Dynamic electron polarization

Pedersen J B and Freed J H 1973 Theory of chemically induced dynamic electron polarization. I J. 

Wong S K, Hutchinson D A and Wan J K S 1973 Chemically induced dynamic electron polarization. II. A general theory for radicals produced by photochemical reactions of excited triplet carbonyl compounded. Chem. Phys. 58 985-9... 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

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. 

Time-Resolved Chemically Induced Dynamic Electron Polarization and Optical Emission Studies... 

Appropriate modifications of the ESR spectrometer and generation of free radicals by flash photolysis allow time-resolved (TR) ESR spectroscopy [71]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They may be enhanced as much as one hundredfold and may appear in absorption, emission, or in a combination of both modes. These spectra indicate the intermediacy of radicals with substantial deviations from equilibrium populations. Significantly, the splitting pattern characteristic for the spin density distribution of the intermediate remains unaffected thus, the CIDEP (chemically induced dynamic electron polarization) enhancement facilitates the detection of short-lived radicals at low concentrations. 

CIDEP chemically induced dynamic electron polarization... 

There is a related effect seen in the EPR spectra of products arising from radical pair reactions. This effect is known as chemically induced dynamic electron polarization, or... 

CIDEP (Chemically Induced Dynamic Electron Polarization) Non-Boltzmann electron spin state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism), and detected by ESR spectroscope... 

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. 

By the late 1960s, more experimental observations of partial emission in esr spectra were reported. By the early 1970s, totally emissive esr spectra were reported (13,95,96,135) in photochemical systems of organic carbonyl compounds. These studies generated much theoretical interest in CIDEP, but the mechanisms of the dynamic electron polarization in various chemical systems remained controversial for a long time. 

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. 

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