Carbon coupling constants bond order

The C-H spin couplings (Jen) have been dealt with in numerous studies, either by determinations on samples with natural abundance (122, 168, 224, 231, 257, 262, 263) or on samples specifically enriched in the 2-, 4-, or 5-positions (113) (Table 1-39). This last work confirmed some earlier measurements and permitted the determination for the first time of JcH 3nd coupling constants. The coupling, between a proton and the carbon atom to which it is bonded, can be calculated (264) with summation rule of Malinovsky (265,266), which does not distinguish between the 4- and 5-positions, and by use of CNDO/2 molecular wave functions the numerical values thus - obtained are much too low, but their order agrees with experiment. The same is true for Jch nd couplings. 

The 13C—H coupling constants of methyl (213 Hz23 ) and phenyl (216 Hz55 ) cyclopropenone are in the order of those obtained for cyclopropene vinylic protons (200/201 218 Hz/221 Hz174 ) and reflect an s-contribution of more than 40% in the carbon hybrid orbital of the vinyl C—H bond. 

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. 

The factor ijhb is the bond order between the interacting hydrogen atoms and is unity for H2. Proton-proton bond orders in substituted benzenes as derived from coupling constants and Eq. (21) are given in Table III. The same quantitative significance should not be attached to the proton-proton bond orders of Table III that one gives to the carbon-carbon... 

In general there is a good correlation between bond distances in fused aromatic compounds and bond orders. Another experimental quantity that correlates well with the bond order of a given bond in an aromatic system is the nmr coupling constant for coupling between the hydrogens on the two carbons of the bond.74... 

Organophosphorus compounds rarely have more than one P atom in the molecule, giving rise to a doublet for any C atom within three bonds (assuming the proton-carbon coupling has been removed) and so these compounds show clearly the connectivity of carbon atoms close to the P atom. The size of the coupling constant is dependent on the number of bonds and the phosphorus oxidation state, such that Vis the largest at about 45-150 Hz, while V and V are of the order of 10-15 Hz. 

The one-bond coupling constants Jcc tabulated so far cover a span of 30 to 180 Hz (Table 3.11 (a)). They increase with increasing s character of the hybrid orbitals contributing to the bond between the coupling carbon nuclei. A rough correlation between Jcc and the product Sj s2 of the s characters of the interacting carbon hybrid orbitals (C-l, C-2), i.e. the s bond order of the C-C bond, is observed (Table 3.11 (a) Fig. 3.17). 

Due to the partial n character of the CN bonds which are, in fact, vinylogous forma-mide or amidine connections in keeping with the resonance formulae, different methyl shift values are observed for some N,N-dimethylamine groups. Moreover, one-bond carbon-proton coupling constants decrease from the end to the center of the polymethine chain. This is explained by CNDO/2 calculations, in which corrected bond orders and bond angles alternating between 125° (a, y, e) and 117.5° (/5, 5) are taken into account [343],... 

Hyperconjugation should raise the bond order between the ipso and benzylic carbons, and lower the bond order between the benzylic carbon and the silicon atom. As the ipso-benzylic bond length decreases, the 13C-13C coupling constant should increase, and as the benzylic carbon-silicon bond length increases, the 13C-29Si coupling constant should... 

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