Intrinsic shift

In a Mdssbauer transmission experiment, the absorber containing the stable Mdssbauer isotope is placed between the source and the detector (cf. Fig. 2.6). For the absorber, we assume the same mean energy q between nuclear excited and ground states as for the source, but with an additional intrinsic shift A due to chemical influence. The absorption Une, or resonant absorption cross-section cr( ), has the same Lorentzian shape as the emission line and if we assume also the same half width , cr( ) can be expressed as ([1] in Chap. 1)... 

C solvent shifts may be extrapolated to infinite dilution, and thence via the postulation of two types (82) and (83) of solvent interaction <74JMR(14)286), a scheme of parameterized solvent shifts can be prepared assuming that each type of interaction produces its own intrinsic shift. 

It is now necessary to relate 0<° to the chemical shift through Equation 47. This requires an assumption about the intrinsic shift of the ith solvation species. It will be assumed (cf. Equation 47) that... 

However, the assumption of Equation 80 is now dubious, for the intrinsic shifts of the solvates will depend on their structure and symmetry. Hence the statistical treatment and NMR assumptions are virtually dependent and it is inconsistent to treat them separately. 

The presence of such isomeric solvates may account for the asymmetry noted (62, 6) when carrying out the curve resolution of the 90-MHz spectra, but, within the accuracy attainable, the effect is barely significant. There are discrepancies in the values for the intrinsic shifts of the solvated species recorded by all three groups of workers (but differences between successive solvates are more satisfactory). Toma and co-workers calculated (62) the theoretically expected and isomeric differences between successive solvates, obtaining reasonable agreement with observed values (Table V). 

It should be noted that, since the H/Br difference is opposite in sign to the H/Cl and Br/Cl differences, the average intrinsic shift difference is considerably smaller than the absolute values of the... 

An important method for measuring intrinsic shifts consists of measuring the shifts, S, relative to an internal standard like tetramethyl silane [Si(CH3)4] in organic solvents for two nuclei at two different lanthanide concentrations [35]. For two nuclei a and b we have... 

As the chemical shifts 6j are mainly sensitive to the immediate environment of the ion, it is obvious that the intrinsic shifts 8, of the various solvated species depend on the fraction of B in the solvation shell. 

In the simplest case, the intrinsic shifts 4ds and direct relationship between the changes in obs and 7 i,s. On the other hand, if this relationship is not found experimentally, then the shift 4ds most likely depends on the coverage. 

If it is assumed that there is no force constant difference and thus no equilibrium isotope effect between the P- and y-olefinic positions on [21], the different upheld shift observed for these carbons is due to different chemical shifts for the two positions in the frozen static spectrum. The isotopic exchange would then occur between a single populated downheld site, the a-position, having different vibrational force constants from the upheld side which has four times the population (the P,P and y,y -positions). The spectra indicate that the sum of the upheld shift of the two olehnic positions is half the size of the downheld shift of the deuteriated a-carbon when corrections for intrinsic shifts are taken into account. 

A plot of S against (S + 5 ) should be linear with a slope of 4b/(4b + 4b)- Thus the relative intrinsic shifts can be obtained even without the knowledge of the shifts in the absence of complexation. 

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