Single quantum coherence transfer pulse sequence

Figure 1. Gradient-enhanced heteronuclear single quantum coherence pulse sequence with coherence transfer selection and artifact suppression gradients. All pulses are of phase x unless otherwise indicated.

The HSQC (Heteronuclear Single Quantum Coherence) experiment is another widely used inverse detection experiment. It provides essentially the same information as HMQC, but relies on a different sequence of pulses to effect the transfer of magnetization between H and the heteronucleus. A direct comparison of HMQC and HSQC in the study of a natural product has indicated some advantages of the latter-sequence, which may provide improved sensitivity and narrower crosspeaks for improved resolution. ... 

In heteronuclear correlation experiments, magnetization transfer between protons and heteronuclei can be via either heteronuclear single quantum coherence (HSQC) or heteronuclear multiple quantum coherence (HMQC) pathways. The HSQC sequence gives rise to narrower lines, but uses more pulses and requires a longer phase cycle than the HMQC. Thus, HSQC is used for 2D experiments where the highest resolution is required and HMQC is preferred for 3D sequences in which the experimental time is limited. 

In addition to the Bloch-Siegert mechanism that is general for all paramagnetic species, another mechanism, specific for high-spin centres, plays an important role in DEER echo reduction on Gd(iii) species. If the frequency of the pump pulse is resonant with a transition that has a level in common with the transition already excited by the detection pulse sequence, then single-quantum coherence created by the detection pulses is partially or fully transferred to non-detectable double-quantum coherence. 

In solution-state NMR, many important experiments incorporate the creation and evolution of MQ coherence (MQC).5,6,84-86 Since MQC cannot be directly detected, experiments that follow the evolution of a MQC are inherently at least two-dimensional. This is the case with H- H DQ MAS spectroscopy. Figure 7 shows a corresponding pulse sequence and coherence transfer pathway diagram first, a DQC is excited, which subsequently evolves during an incremented time period q the DQC is then converted into observable single-quantum (SQ) coherence (SQC), which is detected in the acquisition period, q. To select the desired coherence transfer pathways, e.g., only DQC during q, a phase cycling scheme is employed.79,80 Pure absorption-mode two-dimensional line shapes are ensured by the selection of symmetric pathways such that the time-domain... 

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