The 2D heteronuclear relayed coherence transfer experiment is useful to have information available about protons that are vicinally coupled to a proton attached to a given carbon atom. However, the H,X Relay experiment cannot bridge heteroatoms or quaternary carbon sites, cannot orient the spin systems it identifies and connectivity networks can be break if substancial  disparity between the active and optimized coupling exists.

Easy implementation on any AVANCE spectrometer.

The basic pulse sequence of the heteronuclear relayed coherence transfer experiment ( 83JMR149-53 ) differs of the original sequence ( 82JMR336-48 and 82CPL139 ) by the inclusion of the 180º pulse midway through the D2 interval in order to avoid heteronuclear coupling evolution and to obtain a better sensitivity. The efficiency of the transfer is largely dependent of the disparity between the ideal and the optimized value of the delays ( 83JACS6944 ). Better alternatives to remove direct responses in this experiment have also been proposed by including a low-pass J-filter after the initial 90º proton pulse ( 83JMR157-55 ) or a BIRD operator midway through the evolution period ( 85JMR225-63 ). A much better alternative is to replace the relay block by a TOCSY block ( 85JMR230-63 and 90MRC62 ). A final category of heteronuclear relayed experiments include proton DQ ( 83JMR333-54 and 85JMR169-65 ) relayed transfers, designed to avoid undesirable signal overlap.

The 2D X,H RELAY experiment can be recorded in routine/automation modes. All acquisition parameters as described in 2D HETCOR experiment. except the fixed evolution period must be optimized as described for homonuclear RELAY experiments.

Tutorials: 2D X-detected experiments

Presentation of the H,X Relay spectra is similar to HETCOR spectra. Two types of responses appear (i) at the frequency of the directly attached proton (H_d) and (ii) at the chemical shift of those protons vicinally coupled to H_d. In order to differenciate direct and relayed signals, a previously acquired HETCOR experiment should be required.

Presently, improved sensitivity ratios are obtained from heteronuclear correlation experiments based on proton detection as the HMQC-TOCSY or HSQC-TOCSY experiments.

Related experiments:

2D X-detected experiments