Coupling long range

FIGURE 5.51 Expansions of the ring-proton resonances from the 300-MHz H spectrum of furfuryl alcohol. The resonances of the side chain (-CH2OH) are not shown. 

The magnitude of for W coupling is usually small except in highly strained ring systems. Apparently these rigid strained arrangements somehow have a favorable geometry for the overlaps involved. 

FIGURE 5.56 Two possible overlap mechanisms explaining the transfer of spin information between Ha and Hb when a molecule is arranged in a ( J) W pattern. 

FIGURE 5.57 A steroid ring skeleton showing several possible couplings of the W t)T3e 

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Two-dimensional C//correlations such as C//COSY or HC HMQC and HSQC provide the Jqh connectivities, and thereby apply only to those C atoms which are linked to H and not to non-protonated C atoms. Modifications of these techniques, also applicable to quaternary C atoms, are those which are adjusted to the smaller Jqh and Jqh couplings (2-25 Hz, Tables 2.8 and 2.9) Experiments that probe these couplings include the CH COLOC (correlation via long range couplings) with carbon-13 detection (Fig. 2.16) and HC HMBC (heteronuclear multiple bond coherence) with the much more sensitive proton detection (Fig. 2.17)... 

The long-range coupling of 2.2 Hz which appears in A and B, two quaternary C atoms in the NMR spectrum with appropriate shifts (5c = 76.6 and 83.0) and the two double-bond equivalents (molecular formula C TZ/oO) suggest that a CC triple bond links the two structural fragments. Hence the compound is identified as hex-3-yn-l-ol (C) in accordance with the coupling patterns. 

Not all consequences of fluonne mcorporaton m a molecule aid m structure identification Long range coupling and conformational preferences can comph... 

Long-range coupling information can provide msights into conformational preferences [170] and relative transrmssive abilities of intervenmg atoms or moie-... 

Note the relationship between Ha and the other hydrogens. Long-range coupling to Ha (J 4 Hz) is observed in only one of these molecules. Which molecule do you think this is Identify the proton that is coupled to Ha. 

The widespread occurrence of long-range couplings in both furanose and pyranose derivatives explains why so many of the P.M.R. spectra of carbohydrate derivatives are apparently poorly resolved, even when the resolution of the spectrometer is above reproach. For example, the Hi resonance of the 1,6-anhydro-D-glucose derivative (12) is coupled to all of the other six ring protons. A further example of the line-broadening effect follows a consideration of the spectrum of 5,6-dideoxy-5,6-epithio-l,2-0-isopropylidene-/ -L-idofuranose for which the half-height... 

While the above studies were in progress results from dioxane derivatives were published (19,94) which support all of the comments and cautions we have made above. Additional reports have been made of a long-range coupling across a ring oxygen in an unsaturated pentopy-ranose derivative (17) and in a saturated furanose derivative (20). 

It is evident from the above discussion that more-detailed studies of the P.M.R. spectra of carbohydrate derivatives will reveal an ever-increasing number of long-range couplings. Studies in this laboratory are directed towards the investigation of the occurrence of these couplings in the spectra of both furanose and pyranose derivatives. 

Long-range coupling between protons more than two carbon atoms apart is sometimes observed when v bonds intervene. An example is found in l-methoxy-l-buten-3-yne. Not only does the acetylenic proton, Ha, couple with the vinylic proton H, it also couples with the vinylic proton Hc, four carbon atoms away. The data are ... 

Fiffire 5.38 Pulse sequence for delayed COSY—a modification of the COSY experiment. The fixed delays at the end of the evolution period t and before the acquisition period <2 allow the detection of long-range couplings between protons. 

The H-NMR and C-NMR chemical shifts have been assigned and substractures have been deduced on the basis of COSY-45° (Problem 5.13) and other spectroscopic observations. Interpret the HMBC spectrum and identify the heteronuclear long-range coupling interactions between the H and C nuclei. 

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