The COLOC sequence (COrrelation via LOng range Coupling)
The sequence which is used (Fig 26) allows generally a good correlation between the quaternary carbons and the neighbouring protons.
On the spectrum 12 (COLOC), the correlation between the quaternary carbon (13C=a=156.2) and its vicinal proton (D=7.7) is named Cq. However, it does not give good results for long range correlations involving protonated carbons.
This observation led to write a pulse sequence named XC ORFE (X nucleus proton CORrelation with Fixed Evolution time). This one is very closed to the COLOC sequence.
Fig. 26 : The COLOC pulse sequence
Spectrum 12 : COLOC of DBF