The Kinetic and Mechanistic Evaluation of NMR Spectra. New analytical methods (18)
Tóm tắt
In addition to the static parameters of the chemical shifts and coupling constants, which serve as a source of knowledge for molecular structure and stereochemistry, an NMR spectrum can frequently furnish dynamic quantities characterizing relaxation and exchange phenomena. The information about nuclear switching processes has proved to be particularly useful in practice for the detection of internal molecular motions and for the estimation or determination of the corresponding energy barriers. A plethora of studies of this nature has in the past been performed on simple proton spectra. Methodological developments of recent years have led to a significant reduction of the effort required for the quantitative dynamic evaluation of NMR spectra arising from complex spin systems or involving other nuclei. In many cases it has, moreover, become possible to extract detailed mechanistic information inaccessible by other means. The practical execution of such analyses will be explained and illustrated by a selected number of applications.
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Tài liệu tham khảo
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The fact that at the coalescence point of two equally intense singlets the total width equals the sum of the static splitting and the natural width cannot be exploited for bandshape analyses although it is unequestionably very useful for an estimation of a ΔG+value.
An iterative calculation for an experimental test example of two exchanging singlets with a chemical shift difference of about 50 Hz andW≈ 3W0 in which the dynamic parameterskandW0were confined to the expected domain of 0—5 s−1by means of reflective boundaries [26] yielded that value ofk= 1.26 ± 96424 s−1(correlation coefficient ρ(k W0) = −0.999998); a result that speaks for itself!
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Careful selection of the temperature is critical. The exchange should still be fast enough so as not to be swamped by relaxation but not so fast that saturation is transferred indiscriminately also to the other positions not connected to the irradiated line by an elementary step.