CIDEP

B1.16 Chemically-induced nuclear and electron polarization (CIDNP and CIDEP)... 

As the eleetron eoimterpart to CIDNP, CIDEP ean provide different but eomplementary infonnation on free radieal systems. Wliereas CIDNP involves the observation of diamagnetie produets, the paramagnetie... 

As for CIDNP, the polarization pattern is multiplet (E/A or A/E) for each radical if Ag is smaller than the hyperfme coupling constants. In the case where Ag is large compared with the hyperfmes, net polarization (one radical A and the other E or vice versa) is observed. A set of mles similar to those for CIDNP have been developed for both multiplet and net RPM in CIDEP (equation (B1.16.8) and equation (B1.16.9)) [36]. In both expressions, p is postitive for triplet precursors and negative for singlet precursors. J is always negative for neutral RPs, but there is evidence for positive J values in radical ion reactions [37]. In equation (B 1.16.8),... 

Because the number of grazing encounters is a fimction of the diflfiision coefficient, CIDEP by the RPM mechanism is a strong fiinction of the viscosity of the solvent and, in general, the RPM becomes stronger with increasing viscosity. Pedersen and Freed [39] have developed analytical teclmiques for the fiinctional fonn of the viscosity dependence of the RPM. 

Figure Bl.16.14. Top, the canonical axes for triplet naphthalene. The z-axis is directed out of the plane of the paper. Bottom, energy levels and relative populations during the CIDEP triplet mechanism process. See text...

Figure Bl.16.19. (a) CIDEP spectrum observed in die photolysis of xanthone (1.0 x 10 M) in cyclohexanol at room temperature. The stick spectra of the ketyl and cyclohexanol radicals with RPM polarization are presented, (b) CIDEP spectrum after the addition of hydrochloric acid (4.1 vol% HCl 0.50 M) to the solution above. The stick spectra of the ketyl and cyclohexanol radicals with absorptive TM polarization are presented. The bold lines of the stick spectra of the cyclohexanol radical show the broadened lines due to ring motion of the radical. Reprinted from [62].

Figure Bl.16.22 shows a stick plot siumnary of the various CIDEP mechanisms and the expected polarization patterns for the specific cases detailed in the caption. Each mechanism clearly manifests itself in the spectrum in a different and easily observable fashion, and so qualitative deductions regarding the spin multiplicity of the precursor, the sign of Jin the RP and the presence or absence of SCRPs can innnediately be made by examining the spectral shape. Several types of quantitative infonnation are also available from the spectra.

For example, if the molecular structure of one or both members of the RP is unknown, the hyperfine coupling constants and -factors can be measured from the spectrum and used to characterize them, in a fashion similar to steady-state EPR. Sometimes there is a marked difference in spin relaxation times between two radicals, and this can be measured by collecting the time dependence of the CIDEP signal and fitting it to a kinetic model using modified Bloch equations [64]. 

Figure Bl.16.22. Schematic representations of CIDEP spectra for hypothetical radical pair CH + R. Part A shows the A/E and E/A RPM. Part B shows the absorptive and emissive triplet mechanism. Part C shows the spin-correlated RPM for cases where J and J a.. ...

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... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

Maeda K, Terazima M, Azumi T and Tanimoto Y 1991 CIDNP and CIDEP studies on intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene. Determination of the... 

Goudsmit G H and Paul H 1993 Time-resolved EPR investigation of triplet state Cgg. Triplet-triplet annihilation, CIDEP, and quenching by nitroxide radicals Chem. Phys. Lett. 208 73-8... 

Pulse techniques, coupled with the observation of the decay of enhancement (Atkins et al., 1970a, b Glarum and Marshall, 1970 Smaller etal., 1971) constitute the most sensitive procedure for detecting CIDEP. Both net and multiplet polarization have been described. As with CIDNP, the former is believed to arise essentially from the Zeeman interaction and the latter from the hyperfine term. Qualitative rules analogous to Kaptein s rules should be capable of development. 

As yet there has been little exploitation of CIDEP. The phenomenon is potentially very valuable in the study of transient radicals and should complement CIDNP. 

Jeevarajan, A. S., M. Khaled et al. (1993a). CIDEP studies of carotenoid radical cations. Z. Phys. Chem. 182 51-61. 

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. 

Most CIDEP spectrometers have a time resolution of few microseconds or longer. CIDNP is a slower process and only steady-state measurements... 

The photochemistry of benzaldehyde (90% 13C=0), 519, deoxybenzoin (99%) 13C=0), 521, and / -chloro benzoin (99% 13C=0), 522, in cyclohexane-Dn solution has been studied633 by spectroscopic techniques, such as XH chemically induced dynamic nuclear634 or electron polarization635 (CIDNP/CIDEP) or dynamic nuclear polarization636 (DNP). In all these cases the formation of benzaldehyde-D with emissive 13C=0 polarization has been observed and the results rationalized by intermolecular hydrogen (deuterium) abstraction by the photoexcited ketones from the solvent molecules and by reactions of cage-escaped radicals (equations 303-308), Benzoin, 520, is formed also. 

Time-resolved CIDEP and optical emission studies provide further definitive characterization of the triplet and excited singlet states followed by their primary photochemical reactions producing transient radicals in individual mechanistic steps in the photolysis of a-guaiacoxylacetoveratrone. Both fluorescence and phosphorescence are observed and CIDEP measurements confirm the mainly n,n character of the lowest triplet state. The results indicate a photo triplet mechanism involving the formation of the ketyl radical prior to the P-ether cleavage to form phenacyl radicals and phenols. Indirect evidence of excited singlet photo decomposition mechanism is observed in the photolysis at 77 K. 

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