Trigonal and Tetrahedral Compounds

The electronic structures of these compounds are generally typified by straightforward a frameworks of localized two-electron two-center bonds with occasional additional % bonding geometry and constitution then readily specify familiar concepts of electronic structure which permit empirical correlations with NMR data to be made. 

Theoretical treatments (which so far have not been extended much beyond CNDO-type levels) also show that calculated boron Ip density matrix elements exhibit the best correlations with experimentally determined shieldings. This applies to both trigonal and tetrahedral species. These treatments also indicate that in general there is little reliable correlation with calculated total electron density, which tends to confirm the expected dominance of variations in the excited state term o (Chapter 3, Section 2 and 3) in these compounds. 

Again the overall shielding trends are paralleled in carbon chemistry, and a plot of 5( C) for classical carbenium ions [CXYZ] versus ( B) for the corresponding trigonal boron compounds [BXYZ] has a gradient 5( B) 5( C) of ca. 0.38 1 for the species examined. The deviations from this correlation are somewhat greater 

These generalizations, however, do not appear to hold for couplings B) 

Couplings to fluorine B) are well studied,and those of larger 

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