Abstract
The formation of the F3B–NH3 supermolecule by chemical interaction of its fragment parts, BF3 and NH3, and the dynamics of internal rotation about the ‘B–N’ bond have been studied in terms of parameters provided by the molecular orbital and density functional theories. It is found that the pairs of frontier orbitals of the interacting fragments have matching symmetry and are involved in the charge transfer interaction. The donation process stems from the HOMO of the donor into the LUMO of the acceptor and simultaneously, back donation stems from the HOMO of acceptor into the LUMO of the donor. The density functional computation of chemical activation in the donor and acceptor fragments, associated with the physical process of structural reorganization just prior to the event of chemical reaction, indicates that BF3 becomes more acidic and NH3 becomes more basic, compared to their separate equilibrium states. Theoretically it is observed that the chemical reaction event of the formation of the supermolecule from its fragment parts is in accordance with the chemical potential equalization principle of the density functional theory and the electronegativity equalization principle of Sanderson. The energetics of the chemical reaction, the magnitude of the net charge transfer and the energy of the newly formed bond are quite consistent, both internally and with the principle of maximum hardness, PMH. The dynamics of the internal rotation of one part with respect to the other part of the supermolecule about the ‘B–N’ bond mimics the pattern of the conformational isomerism of the isostructural ethane molecule. It is also observed that the dynamics and evolution of molecular conformations as a function of dihedral angles is also in accordance with the principle of maximum hardness, PMH. Quite consistent with spectroscopic predictions, the height of the molecule’s barrier to internal rotation is very small. A rationale for the low height of the barrier has been put forward in terms of the energy partitioning analysis. On the question of origin of the barrier to internal rotation, we conclude that the conformational barrier to internal rotation does not originate from a particular region of the molecule, but rather it is a result of the subtle conjoint interplay of a number of opposing effects of one- and two-center bonded and nonbonded energy terms involving the entire skeleton of the molecule.
Highlights
Boron trifluoride, BF3, is a Lewis acid [1] and in Pearson’s HSAB classification [2] it is a hard acid
We have studied the formation of a well-known supermolecule like F3B–NH3 by the chemical interaction of its interacting donor and acceptor units, BF3 and NH3
We have invoked density functional theory and molecular orbital theory to rationalize the formation of the complex and to study the dynamics of the internal rotation of the molecule
Summary
BF3, is a Lewis acid [1] and in Pearson’s HSAB classification [2] it is a hard acid. The adduct of BF3 with NH3 was the first known coordination compound of any element [4] Lewis classified this important class of adduct molecules as electron pair donor-acceptor complexes [1]. Ghosh et al [7] have performed a comparative frontier orbital and density functional study of the variation of stability and reactivity of BF3 and BH3 molecules associated with their physical process of D3h to C3v structural reorganization prior to a chemical reaction event according to Figure 1 and have concluded that the hardness or softness is not an invariable static property but rather a dynamic variable property of the molecules
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