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NMR Experiments

NMR Spectroscopy is now 60 years old, Many NMR experiments are routinely used in many fields. Hardware and software development make chemists much easier to perform those experiments that is impossible ten years ago. This web site will provide a detailed NMR experiment setup for rutine application.

1. CycleNOE Experiment

Special NMR Experiments:

2H NMR without lock

Solid State NMR C13 spectrum with H1 and F19 Decoupling

Probe Tuning

The probe must be tuned to the observe frequency with the particular sample of interest. There can be a big difference depending on the solvents and concentration, such as water or organic solvents. When the probe is tuned the power is efficiently transferred from the transmitter to the probe, rather than reflect back to the transmitter, and pulse width is minimized. On the other hand, the detected signal power is efficiently transferred to the preamplifier and the S/N ratio is maximized. Probe tuning is essential for obtaining a good spectrum, and for some advanced experiments to get any meaningful results at all. For most of solid state NMR experiments, the probe must be properly tuned each time, otherwise high power could not efficiently delivered to the probe. It may cause arching or damage the probe.

lthough probe designs are different according to their functions, one has in general two adjustable capacitors. One is called tune and the other called match. The tune capacitor is used to adjust probe circuit to the desired frequency, most broad band probe have additional fixed external capacitors to extend the tune range. The match capacitor is used to adjust probe circuit to meet impedance requirement (50 Ohm). In most case, these capacitors are mutually interactive and therefore we should adjust them in turn.

Shimming

In early sixties when larger electromagnets were used for NMR instrument, the field homogeneity was adjusted by mechanical alignment of the magnet pole faces. By place thin pieces of brass between the magnet and the pole pieces to make the poles more perfectly parallel. The metal piece was called shim and the adjustment was called shimming. A well-shimmed electromagnet (1.4T) could yield line widths of 0.2 Hz. As the magnetic field is increased it is necessary to add electronic shimming which is a small coil placed around the probe. As the field become higher and higher, more and more shimming coils are added into the room temperature shimming coil, the shimming process become much more difficulty.

Spectrum Width (Hz or ppm):The frequency range of the spectrum. For proton, 1ppm is equal to 500 Hz on a 500MHz instrument; 1ppm=400 Hz on a 400MHz instrument. For carbon, 1ppm = 125 Hz on a 500 MHz instrument, 1ppm = 100 Hz on a 400 MHz instrument.	*Pulse Delay* (Seconds): The intervals between observe pulses. It should be set to five times of T 1 if a 90 degree observing pulse is used.
*Pulse Width* (Micro Seconds): The length of time in microseconds used to apply an RF pulse to perturb the spin system. For example, an RF pulse applied along the X-axis of the rotating frame causes the magnetization vector to be rotated in the XY plane.	*Data Points:* It is used to define an FID or a spectrum. The number always equal to the power of two, i.e. data points = 2 n ; n is an integer. (16K=16384, 2 14; 32K=32768, 2 15; 64K=65536, 2 16).
*Number of Scan* (Integer): It is used to set the times of acquisition for increasing the S/N. Since NMR is a relatively insensitive technique by spectroscopic standard, most of spectra require a certain degree of signal averaging. Adding FIDs together results in the coherent addition of the signals. In contrast, the noise is a random function that is not phase coherent. Therefore, addition of FIDs will increase signal intensity. In fact, the S/N will increase by the square root of the number of scans.	*Observe Frequency* (Hz): The Larmor Frequency of the nucleus under observation. This parameter is set in the configure file relative to the strength of the magnetic field. In most instruments, all Larmor frequencies of other nuclei are calculated from proton frequency and they are stored in a table.

NMR

NMR Experiments

Reserved Links

NMR Experiments

Probe Tuning

Shimming