The ge-2D HMQC experiment is the gradient-enhanced version of the conventional 2D HMQC experiment in which coherence selection is achieved by means of PFG. Thus, clean 2D HMQC spectra can be recorded in a single scan per t₁ increment without need for phase cycle when sample concentration is high. Other advantages are the optimal dynamic range, improved water and artefact suppression, and reduced t₁ noise in the minimally required experiment time. The HMQC experiment allows to trace out directly bonded ¹H-X pairs via the large ¹J_HX coupling constant.
 

Easy implementation on any AVANCE spectrometer equipped with pulsed field gradients (PFGs) and inverse probehead.

In addition of the advantages to use PFGs, an important aspect when PFG are incorporated in the HMQC pulse sequence, are the sensitivity and resolution requirements. Thus, in the original ge-2D HMQC pulse sequence ( 91JMR648-91 , 91JACS9688 , 92JMR305-97 , and 92JMR282-100 ) PFGs are used for coherence selection and only one of two desired coherence-transfer pathways are selected, thereby producing magnitude-mode spectra. Thus, defocusing gradients are usually applied during the variable evolution period and the refocusing gradient is applied in the last proton evolution period prior to acquisition.This version can be useful for small or medium-size molecules where resolution and sensitivity are not the limiting factor

Several related HMQC schemes have been reported:

• Alternatively, phase sensitive spectra can be recorded using the echo-antiecho approach ( 92JMR207-98 , 92JMR462-98 , 92JMR660-98 , and 93JB215 ), the PEP methodology ( 98JMR232-135 ), introducing a multiple-quantum filter ( 92JMR215-100 ) or a z-filter ( 93MRC287 ) although that unsuitable signal losses are present due to single-pathway selection when compared to phase cycled versions. Phase sensitive spectra can also be obtained using difusion filters in the defocusing-refocusing periods ( 94JMRA116-108 ).
• The basic HMQC experiment has been modified to include gradient-based water-suppression methods as, for instance, the WET scheme during the preparation period ( 95JMRA295-117 ) and the water flip-back in the so-called FHMQC experiment ( 98JB279 ).
• A Constant-time HMQC version has been described ( 95JB335 ).
• An HMQC-J-resolved version has been reported to measure proton-proton coupling constants for samples at natural abundace ( 01TL899 ). Related experiments were already described to measure J(HA-HN)coupling constants in labelled proteins.
• Simultaneous acquisition of ¹H-¹³C and ¹H-¹⁵N 2D spectra can be obtained using the Time-Shared method ( 91JMR635-93 and 94JB201 ).
• Modified ge-2D HMQC-type sequences have been proposed as a tool to measure J(HN), J(NN) and J(CN) coupling constants ( 96JACS5096 , 97JMR383-124 , 99JMR185-141 ) from both F1 and F2 dimensions in small organic compounds.
• Improved resolution in the F1 dimension can be achieved by applying a band-selective carbon pulse (see Semi-selective HMQC experiment).
• Solvent suppression methods can be included as, for instance, the WATERGATE scheme.
• The HMQC can be recorded by decoupling on other nuclei in a third throughout the experiment and during acquisition.

The ge-2D HMQC experiment is usually recorded in routine/automation modes and minor changes are required if a predefined parameter set is available. Important parameters to consider:

• Version to be used: magnitude mode vs phase sensitive data
• The user need to define the strength, duration, shape of the gradients and the recovery delay.
• Offset and spectral widths in each dimension.
• Optimization of the evolution delay as a function of 1/(2*¹J_XH).

More details on practical implementation of ge-2D HMQC experiments on AVANCE spectrometers can be found in the following tutorials:

Tutorials: 2D inverse experiments

Tutorials: 2D gradient-based inverse experiments

The HMQC spectrum correlates chemical shifts of heteronucleus X (F₁ dimension) and protons (F₂ dimension) via the direct heteronuclear coupling 1J(XH). Broadband carbon decoupling is usually performed during proton acquisition and the corresponding satellites colapse to a single resonance showing all proton-proton couplings. The effective suppression of unwanted ¹H-¹²C or ¹H-¹⁴N magnetization by means of PFGs allows to obtain ultra-clean 2D spectra from which clear analysis can be done.

Related experiments:

2D Inverse experiments

2D Inverse gradient-enhanced experiments