NMR
Experiment Descriptions
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). This may be indicated by a g in front of the experiment name. 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.
HMQC (Heteronuclear Multiple-Quantum Correlation)
- Proton or 'inverse' detected. Correlations indicate heteronuclear spins which are coupled across a single bond.
- The desired correlations in HMQC are between the low intensity 1H-13C (or 1H-15N) signals. The 1H-12C resonance, the main line in the proton spectra, is a source of interference which is suppressed with pulsed field gradients.
HSQC (Heteronuclear Single-Quantum Correlation)
- Proton or 'inverse' detected. Similar to hmqc but can provide higher resolution for larger molecules.
- Can be run multiplicity edited so that CH and CH3 correlations can be easily distinguished from CH2 correlations.
HMBC (Heteronuclear Multiple-Bond Correlation)
- Proton or 'inverse' detected. Set up to detect smaller heteronuclear couplings. Therefore correlations indicate heteronuclear spins which are coupled across a multiple (typically 2 or 3) bonds.
- 1 bond (hmqc) correlations do sometimes break through in hmbc spectra but will appear as doublets and so can usually be easily identified.
H2BC (Heteronuclear 2-Bond
correlation)
- H2BC correlates protons and proton bearing carbons that are separated by two covalent bonds.
- It can solve the problem of distinguishing two and three bond correlations in HSQC-TOCSY or HMBC spectra.
- H2BC can help in cases where there are missing two-bond correlations in HMBC spectra due to small (2)J(CH) coupling constants. This is because the H2BC experiment is independent of (2)J(CH) coupling.
- Be aware that the suppression of (3)J(CH) may not always be perfect.
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 but will proved better resolution in the X (13C) dimension.
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 suitable for for large (>3000Da) or small (<1000Da) molecules.
ROESY (Rotating-Frame NOE Spectroscopy).
- NOE's measure in the "rotating-frame".
- Used for molecules with molecular weights between 1000-3000Da which may not have observable NOE's.
- Can often suffer from coupling artifacts.
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 frequemtly for peptides and sugars where there are isolated spin system.
HSQC-TOCSY
- A hsqc experiment where the proton-carbon correlation signal is then
transferred to other protons with the same spin system.