Difference structure factors, isotope substitutions

It is suggested that the nickel is bound to a low molecular weight factor, different from F430 of methanogens, and which may involve a corrinoid structure. The nickel is thought to cycle between Ni and Ni , with the CO bound to the metal. ESR data have been interpreted in terms of an Ni species with a bound radical derived from either CO or CO2. The involvement of a nickel-carbon bond has been unequivocally established by isotopic substitution (which is shown in the g = 2.08, 2.02 signals). About 40% of the total nickel is present as this species, suggesting that the Ni—C species is a viable intermediate in the catalytic conversion of CO to CO2. There are parallels with industrial and laboratory catalytic processes, but the involvement of Ni seems... 

Figure 4.08 Structural studies of NigiB 9 using isotopic neutron diffraction (a) Structure factor for different isotopically substituted alloys, (b) Partial structure factors obtained form those shown in (a), (c) Partial reduced RDFs obtained by Fourier transformation of the 5ij(0 shown in (b) (After Lamparter et al., 1982).

If one wishes to determine aU the three partial radial distribution functions of a two-component material, like water, then three independent total structure factors must be measured for (chemically) the same material. In this way, the coefficients of the partial structure factors Sy(Q), O Eq. (29.33), will be different, because the scattering lengths wiU be different. As shown by Table 29.1, different isotopes of the same element do possess different scattering amplitudes for neutrons. This property made possible the development of the technique called isotopic substitution (North et al. 1968). For a two-component system, three different isotopic samples have to be prepared, so that the coefficients of the partial structure factors should differ sufficiently weU for being able to solve a set of three linear equations, of the form of Eq. (29.33), for the three unknown partial structure factor. 

It should also be mentioned that not only were the three sample temperatures different by up to 80 K (Table 6.2), the temperature of each run fluctuated by as much as 50 K. Although it is possible that both of these factors might lead to significant variations in the structure which would invalidate the isotopic substitution, the diffraction measurements on iron of Holland-Moritz et al. [17] suggest that the structure will have remained constant within experimental precision. 

The neutron-scattering results then becomes particularly informative. One can vary systematically the scattering length density of the solvent, n by isotopic substitution, and from the corresponding intensity measurements separate the different partial structure factors. This gives access to the detailed polymer profile. 

The PRISM theory can be used to calculate the partial structure factors, which can be input into equation (31) to obtain the SANS chi parameters. In principle, if a molecular model for the polymers is available this comparison could be carried out with no adjustable parameters since the potential parameters can be obtained from fitting to thermodynamic data. Nearly symmetric isotopic blends, where the only difference between the two components is that some of the hydrogen atoms in one component have been substituted with deuterium, are a very good model system because they are fairly easy to model and show interesting behavior, including a strongly composition dependent In most cases... 

Variation of b in three different experiments by varying the isotopic composition permits the three independent linear equations to be solved to yield the three partial structure factors. The Fourier transform of each of these yields the partial pan-correlation functions gu(r) for catimi-anion correlation, and gn(r) and gjj(r) for correlations of ions of the same sign. The Neutron diffraction with isotope substitution (NDIS) method permits this variation of b, because different isotopes of the elements have different b values, some of which may even be negative. 

---