Carbon coupling constants prediction

Spin densities help to predict the observed coupling constants in electron spin resonance (ESR) spectroscopy. From spin density plots you can predict a direct relationship between the spin density on a carbon atom and the coupling constant associated with an adjacent hydrogen. 

Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron-rich, while the carbon has a chemical shift in the range expected for a multiply bonded species. The coupling constant data are difficult to rationalize, as it is not possible to predict the influence of orbital, spin-dipolar, Fermi contact, or higher-order quantum mechanical contributions to the magnitude of the coupling constants. However, classical interpretation of the NMR data indicates that the (phosphino)(silyl)carbenes have a P-C multiple bond character. 

Agreement of predicted and measured carbon-metal coupling constants for metals without lone-pairs is satisfactory or poor, e.g. 

Calculations of the 13C-13C coupling constants for all aldopyranoses of the D-series have allowed some general relationships with respect to stereochemistry and coupling constants to be postulated <2003RJ0663, 2003RJ01194>. Data have been reported for the 13C NMR substituent effects at the a, f3, it, and S positions for 2-halomethyl-2-hydroxy-tetrahydrofurans and 2-halomethyl-5,6-tetrahydro-4/7-pyrans. These studies showed that additivity rules allow the prediction of the chemical shifts for each carbon <1996SPL631>. 

The spectrum which consists of a triplet of triplets is attributed to a major hyperfine interaction with the orlAo-protons and a smaller interaction with the meta-protons. The coupling to the jpara-proton is not resolved and must be less than 3 G. The values of the hyperfine coupling constants are listed in Table 1, together with the theoretical values which have been calculated on the assumption that the unpaired electron is located in an sp -hybrid orbital on the valence carbon. The agreement between the predicted and experimental values is reasonable and shows that the unpaired electron remains in the sp -orbital which is in accord with the conclusion obtained from optical spectroscopic measurements. 

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