HMQC-TOCSY (2D experiment)
This is an experiment that can be useful for tracing out 1H-1H connectivity in crowded spectra with overlap of 1H peaks.
The first step of the pulse sequence is an HMQC, in which 1H magnetization is transferred to the directly-bonded 13C. 13C magnetization evolves during t1 and then is transferred back to the directly-bonded proton(s). The completion of the sequence is a TOCSY mixing period, which transfers magnetization from the "carbon spin-labeled" proton along a chain of coupled protons. Note that this experiment includes a TOCSY mixing period, but not the TOCSY evolution period. Compare to the 3D HMQC-TOCSY experiment.
In the resulting spectrum, the carbon spectrum runs along the vertical dimension. At the chemical shift of each protonated 13C, peaks appear at the chemical shifts of the directly bonded proton(s) and of other protons in the same spin system.
The experiment can be repeated using different mixing times in the TOCSY sequence. As with TOCSY, the intensity of each cross-peak will vary through the series of spectra – long range correlations will "grow in" as the delay value is increased, and shorter range couplings may disappear.
Expanded aliphatic region:
The red dotted lines show that 3 different carbons (11, 16 and 18) correlate to the same 4 Hs (11, 16, 18 and 18′). Those 4 Hs make up an isolated spin system.
The green dotted lines show that 2 carbons (13 and 17) connect to Hs 13′, 17 and 17′ (and C-13 also connects to H-13). CH2s 13 and 17 are another isolated spin system.
The remaining 2 strong peaks are the methyl 1-bond correlations.
The sample is 3.3 mg codeine in ~ .65 ml CDCl3
512 complex points in direct dimension
128 t1 increments
?ms mixing time
2 sec. relaxation delay
Total acquisition time: ~ 1.25 hrs
sine squared window function in both dimensions with 0 degree phase shift
2x zero-fill in the indirect dimension
magnitude calculation (no phasing is required)
final data size 512 x 512
Interactive view – Those who have installed the Chime plug-in can view and manipulate the codeine structure in 3D, examine and expand the 1H spectrum, and interactively correlate 1H peaks with H atoms in the structure.
Last updated: 12/06/2005