Abstract
The nitrogen nuclear quadrupole double-resonance spectra of several amine-trifluoroboranes are reported and compared with the corresponding data of the free amines and amine-boranes. The observed increase in the nitrogen coupling constants with increasing substitution of the nitrogen-bonded hydrogens by organic groups in the amine-borane coordinative compounds is interpreted in analogy to the case of the free amines as being due to the higher electronegativity of carbon compared to hydrogen. Corresponding H 3 B- and F 3 B compounds have similar nitrogen coupling constants in spite of the high electronegativity of fluorine. This experimental finding is in accord with several quantum-chemical calculations of the literature which show that equal charge transfers from the amine to the borane take place on formation of the hydrogen and fluorine compounds from their two constituents. Yet, the higher electronegativity of fluorine results in a stronger polarization of the amine which leads to a uniform increase in the population of all p orbitals centered at the nitrogen at the expense of the N substituents, the nitrogen thus serving as an electron buffer. The experimental nitrogen coupling constants can only give an upper limit of 0.45 e for the charge transfer from N to B which is hardly affected by the N substituents. The figures obtained theoretically vary considerably depending on the method of calculation employed and range from 0.2 e for ab initio methods to 0.4 e for CNDO/2 calculations.
Published Version
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