Nuclear magnetic resonance structure determination, coupling constants

Quadrupole coupling constants for molecules are usually determined from the hyperfine structure of pure rotational spectra or from electric-beam and magnetic-beam resonance spectroscopies. Nuclear magnetic resonance, electron spin resonance and Mossbauer spectroscopies are also routes to the property. There is a large amount of experimental data for and halogen-substituted molecules. Less data is available for deuterium because the nuclear quadrupole is small. 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for structure determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDC13), 6 = H2 7.12, H3 7.34, H4 7.34, and H5 7.12 ppm. Coupling constants occur in well-defined ranges J2 3 = 4.9-5.8 J3 4 = 3.45-4.35 J2 4 = 1.25-1.7 and J2 5 = 3.2-3.65 Hz. The technique can be used quantitatively by comparison with standard spectra of materials of known purity. 13C-nmr spectroscopy of thiophene and thiophene derivatives is also a valuable technique that shows well-defined patterns of spectra. 13C chemical shifts for thiophene, from tetramethylsilane (TMS), are C2 127.6, C3 125.9, C4 125.9, and C5 127.6 ppm. 

Nuclear magnetic resonance (NMR) spectroscopy is the most powerful spectroscopic method for structural elucidation of organic molecules and is routinely used by organic chemists. Summarised below are common NMR active nuclei chemical shift data for NMR solvents, common impurities, and functional groups coupling constants and details of common NMR experiments used to determine the connectivity and stereochemistry of small organic molecules. 

Nuclear magnetic resonance parameters of disubstituted pyrazines have been correlated with chemical structures (867). The coupling constants of 2,3. 2,5-, and 2,6-disubstituted pyrazines were found to be 2.5-3.0, 1.1-1.4, and OHz, respectively, and the values were not influenced by the kind of substituents (867). Methyl-methyl proton spin-spin coupling constants in 2,5- and 2,6-dimethyl-pyrazines have been determined (1502). 

The quantum-mechanical picture of hyperfine structures presented by the spin-spin nuclear magnetic resonance (NMR) and electron-spin resonance (ESR) spectra involves a variety of spin Hamiltonian parameters of molecular origin whose magnitude determines that of the coupling constants. In such an analysis, the most characteristic term arises from the Fermi -or contact -operator ... 

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