HSQC-NOESY (2D experiment)
This is an experiment that can be useful for tracing out 1H-1H NOE connectivity in crowded spectra with overlap of 1H peaks.
The first step of the pulse sequence is an HSQC, 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 NOESY mixing period, which transfers magnetization from the "carbon spin-labeled" proton to other protons via dipolar coupling.
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 to which it has NOEs. The NOE peaks (red) have opposite sign compared to the directly bonded proton peaks (black). Of course, only protonated carbons appear in the spectrum.
Expanded upfield 13C region:
The most upfield carbon is C-18. The 2 black peak along the horizontal line corresponding to ~20 ppm are due to the two non-equivalent H-18 protons, which have NOEs to H-11 (3.3 ppm), H-7 (6.5 ppm) and H-5 (5.3 ppm, weak).
For protons that have strong scalar coupling in addition to NOE, the peaks in the HSQC-NOESY are anti-phase, as shown here for H-3 and H-5.
Note that this is a very insensitive experiment. NOE effects are small to begin with, and in this case, signal is reduced by a factor of 100 due to the 13C natural abundance. The sample is 55 mg codeine in ~ .65 ml CDCl3
1K complex points in direct dimension
128 t1 increments (2 slices acquired per t1 point)
2 sec. relaxation delay
Total acquisition time: ~ 12 hrs
sine squared window function in both dimensions with 90 degree phase shift
2x zero-fill in the indirect dimension
final data size 1K x 512 (complex in both dimensions)
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