Isotopic substitution method

Aside from the question of the precise model by which relaxation times are interpreted there is the more practical problem of isolating that part of the relaxation specifically caused by diffusion. The contributions of exchange processes (see below), spin-rotation interaction (9), and spin diffusion (9) can be identified by temperature dependences different from that which is solely the result of the motionally modulated nuclear dipolar interaction as sketched above, and corrections can be made. The molecular rotation contributions to dipolar relaxation can be removed or corrected for by (a) isotopic substitution methods (19), (b) the fact that rotation is in some cases much faster than diffusion, and its relaxation effects are shifted to much lower temperatures (7, 20), and (c) doping with paramagnetic impurities as outlined above. The last method has been used in almost all cases reported thus far, more by default than by design, because commercial zeolites are thus doped by their method of preparation this... 

Neutron diffraction has been successfully used for structure determinations of aqua complexes of some metal ions, which have isotopes with sufficient differences in scattering lengths to be used for isotopic substitution methods. Not only bond lengths but also coordination numbers and the orientation of the water molecules in the first coordination sphere can then be determined. Several review articles have summarized these results (2-6). 

The most detailed structures have been obtained from neutron diffraction data, when isotopic substitution methods have been possible to use. Neutrons are strongly scattered by hydrogen atoms, making it possible to determine not only bond lengths and coordination number of the metal ion, but also the orientation of the water molecules in its first coordination sphere. A summary of results is given in Table II (19-31). 

More recently, Millen and co-workers 14,17"25) have examined a number of small molecule dimers, primarily with HF as one constituent, using high resolution micro-wave absorption spectroscopy26). As in the above experiments, conventional absorption techniques are used. By examining dimers with small molecule monomers, the sensitivity is adequate to completely resolve rotational spectra. Thus isotopic substitution methods and Stark shift measurements are possible and accurate dimer geometries and electric dipole moments can be calculated. 

The different contributions of Li and Li to the spin-lattice relaxation of neighbouring nuclei also provide, in suitable cases, a means for the determination of Li-C and Li-H distances in aggregates of organolithium compounds through the use of the isotopic substitution method [42], as has... 

The vibrational frequencies are essentially typical values which were taken from the experimental work of Plyler and Benedict (6), Reichman and Overend (7), Morino and Nakamura (8), Jones, et al. (9), Plether (j ), and King, et al. (1 ). The value of Wj as given by Plyler and Benedict (6) appears too low and was not included. Costain (1 ) has calculated the bond distances and angles based on a Isotopic substitution method. From this latter data, the moments of inertia are calculated to... 

An alternative approach to the problem is the isotopic substitution method. Here one uses the same alloy prepared with different isotopes having different neutron scattering factors (Mizoguchi et al., 1978 Kudo et al., 1978). In the amorphous substitution method several alloys A, are used, where x is fixed and B or A is replaced by a component of similar size and chemical affinity but different scattering factor (Chipman et al., 1978 Williams, 1982). In these methods it is tacitly assumed that the atomic distribution functions in the alloy series are the same or, at least, do not differ much. 

Information on the second hydration shell of ions is much poorer than that of the first hydration shell (see Table 3). The quantitative analysis of radical distribution curves in the long r range is difficult and much less reliable. Reliable information in the structure of the second shell may be obtained by the isomorphous substitution method in XRD measurements and the isotopic substitution method in ND measurements. Of course, such methods cannot be applied to every electrolyte solution. In most works in which some attempts have been made to determine the structure of the second hydration shell of ions, suitable structural models have been introduced. In some other cases fitting procedures between experimental and theoretical G r) functions have been continued until a more or less satisfactory agreement is obtained between them. Details about the distance between a central ion and water molecules in the second hydration shell, mo(2), the rmsd, /mo(2), the number of water molecules in the second hydration shell of the ion, mo(2), and /o(i) o(2) and /o(i) o(2) values between water molecules in the first and second hydration shells are given in Ohtaki and Radnai (1993). ... 

In order to Investigate the aromatic residues In detail It Is necessary to resort to Isotopic substitution methods to provide the required spectral simplification. For this reason we have studied enzymes containing selectively deuterlated aromatic residues and also containing 9f-labelled amino acids. The deuterlatlon approach was pioneered some years ago by Jardetzky (13) and Katz (14) and their colleagues. More recently Sykes et al. (15) and other workers (11, 12, 16, 17, 18) have Incorporated fluorine labelled amino acids Into proteins and examined their NMR spectra. 

The is the usual way of defining the bond length in microwave work, although Tj is obtained if the isotopic substitution method is used. Finally, the value is from X-ray diffraction (or other work in crystals, such as neutron diffraction). 

Nitrogen Relaxation in Large Molecules. The Isotopic Substitution Method... 

In indirect methods researchers use experimental dependence of the rates of the process or individual steps on the steric and electronic properties of substrates and inhibitors, on the polarity, donor-acceptor ability of the solvent, ionic strength, various additives reacting with intermediate species (for example, radicals), etc. The methods for quantitative account of some of these effects are presented in chapters considering the factors influencing the rate of chemical reactions and correlations. Let us concentrate our attention on two approaches, which are significant for the establishment of the mechanism isotope substitution method and measurement reaction rates in magnetic fields. The application of the first method is traditional in catalysis. The use of the second method has just been started. 

In highly exothermic reactions such as this, that proceed over deep wells on the potential energy surface, sorting pathways by product state distributions is unlikely to be successful because there are too many opportunities for intramolecular vibrational redistribution to reshuffle energy among the fragments. A similar conclusion is likely as the total number of atoms increases. Therefore, isotopic substitution is a well-suited method for exploration of different pathways in such systems. 

Both the 12C/13C primary KIE and the 14N/15N secondary KIE have been determined (Table 4-2) [19, 20], with the immediate adjacent atoms about the isotopic substitution site quantized as well. To our knowledge, we are not aware of any such simulations prior to our work for a condensed phase reaction with converged secondary heavy isotope effects. This demonstrates the applicability and accuracy of the PI-FEP/UM method. 

This reaction sequence was definitively shown by use of temperature programmed reaction spectroscopy ( 7) The key to the success of this method was that reaction (4) was the rate-limiting step, allowing positive identification of the CH30(a) intermediate by TPRS. Isotopic substitution with b0 and deuterium was used to identify steps (2) and (3). 

The same method of construction of symmetry-adapted operators can be used for any isotopic substitution in which a H atom is replaced by D, for example, the molecule 2,4,6-C6H3D3 of Figure 6.4. This molecule has Dih symmetry (i.e., the symmetry is lowered). The symmetry-adapted operators can be obtained by inspection of Figure 6.4. The nearest-neighbor interactions are all identical (and are of the H-D type). The same is true for the third neighbor interactions. Thus the symmetry-adapted operators S(i) and S(III) are the same as... 

X-Y stretching vibrations of octahedral molecules are thus characterized by four quantities, Ay, Ayy, 7i Y yy- The results of some sample calculations are shown in Table 6.3. Isotopic substitutions can also be considered, using the method of Section 6.9. 

Choosing a method to determine isotope effects on rate constants, and selecting a particular set of techniques and instrumentation, will very much depend on the rate and kind of reaction to be studied, (i.e. does the reaction occur in the gas, liquid, or solid phase , is it 1st or 2nd order , fast or slow , very fast or very slow , etc.), as well as on the kind and position of the isotopic label, the level of enrichment (which may vary from trace amounts, through natural abundance, to full isotopic substitution). Also, does the isotopic substitution employ stable isotopes or radioactive ones, etc. With such a variety of possibilities it is useless to attempt to generate methods that apply to all reactions. Instead we will resort to discussing a few examples of commonly encountered strategies used to study kinetic isotope effects. 

In order to successfully interpret a mass spectrum, we have to know about the isotopic masses and their relation to the atomic weights of the elements, about isotopic abundances and the isotopic patterns resulting therefrom and finally, about high-resolution and accurate mass measurements. These issues are closely related to each other, offer a wealth of analytical information, and are valid for any type of mass spectrometer and any ionization method employed. (The kinetic aspect of isotopic substitution are discussed in Chap. 2.9.)... 

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