Carbon coupling constants distance

Until now, the determination of three-dimensional structures of oligosaccharides in solution was based primarily on proton-proton distance information obtained from n.O.e. data. Here, we discuss the application of three-bond proton-carbon coupling constants. 

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

The X-ray crystal structure determination of bis[2,2,4,4,6,6-hexamethyl-2,4,6-trisilacy-clohexyljzinc (45) shows that this compound exists as a monomer in the solid state (Figure 22). The observed Zn—C bond distance of 1.937(2) A is remarkably short. The zinc atom is located at a crystallographic inversion centre and consequently enforces a perfectly linear C—Zn—C arrangement. The Zn—C(l)—Si and Si—C—Si bond angles of 110 and 117°, respectively, indicate that a considerable planarization of C(l) has occurred. That such a planarization of the carbon atoms bound to zinc is most likely also present in the structure of 45 in solution was concluded from their exceptional low field shift of 13 ppm and the small J( C- H) and 7( C- Si) coupling constants observed in its C NMR spectrum. 

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... 

Using the data shown in Table 3.4, a linear plot of 13C versus 1H solvent shifts is obtained [92], Moreover, 13C solvent shifts correlate linearly with the one-bond carbon-13-proton coupling constants [92]. This is attributed to changes in the average distance of bonding electrons in the C — H bond of chloroform due to intermolecular association [92], Since much smaller solvent shifts of the carbon tetrachloride 13C resonance are found [92], interactions between chlorine and the solvent can be disregarded. Thus,... 

As can be seen from Table 5.9, the 13C — 19F coupling constants decrease drastically with increasing distance of the carbon atoms from the CF group in the molecule. With the exception of C-19, fluorine substitution causes the y-carbon signals to shift upheld (cf. resonances of C-l, C-5, C-7, C-12 and C-14 of the compounds in Table 5.9). 

Addition of the same NHC to Eu(thd)3 (thd — tetramethylheptanedioate) affords the europium(III) adduct Eu(thd)3(NHC). The europium-NHC bond distance of 2.663(4) A is shorter than that of the samarium(II) complex and is consistent with the higher oxidation state of the lanthanide centre. The yttrium(III) analogue was also prepared and characterised by NMR spectroscopy. The C2 carbon resonates at 199 ppm in the 13C NMR spectrum, with a yc coupling constant of 33 Hz. This indicates that the NHC remains bound to the metal centre in solution and does not dissociate on the NMR timescale. 

---