Three-electron atoms, resonance spectrum

The reduction of cyanocobalamin gives three possible oxidation states for the cobalt atom (Fig. 2). Electron spin resonance studies with Bi2-r reveals that this molecule is the only paramagnetic species giving a spectrum expected for a tetragonal low spin Co(II) complex. Controlled potential reduction of cyanocobalamin to Bi2-r proves that this reduction involves one electron, and further reduction of Bi2-r to B12-S requires a second single electron (16—19). At one time B12-S was considered to be a hydride of Co(III), but controlled potential coulometry experiments provided evidence against a stable hydride species (16). However, these experimental data do not exclude the possibility of a stable Co(III) hydride as the functional species in enzyme catalyzed oxidation reduction reactions. 

Figure 2. ESR spectrum of pseudorotating Ks in an Ar matrix at 34.2 K. The stick spectrum shows the predicted intensity distribution for three equivalent nuclei. Also indicated are K atom resonances for several matrix sites. He = 3312.3 G is the resonance field of a free electron...

While many spectroscopic studies have been published on dimers, the most extensive polymer studies have been with Ag, Na, and Cu clusters. As might be expected much of the interest in silver relates to the photographic process where it appears that a four-atom silver cluster on a silver halide surface leads to reduction by developer, whereas a three-atom cluster does not. The electron spin resonance (ESR) spectrum of sodium in argon confirms that the trimer is covalently bonded and not an equilateral triangle. Ultraviolet photoelectron spectroscopy (UPS) of Cu clusters indicates that the d band is separate from the s band, unlike in the bulk or in the Xa calculations mentioned earlier. 

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