On the Temperature Dependence of Lanthanide-Induced NMR Shifts.
ILLINOIS UNIV AT URBANA-CHAMPAIGN DEPT OF CHEMISTRY
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The current theories of the origin of lanthanide-induced NMR shifts are examined and compared with one another and with experimental results in an attempt to understand the temperature dependence of the shifts. It is concluded that shifts arising from a pseudocontact mechanism should in general show a 1T squared behavior as predicted by Bleaney. In most situations with a fixed molecular geometry, theory does not predict a substantial 1T component of the shift but in some instances a fortuitous combination of crystal field parameters can lead to additional 1T to the n power components n greater than or equal to 3 which cause the overall shift to appear to be a linear function of 1T with a large intercept at 1T 0. Extension of Bleaneys method to the positive 1T cubed term shows it to be negligible compared to that in 1T squared for systems with J 32 and relatively small approximately 10 for larger J. In the absence of detailed experimental studies of crystal field parameters in solution, it is likely that for the majority of lanthanide adduct systems the most realistic description of the proton shifts is in terms of the well-established 1T squared dependence.
- Atomic and Molecular Physics and Spectroscopy