Single-quantum coherences

HC HMQC (heteronuclear multiple quantum coherence) and HC HSQC (heteronuclear single quantum coherence) are the acronyms of the pulse sequences used for inverse carbon-proton shift correlations. These sensitive inverse experiments detect one-bond carbon-proton connectivities within some minutes instead of some hours as required for CH COSY as demonstrated by an HC HSQC experiment with a-pinene in Fig. 2.15. 

HSQC Heteronuclear single quantum coherence, e.g. inverse CH correlation via one-bond coupling providing the same result as HMQC but using an alternative pulse sequence... 

Figure 1.33 The underlying principle of the Redfield technique. Complex Fourier transformation and single-channel detection gives spectrum (a), which contains both positive and negative frequencies. These are shown separately in (b), corresponding to the positive and negative single-quantum coherences. The overlap disappears when the receiver rotates at a frequency that corresponds to half the sweep width (SW) in the rotating frame, as shown in (c). After a real Fourier transformation (involving folding about n = 0), the spectrum (d) obtained contains only the positive frequencies.

Apparently, all coherence pathways will therefore start at zero coherence levels and end at -t-1 coherence levels since the quadrature receiver is sensitive only to the +1 polarization, only the single-quantum coherence is detected. 

What is the difference between single-quantum coherence and zero-... 

Single-quantum coherence is the type of magnedzadon that induces a voltage in a receiver coil (i.e., Rf signal) when oriented in the xy-plane. This signal is observable, since it can be amplified and Fourier-transformed into a frequency-domain signal. Zero- or multiple-quantum coherences do not obey the normal selection rules and do not... 

In contrast, in a two-spin system the two nuclei coupled with each other by the coupling constant, J, will have four energy levels available for transitions (Fig. 5.56). Such a system not only has single-quantum coher-... 

Coherence A condition in which nuclei precess with a given phase relationship and can exchange spin states via transitions between two eigenstates. Coherence may be zero-quantum, single-quantum, double-quantum, etc., depending on the AM of the transition corresponding to the coherence. Only single-quantum coherence can be detected directly. 

DEPT (distortionless enhancement by polarization transfer) A onedimensional C-NMR experiment commonly used for spectral editing that allows us to distinguish between CH, CH2, CH, and quaternary carbons. Detectable magnetization The magnetization processing in the x y -plane induces a signal in the receiver coil that is detected. Only single-quantum coherence is directly detectable. 

Double-quantum coherence Coherence between states that are separated by magnetic quantum numbers of 2. This coherence cannot be detected directly, but must be converted to single-quantum coherence before detection. 

Single-quantum coherence Coherence between states whose total quantum numbers differ by 1. The only type of coherence that can be observed directly. 

The most powerful techniques of all are undoubtedly the 2-D proton-carbon experiments (Hetero-nuclear Multiple Quantum Coherence///eteronuclear Single Quantum Coherence, or HMQC/HSQC and //ctcronuclcar Multiple Bond Correlation, or HMBC) as they provide an opportunity to dovetail proton and carbon NMR data directly. 

HSQC //eteronuclear single quantum coherence. As for HMQC but with improved resolution in the carbon dimension. 

Fig. 10.15. Pulse sequence for the multiplicity-edited gradient HSQC experiment. Heteronuclear single quantum coherence is created by the first INEPT step within the pulse sequence, followed by the evolution period, t . Following evolution, the heteronuclear single quantum coherence is reconverted to observable proton magnetization by the reverse INEPT step. The simultaneous 180° XH and 13C pulses flanked by the delays, A = l/2( 1 edits magnetization inverting signals for methylene resonances, while leaving methine and methyl signals with positive phase (Fig. 16A). Eliminating this pulse sequence element affords a heteronuclear shift correlation experiment in which all resonances have the same phase (Fig. 16B).

Finally, the double quantum (DQ) spin spin relaxation time T2d can be determined using the pulse sequence 90° — x — 45° — 90° —t — 45°.51 The first three pulses create the DQ coherence, and the last read pulse converts the DQ to a single quantum coherence for detection. 

Exploitation of the TROSY effect is rather straightforward. In contrast to 15N-HSQC (Heteronuclear Single Quantum Coherence) or standard triple-resonance experiments based on 15N-HSQC, no radio frequency pulses or composite pulse decoupling should be applied on amide protons when HN spin is not in the transverse plane. Likewise the 15N decoupling should be... 

N single quantum coherence. The chemical shift of 15N is recorded in a (semi-)constant-time manner during t2 and ultimately the 15N-1Hn PEP-TROSY element selects the most slowly relaxing 15N Hn multiplet component prior the acquisition. The coherence flow can thus be described as... 

FHSQC fast heteronuclear single-quantum coherence... 

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