Put the tube into the magnet by means of the air lift, adjust the rotation frequency (20 Hz), and display the lock signal on the screen. Perform the lock procedure and optimize the field homogeneity (see Ch. 1, Section 1.4).

The spectrometer is to be adjusted for ¹H observation in quadrature detection mode. Load the ¹H acquisition program, which comprises the following basic commands: zero memory, set the relaxation delay, set the excitation pulse, perform acquisition, and write the data on file.

For acquisition the following parameters have to be set:

td: 32 k

td (time domain) is the number of points at which the free induction decay (FID) is sampled and the data stored. This parameter has to be chosen according to the desired digital resolution of the FID, which in turn depends on the spectral width sw. The settings of td and sw determine the acquisition time aq.

sw: 20 ppm

sw, the spectral width, has to be chosen so that all types of protons are within this spectral window; otherwise folding occurs. Folding of noise is prevented by appropriate filters which are set by the software corresponding to sw. The settings of sw and td determine the acquisition time aq, the time during which the FID data are acquired. The relationship between these three parameters, where aq is usually the dependent variable and sw has to be expressed in Hz, is described by the following fundamental equation (1):

aq = td2 · sw(1)

Thus, with the above settings at 300 MHz, the acquisition time is 2.7 s.

o1: frequency (offset) of the r.f. pulse at the center of the ¹H NMR spectrum:

In quadrature detection mode the frequency of the exciting r.f. pulse (often called transmitter offset) is positioned in the center of sw, e.g. at about δ_H = 7. On older instruments without digital lock, o1 depends on the field position of the deuterium lock and thus on the deuterated solvent used.

p1: 30°

p1, the ¹H transmitter pulse, creates an observable x,y-magnetization by tipping the magnetization vector towards the −y-axis through an angle α, determined by Equation (2):

α = (360°/2π) γ B₁ · p(2)

where γ is the proton gyromagnetic ratio, B₁ the field strength of the radiofrequency field, and p the duration of the pulse. Since p1 is known for a flip angle α = 90° (see Exp. 2.1), the duration of p1 corresponding to 30° is easily obtained.

Although a 90° pulse gives maximum signal intensity (Exp. 2.1), a shorter pulse-length of about 30° is used for routine work with data accumulation in order to avoid a long pulse repetition time of 5 T₁, which is necessary after a 90° pulse (see d1).

It should be mentioned here that between the irradiating pulse and the opening of the detector a preacquisition delay is introduced. It prevents the break-through of the