Isotopic shifts

A second example of a study exploiting chemical shift isotope effects is shown in Fig. 7.15b which reports 1II, 2H and 19F low temperature NMR spectra of the... 

Fig. 12.7 An unusually long range (10-bond) secondary D isotope shift IE on an [19F] NMR. The H/D chemical shift isotope effect due to substitution at an OH — N hydrogen bond sited ten bonds away from a para-F is 11 ppb (Hansen, P. E. et al. Acta Chim. Scandanavia 51,881 (1997))...

Compound C-D signal multi- plicity C-D coupling constants Hz 13C shifts Isotopic H compound ... 

Interpretation of cellulose-interred lake water records would benefit from coupled estimation of lake water in order to clearly decipher shifting isotopic composition of precipitation from changes in water balance (see Fig. 1). Exploratory paleolimnological reconstructions at Austin Lake demonstrate that further studies on the hydrogen isotope composition of kerogen and its isotopic relationship with ambient water is an especially promising new research direction (Krishnamurthy et al., 1995 also see Fig. 7). Mapping temporal shifts in lake water isotopic composition in two-dimensional space... 

Isotope Shifts. - Isotope shifts are intimately related to rovibrational corrections. Thus, with the same shielding surfaces used for rovibrational averaging, one can extract expected changes in chemical shift due to isotope substitution. This is once again illustrated in Dransfeld s work, which is cited near the beginning of the preceding section. Here too, some cautionary words are in... 

Chemical shifts are shifted to higher field when deuterium replaces hydrogen either geminally or vicinally the stereochemical dependence of the vicinal-shift isotope effect has been investigated by means of cis- and trans-[2- H]fluorocyclohexane with spectra obtained under conditions of slow ring-inversion at — 85°C. The observed F isotope shifts (/p.p.m.) for (34) and (35) are shown. Values for (36) and (37) and the norbornyl systems (38) and (39) are also shown these were determined in order to ascertain the stereochemical dependence of the isotopic shift. The smallest... 

Given these observations in natural samples, systematic investigations of which processes can reset metal isotope ratios, how those processes shift isotope records, and what signs can be used to detect, and possibly correct for, the effects of diagenesis are absolutely essential. Some of the experimental and theoretical studies discussed above have yielded results that will help build a fundamental understanding of how diagenesis affects stable isotopic signatures of metals, but to... 

The preceding discussion makes clear the fact that the precise values of chemical shift isotope effects will depend on the choice of isotopic reference. Correct choice of the reference is somewhat subtle. In the context of tritium as an isotope at low abundance, pertritiated TMS would clearly be an incorrect choice partially monotritiated TMS would appear to be best. 

Having given the definition of isotope effects on chemical shifts, isotope effects can be further subdivided into intrinsic or equilibrium. In the latter case, isotope substitution many lead to a change in the chemical equilibrium due to a difference in zero-point energies as illustrated in Figure 6.3 (for an explanation, see later). 

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