2D NMR Experiments

One dimensional NMR experiments are plots of signal intensity against frequency. Two dimensional experiments are usually shown as contour plots of signal intensity against two frequencies. This is similar to the way height is represented on normal maps.

2D experiments are now routinely often run using Pulsed Field Gradients (PFG). For samples of normal concentration, experiments using Pulse Field Gradients give faster results and cleaner spectra than their non-gradient versions.



COSY (Correlation Spectroscopy)

The cross-peaks in a COSY spectrum indicate which nuclei (usually protons) in a molecule are spin-spin coupled.

COSY shows scalar couplings where nuclear spins interact through the electron spins of chemical bonds and is most often used to analyse coupling relationships between protons.



DQF-COSY (Double Quantum Filtered Correlation Spectroscopy)

The double-quantum filter serves to alter the phase properties of the spectrum, enabling a phase-sensitive presentation and thus higher resolution. In practice, this aids the analysis of crowded spectra, reducing cross-peak overlap, and allows the potentially informative fine-structure.

In general, it is a phase sensitive, higher resolution version of the COSY experiment where diagonal cross peaks are suppressed. However, because of the filter, the intensity of signals is reduced and will take longer time compared with COSY.



HMQC (Heteronuclear Multiple-Quantum Correlation)

Correlations indicate heteronuclear spins which are coupled across a single bond. Proton or 'inverse' detected. The desired correlations in HMQC are between the low intensity 1H-13C signals. The 1H-12C resonance, the main line in the proton spectra, is a source of interference which is suppressed with pulsed field gradients.

It also provides a convenient way of identifying diastereotopic geminal protons (which are sometimes difficult to distinguish unambiguously, even in COSY) since only these will produce two correlations to the same carbon.



HSQC (Heteronuclear Single-Quantum Correlation)

Similar to HMQC but can provide higher resolution for molecules especially for larger (biological) molecules. Can be run multiplicity edited so that CH and CH3 correlations can be easily distinguished from CH2 correlations.

For most routine applications the resolution difference between HMQC and HSQC for small molecules is barely noticeable, but where crowding occurs in the X dimension (such as polysaccharide), the HSQC should provide better results (provided sufficiently high digital resolution is used).



HMBC (Heteronuclear Multiple-Bond Correlation)

Set up to detect smaller heteronuclear couplings. Therefore correlations indicate heteronuclear spins which are coupled across a multiple (typically 2 or 3) bonds.

Because it is developed from HMQC pulse, proton or 'inverse' detected 1 bond (HMQC) correlations do sometimes break through in HMBC spectra but will appear as doublets and so can usually be easily identified. HMBC is an extremely powerful tool for piecing together organic structures.



HETCOR

Correlations are of heteronuclear spins across a single bond but the experiment is standard or X (e.g.13C) detected. Used less nowadays due to the lower sensitivity than HMQC & HSQC.

However, HETCOR still find use when very high carbon resolution is required (such as ligands with fused aromatic rings in organometallic chemistry), since this is easier to achieve in the directly observed dimension of a 2D experiment.



NOESY (Nuclear Overhauser Effect spectroscopy)

2D NOESY spectra are similar in appearance to COSY spectra. NOESY shows dipolar couplings where nuclear spins interact via though-space magnetic interactions.

Generally, NOESY experiment is suitable for large (>3000Da) or small (<1000Da) molecules.



ROESY (Rotating-Frame NOE Spectroscopy)

NOE is measured in the "rotating-frame". ROESY is used for molecules with molecular weights between 1000-3000Da which may not have observable NOE's.

It may often suffer from coupling artefacts from TOCSY. However, ROESY will provide a faster choice for mapping NOE correlations (may use 50% time compared with NOESY).



TOCSY (Total Correlation Spectroscopy)

TOCSY shows scalar-coupling between protons within the same spin system. Magnetisation is passed on from one proton to the next. It therefore shows correlations between spins that are not directly coupled (i.e. A to C, not just A to B as in COSY).

Used frequently for peptides and sugars where there are isolated spin system.



Specific Requirements

If you have specific experiments other than those mentioned above, please contact our staff for further assistance.