ABSTRACT
The benefits of polarization transfer are extended to two-dimensional experiments. One of the most straightforward versions of two-dimensional polarization transfer is heteronuclear correlation (HETCOR). The two-dimensional experiment of choice for detecting heteronuclear correlations is one in which the more sensitive nucleus is detected: the double insensitive nucleus enhancement by polarization transfer (INEPT) or heteronuclear single-quantum coherence (HSQC) experiment. Since the insensitive nucleus is detected indirectly, HSQC is always a two-dimensional experiment. An operator analysis of all of the INEPT-based experiments reveals that at no point is there more than one transverse term in any of the product operators. Higher-order coherences can be used to obtain polarization transfer and signal enhancement in heteronuclear nuclear magnetic resonance (NMR) experiments. A one-dimensional NMR experiment providing such enhancement is called distortionless enhancement by polarization transfer (DEPT). Although, HSQC has supplanted HMQC as the standard two-dimensional heteronuclear correlation experiment on modern NMR spectrometers, the HMQC experiment remains an important building block for three-dimensional NMR experiments.
