Use of long-range C-H (ⁿJ n>3) heteronuclear multiple bond connectivity in the assignment of the ¹³C NMR spectra of complex organic molecules

The structural elucidation of complex organic molecules relies heavily on the application of proton detected heteronuclear NMR. Among these techniques, the HMBC NMR experiment is probably the most useful 2D NMR method. The HMBC (C-H) experiment allows the assignment of structural fragments through correlations between protons and carbons separated by more than one bond, usually two or three bonds (²J_CH and ³J_CH) via ¹H, ¹³C-coupling constants. It is also possible to obtain valuable information through longer correlations, ⁿJ_CH n>3, performing several HMBC experiments with different long-range delays and using a deeper threshold in the contour plot. There have been several attempts to improve the results of the HMBC experiment, mainly focused on the question of optimization of the long-range delay, Δ₂. The D-HMBC, 3D-HMBC, CT-HMBC, ACCORD-HMBC, IMPEACH-MBC and CIGAR-HMBC experiments which provide much better experimental access to sample long-range couplings are briefly discussed. These long-range correlations have proven to be crucial in the structure elucidation of molecules with proton deficient skeleton.