In many triple-resonance NMR experiment it is necessary to use several ¹³C frequencies in order to apply band-selective pulses on the CO or the CA (CA/CB) carbon resonances. Thus, for instance, in the 3D HN(CO)CA experiment the ¹³C frequency is initially at the carbonyl-carbon frequency of 173 ppm. When the magnetization has been transfer from ¹³CO to ¹³CA a the frequency is changed to the value of 52 ppm in the middle of the alpha-carbon region. For this purpose you need to create the frequency list. Edit a file /u/exp/stan/nmr/lists/f1/hncoca13c with the following contents:

O
150.902749
26108.18
8008.18

The first line contains the capital letter O and the basic frequency of ¹³C on your instrument. The second line the carbonyl offset in Hz (given by the o2 parameter). The third line corresponds to the alpha-carbon offset used for the spoff5, in the above example 26108.18-18100=8008.18 Hz.

Sometimes, it is also necessary recommended that the proton frequency is set in the middle of aliphatic region. It is shifted to the water frequency or the amide region for the second part, the former results in better water suppression and the latter in higher sensitivity.