Basic Principles of Pulse FT-NMR

Table of Contents – Front

VIII. NMR Basic Principles Test

1. Diagram and label the three components of the 1PULSE FT-NMR experiment. Give the parameter names used for the three components in the Unity+300 and VXR-S 400 NMR spectrometers.

2. What is the result when you apply the FT to an FID (time domain signal)?

3. What is the relationship between number of points, spectral width, acquisition time, and digital resolution? Which of these parameters would you change if you wanted better digital resolution, and why? What are the parameter names for number of points, spectral width and acquisition time in the Unity+300 and VXR-S 400NMR spectrometers?

4. What is the peak shape found in most solution NMR spectra?

5. What shim(s) should be adjusted if the peak shape is asymmetrically distorted? Label the shim probably responsible for the distortions below.

6. What is the single best factor to tell whether a sample is poorly shimmed?

7. Given that after 100 transients (8.5 minutes) the S/N for a sample is 25:1 on the Unity+300, and 35:1 on the VXR-S 400, how long will it take to achieve a S/N of 350:1 on each instrument?

8. What are the six factors that can affect the accuracy of a ¹H integration? Why? Are there any additional factors that affect the accuracy of a ¹³C{¹H} integration? Why?

9. When would you use a homonuclear decoupling experiment?

10. Is there a difference between the 1PULSE FT-NMR experiment used to acquire ¹³C{¹H}NMR spectra and that used to acquire ¹H NMR spectra? If yes, what is the difference?

Take Home Lesson - V

Image via Wikipedia

Obtaining useful ¹³C{¹H} spectra requires knowledge of the same basic principles as needed for obtaining useful ¹H spectra. When your spectrum does not look right, you can save yourself needless frustration on the instrument by taking a quick spectrum of a ¹³C standard and checking the S/N, or seeing if the standard is decoupled properly.

VII. 13C-{1H} NMR Spectra - II

The ¹³C{¹H} spectrum obtained using a standard 1PULSE experiment is not quantitative, i.e., the integration of the peaks will not give a true indication of relative ratios because of the nuclear Overhauser enhancement (nOe) of the ¹³C nuclei due to their attached ¹H nuclei. For the case of ¹³C spectra acquired with proton decoupling, an enhancement of up to 1.98 (i.e., 198%), or an almost threefold improvement in signal-to-noise is expected for those carbon nuclei that are attached to protons.

The ¹H digital resolution given by the default parameters on both the Unity+300 and VXR-S 400 differ from the ¹³C digital resolution by a factor of five. This is mainly due to the need for a larger spectral width used to accomodate the wider chemical shift range for ¹³C and the need to use a fewer points to conserve space on the hard disk. Due to the large spectral width typical of ¹³C{¹H} spectra, it is important that the number of points not be too small, or distortions of the peaks can occur. Figure VII-4 shows a spectrum of 30% menthol in CDCl₃ collected with only 8192 points at 75.432 MHz. Compare it to Figure VII-2 and note the anamolous peak heights in Figure VII-4, as well as the phase distortion in all the peaks. These effects are due to an insufficient number of data points and is not an instrument problem. The distortion can be eliminated by increasing the number of data points as is the case in the standard default parameters for ¹³C NMR spectroscopy on the Unity+300 and VXR-S 400 NMR spectrometers.

VII. 13C-{1H} NMR Spectra - I

This section gives you some useful information about¹³C{¹H}NMR spectroscopy. Examples of spectra of 30% menthol in CDCl₃ taken on the Unity+300 and VXR-S 400 are given in Figures VII-1 and VII-2 respectively. Some of the important default ¹³C NMR acquisition parameter values are given below.

Unity+300	VXR-S 400
Spectrometer Frequency (sf) = 75.432 MHz	Spectrometer Frequency (sf) = 100.580 MHz
Spectral Width (sw) = 18,859 Hz	Spectral Width (sw) = 25000 Hz
Acquisition Time (at) = 0.820 sec	Acquisition Time (at) = 0.819 sec
Number of Points (np) = 30912	Number of Points (np) = 40960
Digital Resolution = 1.22 Hz	Digital Resolution = 1.22 Hz
Line Broadening = 1.00 Hz	Line Broadening = 1.00 Hz

The symbol ¹³C-{¹H} implies a ¹³C NMR spectrum where the ¹H nuclei are decoupled from the ¹³C nuclei. This is a double resonance experiment, just as described in section VI for homonuclear decoupling except that now the observed nucleus (¹³C) and decoupled nucleus (¹H) are not the same. This experiment is called heteronuclear decoupling, and is a 1PULSE experiment, as described in section II, with the addition of the decoupling field (Figure VII-3). It should be noted that the proton decoupling field is left on during the entire experiment.

Take Home Lesson - IV

Homonuclear decoupling is an effective way to establish that two nuclei are spin coupled, and to simplify a complex coupling pattern for further analysis. It can be difficult to obtain definitive data if the two nuclei are closer than 0.5 ppm to each other.

VI. Homonuclear Decoupling

This section will explain what homonuclear decoupling does. Examples of homonuclear decoupled spectra taken on the Unity+300 and VXR-S 400 are given in Figure VI-1.

Homonuclear decoupling is a double-resonance experiment because it uses two RF fields to affect magnetically active nuclei. Homonuclear decoupling involves applying a second ¹H RF field to cause selective saturation of a nucleus A while observing all other nuclei in the molecule; B, C, D, etc. If nucleus A is spin-coupled to nucleus B and if the second RF field is strong enough, the result is that A is effectively prevented from spin-spin interacting with B. The observed B nucleus spectrum will appear as if it is not coupled to A. The A resonance commonly appears as a tall spike or glitch as a result of this experiment. In Figure VI-1, if the triplet is decoupled, the quartet collapses to a singlet. Similarly, if the quartet is decoupled, the triplet collapses to a singlet.