The selective ge-1D HMQC-TOCSY experiment allows to obtain high-quality 1D HMQC-TOCSY spectra from which 1H-1H J-correlation can be elucidated starting from a selected carbon. Thus, in the first part of this experiment, magnetization is transferred from the selected carbon to the directly-bonded protons via a selective HMQC block. After this, the in-phase magnetization of these protons is allowed to evolve by a conventional TOCSY block under the effect of homonuclear J(HH) coupling. The same principles described here are also applied to the selective ge-1D HSQC-TOCSY experiment.

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 HMQC-TOCSY experiment is exactly the same as the conventional ge-2D HMQC-TOCSY pulse train in which the following modifications have been included ( 95JMRA32-114 and 95JMRA106-112 ):

• One of the two 90º 13C pulses in the HMQC block is made selective on a specific resonance. In order to improve the selectivity, proton decoupling is simultaneously applied during the long selective 13C pulse.
• 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 undesired 1H-12C magnetization. In theory, half of the signal is lost with compared to conventional phase-cycled experiment.
• Carbon decoupling during proton acquisition is optional. In the coupled version, the two satellites of the directly.attached protons will appear as in-phase multiplet, allowing the accurate measurement of the 1J(CH). In the decoupled version, these satellites collapse in a single signal.

The selective ge-1D HMQC-TOCSY experiment can be run with minor changes from a predefined parameter set. The HMQC block is optimized as discussed in the selective ge-1D HMQC experiment and the TOCSY mixing time is optimized as usual. 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 in the HMQC block as a function of 1/(2*J(CH)), in order to get in-phase magnetization of the directly-bonded protons.

Optimization of the mixing period (as a function of 1/(J(HH)). When long mixing times are employed, magnetization is transferred to the whole spin subsystem.

The selective ge-1D HMQC-TOCSY experiment affords a simple 1D 13C-edited TOCSY spectrum in which the  protons directly-bonded to the selected carbon appear as large doublet due to 1JCH and the protons J-coupled with them appear as conventional in-phase multiplets. 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 (the selective 90º carbon pulse and the receiver are usually inverted on alternated scans).

 Selective ge-1D HMQC-RELAY experiment