See explanation of the HMBC experiment.
Long range C-H couplings cover a range of values, typically less than 10 Hz. The experiment is optimized for a specific coupling, and selection of that value is a trade-off. Correlations due to couplings of other values are reduced in intensity. To optimize for a specific J value, a delay in the pulse sequence is set to 1/(2J). For very small couplings, this delay becomes so long that much of the magnetization is lost to relaxation. Typically, the delay is optimized for J of 7-8 Hz, the expected value for aromatic 3-bond couplings. However, longer-range couplings can be observed if the delay is optimized for smaller couplings.
The contour plots shown below are of 8 mg codeine in ~ .65 ml CDCl3, in which the experiment was optimized for couplings of 4 Hz.
Compare to the HMBC spectrum optimized for 8 Hz.
Because of the large number of peaks observed, analysis requires several expanded plots. In this case, the spectrum has been divided into 4 sections, each of which is discussed below.
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The discussion below uses the numbering system shown at right.
The numbers were assigned to peaks in the 1D 13C spectrum, starting downfield, moving upfield, and numbering each sequentially. This generates a unique identifier for each Carbon, even before knowing any assignments. |
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Compare to the same quadrant in the HMBC optimized for 8 Hz. Several additional 2-bond couplings are observed, as are 1-bond couplings for 7 and 8. The peaks indicated by red circles are 4-bond couplings: H-8 to C-4 and H-7 to C-1.
Compared to the 8 Hz-optimized spectrum, one additional peak is observed in this quadrant, indicated by the red circle. This is a 4-bond coupling from H-7 to C-15.
In this quadrant, three 4-bond peaks are indicated by red circles (H-18 to C-8 and C-1; H-18' to C-8). In addition, two 5-bond couplings are indicated by blue circles. These are H-18 and H-18' to C-2.
The only additional correlations in the quadrant not seen in the 8 Hz-optimized spectrum are from H-13' (just upfield of the intense N-Me singlet) to carbons 17, 14/15 and 11.
Acquisition Parameters:
512 complex points in direct dimension
128 t1 increments
64 scans
2 sec. relaxation delay
Total acquisition time: 4.5 hrs
Processing:
sine squared window function in both dimensions with 0 degree phase shift in t2 and 90 degree phase shift in t1
2x zero-fill in the indirect dimension
magnitude calculation (no phasing is required)
final data size 512 x 512
See also: detailed description of 2D processing
For readers interested in the details of data acquisition, several good books are available.
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
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