Cyclopropanes upheld shifts

The upheld shifts observed with cyclopropanes are probably related to their large observed magnetic susceptibilities,112 which is similar to that found for benzene. The different sense of the NMR shifts for benzene (downheld) and cyclopropane (upheld) is merely due to the different geometrical arrangement of the hydrogens in the two. The shift in benzene has been attributed... 

CF2 groups within a carbocyclic ring system are unremarkable, generally absorbing slightly downheld from those contained in a straight chain acyclic system, with the remarkable exception of cyclopropane systems, the fluorines of which exhibit a characteristic ( 40 ppm) upheld shift to absorb at about -139 ppm (Scheme 4.4). 

It is now possible to satisfactorily calculate CNMR shifts with an average error of only 5 ppm. The results for the above compounds are given in Table 7, and it can be seen that they are remarkably good. It has not as yet been possible to derive a simple physical model to explain the variation in chemical shifts, but it seems likely that such a model will be forthcoming. It will then be possible to understand a variety of effects found in NMR spectroscopy, including the upheld shifts of cyclopropane hydrogens and carbons. 

A trifluoromethyl group attached to a cyclohexane ring is unremarkable with respect to its chemical shift, absorbing at -75 ppm, with a 3/fh = 8Hz (Scheme 5.2). There are no data available for trifluorometh-ylcyclopentane or cyclobutane. The chemical shift for trifluoromethyl-cyclopropane reflects additional shielding, such CF3 groups appearing the farthest upheld of any CF3-substituted hydrocarbon. 

Some of the more remarkable effects of strain are found in NMR chemical shifts. Cyclopropane derivatives usually have upheld proton chemical shifts with regard to the corresponding cyclohexane derivatives, whereas cyclobutanes commonly have downfield shifts.The upfield shift for cyclopropane protons have sometimes been attributed to a ring current in the three-membered ring, but there is little evidence for such a phenomenon. The unusual shift for these protons has proven valuable in demonstrating the presence of a three membered ring. 

The NMR spectra of cyclopropanes are unique among carbocycles. The HNMR chemical shift (c>H) of cyclopropane is 0.12, considerably upheld from cyclohexane (1.44), whereas the hydrogens of cyclobutane resonate further downheld (1.96).110 Similarly, upheld 13C chemical shifts (<>c) are found for cyclopropane (— 2.9), compared with cyclohexane (27) and cyclobutane (23.). 11... 

The NMR spectra of cyclopropane derivatives are unique among carbocycles. The NMR chemical shift of cyclopropane is 3 0.12, considerably upheld from cyclohexane... 

Probably the earliest manifestations of the induced shielding brought about by the purported ring current of the cyclopropane ring were those of Sauers who reported that the vinyl protons of 26 were shifted upheld by 0.6 ppm with respect to 25 on account of... 

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