Cyclic Compounds and the Karplus Curve

Now let us consider Structure 6.21 and Spectrum 6.18 and see how the Karplus curve can be used to aid assignment of the spectrum. (This compound will be referred to from now on as the morpholine compound as we will use it to demonstrate several different techniques) Note that the aromatic region has been omitted as it contains little of interest and we wish to concentrate on the carbocyclic region of the spectrum. It was acquired in CDCI3. 

It is also worth noting that nine times out of ten, equatorial protons absorb at somewhat lower field than the corresponding axial protons. This can be reversed in certain cases where the specific anisotropies of the substituents predominate over the anisotropies of the rings themselves but this is relatively rare. The difference is typically 0.5-1.0ppm, but may be more. 

Spectrum 6.18 The morpholine compound in CDCI3 with expansion. 

Proton c can be defined by the fact that it is not equatorial and it is highly coupled. The multiplet at 3.82 ppm satisfies these requirements. It is in the right ball park for chemical shift and is highly complex in that this proton is already the X part of an ABX system coupled to both protons alpha to the chlorine (the AB part). It is then further coupled with a 10 Hz, axial-axial coupling (reciprocated in the dd at 2.07 ppm) and with a 2 Hz axial-equatorial coupling which is reciprocated in the ddd at 2.90 ppm. Note that c and d are not fully resolved from each other. Such overlap inevitably complicates the issue. 

The N-benzyl protons are accidentally equivalent, presenting as a singlet at 3.59 ppm and overlap with the two protons alpha to the chlorine atom which present as the heavily roofed AB part of an ABX system (i.e., eight lines) centred at 3.55 ppm. 

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