HMQC (1-bond CH correlation) of codeine

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.

1H 13C Assignment 6.6 113 8 6.5 120 7 5.7 133 3 5.3 128 5 4.8 91 9 4.2 66 10 3.8 56 12 3.3 59 11 3.0 & 2.3 20 18 2.6 40 16 2.6 & 2.4 46 13 2.4 43 14 2.0 & 1.8 36 17

The sample is 3.3 mg codeine in ~ .65 ml CDCl₃

Acquisition Parameters:

512 complex points in direct dimension
128 t₁ increments
2 scans
2 sec. relaxation delay
Total acquisition time: ~ 10 min.

Processing:

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. 

HMQC

Only directly bonded hydrogen and carbons will give cross peaks (quaternary carbons are not seen), which makes interpretation rather straight foreword. As seen in the simulated spectrum below, assignment is made by drawing two lines at a right angle from the 1H spectrum to the 13C spectrum through the cross-peak, which looks like a series of concentric ellipses. Thus, in the spectrum below, C1 is directly attached to H1.