Karplus curves

Aydin, R., Gunther, H. C, H spin-spin coupling. 9. Norbomane a reinvestigation of the Karplus curve for... 

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. 

Figure 6.2 The Karplus curve - for relating the observed splitting between vicinal protons to their dihedral...

Another disadvantage is that for solving certain stereochemistry problems, it is necessary to be able to not only establish connectivity but to measure couplings fairly accurately so that the data can be used in conjunction with the Karplus curve. Whilst this is possible using a phase sensitive COSY (Note -... 

All other factors are constants. The angle 9 is the projection angle between bond vectors and is the sole unknown in Eq. 1, and can be readily and precisely determined by measurement of the cross-correlated dipolar relaxation rate. It should be emphasized that 9 is measured directly, without the need of experimental calibration as in the Karplus curve for J-coupling constants for example. 

Fig. 3. 3J(H,H) Karplus curve for butane at the DFT-B3LYP/6-311G level. Total, -O- FC, - - SD, PSO, DSO.

Instead, we see a strong mutual NOE. Apparently, the H3-C3-C4-H4 dihedral angle is very near the minimum in the Karplus curve (90° angle) so that the vicinal coupling constant is very small. 

Three-membered rings are flat with all bonds eclipsed so the dihedral angle is 0° for cis Hs and 109° for trans Hs. Looking at the Karplus curve, we expect the cis coupling to be larger, and it is. A good example is chrysanthemic acid, which is part of the pyrethrin group of insecticides found in the pyrethrum plant. Both cis and tram chrysanthemic acids are important. 

Figure 8.1 Karplus curve showing dependence of the vicinal coupling constant 3JHa c c Hb (in Hz) on the dihedral angle ...

DFT calculations have been used to construct new Karplus curves for 2./ m. 3/Hh (as mentioned before) 2/CH, Veil- Vnccc. 3/hcoc> and 3/hcsc- A major focus of this investigation62 was DFT computations for the exocyclic hydroxymethyl group of aldohexopyranoside derivatives, particularly in methyl a- and /i-D-gluco- and -galacto-pyranosides, for which coupling constants were also determined experimentally by 2D H -13C heteronuclear zero- and double-quantum, phase-sensitive /-HMBC NMR. DFT on methyl /1-D-glucopyranoside yielded 3/hcoc values that fitted the equation ... 

FIG. 9. Four-quadrant plots of DFT results for an ethyl /J-D-glucopyranoside mimic of a disaccharide, showing phase shifting of 3/Ccoc Karplus curves for C-C-O-C coupling pathways bearing an internal electronegative substituent. The mimic has deoxy functions at C-3, C-4, and C-6. 

Interactive, online programs are available for the calculation of coupling constants, dihedral angles, and the H distance across thr a-linkages of some disaccharides, as well as a program for plotting up to five Karplus curves. 

H. Zhao, I. Carmichael, and A. S. Serianni, Oligosaccharide trans-glycoside. Icroc Karplus curves are not equivalent Effect of internal electronegative substituents,. /. Org. Chem., 73 (2008) 3255-3257. 

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