CIDNP

B1.16 Chemically-induced nuclear and electron polarization (CIDNP and CIDEP)... 

We make one important note here regarding nomenclature. Early explanations of CIDNP invoked an Overhauser-type mechanism, implying a dynamic process similar to spin relaxation hence the word dynamic m the CIDNP acronym. This is now known to be incorrect, but the acronym has prevailed in its infant fomi. 

Figure Bl.16.1. CIDNP spectrum (250 MHz top) observed during irradiation of chloranil with sabinene...

CIDNP involves the observation of diamagnetic products fonned from chemical reactions which have radical intemiediates. We first define the geminate radical pair (RP) as the two molecules which are bom in a radical reaction with a well defined phase relation (singlet or triplet) between their spins. Because the spin physics of the radical pair are a fiindamental part of any description of the origins of CIDNP, it is instmctive to begin with a discussion of the radical-pair spin Hamiltonian. The Hamiltonian can be used in conjunction with an appropriate basis set to obtain the energetics and populations of the RP spin states. A suitable Hamiltonian for a radical pair consisting of radicals 1 and 2 is shown in equation (B1.16.1) below [12]. 

Figure Bl.16.4. Part A is the vector representations of the. S state, an intennediate state, and the Jq state of a radical pair. Part B is the radical reaction scheme for CIDNP.

A general reaction scheme for CIDNP is shown in llgnre B1.16.4B. where the radical dynamics in each region... 

Figure Bl.16.5. An example of the CIDNP net effect for a radical pair with one hyperfme interaction. Initial conditions g > g2, negative and the RP is initially singlet. Polarized nuclear spin states and schematic NMR spectra are shown for the recombination and scavenging products in the boxes.

Figure Bl.16.6. An example of CIDNP net effeet for a radieal pair with two hyperfme interaetions. Part A shows the spin levels and sehematie NMR speetnim for unpolarized prodnet. Part B shows the spin levels and sehematie NMR speetnim for polarized prodnet. Populations are indieated on eaeh level. Initial eonditions ...

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 ...

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. 

Roth et aJ [10] have used CIDNP to study the structures of vinylcyclopropane radical cations fomied from precursors such as sabinene (1). 

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... 

The CIDNP spectrum is shown in figure B 1.16.1 from the introduction, top trace, while a dark spectrum is shown for comparison in figure B 1.16.1 bottom trace. Because the sign and magnitude of the hyperfine coupling constant can be a measure of the spin density on a carbon, Roth et aJ [10] were able to use the... 

DH has only one non-negligible hyperfme, = -1-19.0 G while D has two significant hyperfmes, = -13.96 G and = -1-19.24 G. Clearly, these two radicals will lead to very different polarizations in the CIDNP spectmm of both cage and escape products. 

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].

In an extension of traditional CIDNP methods, Closs and co-workers developed time-resolved CIDNP (TR-CIDNP) m the late 1970s [24, 25 and 26]. The initial time-resolved experiments had a time resolution in the... 

The first application of the time-resolved CIDNP method by Closs and co-workers involved tire Norrish 1 cleavage of benzyl phenyl ketone [24, 25]. Geminate RPs may recombine to regenerate the starting material while escaped RPs may fonn the starting ketone (12), bibenzyl (3), or benzil (4), as shown below. 

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... 

Figure Bl.16.12. Experimental kineties of the CIDNP net eflfeet (A) for the aldehyde proton of the produets II and in of primary biradieal ( ) for the CH CHCOH) protons of the produets IV, V, and VI of seeondary...

Figure Bl.16.13. Kineties of the CIDNP multiplet effeet (frill eurve) the ealeulated CIDNP kineties for the produet of disproportionation of bis-ketyl biradieal (O) experimental kineties for the CH CHCOH) protons of the produets IV, V and VI of the seeondary biradieal. Reprinted from [28].

As the eleetron eoimterpart to CIDNP, CIDEP ean provide different but eomplementary infonnation on free radieal systems. Wliereas CIDNP involves the observation of diamagnetie produets, the paramagnetie... 

As for CIDNP, the polarization pattern is multiplet (E/A or A/E) for each radical if Ag is smaller than the hyperfme coupling constants. In the case where Ag is large compared with the hyperfmes, net polarization (one radical A and the other E or vice versa) is observed. A set of mles similar to those for CIDNP have been developed for both multiplet and net RPM in CIDEP (equation (B1.16.8) and equation (B1.16.9)) [36]. In both expressions, p is postitive for triplet precursors and negative for singlet precursors. J is always negative for neutral RPs, but there is evidence for positive J values in radical ion reactions [37]. In equation (B 1.16.8),... 

Roth H D, Weng H and Herbertz T 1997 CIDNP study and ab initio calculations of rigid vinylcyclopropane systems evidence for delocalized ring-closed radical cations Tetrahedron 53 10 051-70... 

Closs G L and Czeropski M S 1977 Amendment of the CIDNP phase rules. Radical pairs leading to triplet states J. Am. Chem. Soc. 99 6127-8... 

Goez M and Sartorius I 1993 Photo-CIDNP investigation of the deprotonation of aminium cations J. Am. Chem. Soc. 115 11 123-33... 

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