Radical ions, structures. CIDNP

Spectroscopic studies with photo-CIDNP techniques revealed the existence of two distinct radical cations generated from hexamethyldewarben-zene, presumably rapidly interconverting. In one of these, the central carbon—carbon bond is significantly stretched and bears the unpaired spin density. In the second, the spin density is confined to one of the olefinic bonds. This example is the first to show conclusively that two different radical ion structures can correspond to a single minimum on the ground-state surface of the neutral (Roth et al., 1984). 

The structure of these ions rests on the following results (Table 4) detailed CIDNP spectra delineating the hyperfine patterns of both ions [320, 321] ab initio calculations with a 6-31 G basis set for both ions [322] and ESR [323, 324] as well as ENDOR data [324] for the norbornadiene radical cation. The CIDNP results indicate the absolute signs and relative magnitude of the hyperfine coupling constants a comparison with the calculated values (Fig. 22) shows satisfactory... 

Using time-resolved CIDNP [56], the products of the fast geminate reaction may be detected, too. Obviously, the CIDNP effect is predetermined as an analytical tool for the examination of the structure and energetics of radical ions [57], less for ion pairs. 

The ion pair spin multiplicity may be a valuable tool to affect the BET rates and to probe the ion pair dynamics via magnetic field effects [36], Even weak magnetic fields are known to influence relative probabilities of singlet and triplet reactions [34], Chemically induced dynamic nuclear polarization (CIDNP) is a particularly informative technique [12]. Many bond scission reactions and rearrangements in cyclic radical ions have been successfully explored using this approach. Both structural data (spin densities) and approximate kinetic informations are indirectly available from such experiments [12]. 

NMR chemical shifts are usually well understood, and the identity of the coupled nuclei is clearly established. Combined with PET as a method of radical ion generation, the CIDNP technique has been the key to elucidating mechanistic details of important reactions and provided insight into many short-lived radical cations with unusual structures, many of which had previously eluded any other technique. 

Because a structure of A symmetry was established for the prototype [78], it is hardly surprising that the radical ions of many derivatives also belong to that general structure type. Radical cations of the same structure type as those derived from cis- and tran5-l,2-diphenylcyclopropane were established for numerous cyclopropane derivatives, including 1,2-di-, 1,1,2-tri- and 1,1,2,2-tetramethylcyclopropane (Table 4) [104, 109]. Two of these systems provide a direct comparison between the results of CIDNP and ESR experiments. In both instances the ESR spectra observed by Williams and coworkers after y-irradiation in frozen solutions [105, 106] contain splitting patterns supporting the presence of spin-density on two car-... 

PET is the most convenient way to prepare radical ions and use CIDNP to obtain information about their structure. The basis of the identification and structural characterization of intermediates, through the polarization pattern, has been explained above (Section 5.5). Unusual radical cations have continued to attract the attention of CIDNP spectroscopists (Chart 4). 

The analysis of CIDNP effects allows information to be obtained on the structure and reactivity of active short-lived (from nanoseconds to microseconds) paramagnetic species (free radicals and radical ions, triplet excited molecules), and on molecular dynamics in the radical pair. The analysis makes it possible to distinguish between the bulk and in-cage stages of complex chemical reactions. CIDNP data also provide information on the multiplicities of reacting states, which is of utmost importance for understanding photochemical processes. 

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

The analysis of CIDNP effects has also made possible the determination of the structures of a number of radical ions of cyclic compounds (benzonorborna-diene, trialkylcyclopropanes, syn- and awti-para-cyclophanes, etc.). 

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 +. 

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