Unsymmetrical indirect covariance processing of hyphenated and long‐range heteronuclear 2D NMR spectra ‐ Enhanced visualization of 2JCH and 4JCH correlation responses

Recent reports have demonstrated the unsymmetrical indirect covariance combination of discretely acquired 2D NMR experiments into spectra that provide an alternative means of accessing the information content of these spectra. The method can be thought of as being analogous to the Fourier transform conversion of time domain data into the more readily interpreted frequency domain. Hyphenated 2D‐NMR spectra such as GHSQC‐TOCSY, when available, provide an investigator with the means of sorting proton‐proton homonuclear connectivity networks as a function of the ¹³C chemical shift of the carbon directly bound to the proton from which propagation begins. Long‐range heteronuclear chemical shift correlation experiments establish proton‐carbon correlations via heteronuclear coupling pathways, most commonly across three bonds (³J_CH), but in more general terms across two (²J_CH) to four bonds (⁴J_CH). In many instances ³J_CH correlations dominate GHMBC spectra. We demonstrate in this report the improved visualization of ²J_CH and ⁴J_CH correlations through the unsymmetrical indirect covariance processing of GHSQC‐TOCSY and GHMBC 2D spectra.