Nuclear magnetic resonance

The nuclear magnetic resonance (NMR) method is based on the interaction between matter and electromagnetic forces, and can be observed by subjecting a sample simultaneously to two magnetic fields: one stationary and the other varying at a certain radio frequency. At particular combinations of fields, energy is absorbed by the sample, and this absorption can be observed as a change in the signal developed by a radio frequency detector and amplifier290. Nuclear magnetic resonance spectrometry was discovered in 1946, and became one of the basic methods in organic chemistry. In quinolizidine alkaloids, two techniques of NMR are currently used: 1H-NMR and 13C-NMR. In the case of 1H-NMR analysis, the basis is that energy absorption can be related to the magnetic dipolar nature of spinning nuclei. Quantum theory is used in this case. In 1H-NMR analysis, the H-nuclei from the alkaloid molecule is very important. In the case of 13C-NMR, the sensitivity of the 13C isotope is used. 13C-spectra show chemical shifts that are more sensitive to details of structure than proton shifts. 13C-1H spin-spin interaction is capable of being tested. For example, non-protonated 13C gives a singlet, 13CH a doublet, 13CH2 a triplet and so on. Nuclear magnetic resonance spectroscopy is today a basic method in the structural studies of alkaloids.

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