Multiplet effect, CIDNP

The simulated INEPT 13C spectrum of 2-chlorobutane is shown in Figure 12.14. By comparing this to the corresponding undecoupled 13C spectrum (Figure 8.11), note how each multiplet is both intensified and divided at its center into positive and negative lines that no longer show the same relative intensities as in the undisturbed multiplet. The appearance of these multiplets is quite similar to the multiplet effect CIDNP spectra we saw in Section 11.9, and as was true there, the middle leg of a multiplet with an odd number of lines vanishes. 

Figure Bl.16.8. Example of CIDNP multiplet effect for a syimnetric radical pair with two hyperfme interactions on each radical. Part A is the radical pair. Part B shows the spin levels with relative Q values indicated on each level. Part C shows the spm levels with relative populations indicated by the thickness of each level and the schematic NMR spectrum of the recombination product.

One of the most attractive features of the CIDNP multiplet effect is that it allows detennination of the sign of the J coupling, which is often difficult to do by other methods. 

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...

Figure Bl.16.9. Background-free, pseudo-steady-state CIDNP spectra observed in the photoreaction of triethylamine with different sensitizers ((a), antliraquinone (b), xanthone, CIDNP net effect (c), xanthone, CIDNP multiplet effect, amplitudes multiplied by 1.75 relative to the centre trace) in acetonitrile-d3. The stmctiiral formulae of the most important products bearing polarizations (1, regenerated starting material 2, N,N-diethylvinylamine 3, combination product of amine and sensitizer) are given at the top R denotes the sensitizer moiety. The polarized resonances of these products are assigned in the spectra. Reprinted from [21].

Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

Both net and multiplet effects must normally be considered except in two special cases (i) when = 0 and only multiplet effects are observed and (ii) when ai = 0 in which case there is no CIDNP to observe. In addition, if there is no coupling between a given nucleus or nuclei and any other nuclei in the product, the n.m.r. spectrum will be a single peak, which of necessity can show only net polarization. 

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. 

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. 

EXAMPLE 11.9 How would a negative value of J affect the type of multiplet effects in the CIDNP spectra of an AB system ... 

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 ... 

With the popularity of the pulse Fourier transform nmr spectrometers, recent CIDNP experiments were largely performed in a FT spectrometer. The technique has been reviewed by Kaptein (80), who has emphasized the important facts initially pointed out by Ernst et al. (55) in connection with using a FT spectrometer for CIDNP experiments. For homonuclear multiplet effects a small flip angle of less than 20° should be used to exhibit the multiplet features. As a 90° pulse turns the magnetization vector from the z axis to the xy plane, it would eliminate the homonuclear multiplet effects but not the net effects. The... 

If many different products are formed, the QDNP patterns may be quite complex. They may also be of the net or multiplet effect types, or they may represent a superposition of both effects. However, two simplifying rules for CIDNP phenomena have been found experimentally. They serve as starting points for the interpretation and hint at the origin of the effects ... 

Selective trapping of alkyl radicals from the alkyl halide component during the course of the catalytic disproportionation is the same as the previous observation with silver, and it indicates that the prime source of radicals in the Kharasch reaction lies in the oxidative addition of alkyl halide to reduced iron in Equation 47. Separate pathways for reaction of i-propyl groups derived from the organic halide and the Grignard reagent are also supported by deuterium labelling studies which show that they are not completely equilibrated.(49) Furthermore, the observation of CIDNP (AE multiplet effect) In the labelled propane and propene... 

Figure 1 CIDNP net effects (bottom trace) and multiplet effects (top trace) for the olefinic protons of N, N-diethylvinylamine V (for the formula, see Chart 5) formed in the photoreaction of xanthone with triethylamine in acetonitrile. Net and multiplet effects were separated by their flip angle dependence, as described in the text. Left X part (ot proton) of the ABX spectrum right AB part ( protons). The reason for the missing two inner lines from the multiplet of the a protons in the upper spectrum is explained in Ref. 114.

As will be explained in Section 3.2, multiplet and higher multiplet effects are much more prominent at low fields. For that reason, CIDNP net effects prevail on today s high-field spectrometers. 

Figure 6 Explanation of a CIDNP multiplet effect with vector models (projections). From left to right, starting state influence of the first nuclear spin (A) only influence of the second nuclear spin population differences resulting spectrum. The labels on the vector models denote the electron spin of radical 1 or 2, the labels in the NMR spectrum the nuclear transitions. Further explanation, see text.

Figure 14 demonstrates the sensitivity improvement that is obtained by the described co-addition in the spin system, and shows that the method is applicable without distortions to strongly coupled spin systems and to CIDNP multiplet effects. 

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