detecting transient species via their EPR signal intensities, chemically induced dynamic electron polarization (CIDEP) spectra can give information not only about short-lived radical intermediates... 

Electron Spin Resonance. ESR and CIDEP (chemically induced electron spin polarization) studies have been reported for radicals derived from phthalazine and related diazanaphthalenes. ° ... 

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

CIDEP = Chemically Induced Dynamic Electron Polarization. For a review see F. J. Adrian, in Chemically Induced Magnetic Polarization, edited by L T. Muus, P. W. Atkins, K. A. McLauchlan, and J. B. Pedersen (Reidel, Dordrecht, 1977), Chap. 5, pp. 77-105. 

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