Spectroscopy time-resolved EPR

Forbes M D E, Peterson J and Breivogel C S 1991 Simple modification of Varian E-line microwave bridges for fast time-resolved EPR spectroscopy Rev. Sc/. Instrum. 66 2662-5... 

The reactions of cyanoisopropyl radicals with monomers have been widely studied. Methods used include time resolved EPR spectroscopy,352 radical trappingj53 355 and oligomer00 356 and polymer end group determination. 1 Absolute341 and relative reactivity data obtained using the various methods (Table 3.6) are in broad general agreement. 

Time resolved EPR spectroscopy and UV-visible spectophotometry have proved invaluable in determining the absolute rate constants for radical-monomer reactions. The results of many of these studies are summarized in the Tables included in the previous section (3.4), Absolute rate constants for the reactions of carbon-centered radicals are reported in Table 3.6. These include t-butyl374 and cyanoisopropyP2 radicals. 

With this expression, kjkn can be obtained by the measurement of one set of [RI ], [R2 ] values, at full light intensity only. As to kii itself, which is needed to evaluate kc, one must either do a separate experiment by time-resolved EPR spectroscopy (see Chapter 11) or, with less accuracy and reliability, one can assign it the value for the diffusion-controlled rate constant in that solvent. 

Direct detection of DPC is made by time-resolved EPR spectroscopy. In this method, DPC is first generated by photolysis of 30 in a hydrocarbon matrix at 16 K and is excited by a 465-nm laser, which corresponds to a T-T absorption of the To state of DPC. The transient triplet spectrum of the species having a decay rate of 160 ns at 16 K is assigned to the EPR spectrum of DPC. The ZFS parameters are determined by computer simulation to be D = 0.201 m and E = 0.0085 cm The D values observed by different methods are essentially identical. 

Hirota N, Yamauchi S. Short-lived excited triplet states studied by time-resolved EPR spectroscopy. J Photochem Photobiol C Photochem Rev 2003 4 109-24. 

Time-resolved EPR spectroscopy is particularly useful for selective observation of triplet species. By the use of the zero-field splitting parameters D and j obtained from the spectral pattern of each triplet species, the actual conformation can be predicted reasonably well, when those parameters are simulated by theoretical calculation. The time-dependent changes in the spin polarization along the x, y, and z principal axes can provide information concerning the spin-orbit coupling in each direction. 

The time-resolved EPR spectroscopy of photo-excited Rh(III) corroles has been examined at length in two articles [191, 192], Compared with the excited states of main group corroles such as Sn(tpfc)(Cl) [193] and Ga(tpfc)(py) [194], the excited states of Rh(ffl) corroles display broadened signals and greater g tensor anisotropy. This was attributed to spin-orbit coupling effects that effectively admix 3dd and 3jiji excited states. 

This review is divided into the following sections. First, EPR measurements of g-tensors of flavoproteins and the modulation of the principal values of g by the protein surroundings of the cofactor are discussed. Then, two recent examples of application of pulsed ENDOR spectroscopy will be reviewed, and, finally, time-resolved EPR spectroscopy, that is most favorably used to study photo-excited triplet states and radical pairs, will be introduced. 

Hugerat, M., H. Levanon, E. Ojadi, L. Biezok, and H. Linschitz (1991). Multiple decay pathways and electron transfer in excited ion-paired zinc-copper porphyrins Laser photolysis and time-resolved EPR spectroscopy. Chem. Phys. Lett. 181, 400-406. 

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