Specific Radical Ions

The formation of molecular radical ions by electron transfer reactions between alkali metals and a wide variety of aromatic and other organic compounds in polar solvents is well established. A very large number of radical anions have been prepared by this method and extensive studies of their e.s.r. and optical spectra have been made (Bowers, 1965 Gerson, 1967 Kaiser and Kevan, 1968). In solution the electron transfer reaction will be facilitated by the subsequent solvation of the two ions (or ion pair) by the polar solvent molecules. However, we have observed that similar electron transfer reactions occur readily when alkali metal atoms are deposited on a variety of relatively non polar substances at 77°K in the rotating cryostat. In most cases the parent compound acts as the matrix, though for some radical ions an inert matrix of a non-polar hydrocarbon has been used successfully. It is perhaps surprising that the reactions occur so readily as the energy of solvation of the ions must be quite small in most of these systems as compared with that in the polar liquids. 

000=127° and SOS =141°. Both radical anions exhibit hyperfine structure arising from the interaction of the unpaired electron with the alkali metal cation when they are trapped in their parent compounds. This interaction is attributed to the existence of discrete ion pairs of the form 

In contrast no hyperfine structure from the alkali metal is observed when 00 is trapped in a matrix of water. This effect is attributed to heavy solvation of the two ions which results either in an actual physical separation of the two ions, or in stabilization of the ionic structure so strongly that the contribution of a non-ionic structure is negligible. The gf-factors and carbon-13 hyperfine splittings are the same as those for CO trapped in carbon dioxide, which shows that the basic structure of the anion is not influenced by the surrounding matrix. 

The radical anion of molecular oxygen (O ) has been prepared and trapped in a range of alcohols, water and benzene but not in aliphatic hydrocarbons (Bennett et al., 1968a). In contrast to COg the e.s.r. spectrum shows that 0 interacts strongly with its immediate environment. This interaction which alters the separation of the upper molecular orbitals of the anion is strongly dependent on the nature of the matrix. Previously, the Oj radical ion has been stabilized only in ionic materials such as the alkali halides thus it is of particular interest to find that this anion can be trapped successfully in a non-polar matrix (benzene). There is some evidence (Evans, 1961), from optical spectroscopic studies that molecular oxygen can form a weak charge transfer complex with the 77-electron system in benzene and it seems probable that O2 is stabilized in benzene by the formation of a similar complex. 

In contrast to Og the chlorine radical ion (Cl ) has been trapped in aliphatic hydrocarbons such as cyclohexane as well as in benzene and water (Bennett et al., 1968b). This reflects the higher electron affinity of chlorine. Furthermore the e.s.r. spectrum of Cl is identical in all the matrices, showing that its electronic configuration is not affected by its surroundings. 

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The choice of medium in which the radical ions are formed, i. e., the solvent, is determined both by the characteristics of the electrochemical generation method itself and by the properties of the radical ions formed. The requirements for the solvent may be diverse, are frequently difficult to reconcile with each other, and are usually unknown for radical ions which are being produced for the first time. In the latter case the medium for the production of specific radical ions must be found experimentally. 

There are many chemical methods for generating radicals reported in the hterature that do not involve conventional initiators. Specific examples are included in References 64—79. Most of these radical-generating systems carmot broadly compete with the use of conventional initiators in industrial polymer apphcations owing to cost or efficiency considerations. However, some systems may be weU-suited for initiating specific radical reactions or polymerizations, eg, grafting of monomers to cellulose using ceric ion (80). 

Much of the interpretation of electroorganic reactions has assumed the model implied in the above discussion, i.e. conversion of the neutral substrate into a radical ion followed by distinct chemical and/or electrochemical steps. It follows therefore that specific structural effects should be found in the reactions of the intermediates. 

The important role of radicals and radical ions in various branches of chemistry (e.g., electrochemistry, radiation chemistry, macromolecular chemistry), their remarkable physical properties and reactivity, as well as the specific problems in a quantum chemical approach, make this region interesting from the theoretical point of view. 

Farhataziz and Ross, A. B. (1977), Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution. III. Hydroxyl Radical and Perhydroxyl Radical and Their Radical Ions, NSRDS-NBS 59, National Bureau of Standards, Washington, D.C. 

Heterocycles are of great interest in organic chemistry due to their specific properties. Many of these cycles are widely present in natural and pharmaceutical compounds. Electrochemistry appears as a powerful tool for the preparation and the functionalization of various heterocycles because anodic oxidations and cathodic reductions allow the selective preparation of highly reactive intermediates (radicals, radical ions, cations, anions, and electrophilic and nucleophilic groups). In this way, the electrochemical technique can be used as a key step for the synthesis of complex molecules containing heterocycles. A review of the electrolysis of heterocyclic compounds is summarized in Ref. [1]. 

Though the fragmentation is one of the basic reactions of radical ions, this destructive reaction pathway seems to be synthetically useless at first sight. Nevertheless, the electron-transfer-induced bond cleavage can be specific and, for this reason, synthetically useful (e.g., for ring enlargement reactions). 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

In order to investigate hole and electron transfer we have employed a number of techniques to produce holes and electron adducts within DNA [7]. These trapped ion radical species of DNA are produced by y- or UV-irradia-tion at 77 K of DNA in various aqueous matrices. Each technique employed produces specific radical species. 

In contrast to radical ions generated from alkenes or carbonyl compounds, substantially fewer recent reports have appeared which describe the chemistry of radical ions generated from the >C=N— functional group. This situation likely results from the relative obscurity of the >C=N— group (compared to >C=0 and >C=C<), rather than specific problems with the chemistry, per se. Based upon the limited data available, and as might be anticipated, >C=N— + chemistry appears to be analogous to that of >C=C< +, while >C=N— chemistry is reminiscent of >C=0. ... 

Various groups have contributed to clarify the dynamics of radical ion pairs in PET processes. This account cannot discuss all their results. We rather will try to select some examples which emphasize the specific feature of other donor-acceptor pairs different from those discussed above. 

Thus, in the methylanthracene-induced deligation, the trend in the quantum yields for (DUR)2Fe2+ > (HMB)2Fe2+ follows predictably from the lifetimes (/c,-1) of the labile 19-electron radicals (DUR)2Fe+< (HMB)2Fe+, as evaluated by transient electrochemical methods (136). Furthermore, the remarkable trends in the quantum yields to decrease with the increasing strength of the arene donor must take specific cognizance of the rate of back electron transfer (fc, ). Since the latter results in the annihilation of the radical ion pair Ar2Fe+/D+-, it is readily evaluated from the separate redox couples,... 

Common molecules have an even number of electrons. Stable radicals are rare exceptions, such as NO. In classical chemistry, we most often meet active species that are ions with an even number of electrons, or radicals, an uncharged species with an odd number of electrons. In mass spectrometry, we observe ions with an even number of electrons, but we also often meet radical ions, a species uncommon in solution chemistry and having specific characteristics. 

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