Absorption spectra nickel oxide

The nickel in urease is nonmagnetic and appears to be in the oxidation state Ni(II). The broad optical absorption spectrum is influenced by ligands to the metal (Fig. 1). The spectrum obtained in the presence of the competitive inhibitor mercaptoethanol, after correction for Rayleigh scattering by the protein (31), shows absorption peaks at 324,380, and 420 nm, with molar absorption coefficients of 1550,890, and 460 A/-1 cm-1, respectively. These were assigned to sulfur-to-nickel charge transfer transitions. The spectrum is changed by addition of other inhibitors, such as acetohydroxamic acid (Fig. IB). Similar... 

Fig. 7. Optical absorption and magnetic circular dichroism spectra of oxidized hydrogenase from M. thermoautotrophicum (AH strain), nickel concentration 120 pM. (a) Optical absorption spectrum, at room temperature the absorption is predominantly due to iron-sulfur clusters, (b) MCD spectra recorded at 1.53, 4.22, and 8.9 K, in a magnetic field of 4.5 T MCD is predominantly due to Ni(III), which is the only paramagnetic species in the oxidized enzyme. Reproduced, with permission, from Ref. 57.

The cobalt(II) corrole anion prepared as above was characterized primarily by electron spin resonance (esr) and absorption spectroscopy. When prepared via sodium film reduction, the cobalt(II) corrole oxidizes rapidly to the corresponding Co(III) corrole on exposure to air. When prepared by the other methods, it is moderately stable in air in the presence of a reducing agent. Attempts to prepare the neutral form of the initial Co(II) corrole anion, by protonation with perchloric acid, resulted in formal oxidation to the Co(III) derivative. Interestingly, further protonation of the Co(III) corrole with perchloric acid led to what appeared to be a protonated Co(III) corrole. Certainly, the absorption spectrum of this species is similar to that of the corresponding neutral nickel(II) corrole complex. However, the exact nature of this protonated material has not been fully elucidated. 

Attempts to observe the spectrum of CO adsorbed on nickel oxide have not been successful. This result was unexpected on the basis of the adsorption of CO on nickel oxide (4, 5). It is difficult to predict the minimum amount of adsorbed material which would be detectable when the specific absorptivity of that species is not known. On the basis of experience with CO and CO2, it had been assumed that a surface coverage of 10 % of a monolayer of CO on nickel oxide would be sufiBcient to produce a detectable band. Since the purpose of these preliminary experiments was to get information needed to interpret bands which are observed during the oxidation of CO, failure to observe a band in this case was not a serious obstacle in the interpretation of the oxidation experiments. 

The absorption spectrum of an electrochemically grown nickel oxide film shows a peak in the vicinity of 0.5-0.6 pm. Therefore, depending on film thickness, nickel oxide is transparent or pale green in the bleached state and brown-bronze in the coloured state. 

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