Oxidized species, isotopic analysis

In complex systems that involve multiple Fe-bearing species and phases, such as those that are typical of biologic systems (Tables 1 and 2), it is often difficult or impossible to identify and separate all components for isotopic analysis. Commonly only the initial starting materials and one or more products may be analyzed for practical reasons, and this approach may not provide isotope fractionation factors between intermediate components but only assess a net overall isotopic effect. In the discussions that follow on biologic reduction and oxidation, we will conclude that significant isotopic fractionations are likely to occur among intermediate components. 

S. Nanbu, M.S. Johnson, Analysis of the ultraviolet absorption cross sections of six isotopically substituted nitrous oxide species using 3D wave packet propagation, /. Phys. Chem. A 108 (41) (2004) 8905-8913. 

In the mass spectrometric method it is usually most convenient to convert both the substrate and product to the same chemical species for isotopic analysis. This procedure, furthermore, eliminates the difficult corrections which would have to be applied for isotopic discrimination in the mass spectrometer. If the substrate or products contain the isotopes A and A9 in groups or substituents other than the reaction center, there may be complications from isotopic homogeneity. This has been pointed out previously.8 Recently Yankwich and Promislow70 have shown that there is a 1 per cent difference in the C18 content of the methyl and aldehyde carbons of acetaldehyde derived from the air oxidation of propane... 

Other aspects of the report (42) on [Fe3S2(NO)5] are surprising. Elemental analysis of the ammonium salt was reported to distinguish between iron(II) and iron(III) in [Fe3S2(NO)5] , but to find these two types of iron present in equal numbers is most unusual for a triiron complex. Second, the molecular weight of the potassium salt was measured as 420 by mass spectrometry. This value is close to the M/Z of 421 calculated for the most abundant isotopic form of the ion-pair cation [KFe3S2(NO)5] +. Finally, the ESR spectrum reported is that of a dini-trosyliron species, which bears a remarkable resemblance to that reported (22) for a complex formed from Fe(II) and nitric oxide in aqueous alkaline solution. 

Isotopic Exchange/Equilibrium. Chemical steps are required at the outset of the procedure to insure isotopic exchange between the radionuclide to be analyzed (the radioanalyte) and the tracer or carrier that has been added. The carrier or tracer and the radioanalyte must be in the same oxidation state and chemical species in solution. This effort is not required for radionuclides that exist in only a single form, such as Group 1A (Li, Na, K, Rb) elements that are consistently in their +1 state in solution. Other elements (such as I or Ru) that have multiple oxidation states, and also can form stable complexes, will require steps to insure that the added carrier or tracer and the radioanalyte exchange before the analysis is started. 

The mass spectra of 7, obtained from the NADPH/02/P-450 oxidation of 5, and 8 are compared in Table IV. Three isotopic species are expected for the allylic hydroxylation of 5. Oxidation with removal of hydrogen will produce 7-d3and 7a-d3. Extensive controls have demonstrated that 7-d3 and 7a-d3 are indistinguishable by mass spectrometry because of rapid 1,3-migration of the siloxy group upon electron impact. Oxidation with deuterium removal leads to the production of 7-d2 (Scheme 4). By this analysis, the isotope effect is simply the ratio of 5-d3 (and 5a-d3) to 5-d2 in the oxidized sample. Deconvolution of the parent region (with appropriate correction for carbon and silicon... 

Isotopic substitution may help us to obtain rotation constants. It also allows us to define atom positions within a molecule precisely (the effect of change of mass is small if the substituted atom is close to the center of mass, and zero if it is at the center of mass, for example, the rotation constants of BCI3 are unaffected by the boron isotopic mass), and may confirm the presence of a particular type of atom in the molecnle. This latter point is of particular value in studying unstable species where elemental analysis is impracticable. For example, in order to prove that a species under investigation is an oxide we may observe a shift in the position of rotational lines when 0 replaces 0. 

The study of surface-stabilized inorganic radicals by EPR has a long history. This partially arises from their ease of generation and their favorable stabihty on the ionic oxide surfaces. From a catalysis point of view, such radicals are fundamentally important, since they can act as intermediates or oxidants in the catalytic cycle. If isotopic substitution of the radical is facile, then a very thorough description of the electronic and geometric properties of the species can once again be obtained by analysis of the powder EPR pattern. 

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