| Abstrakt: |
Most treatments of solution-state pulse FT NMR ignore relaxation effects during pulses. Since the length of nonselective 90° pulses is on the order of 10 μs, while proton T1values (in solution) are generally >0.5 s, this simplification is valid. The advent of highly selective shaped pulses, however, has resulted in practical pulse lengths of over 100 ms. When using these pulses on samples where T1or T2is of the same order of magnitude as the pulse length, one cannot ignore relaxation during the pulse. Here, a description of the buildup of the NOE during the selective pulse and a quantitative analysis of ID selective NOE difference experiments where shaped pulses have been used is presented. Although this experiment itself is not new, a quantitative analysis of the data has not been presented previously. It is shown that longitudinal, transverse, and cross relaxation duringthe selective pulse cannot be ignored for samples where any of these relaxation times is on the order of the pulse length. This is especially significant in applications to large molecules, such as proteins, where spectral crowding requires high selectivity, i.e., long pulses, and the T1values are frequently less than 0.5 s. |
