Boron-11, chemical shift determination

INDO-parameterised calculations based on eq. 3 give very good results for some electron-deficient boron hydrides (Table 2) (13). Significant contributions to the shielding are obtained from the two- and three-centre terms. Hence current densities in relatively remote parts of the molecule play a significant role in determining the boron chemical shifts. 

The determination of the charge, and thereby the valency, of a boron atom in an organic compound is usually straightforward if its "B NMR chemical shift within the 300+ ppm spectral window is compared to that of a close standard with a firmly established solution stmcture. But, there is a need for caution The structure of many boron-containing compounds depends on the nature of the solvent, and so multisolvent (i.e., aprotic vs. protic) analyses are often essential for a definitive characterization. Sadly, aqueous solution "B NMR spectral analyses are seldom reported—even, surprisingly, for compounds clearly prepared for their potential biological value. 

In order to estimate and compare the magnitude of the M-B interactions in these isoelectronic complexes, a whole set of structural and spectroscopic parameters determined experimentally and/or computed theoretically were considered. This includes the M - B distance the ratio r between the M -B distance and the sum of covalent radii (to take into account the different sizes of the metals involved), the pyramidaliza-tion of the boron environment XB, the rlB NMR chemical shift <5 11B, the difference AqB between the charge at boron in the metal boratrane and the free ligand TPB, the difference A M between the charge at the metal in the metal boratrane and that in the related borane-free complex [M(i-Pr2PPh)3], and the NBO delocalization energy A NBo associated with the main donor-acceptor M-B interaction found at the second order in the NBO analysis (Table 2). Only the conclusions of this detailed analysis will be recalled here ... 

A high-resolution n.m.r. spectrometer has been developed for the determination of reliable nB chemical shifts in solids containing tetrahedr-ally co-ordinated boron.3 The data reported are summarized in Table 1. 

Subsequently, as more data accumulated on systems containing hypercoordinated boron and carbon atoms, it became evident that a roughly hnear relationship of slope of approximately 0.33 existed between the boron-11 and carbon-13 chemical shifts [a relationship to which both Eqs. (5.35) and (5.36) approximate]. This held for a wide range of isoelectronic pairs of boron and carbon compounds, as iUustrated in Figure 5.12 and Table 5.6, and in greater detail in Chapter 6 of the hrst edition of this book, written when the structures of carbocationic systems were stih to be resolved. Now that more structures have been determined, others can be calculated reliably, and disputes about them are few, we need to discuss only a few recent examples below, to illustrate what B- C NMR chemical shift relationships hold for... 

The structure of the alkoxyboron difluorides has recently aroused some interest, and the historical consideration of the structure of these compounds has been outlined. Although the chemical shifts of these compounds are comparatively high (+01 to -f0 8) this does not imply quadricovalency (see above discussion) > however, the formation of the pyridine co-ordination complexes of the alkoxyboron difluorides (11) did not give appreciably higher B chemical shifts, and it was concluded that the boron atom in alkoxyboron difluorides was already tetracovalent. Molecular-weight determination confirmed the trimeric structure similar to the boroxole ring system. 

Holloway (14) has determined chemical shifts and the Jn-c coupling constant for the boron isonitrile complex (CH3)3CNCB(CHs)3 = C = 157.8... 

The role of D-fructose, fluoride, and pH in the structure formed with boronic acid has been investigated with NMR spectroscopy of monomer solutions at different pH values in the presence of D-fructose, with and without fluoride. For example, B and F NMR can be used to determine the structure of the boron (neutral trigonal vs anionic tetrahedral) and the involvement of fluoride in the complex (free vs complexed fluoride). The B chemical shift of a tetrahedral boronate appears approximately 20 ppm upfield from the trigonal boronic acid... 

Figure 3.20 shows the one-dimensional (A) and two-dimensional MQMAS (B) B NMR spectra obtained for heat treated PABA. The broad signal centered at 16.5 ppm and a sharper peak at 1.5 ppm are observed corresponding to three-coordinate and four-coordinate boron, respectively. A two-dimensional MQMAS [107] experiment indicated that two four-coordinate boron sites are present in the sample (Figure 3.20, B), one of which (6.6 ppm) is partly obscured by the quadrupole-broadened, three-coordinate, boron signal in the MAS spectrum. While the precise identities of the four-coordinate boron species were not determined, the chemical shifts were reportedly consistent with...

Table 3.4 B and NMR spectral data for pyridine complexes of B-substituted derivatives of 9-BBN [18], Spectra are recorded in CDClj solvent. No attempt is made to determine the chemical shifts of the very broad signals attributed to boron-bonded carbons...

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