Four bond coupling constants

Coupling Constants. The usual three-bond H—H coupling constant in fluorobenzenes is about 8 Hz, whereas the four-bond coupling constant is between 1 and 3 Hz. Five-bond coupling is usually not observed. Likewise, the three-bond F—H coupling constant is about 8 Hz, the four-bond value 5-6 Hz, and the five-bond coupling constant about 1 Hz. 

Alkorta and co-workers have performed ab initio EOM-CCSD/ (qzp, qz2p) calculations on complexes with intermolecular hydrogen bonds involving N and and molecules with and without intramolecular hydrogen bonds involving these nuclei. The calculations included two-, three-, and four-bond coupling constants. The authors... 

The J( F, F) coupling constant between the two fluorine atoms in compound I in Figure 9 is exceptionally large. It is the highest value reported to date and it also exceeds three- and four-bond coupling constants. This has been attributed to the overlapping nonbonding electron distribution for two F nuclei in close spatial proximity which allows correlation not only of electron-pair bond to electron pair, but also of electron spin. 

Application of the method to 5 -AMP and 3 -AMP gave the results indicated in the remainder of Fig. 3. There were two computed distributions in the 5 -AMP case as shown. The second one is unlikely, however, because it is not consistent with four-bond P-H coupling constants as discussed by Hart (1978). The first computed conformation distribution is reasonably consistent with four-bond coupling constants and with other P-H and P - C coupling-constant analyses reported by Sarma et al. (1973), Evans and Sarma (1974), Lee et al. (1976), Cozzone and Jardetzky (1976), and Schleich and Smith (1976). 

Organic compounds contain four types of carbon atom methyl, methylene, methine and quaternary. And so if we simply record the spectrum as we would a proton spectrum, the result will be a series of quartets, triplets, doublets and singlets, each associated with a carbon-proton one-bond coupling constant of between 125 and 250 Hz. If we are dealing with a complex molecule, these multiplets will overlap and give us spectra which are almost impossible to analyse. In addition, coupling interactions over two or more bonds complicate the picture still further. 

The doublet for the four equatorial fluorines appears at +83.8 ppm, while the pentet for the single axial fluorine appears at 77.6 ppm. The two-bond coupling constant is 161 Hz. 

Three-bond coupling constants (C-6,H-1 and C-2,H-1 ) in the C-n.m.r. spectra of pyrimidine nucleosides were used to obtain information on the glycosidic bond conformation in conjunction with X-ray results, a four-state conformational model was proposed consisting of an equilibrium between two syn and two anti conforma-16 2... 

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