Carbon-Deuterium Coupling

Carbon-proton and carbon-deuterium coupling constants are related to each other by eq. (3.15a), which follows not only from eq. (3.15) but also from Ramsay s theory [5, 10]. 

As can be seen in Table 3.1 for commonly used deuterated solvents, deviations of Jcu/Jcd from the ratio yH/yD are rare and small, so that only J(H or JCD need be determined experimentally. To conclude, all relations found for JCH can also be derived for JCD. The signal multiplicities, however, are different (3 for CD, 5 for CD2 and 7 for CD3, respectively) because of different spin quantum numbers (/D = 1 /H = ). 

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Fig. 5.9. 13Cf1 - NMR spectrum of flavin adenine dinucleotide (FAD), disodium salt, 30 mg in 1 ml, of deuterium oxide 30 °C 100.576/400.133 MHz (13C/ H) 5000 interferograms (32K/15000 Hz) (a) sp3 carbon partial spectrum (20-92 ppm) with expanded output (b) to display carbon-phosphorus couplings (2JCI, for A5 and R5 3Jcr for A4 and R4 carbon nuclei) (c) partial spectrum of heterocyclic carbon nuclei (117-167 ppm). Signals are assigned according to ref. [147].

Something else will happen to the proton NMR spectrum. The signal for the CH3 group was a triplet in the original ketone, but when those two Hs are replaced by Ds, it becomes a singlet. In the carbon spectrum, coupling to deuterium appears rememberthe shape of the CDCI3 peak (Chapters) ... 

Figure 1. Representation of the properties of a nonquadrupolar and a quadrupolar nucleus. The charges around the nucleus represent those of its molecular environment. The electric field gradient tensor, qae, is expressed in the coordinate system of the molecule. Usually, the z-axis lies along the carbon-deuterium bond e is the charge on the electron, h is Planck s constant, and Q is the quadrupole moment. For axial symmetry, only q, is required to express the quadrupole coupling constant.

Because axial and equatorial protons are different, when the flexing of the cyclohexane chair has been frozen out, the geminal protons (i.e., protons on the same carbon) will couple with each other. Furthermore, axial and equatorial protons can also couple with equatorial and axial protons (respectively) on adjacent carbons. Thus, in the experiment described above, all but one proton was replaced by deuterium and the decoupling of deuterium (/ = 1) from protium effected. 

The static deuterium quadrupole coupling constant is (e qQ/h) = 170 kHz for aliphatic carbon-deuterium (C—D) bonds. A change in the residual quadrupole splitting can be caused by two different mechanisms. First, the angle of the molecular fluctuations may increase or decrease, secondly, the molecule may undergo a conformational change which alters the orientation of the C—D bond vector with respect to the bilayer normal. 

Diastereoselective reductive coupling of MVK and p-nitrobenzaldehyde performed under an atmosphere of elemental deuterium provides an aldol adduct incorporating a single deuterium atom at the former enone f>-pos-ition [69]. Deuterium incorporation at the a-carbon is not observed, excluding Morita-Baylis-Hillman pathways en route to product. Incorporation of a single deuterium atom suggests irreversible enone hydrometallation (Scheme 5). 

In this case ylide complexes are not observed and therefore the reactions are very simple. When L 2-methylpyridine or acetonitrile, the product was shown to be (XII) rather than (XIII). Complex (XII) could be characterised directly by lU and 13C NMR spectroscopy or, more simply, treated with triphenylphos-phine to release the alkene. Figure 1 shows the 13C XH NMR spectrum of the released alkene (together with 2-methylpyridine), which clearly shows 1 1 1 triplets for carbon atoms C1 and C due to coupling to deuterium as expected for the alkene from (XII) but not from (XIII). In addition, the 2H 1H NMR spectrum shows approximately equal integration for deuterium at C1 and at C1 ... 

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