Coupling constants over four bonds

The 1H NMR spectrum of the four central protons of 48h was analyzed as an AA BB spin system. The coupling constants between II7/II7 and the deuterium nuclei on C6/C6 (ca 2 Hz) were taken into account as first-order perturbations. In all cases coupling constants over four (—0.58 to —0.87 Hz) and five (+0.32 to +0.69 Hz) bonds were also considered in performing the spectral analysis. Chemical shifts and vicinal coupling constants for 48a-f are reported in Table 14. 

Organic compounds contain four types of carbon atom methyl, methylene, methine and quaternary. And so if we simply record the spectrum as we would a proton spectrum, the result will be a series of quartets, triplets, doublets and singlets, each associated with a carbon-proton one-bond coupling constant of between 125 and 250 Hz. If we are dealing with a complex molecule, these multiplets will overlap and give us spectra which are almost impossible to analyse. In addition, coupling interactions over two or more bonds complicate the picture still further. 

We have recently reported the first observation and measurement of spin-spin interactions in neutral hexacoordinate silicon chelates [1] which extend across the N— Si coordinative bond and over two, three, and even four bonds. These coupling constants were highly sensitive to small geometrical modifications in the complex. The major geometrical requirement found for spin-spin coupling over two bonds (N—>Si-F or N—>Si-H) in hexacoordinate complexes la-lc was that the corresponding bond angle be 90° or very close to it. 

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