An improved chemical-shift-selective filter

Abstract

Two-dimensional NMR correlation experiments (1-4) are now widely used for assigning all but the simplest of NMR spectra. Paradoxically, it is the very virtue of these experiments, the completeness of the set of correlations that they provide, that can in some circumstances be a disincentive to using them. It is often the case in the spectra of small or medium sized molecules that only the assignments of relatively few of the resonances are ambiguous. If a two-dimensional correlation experiment is used in these circumstances much time will inevitably be expended in redetermining what is already known. Consequently it may be much quicker to use a one-dimensional experiment, which performs the same assignment function, but selectively for a single spin. A number of 1 D correlation experiments have been developed (5-1 I), many of which are the 1 D analogs of 2D experiments such as COSY (I-4), designed by replacing the first nonselective “hard” pulse of one of these experiments by a semiselective “soft” one (8-10). The information produced by such 1 D analogs pertains to the spin which is initially excited selectively by the sol% pulse. Unfortunately, these 1 D experiments have a number of drawbacks that restrict the extent of their application, including the condition that the multiplet of the spin under investigation be resolved from the transitions of its neighbors. Even if this criterion is met, it is often necessary to be able to precisely shape the soft pulse if the selected spin is to be excited cleanly and the necessary hardware is not yet widely available. We have recently shown (II) that both of these limitations can be overcome by using a chemical-shift-selective filter (Fig. 1A) which requires only that the chemical shift of the spin, and not its multiple& be resolved from those of its neighbors. Unfortunately the usefulness of that chemical-shift-selective filter is greatly diminished by the need to repeat the experiment many times (up to 90 or more) if it is to provide a high level of rejection. In the present communication we demonstrate that by combining a chemical-shift-selective tilter together with a semiselective pulse in one pulse sequence (Fig. 1B) it is possible to overcome the disadvantages that both techniques have when used separately, thereby making possible the rapid selection of any spin which has a uniquely resolved chemical shift, and without the necessity of shaping the soft pulse. Frequency-selective filtration (12-16) is accomplished by combining the free induction decay signals from a number of experiments in which evolution due to free precession has been allowed to occur for different integer multiples of a period A prior