Hypothetical isotope method

The hypothetical isotope method is aimed at calculating absolute rotational correction terms. An alternative mediod for evaluating relative rotational corrections in a series of isotopically related molecules was proposed by Escribano, del Rio and Orza [71], The formalism of diis rproach will be briefly presented in the second part of the book. 

The value of the kinetic isotope effect method lies mainly in the possibility of making a substitution within tlve reactive center of the molecule, while still retaining the original type of the read ion, thus allowing the cancellation of many poorly defined quantities in the absolute rate equations and permitting a direct comparison between the measured and calculated values of the relative rate constants. Since the magnitude of these latter values depends on the hypothetical transition state model, a diagnostic means is provided by the method for the experimental verification of the nature of tlie transition state in question. 

All of the measurements employed the technique described above that involves the analysis of the isotope composition of 02 released from the carrier complexes in preequilibrated solutions. In addition, an established DFT method (mPWPW91)34 with the atomic orbital basis functions, Co, Fe, and Cl (the compact relativistic effective core potential basis CEP-31G),35 N and O (6-311G ), P (6-311G ), C(6-31G), and H (STO-3G),36 were used to calculate the 180 EIE in terms of actual and model structures. The latter approach has also been employed for hypothetical intermediates in enzymes as described below. 

Often the most relevant characteristics of a separation method is peak capacity (pc)—the number of separable species for a particular sample. The pc is proportional to R, but also to the width of separation space—the range of separation parameters possible for a certain analyte type. As an illustration, an MS system with R = 1000 would produce pc 10 for a mixture of ions uniformly distributed between m/z = 500 and 1300 (a complex proteolytic digest) but only 10 for a mixture comprising m/z = 500-505 only (an isotopic envelope of a hypothetical compound). Hence broader separation space may compensate for lower R, which is typically the case for differential IMS in comparison with conventional IMS (Chapter 4). 

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