HMBC


The HMBC (Heteronuclear Multiple Bond Correlation) experiment gives correlations between carbons and protons that are separated by two, three, and, sometimes in conjugated systems, four bonds. Direct one-bond correlations are suppressed.This gives connectivity information much like a proton-proton COSY. The intensity of cross peaks depends on the coupling constant, which for three-bond couplings follows the Karplus relationship. For dihedral angles near 90 degrees, the coupling is near zero. Thus, the absence of a cross peak doesn't confirm that carbon-proton pairs are many bonds apart.

Because of the wide range (0-14 Hz) of possible carbon-proton couplings, one often does two experiments. One optimized for 5 Hz couplings and the second optimized for 10 Hz. This gives the optimum signal-to-noise. Alternatively, a comprise value of 7-8 Hz can be used. There are also "accordion" versions that attempt to sample the full range of couplings.


The spectrum of sucrose at 500 MHz is shown below. The peak outlined in green shows the two bond correlation between the 2' carbon and the 1' proton. The peak outlined in red correlates the 6 carbon and 4 proton separated by 3 bonds. Note also that the 2' carbon correlates with the 1 proton across the glycosidic bond.