Inverse‐detected two‐dimensional NMR experiments on a system undergoing chemical exchange near coalescence

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Abstract
In systems undergoing intermediate chemical exchange, observation of carbon NMR signals by standard onedimensional methods is often hampered by severe line broadening accompanied by reduced sensitivity. In contrast, the HMQC inverse‐detected heteronuclear chemical shift correlated experiment detects the 13C 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 efficiently detected. The carbon spectrum of 1‐tert‐butoxycarbonyl‐N‐tert‐butyloctahydro‐1H‐indole‐2‐carboxamide serves to demonstrate this phenomenon. Carbons severely broadened in the 1D experiment appear largely unaffected when detected in the HMQC experiment. A straightforward explanation for this observation is presented together with a preliminary discussion of the carbon (F1) line widths in the HMQC spectra of exchanging systems.