Some Observed Coupling Constants

Proton-proton couplings through single bonds are usually attenuated rapidly, so that generally 4J 0.5 and is usually unobservable. However, couplings are 

TABLE 5.1 Typical Proton-Proton Spin Coupling Constants 

Geminal proton—proton coupling constants (2J) depend markedly on substituents, as indicated in Table 5.1. The trend of 2J with substitution has been treated successfully by theory and will be discussed in Section 5.3. 

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The conformation isomerism in a wide range (31 examples) of 1-sub-stituted derivatives of 3,3-dimethylbutane has been examined. In some cases, the temperature variation of the vicinal coupling constants was studied. In 1,1,2-trichloroethane the coupling constants of the gauche and anti isomers are deduced from solvent-dependent changes in the observed coupling constants. In some similar systems (1,1,2-trichloro- and 1,1,2-tribromoethane in carbon tetrachloride and in benzene), a correlation of the dipole moments of the compounds and the vicinal coupling constants was found.Rotational isomerism in the phenylalanine anion and dipolar ion has been studied in deuterium oxide solutions. 

The magnitude of J often provides stmctural clues. For instance, one can usually distinguish between a cis olefin and a trans olefin on the basis of the observed coupling constants for the vinyl protons. Table 3.9 gives the approximate values of some representative coupling constants. A more extensive list of coupling constants appears in Appendix 5. 

A number of MO calculations has been carried out, and these have had mixed success in predicting chemical reactivity or spectroscopic parameters such as NMR chemical shifts and coupling constants. Most early calculations did not take into account the contribution of the sulfur 3d-orbitals to the ground state, and this accounts for some of the discrepancies between calculations and experimental observations. Of the MO methods used, CNDO/2 and CNDO/S have been most successful the INDO approximation cannot be used because of the presence of the sulfur atom. 

If one wishes to obtain a fluorine NMR spectrum, one must of course first have access to a spectrometer with a probe that will allow observation of fluorine nuclei. Fortunately, most modern high field NMR spectrometers that are available in industrial and academic research laboratories today have this capability. Probably the most common NMR spectrometers in use today for taking routine NMR spectra are 300 MHz instruments, which measure proton spectra at 300 MHz, carbon spectra at 75.5 MHz and fluorine spectra at 282 MHz. Before obtaining and attempting to interpret fluorine NMR spectra, it would be advisable to become familiar with some of the fundamental concepts related to fluorine chemical shifts and spin-spin coupling constants that are presented in this book. There is also a very nice introduction to fluorine NMR by W. S. and M. L. Brey in the Encyclopedia of Nuclear Magnetic Resonance.1... 

The values of 3/(NH,H) coupling constant observed for imine proton can be helpful in detection of the proton transfer processes and determination of mole fractions of tautomers in equilibrium. For NH-form, this value is close to 13 Hz, lower values usually indicate the presence of tautomeric equilibrium. It should be mentioned that the values below 2.4 Hz have not been reported. The chemical shift of C—OH (C-2 for imines, derivatives of aromatic ortho-hydroxyaldehydes or C-7 for gossypol derivatives) carbon to some extent can be informative, however, this value depends on type of substituents and should be interpreted with caution. 

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