Examples of CIDNP

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 ...

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 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... 

One of the earliest examples of CIDNP is the proton resonance emission of benzene observed during thermolysis of dibenzoylperoxide (I) in cyclohexanone > which proceeds predominantly via (1) (see Sect. 4.1). 

Other recent examples of CIDNP studies on olefin isomerizations via reverse electron transfer populating the triplet, which all fall into class Ila, concerned all-frans retinal with stilbene as donor or a quinone as an acceptor, and a, P unsaturated ketones with triphenylamine or triphenylphosphine as donors °... 

Purine bases have received much less interest from CIDNP spectro-scopists [135]. The photoreaction of guanine with synthetic water-soluble porphyrins was addressed in [141]. Besides an earlier study of the single-stranded region of tRNA [142], the only reported examples of CIDNP from the corresponding biopolymers are two investigations of oligonucleotide duplexes [143]. 

Figure 4.1 shows a typical example of CIDNP observed during the Grignard reaction of iodoethane with magnesium in benzene and one molar equivalent of THF [79]. 

Examples of CIDNP applications to the structure determination and properties of paramagnetic species (free radicals, radical ions, biradicais, carbenes, macromoiecuies)... 

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.

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. 

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... 

One aspect of both EPR and CIDNP studies that should be kept in mind is that either is capable of detecting very small amounts of radical intermediates. This sensitivity makes both techniques quite useful, but it can also present a pitfall. The most prominent features of either EPR or CIDNP spectra may actually be due to radicals that account for only minor amounts of the total reaction process. An example of this was found in a study of the decomposition of trichloroacetyl peroxide in alkenes. 

The first example of chemically induced multiplet polarization was observed on treatment of a solution of n-butyl bromide and n-butyl lithium in hexane with a little ether to initiate reaction by depolymerizing the organometallic compound (Ward and Lawler, 1967). Polarization (E/A) of the protons on carbon atoms 1 and 2 in the 1-butene produced was observed and taken as evidence of the correctness of an earlier suggestion (Bryce-Smith, 1956) that radical intermediates are involved in this elimination. Similar observations were made in the reaction of t-butyl lithium with n-butyl bromide when both 1-butene and isobutene were found to be polarized. The observations were particularly significant because multiplet polarization could not be explained by the electron-nuclear cross-relaxation theory of CIDNP then being advanced to explain net polarization (Lawler, 1967 Bargon and Fischer, 1967). 

Photoinduced reactions of homolytic addition of bromotrichloromethane C.CI3B1 to allylic derivatives of germanium and tin could be an illustrative example of the potentialities of CIDNP application to study processes where the polarization effects have not been generated at the initiation stage. An earlier proposed46 overall scheme of CChBr addition to the allylic double bond in the R3MCH2CH=CH2 molecule was based on the analysis of the reaction products (Scheme 5). 

For simple carbonyl compounds, the equilibrium between an aldehyde or a ketone and its corresponding enol is usually so shifted towards the keto form that the amount of enol at equilibrium can neither be measured nor detected by spectroscopy. Nevertheless, as recently emphasised by Hart (1979), this does not mean that the enol cannot exist free, not in equilibrium with ketones and aldehydes. Several examples of kinetically stable enols in the gas phase or in aprotic solvents have been reported. Broadly speaking, it appears that enols have relatively large life-times when they are prepared in proton-free media [e.g. the half-life of acetone enol was reported to be 14 s in acetonitrile (Laroff and Fischer, 1973 Blank et al., 1975) and 200 s in the gas phase (MacMillan et al., 1964)]. These life-times are related to an enhanced intramolecular rearrangement, indicated by the very high energies of activation (85 kcal mol-1 for acetaldehyde-vinyl alcohol tautomerization) which have been calculated (Bouma et al., 1977 Klopman and Andreozzi, 1979) It has therefore been possible to determine most of the spectroscopic properties of simple enols [ H nmr,l3C nmr (CIDNP technique), IR and microwave spectra of vinyl alcohol... 

Early explanations of CIDNP were based solely on Over-hauser effects between an unpaired electron and a nucleus (Section 12.3, Example 12.4), but these early theories failed... 

From Example 11.11 you can infer that the greatest values of CIDNP are its ability to confirm the involvement of radical pairs in reactions, to determine the spin state of the radical pair precursor at its birth, and to distinguish between products that arise via recombination and those that arise from escape. 

When peroxide 11-10 (Example 11.12) is ther-molyzed in the presence of PhSH (rather than S-Cl), one of the products is PhS CH2CH v Predict the type of CIDNP effect(s) in the H spectrum of both the quartet and triplet of its ethyl group. 

Stimulated emission That part of the emission which is induced by a resonant perturbing electromagnetic field. The transition between states, n and m, is governed by the Einstein coefficient of stimulated emission, Bnm- CIDNP emission and lasing action are examples of processes which require stimulated emission. 

It is now well established that both CIDEP and CIDNP have their origins in the formation and removal reactions of free radicals. As a result of this, it is now possible to gain information, not normally obtained from magnetic resonance studies, for those photochemical reactions which show CIDEP and CIDNP. An example of this is those photochemical reactions in which the primary radicals react immediately to regenerate the starting compounds. The regenerated compounds may show CIDNP, and this is often the only evidence that this reaction has occurred. In the radical-pair mechanism, spin polarization is caused by the spin-selective reaction. While it is generally not possible to monitor by esr the selective reactivity of the radical pairs as a function of their nuclear spin states, CIDNP has proved to be a valuable tool to probe the small difference in reactivity of the nuclear spin states of the radical pairs. 

Not many of the in-cage cross-recombination products between a semiquinone radical and the hydrogen donor counter radical have been observed. However, one example of such a minor pathway of an in-cage reaction was provided by CIDNP study of the crossrecombination product between a tetrafluorosemiquinone radical and the counter dioxane radical (127). 

The only pieces of hardware needed for photo-CIDNP are a light source and an unmodified NMR spectrometer. Pulsed lasers are most convenient for illumination, as they allow both time-resolved experiments (when the laser flash is followed by an acquisition pulse after a variable time delay) and steady-state ones (when the laser is triggered with a high repetition rate, thus providing quasi-continuous excitation). All the examples of this work draw on the second variant. Nevertheless, they yield kinetic information about much faster processes than would be observable by direct... 

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