Spin-state-selective excitation in selective 1D inverse NMR experiments.

Abstract

A general and very simple strategy for achieving clean spin-state-selective excitation with full sensitivity in carbon-selective gradient-enhanced 1D HMQC and HSQC pulse schemes is presented. The incorporation of an additional hard 90 degrees (13)C pulse applied along a specific orthogonal axis just prior to acquisition into the conventional sequences allows us to select a simultaneous coherence transfer pathway which usually is not detected. The superimposition of this resulting antiphase magnetization to the conventional in-phase magnetization gives the exclusive excitation of the directly attached proton showing only the alpha or beta spin state of the passive (13)C nucleus. The propagation of this particular spin state to other protons can be accomplished by adding any homonuclear mixing process just after this supplementary pulse. Such an approach affords a suite of powerful selective 1D (13)C-edited NMR experiments which are helpful for resonance assignment purposes in overcrowded proton spin systems and also for the accurate determination of the magnitude and sign of long-range proton-carbon coupling constants in CH spin sytems for samples at natural abundance. Such measurements are performed by measuring the relative displacement of relayed signals in the corresponding alpha and beta 1D subspectra.