Structures CIDNP

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 excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

On the basis of reaction-product structures, it might be expected that the reactions of organic halides with sodium naphthalene (Scheme 9) might resemble mechanistically the reactions of organic halides with lithium alkyls. CIDNP studies have shown that they are in fact quite different, in particular in the mechanism by which polarization occurs. The observations are as follows (Garst et al., 1970). 

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. 

Spectra of carbenes are very useful sources of information on the structure of the free carbenes, e,g. the R—C—R angle, or the multiplicity of their lowest state. However, these data were mostly obtained under conditions different from those in solution, where chemical reactions normally occur. The spectra are usually recorded either in matrices at low temperatures, say at 4 or 77 °K, or in the gas phase. Only very few investigations of that type have been carried out in solution. The most important spectroscopic technique used in the investigations of carbenes is ESR. Other spectroscopic methods, such as flash photolysis which produces electronic spectra of carbenes, and infrared and lately CIDNP spectroscopy have been successfully employed. 

Exposure of several methyl-substituted derivatives to y-radiolysis at 77 K in cryogenic matrices gave rise to a family of radical cations of the same structure type, some of which had been previously identified on the basis of CIDNP results. We begin with a discussion of the CIDNP investigations, since they preceded the ESR studies of all species but the prototype. The first CIDNP results attributed to a cyclopropane radical cation were observed during the photoreaction between 1,4-dicyanonaphthalene and cis-l,2-diphenylcyclopropane. However, the nature of the cyclopropane radical cation was characterized by CIDNP effects observed during the reaction of chloranil with cis- and /rans-l,2-diphenylcyclo-propane. ... 

These structures may be viewed as distorted from the Bj-type geometries via a second-order JT-type mechanism or, alternatively, as Aj-type with the substituents at the wrong carbon atom. The calculations suggest that the radical cation state preference can be fine-tuned by appropriate substituents and predict substantial differences in spin-density distributions. These predictions should be verifiable by an appropriate spectroscopic technique (ESR or CIDNP) and might be probed via the chemical reactivity of the radical cations (vide infra). 

Figures, h CIDNP spectra (cyclopropane resonances) observed during the electron transfer photoreaction of chloranil with c/s-1,2-diphenylcyclopropane (fop) and ben-zonorcaradiene (.bottom). The opposite signal directions observed for analogous protons in the two compounds constitute evidence that the two radical cations belong to two different structure types.

On the other hand, the CIDNP approach and recent ab initio calculations have provided what we consider unambiguous evidence for the nature of the vinylcyclo-propane radical cation as well as for some simple derivatives. We have carried out ab initio calculations on the prototype 19 and several simple derivatives, and probed by CIDNP the structures of three simple vinylcyclopropane radical cations in which the two functionalities are locked in a syn configuration. 

Hydrogen migrations have been demonstrated or invoked in several instances, although the detailed course of these reactions has not been fully elucidated. The involvement of hydrogen shifts is postulated based on experimental data, such as EPR or CIDNP spectra, the structure of reaction products, or on theoretical calculations (Section Although some migrations clearly involve a hydride... 

For radical cations of norcaradiene and derivatives, the interaction of the cyclopropane in-plane e orbitals with the butadiene frontier MO favors the type B structure. The assignments are based on ab-initio calculations, CIDNP results, and the ET photochemistry. The norcaradiene radical cation (lla ) has a electronic ground state (Cj symmetry). The Cl—C6 bond is shortened on ionization (—3.4 pm) while the lateral bonds are lengthened (+2.8 pm). The delocalization of spin density to C7 (py = 0.246 py 5 = 0.359) and the hyperfine coupling constants of the cyclopropane moiety a e = 1.36 mT oysyn = —0.057 mT flvanti = —0.063 mT) support a type B structure. 

The structures of the ions rest on CIDNP spectra delineating their hyperfine patterns,ab initio calculations - ESR and ENDOR data for 16 +, and TR-ESR results for 15 +. Ab initio calculations (B3LYP/6-31G //UMP2/6-31G ) reproduce positive and negative hyperfine coupling constants satisfactorily. Each radical cation is related uniquely to the geometry of one of the precursors (Fig. 6.13). 

Dicyclopentadiene forms a radical cation (20 ) in which one of the bonds linking the monomer units is cleaved. The species contains two allyl moieties attached to a C4 spacer . Structure 20 + rests on an unmistakable CIDNP pattem " and is supported by an analysis of the electronic absorption spectmm. The large energy gap in the OS of this ion (AE = 1.67 eV) is incompatible with the photoelectron spectrum of the parent molecule (AE = 0.15 eV), but it fits the ring-opened structure 20 +. 

Radical cations (21 +) derived from semibullvalene + or barbaralane belong to a different structure type. The ESR spectrum of 21 + (a = 3.62 mT, 2H a = 0.77 mT, and strong CIDNP effects support a structure in which... 

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