heteronuclear multiple bond correlation NMR 

Initialism: HMBC
https://doi.org/10.1351/goldbook.08352
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:
  1. Direct one-bond correlations are suppressed.
  2. HMBC gives connectivity information much like a proton-proton COSY.
  3. 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 90 ∘, the coupling is near zero. Thus, the absence of a cross peak does not confirm that carbon-proton pairs are many bonds apart.
  4. Because of the wide range (0 to 14 Hz) of possible carbon-proton couplings, two experiments are often performed. One optimized for 5 Hz couplings and the second optimized for 10 Hz. This gives the optimum signal-to-noise ratio. Alternatively, a comprise value of 7 to 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 [Terms] [Paper]
Citation: 'heteronuclear multiple bond correlation NMR' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.08352 RIS BibTex EndNote
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