Multiplet CIDNP signal

In many cases an NMR spectrum exhibits both net and multiplet CIDNP effects. In many of these cases the net For A effect is superimposed on the multiplet effect of a given signal. For example, when singlet radical pair 11-14 undergoes recombination to 11-15, the H spectrum of the CH2 group in the product shows an E/A multiplet effect superimposed on an E net effect3 ... 

While the stick plot examples already presented show net and multiplet effects as separate phenomena, the two can be observed in the same spectrum or even in the same NMR signal. The following examples from the literature will illustrate real life uses of CIDNP and demonstrate the variety of structural, mechanistic, and spin physics questions which CIDNP can answer. 

Figure B 1.16.9 shows background-free, pseudo-steady-state CIDNP spectra of the photoreaction of triethylamine with (a) anthroquinone as sensitizer and (b) and (c) xanthone as sensitizer. Details of the pseudo-steady-state CIDNP method are given elsewhere [22]. In trace (a), no signals from the p protons of products 1 (recombination) or 2 (escape) are observed, indicating that the products observed result from the radical ion pair. Traces (b) and (c) illustrate a usefiil feature of pulsed CIDNP net and multiplet effects may be separated on the basis of their radiofrequency (RF) pulse tip angle dependence [21]. Net effects are shown in trace (b) while multiplet effects can...

As mentioned in Section 11.7, the most baffling aspect of NMR spectra showing CIDNP effects was the occurrence of both positive and negative signals within the same multiplet. Sometimes the left half of the multiplet was positive and the right half negative (an A/E multiplet effect), while sometimes the reverse was observed (an El A multiplet effect). These are shown in Figure 11.5. 

Figure 14 Signal improvement by multi-flash CIDNP experiments with storage and accumulation in the spin system. Delay At (see, Figure 13) between laser flash and sampling rf pulse, 0 jis. Bottom trace absorptive doublet and emissive AB spin system of 3-fluoro-DL-tyrosine. Top trace multiplet effect in S-NADH. The number of flashes accumulated before one acquisition is given at the upper spectra, 1 denoting the control experiment with the standard sequence of Figure 9. It is seen that neither multiplet signals nor CIDNP multiplet effects are distorted by the method. Further explanation, see text. Adapted from Ref. 75 with permission copyright (2006) Taylor Francis Ltd, http / www.tandfco.uk/journals.

When the ratios of populations within the spin system correspond to thermal equilibrium but the total population does not—this situation is called a CIDNP net effect—only 1-spin order is present. Hence, it is meaningful to speak of the polarization of a particular nucleus in this case. The appearance of the signals is the same as in a normal NMR spectrum, the relative intensities within multiplets being unchanged. However, a signal... 

Figure 1 (A) Schematic presentation of the types of CIDNP effects (1) equilibrium NMR spectrum, (2) enhanced absorption (A), (3) emission ( 2). (4) multiplet effect (A/ ), (5) multiplet effect BA). (B) Experimental example of time-resolved (100 ps after laser pulse) 250 MHz H CIDNP spectrum, detected during the photolysis of substituted 1,2-dihydropyridine in the presence of 2,5-dichlorobenzoquinone line assignment is shown on the structures signal (6) is attributed to HDD admixtures in CD3CN. The equilibrium signals were removed using the presaturation pulse sequence preceding the laser pulse.

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