TECHNICAL REPORT NO. 88 NMR Methods for Determining Chemical Exchange Rates

摘要

Factors which affect the accuracy of high-resolution and spin-echo nmr determinations of chemical exchange rates are analyzed and discussed, including mathematical as well as experimental aspects. Numerical methods show that most of the mathematical approximations commonly used to evaluate exchange rates from high-resolution spectra are subject to systematic error, and circumstances under which the error become serious are presented. A line-width equation is derived which gives accurate exchange rates in the vicinity of the coalescence point for an uncoupled AB system, and from which the chemical shift at the coalescence point can be obtained. The effects of saturation broadening are calculated and compared with experiment. In general, it appears desirable to determine exchange rates from high-resolution spectra by using computer-ized, complete line-shape fitting methods which do not neglect spectral complication unless their effects are demonstrably less than other inaccuracies in the rates. In rate determinations by the spin-echo method which employs a Carr-Purcell train of echoes, it is possible for self diffusion to cause systematic error. Simple criteria are presented for estimating and minimizing such effects. Other presently known factors peculiar to the spin-echo method, including the effects of finite pulse widths, do not appear to present serious problems.