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Wednesday, November 13, 2019

13C-13C Connectivity via 1H-13C 1,1-ADEQUATE

One of the most valuable pieces of information one could obtain in elucidating the structure of a small organic molecule is carbon-carbon connectivity information.  This information can sometimes be indirectly deduced from HMBC and/or H2BC data with reasonable sensitivity.  The same information can be determined directly, albeit with dramatically less sensitivity, using the 13C INADEQUATE technique.  Another option for obtaining carbon-carbon connectivity information is the 1,1-ADEQUATE technique (Adequate sensitivity DoublE QUAnTum spEctroscopy).  This method is proton detected and relies on a 1-bond INEPT transfer between 1H and 13C.  One-bond 13C-13C double quantum coherence between the carbon bound to the proton used for the initial INEPT transfer and adjacent carbons is allowed to evolve in much the same way as in the INADEQUATE technique.  Magnetization is transferred back to single quantum coherence for proton detection.  The 2D NMR data show correlations between the proton resonances and the double quantum frequencies between the carbon attached to the proton and those carbons bound to that carbon.  The carbon-carbon connectivity information is provided in the double quantum carbon frequencies.  One drawback to the 1,1-ADEQUATE technique is that connectivity cannot be established between two quaternary carbon atoms not attached to protonated carbons.  Connectivity information between a quaternary carbon bound to a protonated carbon can however be established.  The sensitivity advantage of the 1,1-ADEQUATE technique compared to the 13C INADEQUATE technique arises from 1H rather than 13C detection and that the recycle delay depends on the proton T1's rather than the 13C T1's.  Here is an example of how one could use the 1,1-ADEQUATE technique with other methods to unambiguously assign the structure of a small organic molecule.  The edited HSQC spectrum of the unknown molecule with separately acquired 1H and 13C NMR spectra as projections is shown in the figure below.
 The 13C spectrum provides all of the 13C frequencies, while the edited HSQC signals provide the 1H-13C one-bond connectivity and multiplicities for each protonated carbon.  Note that the carbon frequencies could also be determined from a high resolution HMBC spectrum if insufficient material is available for a direct 13C measurement.  From the carbon frequencies, one can determine all of the double quantum frequencies as shown in the table below, taking into account the 13C offset frequency expressed in ppm, 'o1p'.
Those  highlighted in pink are those that are present in the 1,1-ADEQUATE spectrum which is shown below.
The spectrum was acquired on a concentrated sample at 600 MHz with a cryoprobe using the standard 'adeq11etgpsp' Bruker pulse program .  The total data collection time was less than 1 hour.  The carbon-carbon connectivity is labelled on the spectrum based on the double quantum frequencies using the numbering scheme from the 13C spectrum presented as the projection on the edited HSQC spectrum above.  From these connectivities, the structure of the compound can unambiguously be assigned to limonene.