Problems, artifacts and solutions in the INADEQUATE NMR experiment

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  • Published: Jul 14, 2010
  • Channels: NMR Knowledge Base
thumbnail image: Problems, artifacts and solutions in the INADEQUATE NMR experiment
Pulse sequence and coherence pathway for the INADEQUATE pulse sequence. D1 is the relaxation delay between acquisitions, and t1 is the incremented delay for a 2D experiment. The second line shows the gradient program, with the two gradients in a 1:2 ratio. The equations in the coherence pathway show the dependence of the transfer on the flip angle of the final pulse, p4. Normally, alpha = beta = p4, but if there are significant offset effects, the effective flip angles, alpha and beta, felt by the two nuclei that make up the DQ coherence may be different.
Problems, artifacts and solutions in the INADEQUATE NMR experiment
Magnetic Resonance in Chemistry 2010, 48, 630-641
Alex D. Bain, Donald W. Hughes, Christopher K. Anand, Zhenghua Nie and Valerie J. Robertson

The INADEQUATE experiment can provide unequalled, detailed information about the carbon skeleton of an organic molecule. However, it also has the reputation of requiring unreasonable amounts of sample. Modern spectrometers and probes have mitigated this problem, and it is now possible to get good structural data on a few milligrams of a typical organic small molecule. In this paper, we analyze the experiment step by step in some detail, to show how each part of the sequence can both contribute to maximum overall sensitivity and can lead to artifacts. We illustrate these methods on three molecules: 1-octanol, the steroid 17-ethynylestradiol and the isoquinoline alkaloid -hydrastine. In particular, we show that not only is the standard experiment powerful, but also a version tuned to small couplings can contribute vital structural information on long-range connectivities. If the delay in the spin echo is long, pairs of carbons with small couplings can create significant double-quantum coherence and show correlations in the spectrum. These are two- and three-bond correlations in a carbon chain or through a heteroatom in the molecule. All these mean that INADEQUATE can play a viable and important role in routine organic structure determination.

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