Cationic structures organic radical ions

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

Occasionally such intermolecular effects are specific and have sufficient lifetime to contribute individually to the spectrum. This is the case, for example, when organic radical-anions are studied in solvents of low ionizing power such as tetrahydrofuran. Ion-pairing then becomes important and, when alkali-metal cations are involved, the effect on the spectrum is to split each hyperfine component into four lines, each having one-quarter of the original intensity. This may convert a complicated spectrum into one that is quite uninterpretable, and can be avoided by using a better solvent or non-magnetic cation. However, it also provides evidence that contact ion-pairs are important in such solutions, and yields structural details unobtainable by other techniques. 

This is one of several reactions of this type in which an organic negative radical-ion and its parent molecule react in the presence of an alkali metal. It is found, rather interestingly, that the rate coefficients depend on the nature of the metal. To account for this, it has been postulated that the metal is involved in a bridging role in the activated complex, e.g., dipy.. K" ". . dipy for the case of 2,2 -dipy-ridyl (dipy) A more extreme case of this association between the radical-ion and the ion of the alkali metal used to form it occurs in the reaction of benzophenone with its negative ion. The spectrum of (benzophenone)" in dme has many hyper-fine lines caused by the interaction of the free electron with the and, when the metal is sodium, the Na nuclei. When benzophenone is added, the structure, due to the proton interaction, disappears and only the lines associated with the sodium interaction remain. To account for this, it has been suggested that the odd electron moves rapidly over all the proton positions too fast for the lines characteristic of the electron in the different proton environments to be seen), but relatively slowly from one sodium nucleus to another. Seen another way, this means that the transfer of an electron from molecule to molecule is associated with the transfer of the cation . ... 

M. B. Inoue, M. Inoue, M. A. Bruck, and Q. Fernando, Structure of bis(ethylenedithio)tetrathiafulva-lenium tribromocuprate(I), (BEDT-TTF )Cu 2Br3 Coordination of the organic radical cation to the metal ions, J. Chem. Soc., Chem. Commun. 7992 515. 

Almost all organic molecules contain an even number of electrons and so removal of an electron by El leads to a radical cation, M +, the molecular ion. The conventional mass spectrum reflects the competitive and consecutive fragmentations with rate constants /c> 10 s" of the molecular ion induced by El. The interpreter s task then is to deduce the structure of the original neutral molecule from the occurrence or absence of ionic fragmenta tions. Obviously, a good understanding of which ion structures can be formed and how they dissociate is critical to the interpreter s success. 

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