HMQC (1-bond CH correlation) of codeine
This is a 2D experiment used to correlate, or connect, 1H and 13C peaks for directly bonded C-H pairs. The coordinates of each peak seen in the contour plot are the 1H and 13C chemical shifts. This is helpful in making assignments by comparing 1H and 13C spectra.
This experiment yields the same information as the older "HETCOR" experiment, but is more sensitive, so can be done in less time and/or with less material. This is possible because in the HMQC experiment, the signal is detected by observing protons, rather than carbons, which is inherently more sensitive, and the relaxation time is shorter. This so-called "inverse detection" experiment is technically more difficult and is possible only on newer model spectrometers. Acorn NMR’s new JEOL Eclipse+ 400 is equipped to perform inverse experiments, and uses Z-gradients for improved spectral quality.
The time required for an HMQC depends on the amount of material, but can be done in 1/2 hour or less, compared to several hours for a HETCOR spectrum.
Contour plot of the HMQC spectrum. Because it is a heteronuclear experiment, the 2 axes are different, and the plot is not symmetrical. Unlike a COSY spectrum, there are no diagonal peaks.
Normal 1D 1H and 13C spectra are shown along the edges. Peaks occur at coordinates in the 2 dimensions corresponding to the chemical shifts of a carbon and its directly bonded proton(s). For example, the contour peak indicated in red shows that the 13C with peak at 91.5 ppm is bonded to the 1H with peak at 4.9 ppm.
Non-equivalent methylene protons are easily identified as 2 peaks located at the same 13C position. There are 3 CH2s in the codeine HMQC spectrum.
The sample is 3.3 mg codeine in ~ .65 ml CDCl3
512 complex points in direct dimension
128 t1 increments
2 sec. relaxation delay
Total acquisition time: ~ 10 min.
sine squared window function in both dimensions with 45 degree phase shift
2x zero-fill in the indirect dimension
magnitude calculation (no phasing is required)
final data size 512 x 512
There are variations on this experiment, including a version in which CH2s have phase opposite of that of CH and CH3 peaks, called an HSQC-DEPT spectrum. Negative peaks are shown in red in the plot below, easily identifying the 3 CH2s in codeine.
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: 01/22/2003