Equatorial protons, coupling

One-bond couplings ( /ch) in saturated systems do not seem to have been investigated extensively. The value for cyclohexane (an average of couplings to axial and equatorial protons) iSk 123 Hz, and is increased by substitution adjacent to the carbon by an electronegative element, as with the aromatic systems discussed above. 

The multiplets and coupling constants of the axial) protons at = 3.15, 3.50 and 4.08 moreover confirm the equatorial positions of all three OH groups, as can be seen in formula B. Here the couplings from 10.0 to 11.5 Hz, respectively, identify vicinal protons in diaxial configurations, whilst values of 4.5 and 5.0 Hz, respectively, are typical for axial-equatorial relationships. As the multiplets show, the protons at 5 = 3.50 and 4.08 couple with two axial and one equatorial proton (triplet of doublets) respectively, whereas the proton at = 3.15 couples with one axial and one equatorial proton (doublet of doublets). 

The same features exist at a reduced level for POCCH couplings in phosphorinanes and these are about 2-6 Hz for an equatorial proton at C-5, while it is less than 1 Hz for an axial proton. Similarly, coupling of phosphorus to an equatorial methyl at C-4 is 2 to 3 Hz while that to axial methyl is less than 1 Hz (1968>83) similar trends have been found in the corresponding phosphonates.<1969,52)... 

A number of interacting factors precludes any generalisations to be made but it should be noted that the clear distinction of the proton couplings to axial and equatorial fluorine atoms in pentavalent compounds is again obtained. The same order is found as for 2/(H-P-F) namely 13/(H-F ax) 3/(H-Feq). ... 

In the proton spectrum, the vinylic hydrogen appears as a pentet at 8 6.14 with a three-bond F-H coupling constant of 9 Hz, which means that only the equatorial fluorines couple to the hydrogen. There appear to be no examples of the axial fluorine of an SF5 group exhibiting... 

More recently, the radical cation of the parent system has been characterized by ESR/ENDOR spectroscopy [340], These experiments revealed medium-sized hfcs for both the bridgehead (—11.4 G) and the equatorial protons (+11.4G) and large positive hfcs (+ 77 G) for the axial protons (which in 75 are replaced by the trimethylene bridge). The large difference between the couplings of the axial and equatorial protons indicates a nonplanar, puckered geometry of the radical cation. No evidence for interconversion is apparent up to 160 K this finding requires an inversion barrier of at least 12 kJ mol-1. 

Some of the extensive experimental data on the one-bond carbon-proton coupling constants in several pyranose derivatives, as well as oligo- and polysaccharides, are summarized in Tables I-V. Several generalizations may be made from inspection of these data. The anomer that has an equatorially disposed hydrogen at C-1 has the larger coupling constant, and the value in such pyranosides is in... 

You can draw a general conclusion from this observation an NMR signal is roughly as wide as the sum of all its couplings. In any given compound, an axial proton will have a much wider signal than an equatorial proton. 

A complete assignment of the H NMR data for tetrahydropyran (THP) 140 has been reported <1998J(P2)1751>. At room temperature the 400 MHz spectrum of THP in 1 1 CDCl3 CFCl3 consists of three multiplets at S 3.63, 1.64, and 1.57 in a ratio of 2 1 2 (H(2), H(4), and H(3)). At -85 °C, all of the resonances are resolved and the assignment of axial and equatorial protons is possible. This data, in conjunction with the previously reported study of the coupling constants of tetrahydropyran and comparison with cyclohexane data (Table 11) <1976JOC1380>, provide a complete picture of the NMR of tetrahydropyran. 

The observation of four carbon resonances now limits the possibilities to just two isomers these can be distinguished by the couplings in the proton spectrum. A is a 3 Hz triplet, so it is coupled to two equatorial protons. D is a 10 Hz triplet, so it is coupled to two axial protons. C is coupled to one axial and one equatorial proton, while B is coupled to two axial protons. The only relative stereochemistry and conformation consistent with these is that shown to the right, myo-inositol. 

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