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

The structural elucidation of complex organic molecules relies heavily on t he application of proton detected heteronuclear NMR. Among these techniques , the HMBC NMR experiment is probably the most useful 2D NMR method The HMB C (C-H) experiment allows the assignment of structural fragments through co rrelations between protons and carbons separated by more than one bond, usu ally two or three bonds ((2)J(CH) and (3)J(CH)) via H-1,C-13-coupIing const ants. It is also possible to obtain valuable information through longer cor relations, (n)JCH n>3, performing several HMBC experiments with different l ong-range delays and using a deeper threshold in the contour plot. There ha ve been several attempts to improve the results of the HMBC experiment, mai nly focused an the question of optimization of the long-range delay, Delta (2). The D-HMBC, 3D-HMBC, CT-HMBC, ACCORD-HMBC, IMPEACH-MBC and CIGAR-HMBC experiments which provide much better experimental access to sample long-ra nge couplings are briefly discussed. These long-range correlations have pro ven to be crucial in the structure elucidation of molecules with proton def icient skeleton.