INVERSE-DETECTED 2-DIMENSIONAL NMR EXPERIMENTS ON A SYSTEM UNDERGOINGCHEMICAL-EXCHANGE NEAR COALESCENCE

In systems undergoing intermediate chemical exchange, observation of c arbon NMR signals by standard one-dimensional methods is often hampere d by severe line broadening accompanied by reduced sensitivity. In con trast, the HMQC inverse-detected heteronuclear chemical shift correlat ed experiment detects the C-13 satellites of the proton lines. If the proton exchange is fast, the width of the satellites is independent of the intramolecular carbon exchange, and hence the carbons are more ef ficiently detected. The carbon spectrum of onyl-N-tert-butyloctahydro- 1H-indole-2-carboxamide serves to demonstrate this phenomenon. Carbons severely broadened in the 1D experiment appear largely unaffected whe n detected in the HMQC experiment. A straightforward explanation for t his observation is presented together with a preliminary discussion of the carbon (F1) line widths in the HMQC spectra of exchanging systems .