Two-dimensional nuclear magnetic resonance spectroscopy that gives correlations between carbons and protons that are separated by two, three, and, sometimes in conjugated systems, four bonds.
Notes: - Direct one-bond correlations are suppressed.
- HMBC gives connectivity information much like a proton-proton COSY.
- The intensity of cross peaks depends on the spin-spin coupling constant, which for three-bond couplings follows the Karplus relationship. For dihedral angles near \(\pu{90\!^{\circ}}\), the coupling is near zero. Thus, the absence of a cross peak does not confirm that carbon-proton pairs are many bonds apart.
- Because of the wide range (\(\pu{0}\) to \(\pu{14 Hz}\)) of possible carbon-proton couplings, two experiments are often performed. One optimized for \(\pu{5 Hz}\) couplings and the second optimized for \(\pu{10 Hz}\). This gives the optimum signal-to-noise ratio. Alternatively, a comprise value of \(\pu{7}\) to \(\pu{8 Hz}\) can be used. There are also "accordion" versions that attempt to sample the full range of couplings.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 690 (https://doi.org/10.1515/pac-2019-0203)