Kaptein

The phase of a transition in a CIDNP speetnim ean be detennined rising niles developed by Kaptein [20]. The nile for the net effeet is shown in equation (Bl.16.6). For eaeh tenn, the sign (-t or -) of that value is inserted, and the final sign detennines the phase of the polarization phis is absorptive and minns is emissive. The variables are defined in the eaption to figure B 1.16.7. 

Figure Bl.16.7. Kaptein s niles for net and multiplet RPM of CIDNP. The variables are defined as follows p = -t for RP fonned from triplet preeursor or F pairs and - for RP fonned from singlet preeursor. e = -t for reeombination (or disproportionation)/eage produets and - for seavenge/eseape produets. + if nuelei ...

Kaptein s rule is applied below to eaeh transition in the example in figure Bl.16.6. It is important to ehoose Ag eorreetly Ag is equal to g - 2 where g deseribes the radieal eontaining the nueleus of interest (often a proton) while 2 is the other radieal in the RP. The rule eorreetly prediets absorptive phase for NMR transitions 1 and 2 and emissive for NMR transitions 3 and 4. 

Kaptein s rule for the multiplet effect is useful for predicting the phase of each transition, and it is similar to... 

The application of Kaptein s rule to the example in figure Bl.16.8 is shown below, and it correctly predicts E/A multiplets. 

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

Kaptein R and Oosterhoff J L 1969 Chemically Induced dynamic polarization II (relation with anomalous ESR spectra) Chem. Phys. Lett. 4 195-7... 

Kaptein R 1971 Simple rules for chemically induced dynamic nuclear polarization J. Chem. See. Chem. Commun. 732-3... 

I de Vlieg, RM Scheek, WE van Gunsteren, HJC Berendsen, R Kaptein, I Thomason. Proteins Stiaict, Eunct, Genet 3 209-218, 1988. 

Van Beeren HC, Jong WMC, Kaptein E et al (2003) Dronedarone acts as a selective inhibitor of 3,5,3 -triiodothyronine binding to thyroid hormone receptor-a. in vitro and in vivo evidence. Endocrinology 144 552-558... 

The origin of CIDNP lies in the microscopic behaviour of radical pairs. Our discussion of this will follow fairly closely the model approach associated with the names of Gloss, Kaptein, OosterhofF, and Adrian, rather than the more formal kinetic treatments of Fischer (1970a) and Buchachenko et al. (1970b). 

Gloss, 1969 Gloss and Trifunac, 1969, 1970a Kaptein and Ooster-hoff, 1969a, b) and this has proved very successful in explaining the experimental findings. 

The problem of estimating the number of encounters that a given pair of radicals will have is not trivial. The most successful approach (Adrian, 1970, 1971 Kaptein, 1971b, 1972a, b Kaptein and den... 

As has been shown (Kaptein, 1971b, 1972a) by application of perturbation theory (Itoh et al., 1969), the spin Hamiltonian in equation (17) can be obtained for S and T radical pairs. 

Kaptein s (1971b, 1972a) equivalent expression contains a further term —... 

Kaptein (1971a, b) has analysed CIDNP spectra in terms of an expression equivalent to equation (38) and the considerations mentioned above for net and multiplet effects. A summary of his predictions follows ... 

Kaptein (1971a, b, 1972a) has further derived relations for predictmg net and multiplet effects on the basis of (i)-(iv) above. The qualitative features of CIDNP spectra can be determined by determining the signs of the functions Dub and Fme given by equation (39a) and (39b) for net and multiplet effects respectively. 

In using Kaptein s rules in later sections, the sequence of signs will be that corresponding to the parameters as given in equations (39a) and (39b). 

Some of the uncertainties and ambiguities of qualitative rules such as Kaptein s and Muller s (1972) can be cleared up by computer simulation of polarized spectra. Alternatively, such quantitative approaches can be used to deduce a- and Jgr-values. 

The predictions of simple rules such as Kaptein s and Muller s can be distorted by relaxation effects. These are particularly noticeable in photochemical experiments. In the pre-steady state (e.g., immediately after irradiation has begun), when build-up of the polarized signals is occurring, relaxation effects in the final product are relatively unimportant and observed spectra accord with the simple theory. Con-... 

The simple rules refer to nuclear spin selection during mixing of Tq and S states of the radical pair. If mixing of T+j and S states occurs instead, entirely different rules apply as discussed in Section IV (Kaptein, 1971b, 1972a Kaptein and den Hollander, 1972 Garst et al., 1971 J. I. Morris et al, 1972). 

For benzoyl and acetyl peroxides, loss of carbon dioxide occurs in a stepwise process. Estimates of the rate constants for step c in Scheme 1 are 7 x 10 sec (benzene, 60°). The corresponding process for acetyl peroxide has k = 2x 10 sec (n-hexane, 60°), so that the lifetime of radical pairs containing acetoxy radicals is comparable to the time necessary for nuclear polarization to take place (Kaptein, 1971b Kaptein and den Hollander, 1972 Kaptein et al., 1972). Propionoxy radicals are claimed to decarboxylate 15-20 times faster than acetoxy radicals (Dombchik, 1969). 

CIDNP studies of the decomposition have centred mainly on thermal decompositions photochemical decomposition has generally been less intensively investigated. While most reports of polarization refer to n.m.r. spectra, a number of papers have described polarization of other nuclei, (Kaptein, 1971b Kaptein et al., 1972), (Lippmaa el al., 1970a, b, 1971 Kaptem, 1971b Kaptein et al., 1972 Kessenikh et al., 1971), and F (Kobrina et al., 1972) contained in the peroxide reactant. Additionally, polarization of P has been reported in the products of decomposition of benzoyl peroxide in phosphorus-containing solvents (Levin et al., 1970). 

The polarization of biphenyl, deserves special comment. If, as indicated in Scheme 2, its immediate precursor is a radical pair consisting of two phenyl radicals, then it should be formed without detectable net polarization since if Ag = 0. Analogous results have been reported in the decomposition of other peroxides for example, ethane formed from acetyl peroxide shows net emission. To account for this, it has been suggested (Kaptein, 1971b, 1972b Kaptein et al., 1972) that nuclear spm selection which occurs in the primary radical pair—in... 

In the decomposition of benzoyl peroxide, the fate of benzoyloxy radicals escaping from polarizing primary pairs remains something of a mystery. Benzoic acid is formed but shows no polarization in and C-spectra, and the carboxylic acid produced in other peroxide decompositions behaves similarly (Kaptein, 1971b Kaptein et al., 1972). Some light is shed on the problem by studies of the thermal decomposition of 4-chlorobenzoyl peroxide in hexachloroacetone containing iodine as... 

Fig. 9. Intensity of maximum polarization (100 MHz) of chloroform produced in the thermolysis (80°C) of isobutyryl peroxide (0-2 m) in hexachlorobutodiene containing bromotriohloromethane. Data of Kaptein et cU., 1971.

Kaptein (1971b Kaptein et al., 1972) has investigated a wide range of dialkanoyl peroxides by CIDNP and has concluded that the rate of... 

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