CIDNP Experiments

Chemical Induced Dynamic Nuclear Polarization is a useful source of information about chemical mechanisms involving free radicals and it has been largely exploited in and spectroscopies. Due to 

N-CIDNP investigations of the decomposition of several unsymmetric diazenes and radical trapping experiments both confirm a mechanism involving initial cleavage of one carbon-nitrogen bond (P 7). In the case of phenylazotriphenylmethane, for example this cleavage leads to the phenyldiazenyl-triphenylmethyl radical pair 

In addition, the polarized signal observed at about 66 ppm from NO zan be  

3 Application of Spectroscopy to the Study of Chemical and Biochemical Mechanisms 

In fact, the investigation of reaction mechanisms is therefore based usually on an identification of the reaction products at equilibrium. However, two formal cases may be distinguished according to whether the species concerned are in fast or slow equilibrium with respect to the NMR time scale  

---

The investigation by Becker et al. (1977 b) also included work on the effect of pyrene added as electron donor. Pyrene has an absorption maximum at 335 nm (e = 55000 M-1cm-1, in petroleum). Much more hydro-de-diazoniation takes place in the presence of pyrene with irradiation at 365 nm, and even more on irradiation with light of wavelength <313 nm. Photoexcited pyrene has a half-life of 300 ns and is able to transfer an electron to the diazonium ion. This electron transfer is diffusion-controlled (k= (2-3) X 1010 m 1s 1, Becker et al., 1977a). The radical pairs formed (ArN2 S +) can be detected by 13C- and 15N-CIDNP experiments (Becker et al., 1983, and papers cited there). 

Barbaralene [85] undergoes a rapid Cope rearrangement with a doublewell potential. The radical cation was studied using CIDNP by Roth (1987) after one-electron oxidation of [85] by y or X-irradiation. On the time-scale of the CIDNP experiment ( 10 8s), a single-minimum potential energy surface was found, i.e. bishomoaromatic structure [156] was suggested. 

Chemically Induced Nuclear Polarization (CIDNP)-Experiments... 

The CIDNP experiments have been carried out to demonstrate an intervention of triplet radical pairs in many hydrogen abstraction reactions involving triplet car-benes. The following examples in reactions with halo compounds and ethers are of particular interest. 

Quotation marks are used to describe the hole-catalyzed Diels-Alder reaction because of the fact that these are Diels-Alder reactions only in the sense that they yield the same products. Mechanistically, however, they are distinct and are generally considered to occur in a stepwise, rather than concerted, manner. Results obtained from CIDNP experiments suggest that an intermediate, ring-opened radical cation may be involved in the reaction (Scheme 40)117. 

Table I shows that in either dioxane or acetonitrile the quantum yield for degradation of I, is unaffected by the presence of 0.1 M of triplet quencher, either sorbic acid, naphthalene or cyclohexadiene. In ethanol, triplet quenchers reduce < >d from 0.34 to 0.14. Quantum yields for intersystem crossing, as determined by a laser opto-acoustic technique ( ), were 0.36 in ethanol and 0.59 in dioxane. These results agree with our earlier report (3), and indicate that significant reactivity occurs from St of I in protic solvents, and that reaction occurs exclusively from Sx in aprotic solvents. While triplet quenching experiments cannot rigorously exclude participation by short-lived higher triplet states, Palm et al (9) have obtained conclusive evidence from CIDNP experiments for singlet-state participation in a series of aryloxy-acetophenones. Note that the triplet state of I is formed in aprotic solvents, and that in deaerated solutions at room temperature it decays by first-order kinetics with a lifetime of 200 ns (3). Remarkably, despite having lifetimes about 100 times longer than other, differently-substituted, aryloxyacetophenones (the longer lifetimes may...

As in the CIDNP experiment, the positive charge is not at all observed, but is an outgrowth of the chemical intuition of the investigator and is supported by appropriate secondary experiments. The potential problems with such an assignment will be illustrated for the secondary intermediate obtained upon warming a matrix containing the tetramethylcyclopropane radical cation (see Chap. 5). 

Tertiary amines have also been employed in electron transfer reactions with a variety of different acceptors, including enones, aromatic hydrocarbons, cyanoaro-matics, and stilbene derivatives. These reactions also provide convincing evidence for the intermediacy of aminoalkyl radicals. For example, the photoinduced electron transfer reactions of aromatic hydrocarbons, viz. naphthalene, with tertiary amines result in the reduction of the hydrocarbon as well as reductive coupling [183, 184]. Vinyl-dialkylamines can be envisaged as the complementary dehydrogenation products their formation was confirmed by CIDNP experiments [185]. 

The electron transfer reaction of gem-diarylmethylenecyclopropanes (60) with singlet sensitizers results in the exchange of the exo-methylene and the secondary cyclopropane carbons [241], The chloranil photo-sensitized reaction generates two unusual cycloaddition products (61, 62) [242], whereas the tetracyanoethylene sensitized oxygenation produces the respective dioxolanes [238]. These reactions are compatible with a ring-opened radical cation, and CIDNP experiments have... 

The lowest state of prismane (2B,) cation lies 16 kcal mol-1 above the 2B2 state of the Dewar benzene cation (at the MP2/6-31 G level). This is considerably less than the corresponding energy difference of the neutral systems (37 kcal mol-1). The ground electronic states of prismane and Dewar benzene ions do not correlate their interconversion is forbidden from both state-symmetry and orbital-symmetry considerations. The CIDNP experiments indicate, however, that the actual barrier is quite small. 

Photo-CIDNP experiments using anthraquinones as photosensitizers for oxidation of a variety of uracil- and thymine-derived cyclobutane dimers, e.g. c,s-l, t,s-l, c,a-1, t,a-l, and c,s-2, 4, and 5, demonstrated the existence of both Pyr +oPyr and its dissociation product, the monomer radical cation Pyr + [6, 7]. 

Following the prophetic speculations of Kharasch and Reinmuth [A] and the crucial CIDNP experiments of the Amsterdam group [22], the main features of the mechanism are now clear. Electron transfer from the magnesium surface to adsorbed organic halide gives rise to alkyl radicals and halide ions. Further recombination steps lead to the alkylmagnesium halide. Two matters especially remain controversial whether the electron transfer step involves a discrete radical anion intermediate, and to what extent radicals leave the metal surface to diffuse into the solution. Recent arguments have been summarized [23, 24], commented on [25], and pursued [26]. The implications for practical work may be summarized briefly as follows ... 

EXAMPLE 12.4 Calculate the maximum signal enhancement due to the Overhauser effect between an unpaired electron and a H nucleus, as takes place in a CIDNP experiment (Section 11.8). 

In contrast, most of the photochemical CIDNP experiments are carried out with conventional or FT spectrometers with slow response time (T n > x > t /2) The recent efforts by Closs (37) and Barbara (21) may beat the nuclear relaxation problem and lead to quantitative measurements of the CIDEP enhancement factor. We will restrict ourselves mainly to deal with photochemical CIDEP and CIDNP experiments, although the reader should consult the excellent papers by Verma and Fessenden (124), Trifunac and Nelson (121) on radiolysis systems, and the flow technique of Lawler and Halfon (90) on thermolysis. 

The majority of the earlier photo-CIDNP experiments were carried out in a cw spectrometer with the sample subjected to continuous uv irradiation. The first consideration was therefore how to get light into the nmr probe as a relatively high light-absorbed intensity was required to generate the radicals at a... 

Figure 6. Schematic diagram of the irradiation arrangement in photo-CIDNP experiments.

Figure 7. "Temperature effect" in a photo-CIDNP experiment using a high-power light source. The upper trace represents polarization observation without adequate cooling of the sample to offset the heat generated by the light source. The lower trace represents polarization observation with air cooling.

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... 

For example, the reversal in sign of the CIDNP observed in the dimerization products of the direct photolysis of benyl-9-azofluorene (singlet precursor) as compared to 9-diazofluorene (triplet precursor) in toluene has been explained in terms of the requirement of triplet-singlet inversion as a step prior to combination (33). Similar reversals in sign in the decomposition of benzoyl peroxide were mentioned in an earlier section. These CIDNP experiments show that spin inversion is possible but is not... 

It has been a most satisfying experience when we could combine the CIDEP and CIDNP techniques to look at the photoreductions of certain quinones. In these experiments CIDEP provides information on the primary reactions involving the excited triplet and the secondary reactions involving the primary polarized radicals. However, esr experiments only "see" those radicals which escape the cage the CIDNP experiments fill in the missing information about the initial radical pair reacting in the primary cage. 

Second, these two steps can take place (and with CIDNP do take place) in different molecular surroundings. Asa result, a CIDNP experiment acts like a double resonance experiment, which greatly facilitates assignments of the observables on a molecular level. An EPR spectrum yields a set of hyperfine coupling constants but does not, by itself, reveal which constant belongs to which nucleus. In contrast, the CIDNP experiment encodes those hyperfine coupling constants as polarization intensities and detects that information in an NMR spectrum of the products, so immediately correlates each hyperfine coupling constant with a particular nucleus. 

The fact that no new instruments are needed is a major enticement to try CIDNP experiments. Conventional high-resolution NMR instruments may be used. Special applications require only minor modifications. 

Recent CIDNP experiments (17) support this view since it was observed that the radical pair formed on irradiation of benzoin was identical to that formed on irradiation of benzaldehyde. 

---