NMR Experiments

General information

SENSITIVITY

Higher field gives better S/N, so 500 > 300 > 200MHz
Probes optimized for a particular nucleus give the best S/N for that nucleus. The VI-500 has an indirect detection probe and is thus most sensitive to proton. The VI-300, VXR-300, the VAC-300, and the VAC-200 all have direct detection probes and give better S/N for other nuclei, e.g., carbon.

RESOLUTION and DISPERSION

Higher field spreads out the signal better, makes coupling patterns simpler, separates different signals better. Again, 500 > 300 > 200MHz.
Linewidths in Hz are similar whatever the field, so the linewidths in ppm are smaller at the higher field strengths, unless a frequency-dependent line broadening mechanism interferes.
Proton-detected experiments
1. Availability
  - H1 experiments are available on all instruments.
2. 1D Proton experiment
  - Default Parameters
    - 2 second acquisition time
    - 1.5 second relaxation delay
    - Spectral width -2ppm to 18ppm
    - 16 scans
    - Total time: 1 minute
  - Variations on the 1D experiment include
    - 1D NOE (subtraction or gradient)
    - Selective decoupling
    - Solvent saturation
    - T1 determination
  - Recommended Concentration
    - > 0.1mM
3. COSY - Proton-proton correlation experiment
  - Gives information about pairs of protons that are J-coupled. This usually indicates that the protons are on adjacent carbons, e.g., 3-bonds away (though protons further apart may in some cases be J-coupled).
  - Default Parameters
    - 1.5 second relaxation delay
    - 512 complex points in t2 and 192 increments in t1
    - 4 scans per increment (no PFG) / 1 scan per increment (PFG)
    - Total time: 20 minutes (no PFG) / 5 minutes (PFG)
  - Recommended Concentration
    - > 10mM
  - Variations on COSY
    - DQF-COSY
      - phase-sensitive experiment
      - diagonal peaks are narrower
      - less-sensitive
    - RelayH
      - 4-5 bond connectivities
    - TOCSY - total correlation spectroscopy
      - gives correlations for all protons within a spin system
4. NOESY - Proton-proton through-space interactions via NOE
  - Gives information about pairs of protons that are close in space (<5 A apart)
  - Default Parameters
    - 1.5 second relaxation delay
    - 512 complex points in t2 and 256 increments in t1
    - 8 scans per increment
    - Total time: 1 hour 15 min.
  - Recommended Concentration
    - > 10mM
  - ROESY - rotating frame NOE
    - Compounds of molecular weight ~1000-2000
    - Exchange peaks are opposite sign from NOE peaks
5. HMQC - Heteronuclear Multiple Quantum Correlation experiment
  - Gives information about strong proton-carbon J-couplings. A strong proton-carbon J-coupling indicates that the proton is directly bonded to the carbon. This experiment gives information that is identical to HETCOR, but because it is proton-detected, it is more sensitive than the standard HETCOR especially on indirect-detection probes.
  - Default Parameters
    - 1.5 second relaxation delay
    - 512 complex points in t2 and 192 increments in t1
    - 4 scans per increment (no PFG) / 1 scan per increment (PFG)
    - Total time: 20 minutes (no PFG) / 5 minutes (PFG)
  - Recommended Concentration
    - > 50mM
6. HMBC - Heteronuclear Multiple Bond Correlation experiment
  - Gives information about weak proton-carbon J-couplings. A weak proton-carbon J-coupling indicates that the proton is two, three, or four bonds away from the carbon. This experiment gives information about which protons are near to (but not directly bonded to) different carbons. This experiment (in conjunction with the HMQC) can give an enormous amount of information about molecular structure, since the long range proton-carbon correlations can include quaternary carbons, in addition to protonated carbons.
  - Default Parameters
    - 1.5 second relaxation delay
    - 512 complex points in t2 and 192 increments in t1
    - 4 scans per increment (no PFG) / 4 scan per increment (PFG)
    - Total time: 20 minutes (no PFG) / 20 minutes (PFG) (NOTE: PFG HMBC experiments usually have less t1 noise than non-PFG HMBC experiments.)
  - Recommended Concentration
    - > 50mM
Carbon-detected experiments
1. General
  - Availability
    - Probes optimized for X-nuclei give better S/N. The VI-300, the VXR-300, the VAC-300, and the VAC-200 have direct detection probes in; these probes give the best carbon S/N.
    - Sensitivity is approximately 5700 times less than for proton, so adequate S/N for a reasonable length experiment (10 minutes, 256 scans) requires a concentration of > 100mM.
2. 1D Carbon experiment
  - Default Parameters
    - 0.8 second acquisition time with proton decoupling
    - 2.0 second relaxation delay (with NOE enhancement)
    - Spectral width -10ppm to 225ppm
    - 192 scans
    - Total time: 10 minutes
  - Recommended Concentration
    - > 100mM
  - Special Considerations
    - Increase d1 for carbons with long T1's (quaternary, carbonyls)
3. DEPT experiment
  - Gives information about the number of protons bonded to each carbon.
  - Default Parameters
    - 0.8 second acquisition time with proton decoupling
    - 2.0 second relaxation delay (with NOE enhancement)
    - Spectral width -10ppm to 225ppm
    - 96 scans per spectrum (2 spectra total)
    - Total time: 10 minute
  - Recommended Concentration
    - > 100mM
4. HETCOR - Proton-carbon correlation experiment
  - Gives information about strong proton-carbon J-couplings. A strong proton-carbon J-coupling indicates that the proton is directly bonded to the carbon. This experiment gives information that is identical to HMQC, but because it is carbon-detected, it is less sensitive than the HMQC experiment.
  - Default Parameters
    - 2.0 second relaxation delay
    - 512 complex points in t2 and 192 increments in t1
    - 4 scans per increment
    - Total time: 30 minutes
  - Recommended Concentration
    - > 100mM
Phosphorus-detected experiments
1. General
  - Availability
    - The VI-300, VAC-300, and VAC-200 have 4-nuclei probes capable of running phosphorus at any time. The VI-500 can run P31 with a cable change.
    - Sensitivity is approximately 15 times less than for proton, so adequate S/N for a reasonable length experiment (10 minutes, 256 scans) requires a concentration of > 0.1mM.
2. 1D Phosphorus experiment
  - Default Parameters
    - 0.8 second acquisition time with proton decoupling
    - 3.0 second relaxation delay (with NOE enhancement)
    - Spectral width -100ppm to 250ppm
    - 32 scans
    - Total time: 2 minute
  - Recommended Concentration
    - > 0.1mM
Fluorine-detected experiments
1. General
  - Availability
    - The VI-300, VAC-300, and VAC-200 have 4-nuclei probes capab le of running fluorine at any time.
    - Sensitivity is approximately the same as for proton, so adequate S/N can be obtained with concentration > 0.1mM.
2. 1D Fluorine experiment
  - Default Parameters
    - 0.8 second acquisition time
    - 3.0 second relaxation delay
    - Spectral width 150ppm to -200ppm
    - 32 scans
    - Total time: 2 minute
  - Recommended Concentration
    - > 0.1mM
philson@nmr.chem.umn.edu
Last modified: Mon Oct 5, 1998

NMR Experiments

General information

Proton-detected experiments

Carbon-detected experiments

Phosphorus-detected experiments

Fluorine-detected experiments