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HMQC

 

HMQC is a 2-D method used to determine 1H-13C connectivities.    An example is shown below using 2,4-nonadien-1-ol.  We are not certain about the two geometrical configurations.

 

 

First the 1H NMR is obtained.

 

 

The assignments for the resonances are as follows:  H1 – 4.2 ppm (d),  H2 and H5 -  5.7 ppm (m),  H3 -  6.25 ppm (dd),  H4- 6.05 ppm (dd),  H6 – 2.1 ppm (q),  H7 and H8 – 1.3 ppm (m),  H9 – 0.8 ppm (t).  Assignments were confirmed using another 2D method similar to COSY, viz. TOCSY, as far as we could overlaps not withstanding.

 

 

 

Next the 13C NMR is obtained.

 

 

We can assign C1 to 63.8 ppm, C8 to 22.4 ppm, and C9 to 14.1 ppm.  C2, C3, C4, C5 generate the three peaks at 129.5, 132.4 and 136.1 ppm, C6 and C7 at 32.5 ppm and 31.6 ppm .   We need to assign the shifts from the six remaining carbon atoms.  C6 and C7 are similar in the 13C spectra but are well separated in the 1H spectra.  The H7 and H8 overlap in the 1H NMR but are well separated in the 13C spectra. 

 

Below is the HMQC spectra of 2,4-nonadien-1-ol.  Along the vertical dimension, F1, the 13C shifts are displayed and along the horizontal dimension, F2, the 1H shifts are displayed.  The observed cross peaks result from direct 1 bond 1H - 13C couplings.  The F2, F1 coordinates, i.e., the 1H and 13C shifts, are given next to each cross peak.

 

 

There are four distinct peaks in the olefinic region resulting from the 4 carbons (C2, C3, C4, C5).  The HMQC

spectrum splits the 129.5 ppm 13C resonance indicating the presence of 2 carbons with distinct 1H resonance, 5.67

and 6.0 ppm.   The 5.67 ppm signals come from H2 and H5 and the 6.0 ppm from H4.  The 13C resonance at

132.4 ppm corresponds to the 1H resonance at 6.2 ppm which is C3.  The 13C resonance at 136.1 ppm

corresponds to the 1H resonance at 5.66 ppm which is either C2 or C5.  Considering 13C chemical shifts it would be

more likely that the 13C resonance at 129.5 ppm would be from C2, b to the OH, and the 136.1 signal from C-5

bearing the 4 carbon chain.  Now a complete assignment of the olefinic carbons has been made:  C2 (5.67, 129.5),

C3 ( 6.2,132.4), C4 (6.0, 129.5) and C5  (5.67, 136.1).

 

The assignments in the aliphatic region can be assigned in a similar manner.  H6 resonates at 2.1 ppm, which correlates to the 13C resonance at 33.4 ppm.  Therefore the peak in the `13C spectrum at 32.5 ppm can be assigned to C6, note the slight variation between the values obtained from the 13C and the HMQC.  H7 and H8 resonances overlap but are well separated in the HMQC spectrum, (1.32, 32.1) and (1.30, 22.4).  Assignments for H7 and H8 can be made by evaluating the 13C resonances.  C7 resonates at 32.1 ppm and C8 resonates at 22.4 ppm.  Complete assignment of the aliphatic carbons has been made:  C6 (2.0, 33.4), C7 (1.32, 32.1), C8 (1.30, 22.4) and C9 (0.84, 14.1).

 

HMQC is a very powerful method to allow complete assignment of peaks, there is direct correlation between the 1 H and 13C resonances.  HMQC is helpful in making assignments and structural elucidation of molecules of pharmaceutical interest.