Put the tube into the magnet by means of the air lift, adjust the rotation frequency (about 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 ¹³C observation in quadrature detection mode. Load the ¹³C acquisition program which comprises the following basic commands: zero memory, set ¹H broad-band decoupling (CPD-mode), 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, which in turn depends on the spectral width sw. The settings of td and sw determine the acquisition time aq.

sw: 250 ppm

sw, the spectral width, has to be chosen so that the resonance frequencies of all types of ¹³C nuclei 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 and a ¹³C resonance frequency of 75 MHz, the acquisition time is 0.9 s.

o1: frequency (offset) of the r.f. pulse at the center of the ¹³C 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_C = 120. On older instruments without digital lock, o1 depends on the field position of the deuterium lock and thus on the deuterated solvent used.

o2: frequency (offset) of the decoupler at the center of the ¹H NMR spectrum:

The frequency of the decoupling r.f. pulse (often called decoupler offset) is positioned in the center of the ¹H NMR spectrum, e.g. about δ_H = 5. On older instruments without digital lock, o2 depends on the field position of the deuterium lock and thus on the deuterated solvent used.

p1: 30°

p1, the ¹³C transmitter pulse, creates an observable x,y-magnetization by tipping the magnetization vector towards the −y-axis through an angle α, determined by eq. (32:

α = (360/2π) γ B₁·p(2)

where γ is the gyromagnetic ratio for ¹³C, B₁ the strength of the radiofrequency field, and p the duration of the pulse. Since p1 is known for α = 90° (see Exp. 2.2), the duration of p1 for a flip angle of 30° is easily obtained. Although a 90° pulse gives maximum signal intensity (Exp. 2.2), a shorter pulse-length of about 30° is used for routine