The selective ge-1D HMBC experiment permits to obtain high-quality 1D HMBC spectra only showing the protons long-range coupled to an specific heteronucleus (for instance, 13C). From the corresponding clean antiphase multiplets, long-range proton-carbon coupling constants can be accurately measured using a fitting algorithm.

Easy implementation on AVANCE spectrometers equipped with pulsed field gradients, selective excitation using shaped pulses and inverse probehead.

The basic pulse sequence of the selective ge-1D HMBC experiment is exactly the same as the conventional ge-2D HMBC pulse train in which the following modifications have been included:

• One of the two 90º 13C pulses is made selective on a specific heteronucleus.  In order to improve the selectivity and to avoid J-coupling evolution during the long selective pulse, a proton spin-lock period can be applied simultaneously with it.
• The variable evolution period of the 2D version is fixed to a  minimum delay (3 microseconds).
• Gradients are usually applied for coherence selection purposes in natural abundance samples to obtain efficient suppression of 1H-12C magnetization.

The original sequences were published in 95JMRA106-112 , 95JMRA32-114 , and 95MRC827 . Recently, a refocused version of this experiment has been proposed ( 98JMR214-132 ) offering the possibility to extract long-range proton-carbon-coupling constants from highly desirable in-phase multiplets. However, careful optimization of the refocusing delay and sensitivity losses due to transverse relaxation are the major drawbacks.

Equivalent selective ge-1D long-range optimized HSQC experiment ( 95JMRA32-114 ) and ge-1D long-range optimized HSQC experiment ( 96MRC377 ) have been also proposed following the same principles mentioned for the HMBC experiment. Alternatively, the same results have been reported using a refocused HSQC pulse train, in which carbon excitation is applied on in-phase magnetization and direct responses are removed using a BIRD cluster in the middle of the refocusing period ( 95MRC827 , 00MRC265 ).

The selective ge-1D HMBC experiment can be run with minor changes from a predefined parameter set. Important parameters to consider are:

Selectivity of the selective excitation 13C pulse: the user must define the offset, the shape, the duration and the power level needed for a defined excitation profile.

Optimization of the J-coupling delay as a function of 1/(2*J(CH)), in order to get anti-phase magnetization of the long-range coupled protons.

In a selective 1D HMBC spectrum, only protons two and/or three bonds away from an specific carbon will be present. The use of gradients allows to obtain a clean, artefact-free spectrum in a short time in which perfect suppression of undesired 1H-12C magnetization is achieved with a single two-step phase cycle.