Couplings geminal

Nonequivalent protons attached to the same carbon (geminal protons) will couple with one another. These coupling constants tend to be large ( 10 Hz) for sp carbon atoms and small ( 4 Hz) for sp carbon atoms. Geminal coupling constants tend to decrease with decreasing ring size, because of hybridization 

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Spm-spm splitting of NMR signals results from coupling of the nuclear spins that are separated by two bonds (geminal coupling) or three bonds vicinal coupling)... 

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

In view of the possible ambiguities which can attend conformational assignments based on vicinal coupling constants, it is fortunate that both long-range and geminal couplings each exhibit stereospecific dependences. 

Geminal coupling constants are presumed to be negative, even if not so stated.1411 "Measured in D20 al 400 MHz. 

Geminal coupling constants are assumed to be negative even if not so stated.147 Measured al 250 MHz in CDC13. 

In another related procedure, the protons that are bound directly to C nuclei can be flipped selectively so that geminal couplings between nonequivalent protons can be measured (Fig. 5.14). This is known as sekctive indirect J-spectroscopy (Rutar et al, 1984a). 

COSY and HETCOR experiments are extremely useful in the structure elucidation of complex organic molecules. The geminal and vicinal protons and their one-bond C-H connectivities are first identified from the HETCOR spectrum, and then the geminal couplings are eliminated from the COSYspectrum, leaving vicinal connectivities. By careful interpretation of the COSY and the one-bond HETCOR spectra, it is then possible to obtain information about the carbon-carbon connectivities of the protonated carbons ( pseudo-INADEQUATE information). In this way the carbon-carbon connectivity information of protonated carbons is obtainable through a combination of COSY and HETCOR experiments. 

The HMQC spectrum of podophyllotoxin shows heteronuclear crosspeaks for all 13 protonated carbons. Each cross-peak represents a one-bond correlation between the C nucleus and the attached proton. It also allows us to identify the pairs of geminally coupled protons, since both protons display cross-peaks with the same carbon. For instance, peaks A and B represent the one-bond correlations between protons at 8 4.10 and 4.50 with the carbon at 8 71.0 and thus represent a methylene group (C-15). Cross-peak D is due to the heteronuclear correlation between the C-4 proton at 8 4.70 and the carbon at 8 72.0, assignable to the oxygen-bearing benzylic C-4. Heteronuclear shift correlations between the aromatic protons and carbons are easily distinguishable as cross-peaks J-L, while I represents C/H interactions between the methylenedioxy protons (8 5.90) and the carbon at 8 101.5. The C-NMR and H-NMR chemical shift assignments based on the HMQC cross-peaks are summarized on the structure. 

Scalar coupling Coupling interaction between nuclei transmitted through chemical bonds (vicinal and geminal couplings). 

The geminal coupling of some azo1ooxaphospho1enes is reported to correlate with A.. 9 1 Vicinal couplings have been used in the conformational analysis of a variety of compounds such as dioxaphosphorinanes,92 phosphorinanes (in combination uith CNDO/2 calculations),93 and bromophosphorinanes (in combination uith dipole moment measurements).99 The various couplings for 1,1-viny1idene-diphosphonic acid and its salts have also been measured.95... 

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