The 2D COLOC (Correlation by Long-Range Coupling) experiment has been the most used carbon-detected 2D long-range correlation experiment and permits to known all long-range proton-carbon connectivities in a single experiment. It is based in the so-called constant time (CT) evolution period that minimizes T2 losses due to relaxation.

Easy implementation on any AVANCE spectrometer.

In the conventional COLOC pulse sequence, broadband homonuclear decoupling in the F1 dimension is achieved by a constant evolution time through which simultaneous proton and carbon 180º pulses are systematically repositioned in successive blocks of the experiment ( 84JMR331-57 , 84ANG444 , and 86ANG342 ). Some improved COLOC pulse sequences have been reported:
 
• By inserting TANGO or/and BIRD operators ( 84JMR526-58 ).
• The COLOC-S experiment ( 87MRC837 ).
• The XCORFE experiment ( 84JMR526-58 ) employs two BIRD clusters. The first is inserted into the fixed evolution period in order to remove one-bond correlations, and the second is inserted in the D3 refocusing period.
• By inserting a low-pass J-filter ( 88JMR280-80 ).
• Varying a specific pulse angle ( 94JMRA220-111 ) into the basic sequence.

However, in modern spectrometers with inverse capability, the proton-detected 2D HMBC experiment has a higher sensitivity than the 2D COLOC experiment ( 87MRC326 ). From COLOC spectra, qualitative knowledgment of heteronuclear coupling constants can be obtained ( 87JACS6927 ).

The 2D COLOC experiment can be recorded in routine/automation modes. All acquisition parameters as described in 2D long-range optimized HETCOR experiment. However, it is not possible to employ the desired sweep width and/or level of digitization. It must accomplish the relationship t1(max)/2=B*IN<=D1 where B is the number of increments of the evolution period and IN is the increment time (IN=1/2*SW). Thus, a compromise value of the sweep width, number of increments, and constant-time period length must be chosen. Specific 1D setup experiments for careful delay optimization in COLOC experiment has been also reported ( 87JMR414-75 ).

More details on practical implementation of the 2D COLOC experiment on AVANCE spectrometers can be found in the Tutorial: 2D COLOC experiment.

A typical 2D map in which cross-peak stands for a two- or three-bond proton-carbon connectivity. However, two major drawbacks are present:

• There is no general suppression of one-bond correlations.
• COLOC experiment can be difficult to optimize and uniformly good signal-to-noise for remote connectivites cannot be obtained. The long-range cross-peak intensity is dependent to the corresponding long-range coupling constant and therefore this is dependent to interpulse delay optimization.

Presently, improved sensitivity ratios are obtained from heteronuclear correlation experiments based on proton detection as the HMBC experiment.

Related experiments:

2D X-detected experiments