The 2D HMBC experiment permits to obtain a 2D heteronuclear Chemical Shift correlation map between long-range coupled 1H and heteronuclei (commonly, 13C). It is widely used because it is based on proton-detection, offering high sensitivity when compared with the carbon-detected COLOC experiment ( 87MRC325 ). In addition, long-range proton-carbon coupling constants can be measured from the resulting spectra.
 
 

Easy implementation on any AVANCE spectrometer equipped with an inverse probehead.

The basic 2D HMBC pulse sequence is closely related to the HMQC pulse sequence but incorporating the following modifications ( 86JACS2093 , 88JACS7926 , and 88JMR186-78 ):

• An optional low-pass J-filter (consisting of a delay-90º(13C) cluster) can be included after the initial 90º 1H pulse to minimize direct responses.
• The defocusing period is optimized to 1/2*ⁿJ(CH) (5-10 Hz).
• The refocusing period is usually omitted.
• Proton acquisition is performed without X decoupling.

Variants of the HMBC experiment:
 
• The incorporation of pulsed-field gradients in the HMBC pulse sequence allows to obtain high-quality spectra in shorter acquisition times (see ge-2D HMBC experiment).
• A refocused and 1H-decoupled version has been reported ( 95TL2817 ).
• 2D versions using band-selective excitation to achieve better resolution in the indirect dimension (see 2D semi-selective HMBC experiment) .
• Analog long-range-optimized 2D HSQC experiments have been reported to measure  ⁿJ(CH) and compared to the HMBC experiment ( 90JMR488-87 , 92JMR411-97 , 93MRC231 , 95JMRB326-109 , and 97JMR503-124 ). Long-range optimized HSQC experiments optimized for 31P have been also applied in nucleic acids (referred as HELCO: 93JMRB81-102 and 00MRC288 ).
Several approaches have been proposed to measure ⁿJ(CH) coupling constants from HMBC Spectra:
 
• From a fitting procedure ( 88CPL545 , 89JMR111-85 , 91JMR533-93 , and 95JMRA43-112 ).
• From resonance intensities compared with a reference spectrum ( 93JMRA353-104 and 94JMRA257-110 ).
• Qualitative estimation based on cross-peak intensities ( 89JMR223-83 and 91BIO10457 ).
• Comparison of proton multiplet widths ( 89JMR223-83 ).
• Direct measurement from phase-sensitive spectra ( 00MRC321 )
• Use of Accordion spectroscopy ( 00MRC452 )

Several processing methods have been applied on HMBC spectra to reduce undesired t1-noise ( 93JMRA351-101 and 98MRC135 ).

The 2D HMBC experiment can be recorded in routine/automation modes. Minor changes are required if a predefined parameter set is available.The interpulse d2 delay is usually optimized to 1/2*nJCH (50-70 ms).
 
More details on practical implementation of ge-2D HMBC experiments on AVANCE spectrometers can be found in

Tutorials: 2D inverse experiments

Tutorials: 2D gradient-based inverse experiments

The HMBC spectrum shows the typical 2D long-range correlation map. A cross-peaks means that the corresponding 1H and heteronucleus are two- or three-bonds away. Residual direct connectivites are usually present as large doublets due to ¹J(CH).

Related experiments:

2D Inverse experiments

2D Inverse gradient-enhanced experiments