Kaptein s rule

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. 

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). 

While authors have generally been cautious in drawing mechanistic conclusions from their observations, application of Kaptein s rules to the n.m.r. results indicates that Ag must be negative in the radical-pair precursor of benzene. Relevant gr-values are as follows PhNN 2-0012 Ph, 2-0020 PhN—NO, 2-0017 HO>, 2-0118. Since the n.m.r. 

One potentially important example of CIDNP in products resulting from a radical pair formed by electron transfer involves a quinone, anthraquinone j5-sulphonic acid (23). When irradiated in the presence of the cis-syn dimer of 1,3-dimethylthymine (24), enhanced absorption due to vinylic protons and emission from the allylic methyls of the monomer (25) produced can be observed (Roth and Lamola, 1972). The phase of the polarizations fits Kaptein s rules for intermediate X... 

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. 

All these regularities are summed up by Kaptein s rule for the phase (sign) T/ of a CIDNP net effect (T/ = +1, enhanced absorption of nucleus i T/ = -1, emission),... 

An in-depth comparison of the last two mechanisms has been given, and sign rules similar to Kaptein s rules have been formulated for them. ... 

All these relationships are summarized by Kaptein s rule [40] for a CIDNP multiplet effect,... 

Owing to their ease of use, Kaptein s rules (Eqs. 62 and 65) are the basis of most chemical applications of CIDNP. On the other hand, they represent the radical pair mechanism in a simplified form only, so the question as to the validity of this approximation is of importance. Salikhov [47a] was the first to show theoretically that these rules can be violated in systems containing more than one magnetic nucleus. This was later analyzed in more detail [47b] and verified experimentally [47b, 47c, 48],... 

Am = 2 is responsible for this polarization. The same authors studied the photolysis of benzoquinones in CDC13 [69] by CIDNP and DNP. They interpreted their results by a hydrogen abstraction from CHC13 present as an impurity in the solvent. On the basis of this chemistry, the polarization phase at low quinone concentrations agrees with Kaptein s rules, whereas the phase is reversed at high concentrations, which was again attributed to Am = 2 cross-relaxation. 

Figure Bl.16.7. Kaptein s rules for net and multiplet RPM of CIDNP. The variables are defined as follows p = + for RP formed from triplet precursor or F pairs and - for RP formed from singlet precursor, s = + for recombination (or disproportionation)/cage products and - for scavenge/escape products. = + if nuclei i and j were on the same radical and - if nuclei i and j were on different radicals. A g- = sign of ig - g )- =...

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