Back to Blog
Spectroscopie rmn terminale s7/12/2023 NMR has been performed in the Earth’s field in order to measure the 1H– 31P and 1H– 14N J-coupling constants 7, 8. The first NMR spectra in the Earth’s field were demonstrated by Packard and Varian 6. Do we come closer to this ideal by performing NMR in low field or in the Earth’s magnetic field (2.5–7.5×10 −5 T)? Important steps towards mobile chemical-shift-resolved low-field NMR in inhomogeneous fields have been shown 4, 5. Single-scan and high-resolution NMR implies that sufficiently high premagnetization of the spins and a very homogeneous magnetic field are required. Low cost and mobile means that heavy electro or superconducting magnets as well as superconducting quantum interference devices (SQUIDs) should be avoided. We define our ideal NMR spectrometer by two requirements: first, it should measure NMR spectra with high resolution and all relevant NMR parameters, such as the longitudinal ( T 1) and transverse ( T 2) relaxation times, the chemical shift and the dipolar and J-coupling, in a single scan and second, the spectrometer should be robust, low cost and mobile. For 1H high-field NMR spectroscopy (1–20 T), it is difficult to measure linewidths with an instrumental broadening below 100 mHz. With the best field homogeneities available (Δ B/ B ∼10 −9 over 1 cm 3, where B is the magnetic field), ultrahigh-resolution carbon ( 13C) NMR spectra at 4.2 T with an instrumental broadening below 50 mHz have been realized 2. Increasing requirements of sensitivity and of spectral dispersion have driven the development of NMR magnets to higher and more homogeneous magnetic fields, which are obtained by immobile and expensive superconducting magnets. The high precision allows the discrimination of similar chemical structures of small molecules as well as of macromolecules. This is at least one order of magnitude better than the precision obtained with superconducting magnets. The accuracy of the measured 1H– 29Si and 1H– 19F J-coupling constants is between a few millihertz up to 20 mHz. The J-coupling constants 3 of silicon-containing ( 29Si) and fluorine-containing molecules are measured with just a single scan. We show that in the Earth’s field the transverse relaxation time T 2 of the 7Li nucleus is very sensitive to its mobility in solution. Here we present the feasibility of liquid-state proton ( 1H), lithium ( 7Li) and fluorine ( 19F) ultrahigh-resolution NMR spectroscopy 2 in the Earth’s magnetic field. Typically, expensive and immobile superconducting magnets are required for chemical analysis by high-resolution NMR spectroscopy. High-resolution NMR spectroscopy is a powerful tool for non-destructive structural investigations of matter 1.
0 Comments
Read More
Leave a Reply. |