NMR techniques and experimental details

The spectra provided here were selected to demonstrate the structural information available from different NMR experiments. A brief description of each technique is given below. The NMR spectra were aquired with the Widener University GE QE-300. All compounds were purchased from Aldrich. Ethyl aldehyde and 2-methyl-1-propanol spectra were aquired in deutero chloroform. Pamoic acid spectra were aquired in d6-DMSO. After the spectra were acquired the data was downloaded to a PC and processed with NUTS (Acorn NMR). The gif images in this paper were prepared by copying and pasting from NUTS into a graphics package. The molecular structures used here were all prepared in Hyperchem release 4.0, Hypercube Inc.

  1. H-1 (Proton NMR)
    1. Experimental Description - 300 MHz proton NMR. 90 degree pulse.
    2. Spectral Interpretation
      1. Splitting Patterns, indicate number of protons on adjacent carbons.
      2. Integration, indicates relative number of protons.
      3. Chemical Shift (ppm), indicates chemical environment.

  2. H-1 Decoupling (Selective Homonuclear Decoupling)
    1. Experimental Description - In this experiment the decoupler is set for a select frequency in the proton spectrum. This irradiates the nuclei at that frequency and selectivly decouples any nuclei coupled to the irradiated nuclei. As a result the splitting patterns will change.
    2. Spectral Interpretation - Select a frequency or chemical shift to decouple at. This will eliminate the signal for the irradiated nuclei (the peak at the decoupler frequency). The splitting pattern for any nuclei coupled to this peak will change.

  3. C-13 (Carbon NMR)
    1. Experimental Description - 75 MHz Carbon NMR. Proton decoupled.
    2. Spectral Interpretation
      1. Chemical Shift (ppm), indicates chemical environment.
      2. These are decoupled spectra so no splitting information is present.
      3. Notice solvent peak (CDCl3) at ca 77 ppm.
      4. Quaternary carbon's frequently give small peaks.
      5. Acquisition conditions NOT optimized for integration

  4. Coupled (Coupled Carbon Spectrum, no decoupler)
    1. Experimental Description - In this experiment the proton decoupler is off. This produces a spectrum where carbons are coupled to adjacent protons. Because the spectral lines are split and there is no NOE, the spectrum has very low signal to noise. This experiment is not typically used for determining splitting patterns because the DEPT spectrum provides the same information, and the polarization transfer in the DEPT experiment results in much higher S/N.
    2. Spectral Interpretation. The chemical shift may be calculated from tables. The number of attached protons is determined by the splitting patterns (n-1). So that a carbon with no protons (quaternary) is observed as a singlet. A carbon with one proton is observed as a doublet. A carbon with two protons is observed as a triplet. And a carbon with three protons is observed as a quartet.

  5. Gated (Gated Carbon Decoupling)
    1. Experimental Description - In this experiment the proton decoupler is pulsed to produce a spectrum where carbons are coupled (peaks are split) but the NOE (Nuclear Overhouser Effect) is allowed to build up. The NOE is produced by turning the decoupler on between pulses. This spectrum show splitting of carbon peaks (like the coupled carbon spectrum), and the NOE enhances the S/N.
    2. Spectral Interpretation. The chemical shift may be calculated from tables. The number of attached protons is determined by the splitting patterns (n-1). So that a carbon with no protons (quaternary) is observed as a singlet. A carbon with one proton is observed as a doublet. A carbon with two protons is observed as a triplet. And a carbon with three protons is observed as a quartet.

  6. Inverse (Inverse Gated Carbon Decoupling)
    1. Experimental Description - In this experiment the proton decoupler is pulsed to produce a spectrum where carbons are decoupled (no splitting), but the NOE (Nuclear Overhouser Effect) does not build up. This spectrum shows each carbon as a single peak. It is used to determine the NOE and for integrating carbon spectra (The NOE may be different for each carbon and this effect is removed in the gated decoupling experiment).
    2. Spectral Interpretation. The chemical shift may be calculated from tables. Compare the signal intensity to the regular carbon spectrum to determine the NOE.

  7. APT (Attached Proton Test)
    1. Experimental Description - APT, Attached Proton Test.
    2. Spectral Interpretation - This pulse sequence produces a carbon spectrum where peaks for carbons with an even number of protons are in the opposite direction of peaks for carbons with an odd number of protons. The spectra here are phased so that carbons with an even number of protons (quaternary carbons and CH2 carbons) are up, while carbons with an odd number of protons (CH and CH3) are down.

  8. DEPT 45 (Distortionless Enhancement of Ploarization Transfer, 45 degree)
    1. Experimental Description - DEPT 45, Distortionless Enhancement of Polarization Transfer using a 45 degree decoupler pulse.
    2. Spectral Interpretation - This pulse sequence produces a carbon spectrum containing only carbons with protons attached (quaternary carbons are not observed). Quaternary carbons are identified by comparing the regular carbon spectrum with the DEPT 45 spectrum.

  9. DEPT 90 (Distortionless Enhancement of Ploarization Transfer, 90 degree)
    1. Experimental Description - DEPT 90, Distortionless Enhancement of Polarization Transfer using a 90 degree decoupler pulse.
    2. Spectral Interpretation - This pulse sequence produces a carbon spectrum containing only carbons with a methyne carbon (CH). Methyl (CH3), methene, CH2, and quaternary (C) carbons are not observed (NOTE: If the decoupler pulse is not well calibrated the methyl and methene carbons may produce small peaks.).

  10. DEPT 135 (Distortionless Enhancement of Ploarization Transfer, 135 degree)
    1. Experimental Description - DEPT 135, Distortionless Enhancement of Polarization Transfer using a 135 degree decoupler pulse.
    2. Spectral Interpretation - This pulse sequence produces a carbon spectrum with methyl (CH3) and methyne (CH) carbons are up. The methene (CH2) carbons are down. CH3 carbons are distinguished from CH carbons by comparison to the DEPT 90 spectrum (which only shows CH carbons).

  11. T1 (T1Inversion Recovery Experiment)
    1. Experimental Description - In this experiment a 180 degree pulse is applied to the system. This causes the net magnitization vector to invert. The net magnitization vector recovers from this inversion at a rate that corresponds to T1. The recovery is measured by applying a 90 degree pulse after a delay period. This 90 degree pulse determines the amount of recovery that has occured. With a short delay, no recovery has occured and the peak is inverted (180 degrees out of phase). After complete recovery the peak has returned to it's initial intensity and phase. By measuring the intensity as a function of delay, the kinetics of the recovery are measured. A useful point is the null point where the system has recovered 50%. This is equivilent to the half life for T1.
    2. Spectral Interpretation - Each nuclei has a unique T1 recovery time. This recovery rate is useful for studying the spin system and it is useful for optimizing data acquisition. For accurate integration, you should wait 5 * T1 between experimental pulses. For other experiments to optimize the S/N you need to know the T1 rate to determine the optimum pulse angle.

  12. COSY (Proton 2D homonuclear corrolation)
    1. Experimental Description - 2D Corelation Spectroscopy Experiment.
    2. Spectral Interpretation - The COSY spectrum plots proton vs proton. The 1D spectrum is plotted along each axis. The 2D data consists of the matrix diagonal(not very useful), and the cross peaks. These peaks show which protons (from the diagonal) are coupled through bonds.

  13. HETCOR (Carbon - Proton Heteronuclear Chemical Shift Correlation)
    1. Experimental Description - 2D Heteronuclear correlation spectroscopy.
    2. Spectral Interpretation - The HETCOR spectrum plots proton vs carbon. The 1D spectra are displayed along the appropriate axis. The 2D peaks show which protons are coupled to which carbons.

  14. NOESY (Nuclear Overhauser and Exchange Spectroscopy)
    1. Experimental Description - 2D Correlation experiment that provides both Proton-Proton correlation (like the COSEY) and correlation from NOE to show protons that are coupled through space.
    2. Spectral Interpretation - The 1D proton spectrum is plotted along each axis. The 2D data consists of the matrix diagonal(not very useful), and the cross peaks. These peaks show which protons (from the diagonal) are coupled. This coupling is either through bonds, as in the COSY, or through space by NOE. These NOE peaks show which protons are close together in the three dimensional structure of the molecule.

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