Multiplet effects

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.

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

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... 

The remainder of equation (38) describes the multiplet effect, and it can be seen that whether an individual line in the multiplet corresponds to emission or absor ption depends on the signs of the hyperfine coupling constants but is independent of Hq. The nature of the hyperfine field is such that the integral over the whole multiplet is zero if Ag = 0. 

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. 

A singlet pair of <-butyl radicals produced by peroxide decomposition disproportionate to yield isobutane and isobutene [equation (41)]. Both products show E/A multiplet effects. 

The spectra shown in Figure 4.4 were obtained from a reaction of fert-butyl-lithium with l-bromobutane. ° One deduces that both the fert-butyl and the butyl radical were produced and that they reacted in disproportionation reactions to give in part isobutylene and 1-butene, respectively. In the spectrum recorded at 30 s, vinyl proton signals from 1-butene are in emission (5 5.0 and 5 6.1) and enhanced absorption (5 5.6). The isobutylene vinyl proton signal at 5 4.6 is in emission on the left side and enhanced absorption is on the right side. This phenomenon is known as a multiplet effect, and it is due to differences in ISC rates for radical pairs containing rcrt-butyl radicals with different proton nuclear spins. Note that the tert-butyl-lithium sample contained an impurity of isobutylene before the reaction, and the amount of isobutylene was increased after the reaction. 

The caged pair 16 can lose the second C02 to yield a new pair 18 18 retains the net polarization of 16, which was emission for the CH2, but now acquires in addition a multiplet effect in the sense EjA for the CH2 group. The CH2 in the product phenylethane (19), group 4 in the spectrum, shows superposition of the net emission, E, and a multiplet effect in the predicted sense EjA. (The CH3 of this product is evidendy obscured by the CH3 of the ethyl benzoate.) Ethyl radicals that escape from the cage either react with iodine to give ethyl iodide, groups 3 (CH3) and 5 (CH2), which shows net polarization just opposite... 

Figure A1.8 Nuclear spin states and spectrum for product 5. At the top the four states are again shown in an energy-level diagram. Heavy lines are the states with enhanced populations. A downward-pointing arrow indicates a net transfer of molecules from an overpopulated higher spin state to a less populated lower one, and corresponds to net emission. The spectrum shows the multiplet effect of type El A. From S. H. Pine, J. Chem. Educ., 49, 664 (1972). Reproduced by permission of the Division of Chemical Education.

The net effect and the multiplet effect may occur together. Figure A 1.10 shows how excess population in the lowest level of our AX system, and a smaller excess in the highest level, will yield a superposition of a net and a multiplet effect, A + EjA. 

Kaptein has worked out simple formulae for deciding the type of spectrum that will be obtained in a given set of circumstances/ Two parameters are defined, Tn and rm. Tn tells whether there will be a net enhanced absorption, A, or a net emission, E, and rm tells whether the multiplet effect will be of the EjA type or of the A/E type. The calculation of Tn and rm and their interpretation are given in Table Al.l. 

Figure A1.9 Nuclear spin states and spectrum for product 6. The conventions of the diagram are the same as for Figure A 1.7. The spectrum shows the multiplet effect of type AjE.

There should be an EjA multiplet effect with no net effect, as in Figure Al. 8. The product of escape from the cage has e negative and an AjE multiplet effect. 

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. 

As with the net E and A effects, the multiplet effects can also be rationalized on the basis of nonequilibrium spin state populations. Recall the AB spin system we discussed in... 

Figure 11.5. Multiplet effects in a doublet, triplet, and quartet. Note how the middle line of the triplet vanishes.

---