Time dependence CIDNP

Figure 8. Time-dependent CIDNP intensities in the photolysis of cyclododecanone 1. The symbols show the H CIDNP signal of the a protons of the ketone, the solid line was calculated by the theoretical model of [55a], [Adapted from ref. [55a] with permission. Copyright 1989 Elsevier Science Publishers B.V.]...

There are various quantitative descriptions of the pair mechanism -34) which are based on different treatments of the time dependences. For high-field polarizations, at least, they all lead to similar agreements of observed with calculated CIDNP patterns. 

Figure 13 Storage and accumulation of a time-resolved CIDNP signal in a spin system. The picture displays the timing of the laser and the spectrometer, and the time dependence of longitudinal and transverse magnetization. Further explanation, see text. Reproduced from Ref 75 with permission copyright (2006) Taylor Francis Ltd, httpr/ www.tandfco.uk/journals.

Figure 18 Time-resolved CIDNP in the system triphenyiamine/fumarodinitriie. Left example spectra taken at three different delays between laser flash and rf pulse. H°, H", are the signals of the aromatic protons of the amine, FN and MN those of the protons of the starting olefin and the isomerized olefin (which are not observed). Right time dependence of the CIDNP signals of FN. Further explanation, see text. Reproduced from Ref. 89 with permission copyright (2006) Verlag Helvetica Chimica Acta AG.

The chemistry underlying the polarization processes of tryptophan, tyrosine, and histidine (electron transfer for the first two amino acids, hydrogen transfer for the third) has long been known. For the frequently used sensitizer 2, 2 -dipyridyl 16, the kinetics of the quenching by tryptophan, tyrosine, and histidine have recently been studied in detail by time-resolved CIDNP experiments the effects of added surfactants on the polarizations of tyrosine with flavins 17 or other sensitizers have been investigated. Several low-field CIDNP investigations of the field-dependence have been carried out, with a view to optimizing... 

Figure 11. Time-resolved CIDNP spectra in a system described by the mechanism of Chart VI photoreaction of the acceptor anthraquinone 12 (8 x 10-4 M) with the donor /V,/V-dimethylaniline 13 (3.2 x 10 4M) in acetonitrile-d,. Experimental parameters T — 257 K, excitation wavelength 343 nm. Shown is the dependence of the signal I of the dimethylamino protons (marked with in the formula) on the delay time f0 between laser flash and acquisition pulse. [Adapted from ref. [86a] with permission. Copyright 1990 Elsevier Science Publishers B.V.]...

Figure 12. Time dependence of the CIDNP intensities P in the system of Chart VIII for different concentrations of the triplet quencher 1,3-cyclohexadiene (diamonds, 9.4 mM triangles, 0.94 mM circles, without quencher). [Adapted from E. Schaffner and H. Fischer, J. Phys. Client., 99, 102 (1995) with permission. Copyright 1995 American Chemical Society.]...

In the TR-CIDNP spectrum, the methyl protons of products IV, V, and VI, formed from the secondary biradical, show a combination of net and multiplet polarizations. Morozova et al [28] measured separately the time dependence of the net and multiplet polarizations for this group of protons, and the results are shown in figure Bl.16.12 and figure B 1.16.13 respectively. Clearly, the net and multiplet polarizations develop on different time scales while the net polarization is constant after approximately 1 ps, the multiplet polarization takes much longer to evolve. It was determined that this difference arises because the net polarization in these products of the secondary biradical is actually inherited from the primary biradical, while the multiplet polarization is generated in the secondary biradical. If chemical transformation (decarbonylation in this case) is fast compared with the rate of intersystem crossing, then a secondary biradical or radical pair may inherit polarization from its precursor. This is known as the memory effect in CIDNP, and this work was the first report of the memory effect in biradicals. The polarization inherited from the primary biradical is net because Ag > 0 in the primary biradical because the secondary biradical is symmetric, A g = 0, and only multiplet polarization can be generated. It was also determined in this study that the kinetics of the net effect reflect the decay of the level while the multiplet effect corresponds to the decay of the 7Y and T levels the reasons for these observations are beyond the scope of this presentation, but the interested reader is directed to the references for additional details. 

CIDNP Time Dependence. We are now in a position to discuss the NMR time dependence expected from a cyclic reaction, such as in Figure 5 when initiated by a short light flash. Let R(t) be the total concentration of A radicals (with a and 3 nuclear spins) and lp(t) and Ij>(t) the polarizations at time t of the single proton in the diamagnetic molecule. A, and in the radical, A , respectively. Suppose that A and B encounter one another and form F-pairs with a bimolecular rate IC2. The time derivatives of It. and I are then (15)... 

Figure 6. CIDNP time dependence arising jrom radical recombination for S and T geminate pairs. Slow relaxation, Equation 24.

Although a reaction scheme involving disproportionation as a significant random recombination route is not strictly cyclic, it can lead to partial cancellation of the A polarization as indicated by the arrows in equation (32). In the limit of very slow relaxation, a maximum of half of the f polarization in A can be cancelled by the opposite phase + polarization in the radicals. The remainder of the escape polarization ends up in the other disproportionation product (A ). This type of reaction leads to much simpler CIDNP time dependence than the A + B back reaction because no net polarization can arise in radical pairs consisting of identical free radicals. 

Figure 8. CIDNP time dependence arising from radical disproportiortation for slow and fast relaxation, Equations 35 and 36, respectively.

However this is not the only source of CIDNP time dependence that can arise from such a reaction. Consider what happens in the limit of very fast nuclear relaxation in A (i.e. no cancellation). Exchange now transfers recombination polarization from A to unpolarized A where it is able to relax very rapidly. This may be called a "relaxation sink" mechanism, and should occur at a maximum rate of k [A ], being most effective when the A radicals are a e to relax completely prior to reconversion to A i.e. 

Figure 9a. CIDNP time dependence arising from an exchange reaction, as predicted by Equations 40-43. k, = 10 Tj = 50 R = 10 M, i) =...

Figure 18. Time dependence of the photo-CIDNP difference intensity of F with 5 mM Fi-acetyltryptophan, 25 mM borate buffer, pH 8.0, and flavin concentrations 1.0 mM (O), 0.4 mM ( ), and 0.1 mM O-...

The enhancement coefficient serves as a basic quantitative feature of CIDNP. One usually distinguishes between the observed enhancement coefficient and the absolute one. The observed enhancement coefficient is defined as the ratio of the intensities of the polarized NMR signal to the equilibrium one recorded after the completion of the reaction. The time dependence of observed polarization on time (d/"7dt) has two components, (dtl reflecting the contributions from chemical reaction and relaxation of CIDNP. As a first approximation, the relaxation of CIDNP occurs through spin-lattice relaxation, which suggests the following expression for the observed polarization ... 

The absolute enhancement coefficient of CIDNP differs from the observed one in that the former is dependent only on the radical pair parameters. At present, the absolute enhancement coefficient can be reliably measured only for the case of photochemical reactions with the employment of specially elaborated time-resolved CIDNP techniques. 

Second, any CIDNP based assignments concerning the sign of hfcs are valid only if the radical pair mechanism (RPM) [93-96] is operative they become invalid if the alternative triplet-Overhauser mechanism (TOM), based on electron nuclear cross relaxation [97-100] is the source of the observed effects. For effects induced via the TOM the signal directions depend on the mechanism of cross relaxation and the polarization intensities are proportional to the square of the hfc. Thus, they do not contain any information related to the signs of the hfcs. However, the TOM requires the precise timing of four consecutive reactions and, thus, is not very likely. In fact, this mechanism has been positively established in only two systems [98-100]. 

The first ist the strong dependence of observed CIDNP intensities on the nuclear spin lattice relaxation times Ti of the products. As is seen from Eqs. 

After the classical studies by Turro and co-workers, who determined the exit rates of radicals from micelles from the changes in steady-state [62a] or time-resolved [62b] CIDNP spectra when an external scavenger was added, and analyzed the magnetic field dependence of the CIDNP signals [62c], activity in this field seems to have quieted down a little. 

With the model systems used in these investigations, dimer splitting can be induced both by reduction and by oxidation, depending on the sensitizer. While it was inferred from the existence of dimer radical anions D that splitting via the former route occurs in two steps, it has been debated for some time whether this also holds for the latter route, that is, whether dimer oxidation and cleavage are concerted or successive. CIDNP spectroscopy is particularly well suited to answer such questions because the intermediates leave their EPR spectrum (their polarization pattern) in the products. Thus, not only can the intermediates be identified by this signature, even if they are rather short lived, but the occurrence of their polarizations in a product... 

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