Induced Dynamic Electron Spin Polarization CIDEP

VI. Chemically Induced Dynamic Electron Spin 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. ... 

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) 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... 

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. 

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... 

Such anomalous NMR spectra as observed in the above reactions have been called Chemically Induced Dynamic Nuclear Polarization (CIDNP) . CINDP should be due to nonequilibrium nuclear spin state population in reaction products. At first, the mechanism of CIDNP was tried to be explained by the electron-nuclear cross relaxation in free radicals in a similar way to the Overhauser effect [4b, 5b]. In 1969, however, the group of Closs and Trifunac [6] and that of Kaptain and Oosterhoff [7] showed independently that all published CIDNP spectra were successfully explained by the radical pair mechanism. CIDEP could also be explained by the radical pair mechanism as CIDNP. In this and next chapters, we will see how CIDNP and CIDEP can be explained by the radical pair mechanism, respectively. 

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). 

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