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2D NMR Experiments

Introduction to 2D experiments

2D 1H Double Quantum Coherence(DQC) CRAMPS NMR

In the 2D DQC CRAMPS Experiment, the excitement and then reconversion of DQC is achieved using a re-coupling element known as POST-C7 sandwiched around the t1 time period. To enhance resolution, CRAMPS is applied in both the direct and indirect time element by use of the DUMBO pulse sequence. To enable acquisition of the FID, the DUMBO pulse applied during t2 is the windowed version, whilst during t1 windowless DUMBO is used. The design of this pulse sequence allows the operator to increase the number of POST C7 elements used, altering the re-coupling time and allowing for the investigation of DQC build up.

The resulting spectrum contains information on the species and coupling between spins. For two coupled spins, Peaks appears at the combined chemical shift of the two coupled spins in the Double Quantum dimension and at each of the chemical shifts of the individual nuclei in the Single quantum dimension. Therefore if both spins are of the same species then only one peak is seen on diagonal, whereas for two spins of different species, two peaks appear either side of the diagonal.

The figure on the right demonstrates the spectrum one would expect for a cluster of spins, 2 of species A and 2 of species B.

In a real spectrum the quantity of spins and the presence of experimental artefacts complicate the results and this may have consequences for determining the packing of unknown structures, however these experiments still yield useful qualitative and semi quantitative information and are an active area of research, and new or improved pulse sequences will appear in the literature in the coming years.

 

example spectrum
DQC sequence

 

2D INEPT

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2D DQC INEPT

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