Boron Chemical Shifts

This ability to accommodate different electronic structures is exemplified by the open ten-vertex C2 cluster structure 5, which is the geometry found both for nido- 

Since initial considerations by Lipscomb and co-workers, who concluded that there was little correlation between 5( B) and total electron density on a polyhedral boron atom, and therefore that o contributions (Chapter 3, Section 1.2) were important, relatively few calculations related to nuclear shieldings in the polyhedral boranes have been attempted (see, for example. Refs. 78-80). Recent 

A major problem in the development of the use and interpretation of boron chemical shifts in polyhedral species is that of assignment. Although the assignments for many of the principal simple basic boranes and carboranes have been established, much smaller proportions of other heteroborane structural classes have been assigned. In simple cases this has been done on the basis of symmetry and relative intensity, but for more ambiguous cases painstaking isotopic labeling and substituent chemistry has had to be carried out. 

Early experiments in this area utilized B — B homonuclear CW INDOR spectroscopy (Chapter 2) at 80.2 MHz, applied to some derivatives of [B oHi4] and ortAo-[C2BjoHi2], and to some somewhat more demanding 18-vertex species. The experiments were very successful for the compounds examined, but unfortunately the work has not since been extended further. However, apart from the assignments, a number of important features, such as an effective absence of coupling 

An apical high-field resonance at 5 ca. — 53 ppm is also apparent for the four-connected B(l) atom in square pyramidal (schematic geometry 11) which 

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The boron-11 chemical shifts of some dimeric iminoboranes relative to F3B 0(C2Hs)2 are listed in Table 1. 

Table 1. Boron-11 chemical shift data of dimeric iminoboranes relative to external tri-fluoroborane-diethyletherate...

A ring current model has been shown to correlate proton and boron-11 chemical shifts in a number of pyramidal boron compounds including pentaborane(9) and its derivatives 180>. This model is analogous to the model used to explain proton chemical shifts in benzene and similar aromatic compounds. 

S. Kroeker and J. F. Stebbins. Three-coordinated Boron-11 chemical shifts in borates. Inorg. Chem. 40, 2001, 6239. 

So far, only one detailed discussion of boron-11 nuclear magnetic resonance spectra of aminoborane systems has been reported 31>. It was found that the 1 lB chemical shifts of aminoborane systems can be described fairly well in terms of a set of additive substituent contributions. In consonance with earlier work on trisubstituted boron compounds 35> these contributions depend on the mesomeric effects of substituents rather than their electronegativity. 1,3,2-diazaboracycloalkanes can be considered as aminoborane derivatives and in the case of the known heterocycles the exocyclic boron substituent will govern primarily the boron chemical shifts and will do so by mesomeric effects. However, the available data are rather limited and it may be possible that additional factors must be considered. Steric effects appear to be negligible, however, since the heterocycles with either six or seven annular atoms have almost identical shifts (Table 5). 

If 6 is reacted with chlorodimethoxyborane instead of dimethoxyborane, the primary product is 8d (established by comparison of its, 3C chemical shifts to those of 8a-c). In CHC13, 8d transforms spontaneously to 9a, probably via 5d as intermediate (compare 8a - 5b). The boron chemical shifts of 9a are close to those calculated for 9auat GIAO- SCF/6-31G //MP2/6-31G. 7... 

Tickling experiments on and B in BH4 and BF4 have been reported. (372) They show that the primary isotope effect upon the boron chemical shift is only 0-11 0-03 ppm, which is negligible for most practical purposes. 

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. 

P. D. Ellis, Y. C. Chou, and P. A. Dobosh, /. Magn. Reson., 1980, 39, 529. Semiempirical theory of boron chemical shifts utilising gauge-invariant atomic orbitals. 

Table 5. Boron Chemical Shifts of Perhaloborane Species"...

D. L. Bryce, R. E. Wasylishen and M. Gee. Characterization of tricoordinate boron chemical shift tensors Definitive high-field solid-state NMR evidence for anisotropic boron shielding. J. Phys. Chem. A 105, 2001, 3633. 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

The 3IP NMR chemical shifts, shown in Table IV, should be a useful indicator of the electronic environment at phosphorus. Almost all the 3IP chemical shifts are upfield (negative 8 values) in contrast to the boron-phosphorus compounds where downfield shifts (positive 8 values) are observed. This can be interpreted in terms of weak gallium-phosphorus it-... 

From the chemical shifts of the carbon atom of the double bond and the directly substituted boron atoms, the bis[bis(trimethylsilyl)amino]-substituted germene, shown below, appears to have significant ylide character.12 This molecule is also significantly twisted by 36° about the Ge—E axis. 

Although no 73Ge NMR spectra of stable doubly bonded germanium compounds have been recorded, NMR chemical shift data for the heteroatom of the double bond are available these are listed in Table II. The chemical shift of the, sp2-hybridized carbon atom of dimesitylneopentylger-mene (entry 1, Table II), without any polar substituents, falls at 8 124.2.26 The chemical shifts of the boron-substituted sp2-hybridized carbon atoms of two stable germenes (entry 2, Table II) fall in the upfield region of the... 

Table 4 Boron and carbon chemical shifts in bridged bis-1 -boraadamantane complexes...

The boron atom has tetrahedral coordination in pyridine complexes of 1,3,2,5-dioxaboraphosphorinane sulfides and selenides. These compounds exist as a mixture of conformers and stereoisomers [Eq. (86)]. In some cases it appeared possible to isolate three individual stereoisomers of one substance. The chemical shift in 31P NMR spectra was shown to be stereospecific. 

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