Electron spin resonance and other spectral methods

2 ELECTRON SPIN RESONANCE AND OTHER SPECTRAL METHODS 

The negative ion of naphthalene can be made in a suitable solvent, such as di-methoxyethane (dme), by treatment with an alkali metal in the absence of oxygen. This stable free radical-ion has an esr spectrum with hyperfine structure. If more naphthalene is added, a broadening of the spectral lines occurs which is attributed to the reaction 

This reaction is of the second order with rate coefficient k, and it may be shown that 

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 .  

There have been recent attempts to apply the same type of argument to the broadening of Raman lines as a result of chemical reaction. Thus, for example, the 1435 cm band of the trifluoroacetate ion, CF3CO2, is broadened considerably in acid as compared with neutral solution. This has been interpreted in terms of the shortness of the lifetime of the species on account of the exchange reaction 

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